- Professor, Hydrology / Atmospheric Sciences
- Professor, Global Change - GIDP
- Professor, Remote Sensing / Spatial Analysis - GIDP
Dr. Xubin Zeng is the Agnese N. Haury Endowed Chair in Environment and professor of atmospheric science at the University of Arizona (UA). He is also the Founding Director of the UA Climate Dynamics and Hydrometeorology Center (CDHC). Through over 180 peer-reviewed publications, Dr. Zeng's research interests include land-atmosphere-ocean interface processes, climate modeling, hydrometeorology, remote sensing, nonlinear dynamics, and big data analytics. He has given over 140 invited talks at conferences and institutions. His research products (including computer models, algorithms, and value-added global datasets) have been used by major national and international research centers and numerous groups worldwide. He also co-founded the Hydrometeorology M.S. and Ph.D. Program, which is the first such program in the U.S. He is a Fellow of the American Meteorological Society (AMS) and American Association for the Advancement of Science (AAAS), a Corresponding Member of the China Science Center of the International Eurasian Academy of Sciences, a Galileo Circle Fellow of the UA College of Science, and a Fellow of the UA Academic Leadership Institute. Dr. Zeng also received the Special Creativity Award in 2014 from the National Science Foundation. He has held numerous leadership positions at national and international organizations and committees, including service on the National Academies Board on Atmospheric Science and Climate.
- Ph.D. Atmospheric Science
- Colorado State University, Tucson, Colorado, USA
- M.S. Meteorology
- Chinese Academy of Sciences, Beijing, China
- B.S. Atmospheric Physics
- University of Nanjing, Nanjing, China
- University of Arizona, Tucson, Arizona (2014 - Ongoing)
- University of Arizona, Tucson, Arizona (2009 - 2011)
- University of Arizona, Tucson, Arizona (2008 - Ongoing)
- University of Arizona, Tucson, Arizona (2004 - Ongoing)
- University of Arizona, Arizona (2000 - 2004)
- University of Arizona, Arizona (1994 - 2000)
- Colorado State University, Ft. Collins, Colorado (1992 - 1994)
- Colorado State University, Ft. Collins, Colorado (1989 - 1992)
- Colorado State University, Ft. Collins, Colorado (1988 - 1989)
- American Association for the Advancement of Science (AAAS), Fall 2018
- American Meteorological Society, Spring 2011
- Outstanding Alum
- Colorado State University Department of Atmospheric Science, Fall 2018
- Fellow, UA Academic Leadership Institute
- University of Arizona, Fall 2015
- Agnese N. Haury Endowed Chair in Environment and Social Justice
- Agnese N. Haury Program, Fall 2014
- Outstanding Faculty Award
- University of Arizona's Asian American Faculty, Staff and Alumni Association (AAFSAA), Fall 2014
- NSF Special Creativity Award
- National Science Foundation, Spring 2014
- Elected member
- Executive Committee of American Meteorological Society, 2011-2013, Spring 2013
- Council (governing body) of American Meteorological Society, Spring 2010
- Honorary Commander
- 25th Operational Weather Squadron, Davis-Monthan Air Force Base, Tucson, AZ, January 2012- December 2014, Spring 2012
- Galileo Circle Fellow
- UA College of Science, Fall 2011
- Corresponding Member
- China Science Center of the International Eurasian Academy of Sciences, elected in 2008, Spring 2008
- Overseas Assessor
- appointed by the Chinese Academy of Sciences President, Spring 2007
Introduction to weather and climate, dynamic meteorology, micrometeorology, climate observations and modeling
Land-atmosphere-ocean interface processes, climate modeling, climate change, hydrometeorology, remote sensing, nonlinear dynamics, and big data analytics
DissertationATMO 920 (Fall 2020)
Dynamic Meteorology IATMO 441A (Fall 2020)
Dynamic Meteorology IATMO 541A (Fall 2020)
ResearchATMO 900 (Fall 2020)
Boundary Layer MeteorologyATMO 579 (Spring 2020)
Boundary Layer MeteorologyPTYS 579 (Spring 2020)
DissertationATMO 920 (Spring 2020)
ResearchATMO 900 (Spring 2020)
ThesisATMO 910 (Spring 2020)
ThesisHWRS 910 (Spring 2020)
DissertationATMO 920 (Fall 2019)
Dynamic Meteorology IATMO 441A (Fall 2019)
Dynamic Meteorology IATMO 541A (Fall 2019)
ResearchATMO 900 (Fall 2019)
ThesisHWRS 910 (Fall 2019)
DissertationATMO 920 (Spring 2019)
Gen Circulation Obsv+MdlATMO 595C (Spring 2019)
Independent StudyHWRS 499 (Spring 2019)
ResearchATMO 900 (Spring 2019)
ThesisATMO 910 (Spring 2019)
DissertationATMO 920 (Fall 2018)
Dynamic Meteorology IATMO 441A (Fall 2018)
Dynamic Meteorology IATMO 541A (Fall 2018)
Dynamic Meteorology IPTYS 541A (Fall 2018)
Independent StudyHWRS 499 (Fall 2018)
ResearchATMO 900 (Fall 2018)
DissertationATMO 920 (Spring 2018)
Dynamic Meteorology IATMO 441A (Fall 2017)
Dynamic Meteorology IATMO 541A (Fall 2017)
Dynamic Meteorology IPTYS 541A (Fall 2017)
DissertationATMO 920 (Spring 2017)
Gen Circulation Obsv+MdlATMO 595C (Spring 2017)
DissertationATMO 920 (Fall 2016)
Dynamic Meteorology IATMO 441A (Fall 2016)
Dynamic Meteorology IATMO 541A (Fall 2016)
ResearchATMO 900 (Fall 2016)
ThesisATMO 910 (Fall 2016)
Boundary Layer MeteorologyATMO 579 (Spring 2016)
DissertationATMO 920 (Spring 2016)
ResearchATMO 900 (Spring 2016)
- Troch, P. A., Zeng, X., Van Haren, J. L., Tuller, M., Sibayan, M., Schaap, M. G., Saleska, S. R., Ruiz, J., Rasmussen, C., Pohlmann, M. A., Pelletier, J. D., Monson, R. K., Maier, R. M., Kim, M., Huxman, T. E., Ferre, P. A., Durcik, M., DeLong, S. B., Cueva, A., , Chorover, J. D., et al. (2018). Controlled Experiments of Hillslope Coevolution at the Biosphere 2 Landscape Evolution Observatory: Toward Prediction of Coupled Hydrological, Biogeochemical, and Ecological Changes. In Hydrology of Artificial and Controlled Experiments. Rijeka, Croatia: IN TECH d.o.o.
- Tuller, M., Sibayan, M., Schaap, M. G., Saleska, S. R., Ruiz, J., Rasmussen, C., Pohlmann, M. A., Pelletier, J. D., Monson, R. K., Maier, R. M., Kim, M., Huxman, T. E., Ferre, P. A., Durcik, M., DeLong, S. B., Cueva, A., Chorover, J. D., Bugaj, A., Breshears, D. D., , Adams, J. R., et al. (2018). Controlled Experiments of Hillslope Coevolution at the Biosphere 2 Landscape Evolution Observatory: Toward Prediction of Coupled Hydrological, Biogeochemical, and Ecological Changes. In Hydrology of Artificial and Controlled Experiments(pp 25-74). Rijeka, Croatia: IntechOpen Limited.
- Volkmann, T. H., Sengupta, A., Pangle, L. A., Dontsova, K. M., Barron-Gafford, G. A., Harman, C. J., Niu, G., Meredith, L., Abramson, N., Alves Meira Neto, A., Wang, Y., Adams, J. R., Breshears, D. D., Bugaj, A., Chorover, J. D., Cueva, A., DeLong, S. B., Durcik, M., Ferre, P. A., , Huxman, T. E., et al. (2018). Controlled Experiments of Hillslope Coevolution at the Biosphere 2 Landscape Evolution Observatory: Toward Prediction of Coupled Hydrological, Biogeochemical, and Ecological Changes. In Hydrology of Artificial and Controlled Experiments. Rijeka, Croatia: IN TECH d.o.o. doi:10.5772/intechopen.72325More infoVolkmann, T.H.M., co-authors, X. Zeng, ad P.A. Troch, 2018: Controlled Experiments of Hillslope Coevolution at the Biosphere 2 Landscape Evolution Observatory: Toward Prediction of Coupled Hydrological, Biogeochemical, and Ecological Change. Chapter 2 in the book "Hydrology of Artificial and Controlled Experiments", doi: 10.5772/intechopen.72325.
- Huxman, T. E., Sibayan, M., Abramson, N., Bugaj, A., Barron-Gafford, G. A., Van Haren, J. L., Zeng, X., Chorover, J. D., Hunt, E., Neilson, J. W., Meira, A. A., Troch, P. A., Dontsova, K. M., Volkmann, T., Pangle, L., & Sengupta, A. (2017). Advancing understanding of hydrological and biogeochemical interactions in evolving landscapes through controlled experimentation and monitoring at the Landscape Evolution Observatory. In Terrestrial Ecosystem Research Infrastructures: Challenges, New Developments and Perspectives(pp 83-118). CRC Press.
- Volkmann, T. H., Troch, P. A., Sengupta, A., Zeng, X., Pangle, L. A., Van Haren, J. L., Tuller, M., Dontsova, K. M., Sibayan, M., Barron-Gafford, G. A., Harman, C. J., Schaap, M. G., Saleska, S. R., Niu, G., Ruiz, J., Meredith, L., Abramson, N., Rasmussen, C., Pohlmann, M. A., , Alves Meira Neto, A., et al. (2018). Controlled Experiments of Hillslope Coevolution at the Biosphere 2 Landscape Evolution Observatory: Toward Prediction of Coupled Hydrological, Biogeochemical, and Ecological Changes. In Hydrology of Artificial and Controlled Experiments. Rijeka, Croatia: IN TECH d.o.o. doi:10.5772/intechopen.72325More infoVolkmann, T.H.M., co-authors, X. Zeng, ad P.A. Troch, 2018: Controlled Experiments of Hillslope Coevolution at the Biosphere 2 Landscape Evolution Observatory: Toward Prediction of Coupled Hydrological, Biogeochemical, and Ecological Change. Chapter 2 in the book "Hydrology of Artificial and Controlled Experiments", doi: 10.5772/intechopen.72325.
- Yang, Z., Dominguez, F., Gupta, H. V., Zeng, X., & Norman, L. (2017). Potential impacts of the continuing urbanization on regional climate: The developing Phoenix-Tucson "Sun Corridor".. In Water Bankruptcy In The Land Of Plenty: Steps towards a transatlantic and transdisciplinary assessment of water scarcity in Southern Arizona. CRC Press.More infoYang, Z., F. Dominguez, H. Gupta, X. Zeng, and L. Norman, 2017: Potential impacts of the continuing urbanization on regional climate: The developing Phoenix-Tucson "Sun Corridor". Chapter 11 (p. 179-193) of the book "Water Bankruptcy In The Land Of Plenty: Steps towards a transatlantic and transdisciplinary assessment of water scarcity in Southern Arizona" edited by F. Poupeau, H. Gupta, A. Serrat-Capdevilla, M.A. Sans-Fuentes, S. Harris, and L. G. Hayde, CRC Press, Boca Raton, FL, p. 437.
- Poupeau, F., Gupta, H. V., Sans-Fuentes, M. A., & Serrat-Capdevilla, A. (2016). Next Steps: Collaborative research and training towards transdisciplinarity. In Water Bankruptcy In The Land Of Plenty: Steps towards a transatlantic and transdisciplinary assessment of water scarcity in Southern Arizona. CRC Press.More infoF Poupeau, Gupta HV, MA Sans-Fuentes & A Serrat-Capdevilla (2016 Invited), Next Steps: Collaborative research and training towards transdisciplinarity; Chapter 22 in: Water Bankruptcy In The Land Of Plenty: Steps towards a transatlantic and transdisciplinary assessment of water scarcity in Southern Arizona, Eds: Gupta HV, F Poupeau, MA Sans-Fuentes & A Serrat-Capdevilla, CRC Press.
- Yang, Z., Dominguez, F., Gupta, H. V., Zeng, X., & Norman, L. (2016). Potential Impacts of the developing Phoenix-Tucson ‘Sun’ Corridor on Regional Climate. In Water Bankruptcy In The Land Of Plenty: Steps towards a transatlantic and transdisciplinary assessment of water scarcity in Southern Arizona. CRC Press.More infoYang Z, F Dominguez, H Gupta, X Zeng and L Norman (2016 Invited) Potential Impacts of the developing Phoenix-Tucson ‘Sun’ Corridor on Regional Climate; Chapter 11 in: Water Bankruptcy In The Land Of Plenty: Steps towards a transatlantic and transdisciplinary assessment of water scarcity in Southern Arizona, Eds: Gupta HV, F Poupeau, MA Sans-Fuentes & A Serrat-Capdevilla, CRC Press.
- Niu, G., & Zeng, X. (2012). The Earth System Model, Modeling the land component of. In Encyclopedia of Sustainability Science and Technology. doi: 10.1007/978-1-4419-0851-3.More infoR.A. Meyers, Ed.
- Claussen, M., Cox, P., Zeng, X., & other coauthors, s. (2003). The global climate. In Vegetation, Water, Humans and the Climate(pp 38-72). Springer.More infoP. Kabat, and nine co-editors, Eds.
- Zeng, X., & He, Y. (2001). Observational and modeling study on the relationship between sea surface temperature and evaporation over the tropical Pacific. In Dynamics of Atmospheric and Oceanic Circulation and Climate(pp 508-520). China Meteorological Press.
- Pielke, R., Dalu, G., Eastman, J., Vidale, P., & Zeng, X. (1998). Boundary layer processes and land surface interactions on the mesoscale. In Clear and Cloudy Boundary Layers(pp 155-176). Royal Netherlands Academy of Arts and Sciences.More infoA. A. M. Holtslag and P. G. Duynkerke, Eds.
- Pielke, R., Nicholls, M., Nygaard, T., Walko, R., & Zeng, X. (1997). Several unresolved issues in numerical modeling of geophysical flows. In Numerical Methods in Atmospheric and Oceanic Modeling, The Andre J. Robert Memorial Volume(pp 557-581).More infoC. A. Lin, R. Laprise, and H. Ritchie, Eds.
- Pielke, R., other coauthors, s., & Zeng, X. (1996). Use of mesoscale models for simulation of seasonal weather and climate change for the Rocky Mountain States. In GIS and Environmental Modeling: Progress and Research Issues(pp 99-103).More infoM.F. Goodchild, Ed., GIS World
- Pielke, R., Eastman, J., Grasso, L., Knowles, J., Nicholls, M., Walko, R., & Zeng, X. (1995). Atmospheric Vortices. In Fluid Vortices(pp 617-650). Kluwer Academic Publishing.More infoS.I. Green, Ed.
- Balsamo, G., Agusti-Panareda, A., Albergel, C., Arduini, G., Beljaars, A., Bidlot, J., Blyth, E., Bousserez, N., Boussetta, S., Brown, A., Buizza, R., Buontempo, C., Chevallier, F., Choulga, M., Cloke, H., Cronin, M. F., Dahoui, M., De, R. P., Dirmeyer, P. A., , Drusch, M., et al. (2019). Observations for Advancing Global Earth Surface Modelling: A Review (vol 10, 2038, 2018). REMOTE SENSING, 11(8).
- Brunke, M. A., Ma, P., Eyre, J., Rasch, P. J., Sorooshian, A., & Zeng, X. (2019). Subtropical Marine Low Stratiform Cloud Deck Spatial Errors in the E3SMv1 Atmosphere Model. GEOPHYSICAL RESEARCH LETTERS, 46(21), 12598-12607.
- Davis, K., & Zeng, X. (2019). Seasonal Prediction of North Atlantic Accumulated Cyclone Energy and Major Hurricane Activity. WEATHER AND FORECASTING, 34(1), 221-232.
- Eyre, J., Van, R. L., Zeng, X., Brunke, M. A., & Golaz, J. (2019). Ocean Barrier Layers in the Energy Exascale Earth System Model. GEOPHYSICAL RESEARCH LETTERS, 46(14), 8234-8243.
- Fan, Y., Clark, M., Lawrence, D. M., Swenson, S., Band, L. E., Brantley, S. L., Brooks, P. D., Dietrich, W. E., Flores, A., Grant, G., Kirchner, J. W., Mackay, D. S., McDonnell, J. J., Milly, P., Sullivan, P. L., Tague, C., Ajami, H., Chaney, N., Hartmann, A., , Hazenberg, P., et al. (2019). Hillslope Hydrology in Global Change Research and Earth System Modeling. WATER RESOURCES RESEARCH, 55(2), 1737-1772.
- Golaz, J., Caldwell, P. M., Van, R., Petersen, M. R., Tang, Q. i., Wolfe, J. D., Abeshu, G., Anantharaj, V., Asay-Davis, X. S., Bader, D. C., Baldwin, S. A., Bisht, G., Bogenschutz, P. A., Branstetter, M., Brunke, M. A., Brus, S. R., Burrows, S. M., Cameron-Smith, P. J., Donahue, A. S., , Deakin, M., et al. (2019). The DOE E3SM Coupled Model Version 1: Overview and Evaluation at Standard Resolution. JOURNAL OF ADVANCES IN MODELING EARTH SYSTEMS, 11(7), 2089-2129.
- Lawrence, D. M., Fisher, R. A., Koven, C. D., Oleson, K. W., Swenson, S. C., Bonan, G., Collier, N., Ghimire, B., van, K. L., Kennedy, D., Kluzek, E., Lawrence, P. J., Li, F., Li, H., Lombardozzi, D., Riley, W. J., Sacks, W. J., Shi, M., Vertenstein, M., , Wieder, W. R., et al. (2019). The Community Land Model Version 5: Description of New Features, Benchmarking, and Impact of Forcing Uncertainty. JOURNAL OF ADVANCES IN MODELING EARTH SYSTEMS.
- Liu, S., Zeng, X., Dai, Y., & Shao, Y. (2019). Further Improvement of Surface Flux Estimation in the Unstable Surface Layer Based on Large-Eddy Simulation Data. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 124(17-18), 9839-9854.
- Sorooshian, A., Anderson, B., Bauer, S. E., Braun, R. A., Cairns, B., Crosbie, E., Dadashazar, H., Diskin, G., Ferrare, R., Flagan, R. C., Hair, J., Hostetler, C., Jonsson, H. H., Kleb, M. M., Liu, H., MacDonald, A. B., McComiskey, A., Moore, R., Painemal, D., , Russell, L. M., et al. (2019). Aerosol-Cloud-Meteorology Interaction Airborne Field Investigations: Using Lessons Learned from the US West Coast in the Design of ACTIVATE off the US East Coast. BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY, 100(8), 1511-1528.
- Wang, Y., Broxton, P., Fang, Y., Behrangi, A., Barlage, M., Zeng, X., & Niu, G. (2019). A Wet-Bulb Temperature-Based Rain-Snow Partitioning Scheme Improves Snowpack Prediction Over the Drier Western United States. GEOPHYSICAL RESEARCH LETTERS.
- Yang, Z., Dominguez, F., & Zeng, X. (2019). Large and local-scale features associated with heat waves in the United States in reanalysis products and the NARCCAP model ensemble. CLIMATE DYNAMICS, 52(3-4), 1883-1901.
- Yu, L., Yang, Q., Zhou, M., Zeng, X., Lenschow, D. H., Wang, X., & Han, B. o. (2019). The Intraseasonal and Interannual Variability of Arctic Temperature and Specific Humidity Inversions. ATMOSPHERE, 10(4).
- Balsamo, G., Coauthors, ., & Zeng, X. (2018). Satellite and in-situ observations for advancing global Earth surface modelling: a review. Remote Sens.. doi:10.3390/rs10122038More infoBalsamo, G., and coauthors, 2018: Satellite and in-situ observations for advancing global Earth surface modelling: a review, Remote Sens., 10(12), 2038; https://doi.org/10.3390/rs10122038.
- Brunke, M. A., Cassano, J. J., Dawson, N., DuVivier, A. K., Gutowski, Jr., W. J., Hamman, J., Maslowski, W., Nijssen, B., Renteria, J. C., Roberts, A., & Zeng, X. (2018). Evaluation of Atmosphere-Land-Ocean-Sea Ice Interface Processes in the Regional Arctic System Model Version 1.0 (RASM1.0). JOURNAL OF ADVANCES IN MODELING EARTH SYSTEMS, 11. doi:10.5194/gmd-11-4817-2018
- Davis, K., & Zeng, X. (2018). Seasonal Prediction of North Atlantic Accumulated Cyclone Energy and Major Hurricane Activity. Weather and Forecasting. doi:10.1175/WAF-D-18-0125.1
- Dawson, N., Broxton, P., & Zeng, X. (2018). Evaluation of Remotely Sensed Snow Water Equivalent and Snow Cover Extent over the Contiguous United States. JOURNAL OF HYDROMETEOROLOGY, 19(11), 1777-1791.
- Qi, J., Zhang, X., McCarty, G. W., Sadeghi, A. M., Cosh, M. H., Zeng, X., Gao, F., Daughtry, C., Huang, C., Lang, M. W., & Arnold, J. G. (2018). Assessing the performance of a physically-based soil moisture module integrated within the Soil and Water Assessment Tool. ENVIRONMENTAL MODELLING & SOFTWARE, 109, 329-341.
- Stillman, S., & Zeng, X. (2018). Evaluation of SMAP Soil Moisture Relative to Five Other Satellite Products Using the Climate Reference Network Measurements Over USA. IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, 56(11), 6296-6305.
- Wang, A., & Zeng, X. (2018). 10.3390/rs10122038. Atmospheric and Oceanic Science Letters. doi:10.1080/16742834.2017.1379865
- Welty, J., & Zeng, X. (2018). Does Soil Moisture Affect Warm Season Precipitation Over the Southern Great Plains?. GEOPHYSICAL RESEARCH LETTERS, 45(15), 7866-7873.
- Yang, Z., Dominguez, F., & Zeng, X. (2018). Large and Local Scale Features Associated with Heat Waves in the United States in MERRA-2 and the NARCCAP model Ensemble. Climate Dynamics. doi:10.1007/s00382-018-4414-xMore infoYang, Z., F. Dominguez, and X. Zeng, 2018: Large and Local Scale Features Associated with Heat Waves in the United States in MERRA-2 and the NARCCAP model Ensemble, Climate Dynamics, doi: 10.1007/s00382-018-4414-x.
- Zeng, X., Broxton, P., & Dawson, N. (2018). Snowpack Change from 1982-2016 over Conterminous United States. Geosphys. Res. Lett.. doi:10.1029/2018GL079621
- Zeng, X., Klocke, D., Shipway, B. J., Singh, M. S., Sandu, I., Hannah, W., Bogenschutz, P., Zhang, Y., Morrison, H., Pritchard, M., & Rio, C. (2018). FUTURE COMMUNITY EFFORTS IN UNDERSTANDING AND MODELING ATMOSPHERIC PROCESSES. BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY, 99(9), ES159-ES162.
- Zeng, X., Roberts, A., Renteria, J. C., Reeves Eyre, J. E., Nijssen, B., Maslowski, W., Hamman, J., Gutowski, Jr., W. J., DuVivier, A. K., Dawson, N., Cassano, J. J., & Brunke, M. A. (2018). Evaluation of Atmosphere-Land-Ocean-Sea Ice Interface Processes in the Regional Arctic System Model Version 1.0 (RASM1.0) Using Local and Globally Gridded Observations. Geoscientific Model Development, 11, 4817-4841. doi:10.5194/gmd-11-4817-2018
- Bashir, F., Bashir, F., Zeng, X., Zeng, X., Gupta, H. V., Gupta, H. V., Hazenberg, P., & Hazenberg, P. (2017). A Hydro-meteorological Perspective on the Karakoram Anomaly using Unique Valley-based Synoptic Weather Observations. Geophysical Research Letters.More infoBashir F, X Zeng, H Gupta and P Hazenberg (2017), A Hydro-meteorological Perspective on the Karakoram Anomaly using Unique Valley-based Synoptic Weather Observations, Geophysical Research Letters, Article ID: GRL56536, DOI: 10.1002/2017GL075284
- Bashir, F., Zeng, X., Gupta, H., & Hazenberg, P. (2017). A Hydrometeorological Perspective on the Karakoram Anomaly Using Unique Valley-Based Synoptic Weather Observations. GEOPHYSICAL RESEARCH LETTERS, 44(20), 10470-10478.
- Broxton, P. D., Zeng, X., & Dawson, N. (2017). The Impact of a Low Bias in Snow Water Equivalent Initialization on CFS Seasonal Forecasts. JOURNAL OF CLIMATE, 30(21), 8657-8671.
- Cassano, J. J., DuVivier, A., Roberts, A., Hughes, M., Seefeldt, M., Brunke, M., Craig, A., Fisel, B., Gutowski, W., Hamman, J., Higgins, M., Maslowski, W., Nijssen, B., Osinski, R., & Zeng, X. (2017). Development of the Regional Arctic System Model (RASM): Near-Surface Atmospheric Climate Sensitivity. JOURNAL OF CLIMATE, 30(15), 5729-5753.
- Dadashazar, H., Wang, Z., Crosbie, E., Brunke, M., Zeng, X., Jonsson, H., Woods, R. K., Flagan, R. C., Seinfeld, J. H., & Sorooshian, A. (2017). Relationships between giant sea salt particles and clouds inferred from aircraft physicochemical data. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 122(6), 3421-3434.
- Dawson, N., Broxton, P., & Zeng, X. (2017). A New Snow Density Parameterization for Land Data Initialization. JOURNAL OF HYDROMETEOROLOGY, 18(1), 197-207.
- Eyre, J., & Zeng, X. (2017). Evaluation of Greenland near surface air temperature datasets. CRYOSPHERE, 11(4), 1591-1605.
- Restrepo-Coupe, N., Levine, N. M., Christoffersen, B. O., Albert, L. P., Wu, J., Costa, M. H., Galbraith, D., Imbuzeiro, H., Martins, G., da, A., Malhi, Y. S., Zeng, X., Moorcroft, P., & Saleska, S. R. (2017). Do dynamic global vegetation models capture the seasonality of carbon fluxes in the Amazon basin? A data-model intercomparison. GLOBAL CHANGE BIOLOGY, 23(1), 191-208.
- Wang, A., & Zeng, X. (2017). Impacts of the internal climate variability on the meteorological drought changes in China. Atmospheric and Oceanic Science Letters. doi:10.1080/16742834.2017.1379865
- Yang, Z., Dominguez, F., Zeng, X., Hu, H., Gupta, H. V., & Yang, B. (2017). Impact Of Irrigation Over The California Central Valley On Regional Climate. Journal of Hydrometeorology.More infoYang Z, F Dominguez, X Zeng, H Hu, H Gupta and B Yang (In review), Impact Of Irrigation Over The California Central Valley On Regional Climate, Journal of Hydrometeorology
- Yang, Z., Dominguez, F., Zeng, X., Hu, H., Gupta, H., & Yang, B. (2017). Impact of Irrigation over the California Central Valley on Regional Climate. JOURNAL OF HYDROMETEOROLOGY, 18(5), 1341-1357.
- van, H. J., Dontsova, K., Barron-Gafford, G. A., Troch, P. A., Chorover, J., Delong, S. B., Breshears, D. D., Huxman, T. E., Pelletier, J. D., Saleska, S. R., Zeng, X., & Ruiz, J. (2017). CO2 diffusion into pore spaces limits weathering rate of an experimental basalt landscape. GEOLOGY, 45(3), 203-206.
- Broxton, P. D., Dawson, N., & Zeng, X. (2016). Linking snowfall and snow accumulation to generate spatial maps of SWE and snow depth. EARTH AND SPACE SCIENCE, 3(6), 246-256.
- Broxton, P. D., Zeng, X., & Dawson, N. (2016). Why Do Global Reanalyses and Land Data Assimilation Products Underestimate Snow Water Equivalent?. JOURNAL OF HYDROMETEOROLOGY, 17(11), 2743-2761.
- Brunke, M. A., Broxton, P., Pelletier, J., Gochis, D., Hazenberg, P., Lawrence, D. M., Leung, L. R., Niu, G., Troch, P. A., & Zeng, X. (2016). Implementing and Evaluating Variable Soil Thickness in the Community Land Model, Version 4.5 (CLM4.5). JOURNAL OF CLIMATE, 29(9), 3441-3461.
- Crosbie, E., Wang, Z., Sorooshian, A., Chuang, P. Y., Craven, J. S., Coggon, M. M., Brunke, M., Zeng, X., Jonsson, H., Woods, R. K., Flagan, R. C., & Seinfeld, J. H. (2016). Stratocumulus Cloud Clearings and Notable Thermodynamic and Aerosol Contrasts across the Clear-Cloudy Interface. JOURNAL OF THE ATMOSPHERIC SCIENCES, 73(3), 1083-1099.
- Dawson, N., Broxton, P., Zeng, X., Leuthold, M., Barlage, M., & Holbrook, P. (2016). An Evaluation of Snow Initializations in NCEP Global and Regional Forecasting Models. JOURNAL OF HYDROMETEOROLOGY, 17(6), 1885-1901.
- Hamman, J., Nijssen, B., Brunke, M., Cassano, J., Craig, A., DuVivier, A., Hughes, M., Lettenmaier, D. P., Maslowski, W., Osinski, R., Roberts, A., & Zeng, X. (2016). Land Surface Climate in the Regional Arctic System Model. JOURNAL OF CLIMATE, 29(18), 6543-6562.
- Hazenberg, P., Broxton, P., Gochis, D., Niu, G. -., Pangle, L. A., Pelletier, J. D., Troch, P. A., & Zeng, X. (2016). Testing the hybrid-3-D hillslope hydrological model in a controlled environment. WATER RESOURCES RESEARCH, 52(2), 1089-1107.
- Lytle, W., & Zeng, X. (2016). Coupled Evaluation of Below-and Aboveground Energy and Water Cycle Variables from Reanalysis Products over Five Flux Tower Sites in the United States. JOURNAL OF HYDROMETEOROLOGY, 17(7), 2105-2119.
- Pelletier, J. D., Broxton, P. D., Hazenberg, P., Zeng, X., Troch, P. A., Niu, G., Williams, Z., Brunke, M. A., & Gochis, D. (2016). A gridded global data set of soil, intact regolith, and sedimentary deposit thicknesses for regional and global land surface modeling. JOURNAL OF ADVANCES IN MODELING EARTH SYSTEMS, 8(1), 41-65.
- Sakaguchi, K., Zeng, X., Leung, R. B., & Shao, P. (2016). Influence of dynamic vegetation on carbon-nitrogen cycle feedback in the Community Land Model (CLM4). Environ. Res. Lett., 11. doi:10.1088/1748-9326/aa51d9
- Sengupta, A., Wang, Y., Meira, A. A., Matos, K., Dontsova, K. M., Root, R., Neilson, J. W., Chorover, J. D., Maier, R. M., & Troch, P. A. (2016). Soil Lysimeter Excavation for Coupled Hydrological, Geochemical, and Microbiological Investigations. JoVE, e54536. doi:10.3791/54536
- Stillman, S., & Zeng, X. (2016). Development of a 0.5 degrees global monthly raining day product from 1901 to 2010. GEOPHYSICAL RESEARCH LETTERS, 43(18), 9704-9711.
- Stillman, S., Zeng, X., & Bosilovich, M. G. (2016). Evaluation of 22 Precipitation and 23 Soil Moisture Products over a Semiarid Area in Southeastern Arizona. JOURNAL OF HYDROMETEOROLOGY, 17(1), 211-230.
- Wang, A., Zeng, X., & Guo, D. (2016). Estimates of Global Surface Hydrology and Heat Fluxes from the Community Land Model (CLM4.5) with Four Atmospheric Forcing Datasets. JOURNAL OF HYDROMETEOROLOGY, 17(9), 2493-2510.
- Yang, Z., Dominguez, F., Gupta, H. V., Zeng, X., & Norman, L. (2016). Urban Effects on Regional climate: A Case Study in the Phoenix and Tucson ‘Sun’ Corridor. Earth Interactions, 21. doi:doi:10.1175/EI-D-15-0027.1More infoYang, Z., F. Dominguez, H. Gupta, X. Zeng, and L. Norman, 2016: Urban Effects on Regional Climate: A Case Study in the Phoenix and Tucson 'Sun' Corridor. Earth Interactions, 21, doi: 10.1175/EI-D-15-0027.1
- Yang, Z., Dominguez, F., Gupta, H., Zeng, X., & Norman, L. (2016). Urban Effects on Regional Climate: A Case Study in the Phoenix and Tucson "Sun Corridor''. EARTH INTERACTIONS, 20.
- Zeng, X., & Geil, K. (2016). Global warming projection in the 21st century based on an observational data-driven model. GEOPHYSICAL RESEARCH LETTERS, 43(20), 10947-10954.
- Zeng, X., Ackerman, S., Ferraro, R. D., Lee, T. J., Murray, J. J., Pawson, S., Reynolds, C., & Teixeira, J. (2016). Challenges and opportunities in NASA weather research. Bull. Amer. Meteor. Soc, 97, ES137-140. doi:10.1175/BAMS-D-15-00195
- Brunke, M., Stegall, S., & Zeng, X. (2015). A climatology of tropospheric humidity inversions in five reanalyses. Atmos. Res, 153, 165-187. doi:10.1016/j.atmosres.2014.08.005
- Davis, K., Zeng, X., & Ritchie, E. A. (2015). A New Statistical Model for Predicting Seasonal North Atlantic Hurricane Activity. WEATHER AND FORECASTING, 30(3), 730-741.
- Geil, K. L., & Zeng, X. (2015). Quantitative characterization of spurious numerical oscillations in 48 CMIP5 models. GEOPHYSICAL RESEARCH LETTERS, 42(12), 5066-5073.
- Hazenberg, P., Fang, Y., Broxton, P., Gochis, D., Niu, G. -., Pelletier, J. D., Troch, P. A., & Zeng, X. (2015). A hybrid-3D hillslope hydrological model for use in Earth system models. WATER RESOURCES RESEARCH, 51(10), 8218-8239.
- Pangle, L. A., Pangle, L. A., Delong, S. B., Delong, S. B., Abramson, N., Abramson, N., Adams, J., Adams, J., Barron-Gafford, G. A., Barron-Gafford, G. A., Breshears, D. D., Breshears, D. D., Brooks, P. D., Brooks, P. D., Chorover, J. D., Chorover, J. D., Dietrich, W. E., Dietrich, W. E., Dontsova, K. M., , Dontsova, K. M., et al. (2015). The Landscape Evolution Observatory: A large-scale controllable infrastructure to study Earth-surface processes.. Geomorphology, 244, 190-203.
- Pangle, L., DeLong, S., Abramson, N., Adams, J., Barron-Gafford, G. A., Breshears, D. D., Brooks, P. D., Chorover, J. D., Dietrich, W. E., Dontsova, K. M., Durcik, M., Espeleta, J., Ferre, P. A., Ferriere, R. H., Henderson, W., Hunt, E., Huxman, T. E., Millar, D., Murphy, B., , Niu, Y., et al. (2015). The Landscape Evolution Observatory: A large-scale controllable infrastructure to study coupled Earth-surface processes. Geomorphology.
- Wang, A., & Zeng, X. (2015). Global hourly land surface air temperature datasets: inter-comparison and climate change. INTERNATIONAL JOURNAL OF CLIMATOLOGY, 35(13), 3959-3968.
- Wang, A., & Zeng, X. (2015). Range of monthly mean hourly land surface air temperature diurnal cycle over high northern latitudes. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 119(10), 5836-5844.More infoDaily maximum and minimum temperatures over global land are fundamental climate variables, and their difference represents the diurnal temperature range (DTR). While the differences between the monthly averaged DTR (MDTR) and the range of monthly averaged hourly temperature diurnal cycle (RMDT) are easy to understand qualitatively, their differences have not been quantified over global land areas. Based on our newly developed in situ data (Climatic Research Unit) reanalysis (Modern-Era Retrospective analysis for Research and Applications) merged hourly temperature data from 1979 to 2009, RMDT in January is found to be much smaller than that in July over high northern latitudes, as it is much more affected by the diurnal radiative forcing than by the horizontal advection of temperature. In contrast, MDTR in January is comparable to that in July over high northern latitudes, but it is much larger than January RMDT, as it primarily reflects the movement of lower frequency synoptic weather systems. The area-averaged RMDT trends north of 40 degrees N are near zero in November, December, and January, while the trends of MDTR are negative. These results suggest the need to use both the traditional MDTR and RMDT suggested here in future observational and modeling studies. Furthermore, MDTR and its trend are more sensitive to the starting hour of a 24 h day used in the calculations than those for RMDT, and this factor also needs to be considered in model evaluations using observational data.
- Broxton, P. D., Zeng, X., Scheftic, W., & Troch, P. A. (2014). A MODIS-Based Global 1-km Maximum Green Vegetation Fraction Dataset. JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY, 53(8), 1996-2004.More infoGlobal land-cover data are widely used in regional and global models because land cover influences land atmosphere exchanges of water, energy, momentum, and carbon. Many models use data of maximum green vegetation fraction (MGVF) to describe vegetation abundance. MGVF products have been created in the past using different methods, but their validation with ground sites is difficult. Furthermore, uncertainty is introduced because many products use a single year of satellite data. In this study, a global 1-km MGVF product is developed on the basis of a "climatology" of data of Moderate Resolution Imaging Spectroradiometer (MODIS) normalized difference vegetation index and land-cover type, which removes biases associated with unusual greenness and inaccurate land-cover classification for individual years. MGVF shows maximum annual variability from 2001 to 2012 for intermediate values of average MGVF, and the standard deviation of MGVF normalized by its mean value decreases nearly monotonically as MGVF increases. In addition, there are substantial differences between this climatology and MGVF data from the MODIS Continuous Fields (CF) Collection 3, which is currently used in the Community Land Model. Although the CF data only use 2001 MODIS data, many of these differences cannot be explained by usage of different years of data. In particular, MGVF as based on CF data is usually higher than that based on the MODIS climatology from this paper. It is difficult to judge which product is more realistic because of a lack of ground truth, but this new MGVF product is more consistent than the CF data with the MODIS leaf area index product (which is also used to describe vegetation abundance in models).
- Broxton, P. D., Zeng, X., Sulla-Menashe, D., & Troch, P. A. (2014). A Global Land Cover Climatology Using MODIS Data. JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY, 53(6), 1593-1605.More infoGlobal land cover data are widely used in weather, climate, and hydrometeorological models. The Collection 5.1 Moderate Resolution Imaging Spectroradiometer (MOD'S) Land Cover Type (MCD12Q1) product is found to have a substantial amount of interannual variability, with 40% of land pixels showing land cover change one or more times during 2001-10. This affects the global distribution of vegetation if any one year or many years of data are used, for example, to parameterize land processes in regional and global models. In this paper, a value-added global 0.5-km land cover climatology (a single representative map for 2001-10) is developed by weighting each land cover type by its corresponding confidence score for each year and using the highest-weighted land cover type in each pixel in the 2001-10 MODIS data. The climatology is validated by comparing it with the System for Terrestrial Ecosystem Parameterization database as well as additional pixels that are identified from the Google Earth proprietary software database. When compared with the data of any individual year, this climatology does not substantially alter the overall global frequencies of most land cover classes but does affect the global distribution of many land cover classes. In addition, it is validated as well as or better than the MODIS data for individual years. Also, it is based on higher-quality data and is validated better than the Global Land Cover Characteristics database, which is based on 1 year of Advanced Very High Resolution Radiometer data and represents a widely used first-generation global product.
- Brunke, M. A., Stegall, S. T., & Zeng, X. (2014). A climatology of tropospheric humidity inversions in five reanalyses. ATMOSPHERIC RESEARCH, 153, 165-187.More infoSpecific humidity is generally thought to decrease with height in the troposphere. However, here we document the existence of specific humidity inversions in five reanalyses: the National Centers for Environmental Prediction (NCEP) second reanalysis (NCEP-2), the European Centre for Medium-Range Forecasts (ECMWF) 40-year reanalysis (ERA-40), the Modem Era Retrospective Analysis for Research Applications (MERRA), NCEP's Climate Forecast System Reanalysis (CFSR), and the ECMWF interim reanalysis (ERA-Interim). These inversions are most frequent in the polar regions. Inversions do occur elsewhere, most notably over the subtropical stratus regions, but are less frequent and likely overproduced depending on the location. Polar inversions are the most persistent in winter and the strongest (as defined by the humidity difference divided by the pressure-difference across the inversion) in summer or autumn with low bases (at pressures > 900 hPa). Winter humidity inversions are lower, being near-surface, due to the persistence of low-level temperature inversions associated with these humidity inversions, while summer humidity inversions tend to be located near cloud top providing moisture to prevent the melt season stratus from evaporating. The most important contributions to affect humidity inversions in MERRA are dynamics, turbulence, and moist physics. However, local advection may not play as much of a role as regional humidity convergence. The subtropical stratus inversions are as thick as polar humidity inversions but with higher bases generally at pressures
- Chorover, J. D., Gavaert, A., Tueling, A. J., Uijlenhoet, R., DeLong, S. B., Huxman, T. E., Pangle, L., Brashears, D. D., Pelletier, J. D., Saleska, S. R., Zeng, X., & Troch, P. A. (2014). Hillslope-scale experiment demonstrates the role of convergence during two-step saturation. Hydrol. Earth Syst. Sci., 18, 1-12.
- Christoffersen, B. O., Restrepo-Coupe, N., Arain, M. A., Baker, I. T., Cestaro, B. P., Ciais, P., Fisher, J. B., Galbraith, D., Guan, X., Gulden, L., van den Hurk, B., Ichii, K., Imbuzeiro, H., Jain, A., Levine, N., Miguez-Machor, G., Poulter, B., Roberti, D. R., Sakaguchi, K., , Sahoo, A., et al. (2014). Mechanisms of water supply and vegetation demand govern the seasonality and magnitude of evapotranspiration in Amazonia and Cerrado. AGRICULTURAL AND FOREST METEOROLOGY, 191, 33-50.More infoEvapotranspiration (E) in the Amazon connects forest function and regional climate via its role in precipitation recycling However, the mechanisms regulating water supply to vegetation and its demand for water remain poorly understood, especially during periods of seasonal water deficits In this study, we address two main questions: First, how do mechanisms of water supply (indicated by rooting depth and groundwater) and vegetation water demand (indicated by stomatal conductance and intrinsic water use efficiency) control evapotranspiration (E) along broad gradients of climate and vegetation from equatorial Amazonia to Cerrado, and second, how do these inferred mechanisms of supply and demand compare to those employed by a suite of ecosystem models? We used a network of eddy covariance towers in Brazil coupled with ancillary measurements to address these questions With respect to the magnitude and seasonality of E, models have much improved in equatorial tropical forests by eliminating most dry season water limitation, diverge in performance in transitional forests where seasonal water deficits are greater, and mostly capture the observed seasonal depressions in E at Cerrado However, many models depended universally on either deep roots or groundwater to mitigate dry season water deficits, the relative importance of which we found does not vary as a simple function of climate or vegetation In addition, canopy stomatal conductance (g's) regulates dry season vegetation demand for water at all except the wettest sites even as the seasonal cycle of E follows that of net radiation In contrast, some models simulated no seasonality in gs, even while matching the observed seasonal cycle of E. We suggest that canopy dynamics mediated by leaf phenology may play a significant role in such seasonality, a process poorly represented in models Model bias in gs and E, in turn, was related to biases arising from the simulated light response (gross primary productivity, GPP) or the intringic water use efficiency of photosynthesis (iWUE). We identified deficiencies in models which would not otherwise be apparent based on a simple comparison of simulated and observed rates of E. While some deficiencies can be remedied by parameter tuning, in most models they highlight the need for continued process development of belowground hydrology and in particular, the biological processes of root dynamics and leaf phenology, which via their controls on E, mediate vegetation-climate feedbacks in the tropics. (C) 2014 Elsevier B.V. All rights reserved.
- Niu, G., Paniconi, C., Troch, P. A., Scott, R. L., Durcik, M., Zeng, X., Huxman, T., & Goodrich, D. C. (2014). An integrated modelling framework of catchment- scale ecohydrological processes: 1. Model description and tests over an energy- limited watershed. ECOHYDROLOGY, 7(2), 427-439.More infoThe interactions between atmospheric, hydrological, and ecological processes at various spatial and temporal scales are not fully represented in most ecohydrological models. This first of a two-part paper documents a fully integrated catchment-scale ecohydrological model consisting of a three-dimensional physically based hydrological model and a land surface model. This first part also presents a first application to test the model over an energy-limited catchment (8.4km(2)) of the Sleepers River watershed in Vermont. The physically based hydrological model (CATchment HYdrology, CATHY) describes three-dimensional subsurface flow in variably saturated porous media and surface routing on hillslopes and in stream channels, whereas the land surface model (LSM), an augmented version of Noah LSM with multiple parameterization schemes (NoahMP), accounts for energy, water, and carbon flux exchanges between various land surface elements and the atmosphere. CATHY and NoahMP are coupled through exchanges of water fluxes and states. In the energy-limited catchment of the Sleepers River watershed, where snowmelt runoff generation is the dominant hydrologic flux, the coupled CATHY/NoahMP model at both 90 and 30-m surface grid resolutions, with minimal calibration, performs well in simulating the observed snow accumulation, and melt and subsequent snowmelt discharge. The Nash-Sutcliffe model efficiency of daily discharge is above 0.82 for both resolutions. The simulation at 90-m resolution shows a marginal improvement over that at 30-m resolution because of more elaborate calibration of model parameters. The coupled CATHY/NoahMP also shows a capability of simulating surface-inundated area and distributed surface water height, although the accuracy of these simulations needs further evaluation. The CATHY/NoahMP model is thus also a potentially useful research tool for predicting flash flood and lake dynamics under climatic change. Copyright (c) 2013 John Wiley & Sons, Ltd.
- Niu, G., Pasetto, D., Scudeler, C., Paniconi, C., Putti, M., Troch, P. A., DeLong, S. B., Dontsova, K., Pangle, L., Breshears, D. D., Chorover, J., Huxman, T. E., Pelletier, J., Saleska, S. R., & Zeng, X. (2014). Incipient subsurface heterogeneity and its effect on overland flow generation – insight from a modeling study of the first experiment at the Biosphere 2 Landscape Evolution Observatory. Hydrology and Earth System Sciences.
- Niu, G., Troch, P. A., Paniconi, C., Scott, R. L., Durcik, M., Zeng, X., Huxman, T., Goodrich, D., & Pelletier, J. (2014). An integrated modelling framework of catchment- scale ecohydrological processes: 2. The role of water subsidy by overland flow on vegetation dynamics in a semi- arid catchment. ECOHYDROLOGY, 7(2), 815-827.More infoIn water-limited regions, surface water and carbon fluxes are strongly controlled by soil water availability, which may be highly variable at very small spatial scales (e.g. metres) because of variations in terrain, soils, and vegetation conditions and to processes of water redistribution along hillslopes. This second of a two-part paper first evaluates the performance of a newly developed ecohydrological model over a small semi-arid experimental catchment (792ha) in southeastern Arizona. Secondly, it investigates the effects of soil properties on water subsidy resulting from lateral overland flow re-infiltration and on overall ecohydrological response. With optimized parameters, the model shows a higher ability to simulate surface energy and water fluxes than CO2 fluxes at all temporal scales. The model simulates observed CO2 fluxes fairly well at diurnal scales during the main growing seasons and the interannual variability of these fluxes in response to soil moisture variations from drought years to wet years. However, the model reproduces less well carbon assimilation in spring and positive CO2 flux pulses following early monsoon rain events, suggesting a need for further development of the model's representations of multiple plant species and soil carbon decomposition. The model simulates soil moisture at 5cm much better than at 15cm mainly because of heterogeneous soil properties. Through five numerical experiments with varying saturated hydraulic conductivity values, it is revealed that the discharge at the outlet of this semi-arid catchment is essentially attributed to lateral overland flow that is generated mainly by infiltration-excess runoff. Subsurface flow plays a minor role in this semi-arid catchment with a very deep groundwater table (>100m). The model produces wetter soils in lowland areas along stream rills and channels through re-infiltration of lateral overland flow. This water subsidy provides plants with favourable conditions to produce more leaves, CO2, and ET fluxes in lowland areas. Re-infiltration of overland flow over complex terrain may play a role in buffering climatic impacts in a warming climate with fewer but more intense rainfall events in the Southwestern United States. Copyright (c) 2013 John Wiley & Sons, Ltd.
- Niu, Y., Pasetto, D., Scudeler, C., Paniconi, C., Putti, M., Troch, P. A., DeLong, S., Dontsova, K. M., Pangle, L., Breshears, D. D., Chorover, J. D., Huxman, T. E., Pelletier, J. D., Saleska, S. R., & Zeng, X. (2014). Incipient subsurface heterogeneity and its effect on overland flow generation -- insight from a modeling study of the first experiment at the Biosphere 2 Landscape Evolution Observatory. Hydrol. Earth Syst. Sci., 18, 1873-1883.
- Pangle, L. A., Delong, S. B., Abramson, N., Adams, J., Barron-Gafford, G. A., Breshears, D. D., Brooks, P. D., Chorover, J. D., Dietrich, W. E., Dontsova, K. M., Durcik, M., Espleta, J., Ferre, P. A., Ferriere, R. H., Henderson, W., Hunt, E. A., Huxman, T. E., Millar, D., Murphy, B., , Niu, G., et al. (2015). The Landscape Evolution Observatory: A large-scale controllable infrastructure to study Earth-surface processes.. Geomorphology, 244, 190-203. doi:10.1016/j.geomorph.2015.01.020
- Rosolem, R., Hoar, T., Arellano, A., Anderson, J. L., Shuttleworth, W. J., Zeng, X., & Franz, T. E. (2014). Translating aboveground cosmic-ray neutron intensity to high-frequency soil moisture profiles at sub-kilometer scale. HYDROLOGY AND EARTH SYSTEM SCIENCES, 18(11), 4363-4379.More infoAbove-ground cosmic-ray neutron measurements provide an opportunity to infer soil moisture at the sub-kilometer scale. Initial efforts to assimilate those measurements have shown promise. This study expands such analysis by investigating (1) how the information from aboveground cosmic-ray neutrons can constrain the soil moisture at distinct depths simulated by a land surface model, and (2) how changes in data availability (in terms of retrieval frequency) impact the dynamics of simulated soil moisture profiles. We employ ensemble data assimilation techniques in a "nearly-identical twin" experiment applied at semi-arid shrubland, rainfed agricultural field, and mixed forest biomes in the USA. The performance of the Noah land surface model is compared with and without assimilation of observations at hourly intervals, as well as every 2 days. Synthetic observations of aboveground cosmic-ray neutrons better constrain the soil moisture simulated by Noah in root-zone soil layers (0-100 cm), despite the limited measurement depth of the sensor (estimated to be 12-20 cm). The ability of Noah to reproduce a "true" soil moisture profile is remarkably good, regardless of the frequency of observations at the semi-arid site. However, soil moisture profiles are better constrained when assimilating synthetic cosmic-ray neutron observations hourly rather than every 2 days at the cropland and mixed forest sites. This indicates potential benefits for hydrometeorological modeling when soil moisture measurements are available at a relatively high frequency. Moreover, differences in summertime meteorological forcing between the semi-arid site and the other two sites may indicate a possible control-ling factor to soil moisture dynamics in addition to differences in soil and vegetation properties.
- Scheftic, W., Zeng, X., Broxton, P., & Brunke, M. (2014). Intercomparison of seven NDVI products over the United States and Mexico. Remote Sensing, 6(2), 1057-1084.More infoAbstract: Satellites have provided large-scale monitoring of vegetation for over three decades, and several satellite-based Normalized Difference Vegetation Index (NDVI) datasets have been produced. Here we intercompare four long-term NDVI datasets based largely on the AVHRR sensor (NDVIg, NDVI3g, STAR, VIP) and three datasets based on newer sensors (SPOT, Terra, Aqua) and evaluate the effectiveness of homogenizing the datasets using the green vegetation fraction (GVF) and the impact it has on phenology trends. Results show that all NDVI datasets are highly correlated with each other. However, there are significant differences in the regression slopes that vary spatially and temporally. There is a general trend towards higher maximum annual NDVI over much of the temperate forests of the US and a longer greening period due mostly to a delayed end of the season. These trends are less well-defined over rainfall dependent ecosystems in Mexico and the southwest US Compared with the NDVI datasets, the derived GVF datasets show more one-to-one relationships, have reduced interannual variation, preserve their relationships better over the entire time period and are characterized by weaker trends. Finally, weak agreement between the trends in the datasets stresses the importance of using multiple datasets to evaluate changes in vegetation and its phenology. © 2014 by the authors; licensee MDPI, Basel, Switzerland.
- Stillman, S., Ninneman, J., Zeng, X., Franz, T. E., Scott, R. L., Shuttleworth, W. J., & Cummins, K. (2014). Summer Soil Moisture Spatiotemporal Variability in Southeastern Arizona. JOURNAL OF HYDROMETEOROLOGY, 15(4), 1473-1485.More infoSoil moisture is important for many applications, but its measurements are lacking globally and even regionally. The Walnut Gulch Experimental Watershed (WGEW) in southeastern Arizona has measured near-surface 5-cm soil moisture with 19 in situ probes since 2002 within its 150 km(2) area. Using various criteria to identify erroneous data, it is found that in any given period from 1 July to 30 September from 2002 to 2011, 13-17 of these probes were producing reasonable data, and this is sufficient to estimate area-averaged seasonal soil moisture. A soil water balance model is then developed using rainfall as its only input to spatially extrapolate soil moisture estimates to the 88 rain gauges located within the watershed and to extend the measurement period to 56 years. The model is calibrated from 2002 to 2011 so that the daily in situ and modeled soil moisture time series have a high average correlation of 0.89 and a root-mean-square deviation of 0.032 m(3) m(-3). By interpolating modeled soil moisture from the 88 rain gauges to a 100-m gridded domain over WGEW, it is found that spatial variability often increases when 88 (rather than 13-17) estimates are taken. While no trend in the spatial average surface soil moisture is found, large variability in the spatial average soil moisture from 1 July to 30 September is observed from year to year, ranging from 0.05 to 0.09 m(3) m(-3). In addition to spatiotemporal analysis of WGEW, this gridded soil moisture product from 1956 to 2011 can be used for validation of satellite-based and reanalysis products and land surface models.
- Wang, A., Barlage, M., Zeng, X., & Draper, C. S. (2014). Comparison of land skin temperature from a land model, remote sensing, and in situ measurement. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 119(6), 3093-3106.More infoLand skin temperature (Ts) is an important parameter in the energy exchange between the land surface and atmosphere. Here hourly Ts from the Community Land Model version 4.0, Moderate Resolution Imaging Spectroradiometer (MODIS) satellite observations, and in situ observations from the Coordinated Energy and Water Cycle Observation Project in 2003 were compared. Both modeled and MODIS Ts were interpolated to the 12 station locations, and comparisons were performed under MODIS clear-sky condition. Over four semiarid stations, both MODIS and modeled Ts show negative biases compared to in situ data, but MODIS shows an overall better performance. Global distribution of differences between MODIS and modeled Ts shows diurnal, seasonal, and spatial variations. Over sparsely vegetated areas, the model Ts is generally lower than the MODIS-observed Ts during the daytime, while the situation is opposite at nighttime. The revision of roughness length for heat and the constraint of minimum friction velocity from Zeng et al. (2012) bring the modeled Ts closer to MODIS during the day and have little effect on Ts at night. Five factors contributing to the Ts differences between the model and MODIS are identified, including the difficulty in properly accounting for cloud cover information at the appropriate temporal and spatial resolutions, and uncertainties in surface energy balance computation, atmospheric forcing data, surface emissivity, and MODIS Ts data. These findings have implications for the cross evaluation of modeled and remotely sensed Ts, as well as the data assimilation of Ts observations into Earth system models.
- Yu, Y., Xie, Z., & Zeng, X. (2014). Impacts of modified Richards equation on RegCM4 regional climate modeling over East Asia. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 119(22), 12642-12659.More infoTo remove the deficiency of the numerical solution of the mass conservation-based Richards equation for soil moisture in a regional climate model (RegCM4 with its land surface component Community Land Model 3.5 (CLM3.5)), a revised numerical algorithm that is used in CLM4.5 is implemented into CLM3.5. Compared with in situ measurements, the modified numerical method improves the ground water table depth simulations in RegCM4. It also improves the temporal and spatial variability of soil moisture to some extent. Its impact on simulated summer precipitation is mixed, with improvements over three subregions in China but with increased errors in three other subregions. The impact on the simulated summer temperature is relatively small (with the mean biases changed by less than 10% over most subregions). The evapotranspiration differences between modified and control land-atmosphere coupled simulations are enhanced over the northwest subregion and Tibetan Plateau compared to offline simulations due to land surface feedbacks to the atmosphere (in coupled simulations). Similarly the soil moisture differences in coupled simulations are geographically different from those in offline simulations over the eastern monsoon area. The summer precipitation differences between modified and control coupled simulations are found to be explained by the differences of both surface evapotranspiration and large-scale water vapor flux convergence which have opposite signs over the northwest subregion and Tibetan Plateau but have the same signs over other subregions.
- Zeng, X., & Wang, A. (2014). Global Hourly Land Surface Air Temperature Data Sets Covering 61 Years Now Available. GEWEX News, 24(3), 6-7.
- Geil, K. L., Serra, Y. L., & Zeng, X. (2013). Assessment of CMIP5 model simulations of the North American monsoon system. Journal of Climate, 26(22), 8787-8801.More infoAbstract: Precipitation, geopotential height, and wind fields from 21 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are examined to determine how well this generation of general circulation models represents the North American monsoon system (NAMS). Results show no improvement since CMIP3 in the magnitude (root-mean-square error and bias) of the mean annual cycle of monthly precipitation over a core monsoon domain, but improvement in the phasing of the seasonal cycle in precipitation is notable. Monsoon onset is early for most models but is clearly visible in daily climatological precipitation, whereas monsoon retreat is highly variable and unclear in daily climatological precipitation. Models that best capture large-scale circulation patterns at a low level usually have realistic representations of the NAMS, but even the best models poorly represent monsoon retreat. Difficulty in reproducing monsoon retreat results from an inaccurate representation of gradients in low-level geopotential height across the larger region, which causes an unrealistic flux of low-level moisture from the tropics into the NAMS region that extends well into the postmonsoon season. Composites of the models with the best and worst representations of the NAMS indicate that adequate representation of the monsoon during the early to midseason can be achieved even with a large-scale circulation pattern bias, as long as the bias is spatially consistent over the larger region influencing monsoon development; in other words, as with monsoon retreat, it is the inaccuracy of the spatial gradients in geopotential height across the larger region that prevents some models from realistic representation of the early and midseason monsoon system. © 2013 American Meteorological Society.
- Gustavo, L., Borak, J. S., Costa, M. H., Saleska, S. R., Baker, I., Restrepo-Coupe, N., Muza, M. N., Poulter, B., Verbeeck, H., Fisher, J. B., Arain, M. A., Arkin, P., Cestaro, B. P., Christoffersen, B., Galbraith, D., Guan, X., J.J.M., B., Ichii, K., M., H., , Jain, A. K., et al. (2013). Overview of the large-scale biosphere-atmosphere experiment in amazonia data model intercomparison project (LBA-DMIP). Agricultural and Forest Meteorology, 182-183, 111-127.More infoAbstract: A fundamental question connecting terrestrial ecology and global climate change is the sensitivity of key terrestrial biomes to climatic variability and change. The Amazon region is such a key biome: it contains unparalleled biological diversity, a globally significant store of organic carbon, and it is a potent engine driving global cycles of water and energy. The importance of understanding how land surface dynamics of the Amazon region respond to climatic variability and change is widely appreciated, but despite significant recent advances, large gaps in our understanding remain. Understanding of energy and carbon exchange between terrestrial ecosystems and the atmosphere can be improved through direct observations and experiments, as well as through modeling activities. Land surface/ecosystem models have become important tools for extrapolating local observations and understanding to much larger terrestrial regions. They are also valuable tools to test hypothesis on ecosystem functioning. Funded by NASA under the auspices of the LBA (the Large-Scale Biosphere-Atmosphere Experiment in Amazonia), the LBA Data Model Intercomparison Project (LBA-DMIP) uses a comprehensive data set from an observational network of flux towers across the Amazon, and an ecosystem modeling community engaged in ongoing studies using a suite of different land surface and terrestrial ecosystem models to understand Amazon forest function. Here an overview of this project is presented accompanied by a description of the measurement sites, data, models and protocol. © 2013 Elsevier B.V.
- Pangle, L. A., Delong, S. B., Abramson, N., Adams, J., Barron-Gafford, G. A., Breshears, D. D., Brooks, P. D., Chorover, J. D., Dietrich, W. E., Dontsova, K. M., Durcik, M., Espleta, J., Ferre, P. A., Ferriere, R. H., Henderson, W., Hunt, E. A., Huxman, T. E., Millar, D., Murphy, B., , Niu, G., et al. (2015). The Landscape Evolution Observatory: A large-scale controllable infrastructure to study Earth-surface processes.. Geomorphology, 244, 190-203.
- Rosolem, R., Gupta, H. V., Shuttleworth, W. J., Gustavo, L., & Zeng, X. (2013). Towards a comprehensive approach to parameter estimation in land surface parameterization schemes. Hydrological Processes, 27(14), 2075-2097.More infoAbstract: In climate models, the land-atmosphere interactions are described numerically by land surface parameterization (LSP) schemes. The continuing improvement in realism in these schemes comes at the expense of the need to specify a large number of parameters that are either directly measured or estimated. Also, an emerging problem is whether the relationships used in LSPs are universal and globally applicable. One plausible approach to evaluate this is to first minimize uncertainty in model parameters by calibration. In this paper, we conduct a comprehensive analysis of some model diagnostics using a slightly modified version of the Simple Biosphere 3 model for a variety of biomes located mainly in the Amazon. First, the degree of influence of each individual parameter in simulating surface fluxes is identified. Next, we estimate parameters using a multi-operator genetic algorithm applied in a multi-objective context and evaluate simulations of energy and carbon fluxes against observations. Compared with the default parameter sets, these parameter estimates improve the partitioning of energy fluxes in forest and cropland sites and provide better simulations of daytime increases in assimilation of net carbon during the dry season at forest sites. Finally, a detailed assessment of the parameter estimation problem was performed by accounting for the decomposition of the mean squared error to the total model uncertainty. Analysis of the total prediction uncertainty reveals that the parameter adjustments significantly improve reproduction of the mean and variability of the flux time series at all sites and generally remove seasonality of the errors but do not improve dynamical properties. Our results demonstrate that error decomposition provides a meaningful and intuitive way to understand differences in model performance. To make further advancements in the knowledge of these models, we encourage the LSP community to adopt similar approaches in the future. © 2012 John Wiley & Sons, Ltd.
- Rosolem, R., Shuttleworth, W. J., Zreda, M. G., Franz, T. E., Zeng, X. -., & Kurc, S. A. (2013). The effect of atmospheric water vapor on neutron count in the cosmic-ray soil moisture observing system. Journal of Hydrometeorology, 14(5), 1659-1671.
- Shao, P., Zeng, X., J., D., & Zeng, X. (2013). Soil microbial respiration from observations and Earth System Models. Environmental Research Letters, 8(3).More infoAbstract: Soil microbial respiration (Rh) is a large but uncertain component of the terrestrial carbon cycle. Carbon-climate feedbacks associated with changes to Rh are likely, but Rh parameterization in Earth System Models (ESMs) has not been rigorously evaluated largely due to a lack of appropriate measurements. Here we assess, for the first time, Rh estimates from eight ESMs and their environmental drivers across several biomes against a comprehensive soil respiration database (SRDB-V2). Climatic, vegetation, and edaphic factors exert strong controls on annual Rh in ESMs, but these simple controls are not as apparent in the observations. This raises questions regarding the robustness of ESM projections of Rh in response to future climate change. Since there are many more soil respiration (Rs) observations than Rh data, two 'reality checks' for ESMs are also created using the Rs data. Guidance is also provided on the Rh improvement in ESMs. © 2013 IOP Publishing Ltd.
- Shao, P., Zeng, X., Sakaguchi, K., Monson, R. K., & Zeng, X. (2013). Terrestrial carbon cycle: Climate relations in eight CMIP5 earth system models. Journal of Climate, 26(22), 8744-8764.More infoAbstract: Eight Earth System Models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are evaluated, focusing on both the net carbon dioxide flux and its components and their relation with climatic variables (temperature, precipitation, and soil moisture) in the historical (1850-2005) and representative concentration pathway 4.5 (RCP4.5; 2006-2100) simulations. While model results differ, their median globally averaged production and respiration terms from 1976 to 2005 agree reasonably with available observation-based products. Disturbances such as land use change are roughly represented but crucial in determining whether the land is a carbon source or sink over many regions in both simulations. While carbon fluxes vary with latitude and between the two simulations, the ratio of net to gross primary production, representing the ecosystem carbon use efficiency, is less dependent on latitude and does not differ significantly in the historical and RCP4.5 simulations. The linear trend of increased land carbon fluxes (except net ecosystem production) is accelerated in the twenty-first century. The cumulative net ecosystem production by 2100 is positive (i.e., carbon sink) in all models and the tropical and boreal latitudes become major carbon sinks in most models. The temporal correlations between annual-mean carbon cycle and climate variables vary substantially (including the change of sign) among the eight models in both the historical and twenty-first-century simulations. The ranges of correlations of carbon cycle variables with precipitation and soil moisture are also quite different, reflecting the important impact of the model treatment of the hydrological cycle on the carbon cycle. © 2013 American Meteorological Society.
- Stillman, S., Zeng, X., Shuttleworth, W. J., Goodrich, D. C., Unkrich, C. L., & Zreda, M. (2013). Spatiotemporal Variability of Summer Precipitation in Southeastern Arizona. JOURNAL OF HYDROMETEOROLOGY, 14(6), 1944-1951.More infoThe Walnut Gulch Experimental Watershed (WGEW) in southeastern Arizona covers similar to 150 km(2) and receives the majority of its annual precipitation from highly variable and intermittent summer storms during the North American monsoon. In this study, the patterns of precipitation in the U.S. Department of Agriculture-Agricultural Research Service (USDA-ARS) 88-rain-gauge network are analyzed for July through September from 1956 to 2011. Because small-scale convective systems generate most of this summer rainfall, the total (T), intensity (I), and frequency (F) exhibit high spatial and temporal variability. Although subsidiary periods may have apparent trends, no significant trends in T, I, and F were found for the study period as a whole. Observed trends in the spatial coverage of storms change sign in the late 1970s, and the multidecadal variation in I and spatial coverage of storms have statistically significant correlation with the Pacific decadal oscillation and the Atlantic multidecadal oscillation indices. Precipitation has a pronounced diurnal cycle with the highest T and F occurring between 1500 and 2200 LT, and its average fractional coverage over 2- and 12-h periods is less than 40% and 60% of the gauges, respectively. Although more gauges are needed to estimate area-averaged daily precipitation, 5-11 gauges can provide a reasonable estimate of the area-averaged monthly total precipitation during the period from July through September.
- Wang, A., & Zeng, X. (2013). Development of global hourly 0.58 land surface air temperature datasets. Journal of Climate, 26(19), 7676-7691.More infoAbstract: Land surface air temperature (SAT) is one of the most important variables in weather and climate studies, and its diurnal cycle is also needed for a variety of applications. Global long-term hourly SAT observational data, however, do not exist. While such hourly products could be obtained from global reanalyses, they are found to be unrealistic in representing the SAT diurnal cycle. Global hourly 0.5° SAT datasets are developed here based on four reanalysis products [Modern-Era Retrospective Analysis for Research and Applications (MERRA for 1979-2009), 40-yr ECMWF Re- Analysis (ERA-40 for 1958-2001),ECMWFInterim Re-Analysis (ERA-Interim for 1979-2009), and NCEP- NCAR reanalysis for 1948-2009)] and the Climate Research Unit Time Series version 3.10 (CRU TS3.10) for 1948-2009. The three-step adjustments include the spatial downscaling to 0.5° grid cells, the temporal interpolation from 6-hourly (in ERA-40 and NCEP-NCAR reanalysis) to hourly using the MERRA hourly SAT climatology for each day (and the linear interpolation from 3-hourly in ERA-Interim to hourly), and the bias correction in both monthly-mean maximum (Tmax) and minimum (Tmin) SAT using the CRU data. The final products have exactly the same monthly Tmax and Tmin as the CRU data, and perform well in comparison with in situ hourly measurements over six sites and with a regional daily SAT dataset over Europe. They agree with each other much better than the original reanalyses, and the spurious SAT jumps of reanalyses over some regions are also substantially eliminated. One of the uncertainties in the final products can be quantified by their differences in the true monthly mean (using 24-hourly values) and the monthly averaged diurnal cycle. © 2013 American Meteorological Society.
- Decker, M., Brunke, M. A., Wang, Z., Sakaguchi, K., Zeng, X., & Bosilovich, M. G. (2012). Evaluation of the reanalysis products from GSFC, NCEP, and ECMWF using flux tower observations. Journal of Climate, 25(6), 1916-1944.More infoAbstract: Reanalysis products produced at the various centers around the globe are utilized formany different scientific endeavors, including forcing land surface models and creating surface flux estimates. Here, flux tower observations of temperature, wind speed, precipitation, downward shortwave radiation, net surface radiation, and latent and sensible heat fluxes are used to evaluate the performance of various reanalysis products [NCEP-NCAR reanalysis and Climate Forecast System Reanalysis (CFSR) from NCEP; 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) and ECMWF Interim Re-Analysis (ERA-Interim) from ECMWF; and Modern-Era Retrospective Analysis for Research and Applications (MERRA) and Global Land Data Assimilation System (GLDAS) from the Goddard Space Flight Center (GSFC)]. To combine the biases and standard deviation of errors from the separate stations, a ranking systemis utilized. It is found thatERA-Interimhas the lowest overall bias in 6-hourly air temperature, followed closely by MERRA and GLDAS. The variability in 6-hourly air temperature is again most accurate in ERA-Interim. ERA-40 is found to have the lowest overall bias in latent heat flux, followed closely by CFSR, while ERA-40 also has the lowest 6-hourly sensible heat bias. MERRA has the second lowest and is close to ERA-40. The variability in 6-hourly precipitation is best captured byGLDAS and ERA-Interim, and ERA-40 has the lowest precipitation bias. It is also found that at monthly time scales, the bias term in the reanalysis products are the dominant cause of the mean square errors, while at 6-hourly and daily time scales the dominant contributor to the mean square errors is the correlation term. Also, it is found that the hourly CFSR data have discontinuities present due to the assimilation cycle, while the hourly MERRA data do not contain these jumps. © 2012 American Meteorological Society.
- Franz, T. E., Zreda, M., Ferre, T. P., Rosolem, R., Zweck, C., Stillman, S., Zeng, X., & Shuttleworth, W. J. (2012). Measurement depth of the cosmic ray soil moisture probe affected by hydrogen from various sources. WATER RESOURCES RESEARCH, 48.More infoWe present here a simple and robust framework for quantifying the effective sensor depth of cosmic ray soil moisture neutron probes such that reliable water fluxes may be computed from a time series of cosmic ray soil moisture. In particular, we describe how the neutron signal depends on three near-surface hydrogen sources: surface water, soil moisture, and lattice water (water in minerals present in soil solids) and also their vertical variations. Through a combined modeling study of one-dimensional water flow in soil and neutron transport in the atmosphere and subsurface, we compare average water content between the simulated soil moisture profiles and the universal calibration equation which is used to estimate water content from neutron counts. By using a linear sensitivity weighting function, we find that during evaporation and drainage periods the RMSE of the two average water contents is 0.0070 m(3) m(-3) with a maximum deviation of 0.010 m(3) m(-3) for a range of soil types. During infiltration, the RMSE is 0.011 m(3) m(-3) with a maximum deviation of 0.020 m(3) m(-3), where piston like flow conditions exists for the homogeneous isotropic media. Because piston flow is unlikely during natural conditions at the horizontal scale of hundreds of meters that is measured by the cosmic ray probe, this modeled deviation of 0.020 m(3) m(-3) represents the worst case scenario for cosmic ray sensing of soil moisture. Comparison of cosmic ray soil moisture data and a distributed sensor soil moisture network in Southern Arizona indicates an RMSE of 0.011 m(3) m(-3) over a 6 month study period.
- Rosolem, R., Gupta, H. V., Shuttleworth, W. J., Zeng, X., & Goncalves de Goncalves, L. G. (2012). A fully multiple-criteria implementation of the Sobol' method for parameter sensitivity analysis. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 117.More infoWe present a novel rank-based fully multiple-criteria implementation of the Sobol' variance-based sensitivity analysis approach that implements an objective strategy to evaluate parameter sensitivity when model evaluation involves several metrics of performance. The method is superior to single-criterion approaches while avoiding the subjectivity observed in "pseudo" multiple-criteria methods. Further, it contributes to our understanding of structural characteristics of a model and simplifies parameter estimation by identifying insensitive parameters that can be fixed to default values during model calibration studies. We illustrate the approach by applying it to the problem of identifying the most influential parameters in the Simple Biosphere 3 (SiB3) model using a network of flux towers in Brazil. We find 27-31 (out of 42) parameters to be influential, most (similar to 78%) of which are primarily associated with physiology, soil, and carbon properties, and that uncertainties in the physiological properties of the model contribute most to total model uncertainty in regard to energy and carbon fluxes. We also find that the second most important model component contributing to the total output uncertainty varies according to the flux analyzed; whereas morphological properties play an important role in sensible heat flux, soil properties are important for latent heat flux, and carbon properties (mainly associated with the soil respiration submodel) are important for carbon flux (as expected). These distinct sensitivities emphasize the need to account for the multioutput nature of land surface models during sensitivity analysis and parameter estimation. Applied to other similar models, our approach can help to establish which soil-plant-atmosphere processes matter most in land surface models of Amazonia and thereby aid in the design of field campaigns to characterize and measure the associated parameters. The approach can also be used with other sensitivity analysis procedures that compute at least two model performance metrics.
- Rosolem, R., Gupta, H. V., Shuttleworth, W. J., Zeng, X., & Gustavo, L. (2012). A fully multiple-criteria implementation of the Sobol' method for parameter sensitivity analysis. Journal of Geophysical Research D: Atmospheres, 117(7).More infoAbstract: We present a novel rank-based fully multiple-criteria implementation of the Sobol variance-based sensitivity analysis approach that implements an objective strategy to evaluate parameter sensitivity when model evaluation involves several metrics of performance. The method is superior to single-criterion approaches while avoiding the subjectivity observed in "pseudo" multiple-criteria methods. Further, it contributes to our understanding of structural characteristics of a model and simplifies parameter estimation by identifying insensitive parameters that can be fixed to default values during model calibration studies. We illustrate the approach by applying it to the problem of identifying the most influential parameters in the Simple Biosphere 3 (SiB3) model using a network of flux towers in Brazil. We find 27-31 (out of 42) parameters to be influential, most (∼78%) of which are primarily associated with physiology, soil, and carbon properties, and that uncertainties in the physiological properties of the model contribute most to total model uncertainty in regard to energy and carbon fluxes. We also find that the second most important model component contributing to the total output uncertainty varies according to the flux analyzed; whereas morphological properties play an important role in sensible heat flux, soil properties are important for latent heat flux, and carbon properties (mainly associated with the soil respiration submodel) are important for carbon flux (as expected). These distinct sensitivities emphasize the need to account for the multioutput nature of land surface models during sensitivity analysis and parameter estimation. Applied to other similar models, our approach can help to establish which soil-plant-atmosphere processes matter most in land surface models of Amazonia and thereby aid in the design of field campaigns to characterize and measure the associated parameters. The approach can also be used with other sensitivity analysis procedures that compute at least two model performance metrics.
- Sakaguchi, K., Zeng, X., & Brunke, M. A. (2012). Temporal- and spatial-scale dependence of three CMIP3 climate models in simulating the surface temperature trend in the twentieth century. Journal of Climate, 25(7), 2456-2470.More infoAbstract: Motivated by increasing interests in regional- and decadal-scale climate predictions, this study systematically analyzed the spatial- and temporal-scale dependence of the prediction skill of global climate models in surface air temperature (SAT) change in the twentieth century. The linear trends of annual mean SAT over moving time windows (running linear trends) from two observational datasets and simulations by three global climate models [Community Climate System Model, version 3.0 (CCSM3.0), Climate Model, version 2.0 (CM2.0), and Model E-H] that participated in CMIP3 are compared over several temporal (10-, 20-, 30-, 40-, and 50-yr trends) and spatial (5° × 5°, 10° × 10°, 15° × 15°, 20° × 20°, 30° × 30°, 30° latitudinal bands, hemispheric, and global) scales. The distribution of root-mean-square error is improved with increasing spatial and temporal scales, approaching the observational uncertainty range at the largest scales. Linear correlation shows a similar tendency, but the limited observational length does not provide statistical significance over the longer temporal scales. The comparison of RMSE to climatology and a Monte Carlo test using preindustrial control simulations suggest that the multimodel ensemble mean is able to reproduce robust climate signals at 30° zonal mean or larger spatial scales, while correlation requires hemispherical or global mean for the twentieth-century simulations. Persistent lower performance is observed over the northern high latitudes and the North Atlantic southeast of Greenland. Although several caveats exist for the metrics used in this study, the analyses across scales and/or over running time windows can be taken as one of the approaches for climate system model evaluations. © 2012 American Meteorological Society.
- Sakaguchi, K., Zeng, X., & Brunke, M. A. (2012). The hindcast skill of the CMIP ensembles for the surface air temperature trend. Journal of Geophysical Research D: Atmospheres, 117(16).More infoAbstract: Linear trends of the surface air temperature (SAT) simulated by selected models from the Coupled Model Intercomparison Project (CMIP3 and CMIP5) historical experiments are evaluated using observations to document (1) the expected range and characteristics of the errors in hindcasting the change in SAT at different spatiotemporal scales, (2) if there are threshold spatiotemporal scales across which the models show substantially improved performance, and (3) how they differ between CMIP3 and CMIP5. Root Mean Square Error, linear correlation, and Brier score show better agreement with the observations as spatiotemporal scale increases but the skill for the regional (5 × 5 - 20 × 20 grid) and decadal (10 - ∼30-year trends) scales is rather limited. Rapid improvements are seen across 30 × 30 grid to zonal average and around 30 years, although they depend on the performance statistics. Rather abrupt change in the performance from 30 × 30 grid to zonal average implies that averaging out longitudinal features, such as land-ocean contrast, might significantly improve the reliability of the simulated SAT trend. The mean bias and ensemble spread relative to the observed variability, which are crucial to the reliability of the ensemble distribution, are not necessarily improved with increasing scales and may impact probabilistic predictions more at longer temporal scales. No significant differences are found in the performance of CMIP3 and CMIP5 at the large spatiotemporal scales, but at smaller scales the CMIP5 ensemble often shows better correlation and Brier score, indicating improvements in the CMIP5 on the temporal dynamics of SAT at regional and decadal scales. © 2012. American Geophysical Union. All Rights Reserved.
- Wang, A., & Zeng, X. (2012). Evaluation of multireanalysis products with in situ observations over the Tibetan Plateau. Journal of Geophysical Research D: Atmospheres, 117(5).More infoAbstract: As the highest plateau in the world, the Tibetan Plateau (TP) strongly affects regional weather and climate as well as global atmospheric circulations. Here six reanalysis products (i.e., MERRA, NCEP/NCAR-1, CFSR, ERA-40, ERA-Interim, and GLDAS) are evaluated using in situ measurements at 63 weather stations over the TP from the Chinese Meteorological Administration (CMA) for 1992-2001 and at nine stations from field campaigns (CAMP/Tibet) for 2002-2004. The measurement variables include daily and monthly precipitation and air temperature at all CMA and CAMP/Tibet stations as well as radiation (downward and upward shortwave and longwave), wind speed, humidity, and surface pressure at CAMP stations. Four statistical quantities (correlation coefficient, ratio of standard deviations, standard deviation of differences, and bias) are computed, and a ranking approach is also utilized to quantify the relative performance of reanalyses with respect to each variable and each statistical quantity. Compared with measurements at the 63 CMA stations, ERA-Interim has the best overall performance in both daily and monthly air temperatures, while MERRA has a high correlation with observations. GLDAS has the best overall performance in both daily and monthly precipitation because it is primarily based on the merged precipitation product from surface measurements and satellite remote sensing, while ERA-40 and MERRA have the highest correlation coefficients for daily and monthly precipitation, respectively. Compared with measurements at the nine CAMP stations, CFSR shows the best overall performance, followed by GLDAS, although the best ranking scores are different for different variables. It is also found that NCEP/NCAR-1 reanalysis shows the worst overall performance compared with both CMA and CAMP data. Since no reanalysis product is superior to others in all variables at both daily and monthly time scales, various reanalysis products should be combined for the study of weather and climate over the TP. Copyright 2012 by the American Geophysical Union.
- Zeng, X., & Wang, A. (2012). What is monthly mean land surface air temperature?. Eos, 93(15), 156-.More infoAbstract: Land surface air temperature is one of the fundamental variables in weather and climate observations, modeling, and applications. Its monthly mean has been computed as the average of daily maximum and minimum temperatures [Jones et al., 1999]. This is different from the true monthly mean temperature, which is defined as the integral of the continuous temperature measurements in a month divided by the integration period and can be very accurately represented using hourly data, as has long been recognized [e.g., Brooks, 1921]. We argue, from scientific, technological, and historical perspectives, that it is time to compute the true monthly mean using hourly data for the national and international climate data record. © 2012 by the American Geophysical Union.
- Zeng, X., Kiviat, K. L., Sakaguchi, K., & Mahmoud, A. M. (2012). A toy model for monthly river flow forecasting. Journal of Hydrology, 452-453, 226-231.More infoAbstract: River flow forecasting depends on land-atmosphere coupled processes, and is relevant to hydrological applications and land-ocean coupling. A toy model is developed here for monthly river flow forecasting using the river flow and river basin averaged precipitation in prior month. Model coefficients are calibrated for each month using historical data. The toy model is based on water balance, easy to use and reproduce, and robust to calibrate with a short period of data. For five major rivers in the world, its results agree with observations very well. Its prediction uncertainty can be quantified using the model's error statistics or using a dynamic approach, but not by the dispersion of 10,000 ensemble members with different sets of coefficients in the model. Its results are much better than those from a physically based land model even after the mean bias correction. The toy model and a standard neural network available from the MATLAB give similar results, but the latter is more sensitive to the length of calibration period. For the monthly prediction of river flow with a strong seasonal cycle, a modified Nash-Sutcliffe coefficient of efficiency is introduced and is found to be more reliable in model evaluations than the original coefficient of efficiency or the correlation coefficient. © 2012 Elsevier B.V.
- Zeng, X., Wang, Z., & Wang, A. (2012). Surface skin temperature and the interplay between sensible and ground heat fluxes over arid regions. Journal of Hydrometeorology, 13(4), 1359-1370.More infoAbstract: Over arid regions, two community landmodels [Noah andCommunityLandModel (CLM)] still have difficulty in realistically simulating the diurnal cycle of surface skin temperature. Based on theoretical arguments and synthesis of previous observational and modeling efforts, three revisions are developed here to address this issue. The revision of the coefficients in computing roughness length for heat significantly reduces the underestimate of daytime skin temperature but has a negligible effect on nighttime skin temperature. The constraints of the minimum friction velocity and soil thermal conductivity help improve nighttime skin temperature under weak wind and dry soil conditions. These results are robust in bothNoah andCLM, aswell as inNoah, with 4 versus 10 soil layers based on in situ data at the Desert Rock site in Nevada with a monthly averaged diurnal amplitude of 31.7 K and the Gaize site over Tibet, China, with an amplitude of 44.6 K. While these revisions can be directly applied to CLM or other land models with subgrid tiles (including bare soil), suggestions are also made on their application to Noah and other land models that treat bare soil and vegetated area together in amodel grid cell. It is suggested that the challenging issue ofmeasuring and simulating surface sensible heat flux under stable conditions should be treated as a land-atmosphere coupled issue, involving the interplay of ground and sensible heat fluxes in balancing the net radiation over arid regions, rather than as an atmospheric turbulence issue alone. The implications of such a coupling perspective are also discussed. © 2012 American Meteorological Society.
- Zheng, W., Wei, H., Wang, Z., Zeng, X., Meng, J., Michael, E. k., Mitchell, K., & Derber, J. (2012). Improvement of daytime land surface skin temperature over arid regions in the NCEP GFS model and its impact on satellite data assimilation. Journal of Geophysical Research D: Atmospheres, 117(6).More infoAbstract: Comparison of the land surface skin temperature (LST) from the National Centers for Environmental Prediction (NCEP) operational Global Forecast System (GFS) against satellite and in situ data in summer 2007 indicates that the GFS has a large and cold bias in LST over the arid western continental United States (CONUS) during daytime. This LST bias contributes to large errors in simulated satellite brightness temperatures over land by the Community Radiative Transfer Model (CRTM) and hence the rejection of satellite data in the NCEP Gridpoint Statistical Interpolation (GSI) system, especially for surface-sensitive satellite channels. The new vegetation-dependent formulations of momentum and thermal roughness lengths are tested in the GFS. They substantially reduce the large cold bias of daytime LST over the arid western CONUS in the warm season. This, in turn, significantly reduces the large biases of calculated satellite brightness temperatures found for infrared and microwave sensors in window or near-window channels, so that many more satellite data can be assimilated in the GSI system. In the arid western CONUS, the calculation of surface emissivity for microwave sensors in the CRTM can be further improved, and the new microwave land emissivity model together with increased LST via changes in surface roughness length formulations reduces biases and root-mean-square errors in the calculated brightness temperature. © 2012 by the American Geophysical Union.
- Zreda, M., Shuttleworth, W. J., Zeng, X., Zweck, C., Desilets, D., Franz, T. E., & Rosolem, R. (2012). COSMOS: the COsmic-ray Soil Moisture Observing System. HYDROLOGY AND EARTH SYSTEM SCIENCES, 16(11), 4079-4099.More infoThe newly-developed cosmic-ray method for measuring area-average soil moisture at the hectometer horizontal scale is being implemented in the COsmic-ray Soil Moisture Observing System (or the COSMOS). The stationary cosmic-ray soil moisture probe measures the neutrons that are generated by cosmic rays within air and soil and other materials, moderated by mainly hydrogen atoms located primarily in soil water, and emitted to the atmosphere where they mix instantaneously at a scale of hundreds of meters and whose density is inversely correlated with soil moisture. The COSMOS has already deployed more than 50 of the eventual 500 cosmic-ray probes, distributed mainly in the USA, each generating a time series of average soil moisture over its horizontal footprint, with similar networks coming into existence around the world. This paper is written to serve a community need to better understand this novel method and the COSMOS project. We describe the cosmic-ray soil moisture measurement method, the instrument and its calibration, the design, data processing and dissemination used in the COSMOS project, and give example time series of soil moisture obtained from COSMOS probes.
- Brunke, M. A., Wang, Z., Zeng, X., Bosilovich, M., & Shie, C. (2011). An assessment of the uncertainties in ocean surface turbulent fluxes in 11 reanalysis, satellite-derived, and combined global datasets. Journal of Climate, 24(21), 5469-5493.More infoAbstract: Ocean surface turbulent fluxes play an important role in the energy and water cycles of the atmosphere- ocean coupled system, and several flux products have become available in recent years. Here, turbulent fluxes from 6 widely used reanalyses, 4 satellite-derived flux products, and 2 combined product are evaluated by comparison with direct covariance latent heat (LH) and sensible heat (SH) fluxes and inertial-dissipation wind stresses measured from 12 cruises over the tropics and mid- and high latitudes. The biases range from -3.0 to 20.2 W m-2 for LH flux, from -1.4 to 6.0 W m-2 for SH flux, and from -7.6 to 7.9 × 10-3 N m-2 for wind stress. These biases are small for moderate wind speeds but diverge for strong wind speeds (>10 m s-1). The total flux biases are then further evaluated by dividing them into uncertainties due to errors in the bulk variables and the residual uncertainty. The bulk-variable-caused uncertainty dominates many products' SH flux and wind stress biases. The biases in the bulk variables that contribute to this uncertainty can be quite high depending on the cruise and the variable. Onthe basis of a ranking of each product's flux, it is found that the Modern-Era Retrospective Analysis for Research and Applications (MERRA) is among the "best performing" for all three fluxes. Also, the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-Interim) and the National Centers for Environmental Prediction-Department of Energy (NCEP-DOE) reanalysis are among the best performing for two of the three fluxes. Of the satellite-derived products, version 2b of the Goddard Satellite-Based Surface Turbulent Fluxes (GSSTF2b) is among the best performing for two of the three fluxes. Also among the best performing for only one of the fluxes are the 40-yr ERA (ERA-40) and the combined product objectively analyzed air-sea fluxes (OAFlux). Direction for the future development of ocean surface flux datasets is also suggested. © 2011 American Meteorological Society.
- Decker, M., & Zeng, X. (2011). Impact of Modified Richards Equation on Global Soil Moisture Simulation in the Community Land Model (CLM3.5). JOURNAL OF ADVANCES IN MODELING EARTH SYSTEMS, 1.More infoA fundamental deficiency has been found in the numerical solution of the soil moisture-based Richards equation using the mass-conservative scheme in the Community Land Model (CLM) in the first part of our efforts (Zeng and Decker 2009). This study implements the revised form of the Richards equation from that study (which doesn't change the property of the differential equation but does remove the deficiency of the numerical solution) along with a new bottom boundary condition into the current version of CLM (CLM3.5) for global offline modeling evaluations. CLM3.5 represents a significant improvement over its earlier version (CLM3.0), but it also introduces a new deficiency in the vertical distribution of the soil moisture variability. Mean soil moisture in CLM3.5 is also too wet. It is found that the new treatments (primarily a numerically correct solution of Richards equation with a new bottom boundary condition) with minimal tuning are able to maintain the improvements of the CLM3.5 over CLM3.0 and, at the same time, remove the new deficiencies of CLM3.5 based on in situ and satellite data analysis. Because the deficiency in the numerical solution of the soil moisture-based Richards equation is also expected in other land models, implementation details are provided to facilitate similar tests using other land models in the future.
- Sakaguchi, K., Zeng, X., Christoffersen, B. J., Restrepo-Coupe, N., Saleska, S. R., & Brando, P. M. (2011). Natural and drought scenarios in an east central Amazon forest: Fidelity of the Community Land Model 3.5 with three biogeochemical models. Journal of Geophysical Research G: Biogeosciences, 116(1).More infoAbstract: Recent development of general circulation models involves biogeochemical cycles: flows of carbon and other chemical species that circulate through the Earth system. Such models are valuable tools for future projections of climate, but still bear large uncertainties in the model simulations. One of the regions with especially high uncertainty is the Amazon forest where large-scale dieback associated with the changing climate is predicted by several models. In order to better understand the capability and weakness of global-scale land-biogeochemical models in simulating a tropical ecosystem under the present day as well as significantly drier climates, we analyzed the off-line simulations for an east central Amazon forest by the Community Land Model version 3.5 of the National Center for Atmospheric Research and its three independent biogeochemical submodels (CASA', CN, and DGVM). Intense field measurements carried out under Large Scale Biosphere-Atmosphere Experiment in Amazonia, including forest response to drought from a throughfall exclusion experiment, are utilized to evaluate the whole spectrum of biogeophysical and biogeochemical aspects of the models. Our analysis shows reasonable correspondence in momentum and energy turbulent fluxes, but it highlights three processes that are not in agreement with observations: (1) inconsistent seasonality in carbon fluxes, (2) biased biomass size and allocation, and (3) overestimation of vegetation stress to short-term drought but underestimation of biomass loss from long-term drought. Without resolving these issues the modeled feedbacks from the biosphere in future climate projections would be questionable. We suggest possible directions for model improvements and also emphasize the necessity of more studies using a variety of in situ data for both driving and evaluating land-biogeochemical models. Copyright 2011 by the American Geophysical Union.
- Wang, A., & Zeng, X. (2011). Sensitivities of terrestrial water cycle simulations to the variations of precipitation and air temperature in China. Journal of Geophysical Research D: Atmospheres, 116(2).More infoAbstract: The quality of simulated soil hydrological variables (i.e., soil moisture, evapotranspiration, and runoff) is largely dependent on the accuracy of meteorological forcing data, especially precipitation and air temperature. This issue is quantitatively addressed here by running the Community Land Model (CLM3.5) over China from 1993 to 2002 using the reanalysis-based precipitation and air temperature and in situ observations in the meteorological forcing data set. Compared to the in situ measured soil moisture data, the CLM3.5 simulation can generally capture the spatial and seasonal variations of soil moisture but produces too-wet soil in northeastern and eastern China and too-dry soil in northwestern China. This deficiency is significantly reduced when the in situ measured precipitation data are used to drive the model. An index is also constructed to quantify the sensitivities of soil hydrological variables to variations of precipitation and air temperature. The highest sensitivity of surface hydrological variables to precipitation appears over semiarid regions, while the sensitivity to air temperature for different variables varies regionally (semiarid regions for runoff and soil moisture and humid regions for evapotranspiration (ET)). Over semiarid regions, precipitation and air temperature are equally important to the simulations of soil hydrological variables. Over humid regions, in contrast, ET is more dependent on air temperature than on precipitation, while soil moisture and runoff are less affected by air temperature. Copyright 2011 by the American Geophysical Union.
- Zreda, M., Zeng, X., & other coauthors, e. (2011). Cosmic-Ray Neutrons, An Innovative Method for Measuring Area-Average Soil Moisture. GEWEX News, 21(3), 6-10.
- Brunke, M. A., Szoeke, S. D., Zuidema, P., & Zeng, X. (2010). A comparison of ship and satellite measurements of cloud properties with global climate model simulations in the southeast Pacific stratus deck. Atmospheric Chemistry and Physics, 10(14), 6527-6536.More infoAbstract: Here, liquid water path (LWP), cloud fraction, cloud top height, and cloud base height retrieved by a suite of A-train satellite instruments (the CPR aboard CloudSat, CALIOP aboard CALIPSO, and MODIS aboard Aqua) are compared to ship observations from research cruises made in 2001 and 2003-2007 into the stratus/stratocumulus deck over the southeast Pacific Ocean. It is found that Cloud-Sat radar-only LWP is generally too high over this region and the CloudSat/CALIPSO cloud bases are too low. This results in a relationship (LWP∼h9) between CloudSat LWP and CALIPSO cloud thickness (h) that is very different from the adiabatic relationship (LWP∼h2) from in situ observations. Such biases can be reduced if LWPs suspected to be contaminated by precipitation are eliminated, as determined by the maximum radar reflectivity Zmax>-15 dBZ in the apparent lower half of the cloud, and if cloud bases are determined based upon the adiabatically-determined cloud thickness (h∼LWP1/2). Furthermore, comparing results from a global model (CAM3.1) to ship observations reveals that, while the simulated LWP is quite reasonable, the model cloud is too thick and too low, allowing the model to have LWPs that are almost independent of h. This model can also obtain a reasonable diurnal cycle in LWP and cloud fraction at a location roughly in the centre of this region (20° S, 85°W) but has an opposite diurnal cycle to those observed aboard ship at a location closer to the coast (20deg; S, 75deg; W). The diurnal cycle at the latter location is slightly improved in the newest version of the model (CAM4). However, the simulated clouds remain too thick and too low, as cloud bases are usually at or near the surface. © 2010 Author(s).
- Rosolem, R., Shuttleworth, W. J., Zeng, X., Saleska, S. R., & Huxman, T. E. (2010). Land surface modeling inside the Biosphere 2 tropical rain forest biome. Journal of Geophysical Research G: Biogeosciences, 115(4).More infoAbstract: Tropical rain forests contribute substantially to regional and global energy, water, and carbon exchanges between the land surface and the atmosphere, and better understanding of the mechanisms of vegetation response to different environmental stresses is needed. The Biosphere 2 facility provides an opportunity to link laboratory-scale and plot-scale studies in a controllable environment. We compiled a consistent quality-controlled time series of climate data from Biosphere 2 and used it to drive the Simple Biosphere model (SiB3) to test how well it represented the behavior of soils and vegetation inside the tropical rain forest biome of Biosphere 2 (B2-TRF). We found that soil respiration parameterization in SiB3 was not suitable for use in B2-TRF, so several alternative parameterizations were tested. None gave outstanding results, but a modified version of the parameterization originally proposed for SiB3 gave the best results. With this modification, SiB3 well simulated the observed net ecosystem exchange in B2-TRF but, significantly, only after additionally modifying parameters describing the thermal tolerance of plants so that photosynthetic capacity was reduced on average but maintained to higher temperatures. This implies either that tropical rain forest species can acclimate to higher temperatures than allowed for by vegetation models or that the plant community assembly in B2-TRF has shifted to allow continued functioning at higher temperatures, and plants in natural ecosystems could also. In either case, this suggests that the Amazon rain forest may be more resilient to climate change than hitherto thought. Copyright 2010 by the American Geophysical Union.
- Wang, Z., & Zeng, X. (2010). Evaluation of snow albedo in land models for weather and climate studies. Journal of Applied Meteorology and Climatology, 49(3), 363-380.More infoAbstract: Snow albedo plays an important role in land models for weather, climate, and hydrometeorological studies, but its treatment in various land models still contains significant deficiencies. Complementary to previous studies that evaluated the snow albedo as part of an overall land model study, the snow albedo formulations as used in four major weather forecasting and climate models [European Centre for Medium-Range Weather Forecasts (ECMWF), National Centers for Environmental Prediction (NCEP) "Noah" land model, National Center for Atmospheric Research (NCAR) Community Land Model (CLM3), and NCEP global model] were directly evaluated here using multiyear Boreal Ecosystem-Atmosphere Study (BOREAS) in situ data over grass and forest sites. First, four idealized cases over grass and forest sites were designed to understand better the different albedo formulations in these models. Then the BOREAS data were used to evaluate snow albedo and relevant formulations and to identify deficiencies of each model. Based on these analyses, suggestions that involve only minor changes in parameters or formulations were made to significantly reduce these deficiencies of each model. For theECMWFland model, using the square root of snow water equivalent (SWE), rather than SWE itself, in the computation of snow fraction would significantly reduce the under-estimation of albedo over grass. For the NCEP Noah land model, reducing (increasing) the critical SWE for full snow cover over short (tall) vegetation would reduce the underestimate (overestimate) of snow albedo over the grass (forest) site. For the NCAR CLM3, revising the coefficient used in the ground snow-fraction computation would substantially reduce the albedo underestimation over grass. For the albedo formulations in the NCEP global model, replacing the globally constant fresh snow albedo by the vegetation-type-dependent Moderate-Resolution Imaging Spectroradiometer (MODIS) maximum snow albedo would significantly improve the overestimation of model albedo over forest. © 2010 American Meteorological Society.
- Wang, Z., Zeng, X., & Decker, M. (2010). Improving snow processes in the Noah land model. Journal of Geophysical Research D: Atmospheres, 115(20).More infoAbstract: Snow is one of the most crucial land surface processes over middle and high latitudes. A widely known deficiency of the Noah land model as used in the National Centers for Environmental Prediction (NCEP) operational models and the Weather Research and Forecasting model (WRF) at the National Center for Atmospheric Research is that snowmelt occurs much too early. Through detailed diagnostics of the Noah output over the high-altitude Niwot Ridge forest site (40.03°N, 105.55°W) and a boreal forest site (53.9°N, 104.7°W), six deficiencies in Noah model physics are identified along with improved formulations that (1) consider the vegetation shading effect on snow sublimation and snowmelt; (2) consider under-canopy resistance; (3) revise the ground heat flux computation when snow is deep; (4) revise the momentum roughness length computation when snow is present; (5) revise the snow density computation near 0°C; and (6) increase the maximum iteration number from five to 30 in the turbulence computation. These revisions significantly improve Noah simulations of all snow processes such as snow water equivalent (SWE), snow depth, and sensible and latent heat fluxes over these two forest sites. The revisions were also evaluated (without tunings) with an independent forest site and a grassland site, further confirming the robust and positive impacts of these revisions on Noah snow simulations. These modifications maintain the Noah model structure and do not introduce new prognostic variables, allowing easy implementation into NCEP operational models and into WRF. Furthermore, they are found to be as good as, or slightly better than, a much more complicated land model in the snow simulation over the three forest sites. Copyright 2010 by the American Geophysical Union.
- Zeng, X. (2010). Reply to comments on "what is the atmosphere's effect on earth's surface temperature?". Eos, 91(46), 432-.
- Zeng, X. (2010). What is the atmospheres effect on Earth's surface temperature?. Eos, 91(15), 134-135.More infoAbstract: It is frequently stated in textbooks and scholarly articles that the surface temperature of Earth is 33C warmer than it would be without the atmosphere and that this difference is due to the greenhouse effect. This Forum shows that the atmosphere effect leads to warming of only 20C. This new conclusion requires a revision to all of the relevant literature in K-12, undergraduate, and graduate education material and to science papers and reports. The greenhouse effect on Earth's surface temperature is well understood qualitatively and is regarded as basic knowledge about Earth's climate and climate change. The 33C warming has been used to quantify the greenhouse effect of greenhouse gases, or of greenhouse gases and clouds, in K-12 educational material (e.g., http://epa.gov/climatechange/kids/greenhouse.html), undergraduate freshman introductory textbooks on weather and climate [e.g., Ahrens, 2008], and graduate textbooks on climate [e.g., Peixoto and Oort, 1992]. Some textbooks and various other publications have less stringently attributed the warming to the greenhouse effect [e.g., Wallace and Hobbs, 2006; Le Treut et al., 2007; American Meteorological Society, 2000].
- Zeng, X., & Decker, M. (2010). Reply. Journal of Hydrometeorology, 11(4), 1051-1054.
- Zeng, X., Barlage, M., Castro, C., & Fling, K. (2010). Comparison of land-precipitation coupling strength using observations and models. Journal of Hydrometeorology, 11(4), 979-994.More infoAbstract: Numerous studies have attempted to address the land-precipitation coupling, but scientists' understanding remains limited and discrepancies still exist from different studies. A new parameter Γ is proposed here to estimate the land-precipitation coupling strength based on the ratio of the covariance between monthly or seasonal precipitation and evaporation anomalies (from their climatological means) over the variance of precipitation anomalies. The Γ value is easy to compute and insensitive to the horizontal scales used; however, it does not provide causality. A relatively high Γ is a necessary-but not sufficient-condition for a relatively strong land-precipitation coupling. A computation of Γ values using two global reanalyses (ECMWF and NCEP), one regional reanalysis [North American Regional Reanalysis (NARR)], and observed precipitation along with Variable Infiltration Capacity (VIC)-derived evaporation data indicates that the land-precipitation coupling is stronger in summer and weaker in winter. The strongest coupling (i.e., hot spots) occurs over the western and central parts of North America, part of the Eurasia midlatitude, and Sahel in boreal summer and over most of Australia, Argentina, and South Africa in austral summer. The Community Climate System Model, version 3 (CCSM3) shows much higher Γ values, consistent with the strong coupling shown by its atmosphere-land coupled components in previous studies. Its overall spatial pattern of Γ values is not affected much over most regions by the doubling of CO2 in CCSM3. The Γ values from the Regional Atmospheric Modeling System (RAMS) are more realistic than those from CCSM3; however, they are still higher than those from observations over North America. © 2010 American Meteorological Society.
- Zhang, S., Zeng, X., Zhang, W., & Barlage, M. (2010). Revising the ensemble-based kalman filter covariance for the retrieval of deep-layer soil moisture. Journal of Hydrometeorology, 11(1), 219-227.More infoAbstract: Previous studies have demonstrated that soil moisture in the top layers (e.g., within the top 1-m depth) can be retrieved by assimilating near-surface soil moisture observations into a land surface model using ensemblebased data assimilation algorithms. However, it remains a challenging issue to provide good estimates of soil moisture in the deep layers, because the error correlation between the surface and deep layers is low and hence is easily influenced by the physically limited range of soil moisture, probably resulting in a large noiseto-signal ratio. Furthermore, the temporally correlated errors between the surface and deep layers and the nonlinearity of the system make the retrieval even more difficult. To tackle these problems, a revised ensemble-based Kalman filter covariance method is proposed by constraining error covariance estimates in deep layers in two ways: 1) explicitly using the error covariance at the previous time step and 2) limiting the increase of the soil moisture error correlation with the increase of the vertical distance between the two layers. This method is then tested at three separate point locations representing different precipitation regimes. It is found that the proposed method can effectively control the abrupt changes of error covariance estimates between the surface layer and two deep layers. It significantly improves the estimates of soil moisture in the two deep layers with daily updating. For example, relative to the initial background error, after 150 daily updates, the error in the deepest layer reduces to 11.4%, 32.3%, and 27.1% at the wet, dry, and medium wetness locations, only reducing to 62.3%, 80.8%, and 47.5% with the original method, respectively. However, the improvement of deep-layer soil moisture retrieval is very slight when the updating frequency is reduced to once every three days. © 2010 American Meteorological Society.
- Er, L. u., Zeng, X., Jiang, Z., Wang, Y., & Zhang, Q. (2009). Precipitation and precipitable water: Their temporal-spatial behaviors and use in determining monsoon onset/retreat and monsoon regions. Journal of Geophysical Research D: Atmospheres, 114(23).More infoAbstract: Precipitation (P) is conventionally used for determining monsoon onset/retreat. It can roughly separate monsoon regions from nonmonsoon regions, Zeng and Lu (2004) found that precipitable water (W) can also determine the monsoon onset/retreat but cannot determine the monsoon regions. Temporal-spatial behaviors of P and W are compared in this article with observed data and results of previous theoretical analyses, and the comparison is used to understand the performances of P and W in determining the monsoon onset/retreat and monsoon regions. It is shown that W increases everywhere from winter to summer, dominated by the large seasonal change of temperature. P increases from winter to summer mainly in monsoon regions; it decreases or does not change much in most of the nonmonsoon regions. Whether P increases or not from winter to summer depends on whether the increase of W is greater than the increase of temperature. Synoptic variations of P and W have positive correlations everywhere. The increases of P and W from winter to summer in monsoon regions make both able to determine the climatic monsoon onset and retreat. The positive correlations of daily P and W in monsoon onset and retreat seasons make the interannual variations of the monsoon onset and retreat determined from P able to be determined from W. The decrease or small change of P from winter to summer in most of the nonmonsoon regions and the increase of W in nonmonsoon regions make P able to roughly determine the monsoon regions while W fails to. Copyright 2009 by the American Geophysical Union.
- Huxman, T., Troch, P., Chorover, J., Breshears, D. D., Saleska, S., Pelletier, J., Zeng, X., & Espeleta, J. (2009). The hills are alive: Earth science in a controlled environment. Eos, 90(14), 120-.
- Lawrence, D. M., Oleson, K. W., Flanner, M. G., Thornton, P. E., Swenson, S. C., Lawrence, P. J., Zeng, X., Yang, Z., Levis, S., Sakaguchi, K., Bonan, G. B., & Slater, A. G. (2009). Parameterization Improvements and Functional and Structural Advances in Version 4 of the Community Land Model. JOURNAL OF ADVANCES IN MODELING EARTH SYSTEMS, 3.More infoThe Community Land Model is the land component of the Community Climate System Model. Here, we describe a broad set of model improvements and additions that have been provided through the CLM development community to create CLM4. The model is extended with a carbon-nitrogen (CN) biogeochemical model that is prognostic with respect to vegetation, litter, and soil carbon and nitrogen states and vegetation phenology. An urban canyon model is added and a transient land cover and land use change (LCLUC) capability, including wood harvest, is introduced, enabling study of historic and future LCLUC on energy, water, momentum, carbon, and nitrogen fluxes. The hydrology scheme is modified with a revised numerical solution of the Richards equation and a revised ground evaporation parameterization that accounts for litter and within-canopy stability. The new snow model incorporates the SNow and Ice Aerosol Radiation model (SNICAR) - which includes aerosol deposition, grain-size dependent snow aging, and vertically-resolved snowpack heating - as well as new snow cover and snow burial fraction parameterizations. The thermal and hydrologic properties of organic soil are accounted for and the ground column is extended to similar to 50-m depth. Several other minor modifications to the land surface types dataset, grass and crop optical properties, surface layer thickness, roughness length and displacement height, and the disposition of snow-capped runoff are also incorporated.
- Sakaguchi, K., & Zeng, X. (2009). Effects of soil wetness, plant litter, and under-canopy atmospheric stability on ground evaporation in the Community Land Model (CLM3.5). Journal of Geophysical Research D: Atmospheres, 114(1).More infoAbstract: The National Center for Atmospheric Research (NCAR) Community Land Model Version 3.5 (CLM3.5) has significantly improved the simulation of hydrologic cycles compared to its earlier version (CLM3.0) owing to a series of new and modified parameterizations for canopy and soil processes. One of the key elements is the addition of a soil resistance to effectively reduce soil evaporation (Es) and improve the partitioning of evapotranspiration. This soil resistance, however, is found to be physically inconsistent under wet soil conditions and implicitly include the effects of dead leaves. A new treatment with three components are proposed here: (1) two different approaches to better reflect the soil moisture limitation to Es, the so-called α and β methods combined and a new soil resistance; (2) anew surface resistance to explicitly represent the effect of plant litter cover on water vapor transfer; and (3) an explicit consideration of the effect of under-canopy atmospheric stability on the under-canopy turbulent resistance. The effects of each modification vary locally and seasonally, and their combination leads to regional differences between CLM3.5 and our new formulations. Our new formulations tend to have higher Es over high latitudes and similar or slightly higher Es in dry regions. A larger reduction of Es by the new formulations is also found over regions with relatively wet soil and more vegetation, in better agreement with previous ET partitioning studies. Copyright 2009 by the American Geophysical Union.
- Wang, A., & Zeng, X. (2009). Improving the treatment of the vertical snow burial fraction over short vegetation in the NCAR CLM3. Advances in Atmospheric Sciences, 26(5), 877-886.More infoAbstract: One deficiency of the NCAR Community Land Model (CLM3) is the disappearance of the simulated snow even in the middle of winter over a boreal grassland site due to unrealistically modeled high downward turbulent fluxes. This is caused by the inappropriate treatment of the vertical snow burial fraction for short vegetation. A new snow burial fraction formulation for short vegetation is then proposed and validated using in situ observations. This modification in the CLM3 largely removes the unrealistic surface turbulent fluxes, leading to a more reasonable snowmelt process, and improves the snow water equivalent (SWE) simulation. Moreover, global offline simulations show that the proposed formulation decreases sensible and latent heat fluxes as well as the ground temperature during the snowmelt season over short vegetation dominant regions. Correspondingly, the SWE is enhanced, leading to the increase in snowmelt-induced runoff during the same period. Furthermore, sensitivity tests indicate that these improvements are insensitive to the exact functional form or parameter values in the proposed formulation. © 2009 Chinese National Committee for International Association of Meteorology and Atmospheric Sciences, Institute of Atmospheric Physics, Science Press and Springer Berlin Heidelberg.
- Zeng, X., & Decker, M. (2009). Improving the numerical solution of soil moisture-based Richards equation for land models with a deep or shallow water table. Journal of Hydrometeorology, 10(1), 308-319.More infoAbstract: The soil moisture-based Richards equation is widely used in land models for weather and climate studies, but its numerical solution using the mass-conservative scheme in the Community Land Model is found to be deficient when the water table is within the model domain. Furthermore, these deficiencies cannot be reduced by using a smaller grid spacing. The numerical errors are much smaller when the water table is below the model domain. These deficiencies were overlooked in the past, most likely because of the more dominant influence of the free drainage bottom boundary condition used by many land models. They are fixed here by explicitly subtracting the hydrostatic equilibrium soil moisture distribution from the Richards equation. This equilibrium distribution can be derived at each time step from a constant hydraulic (i.e., capillary plus gravitational) potential above the water table, representing a steady-state solution of the Richards equation. Furthermore, because the free drainage condition has serious deficiencies, a new bottom boundary condition based on the equilibrium soil moisture distribution at each time step is proposed that also provides an effective and direct coupling between groundwater and surface water. © 2009 American Meteorological Society.
- Brunke, M. A., Zeng, X., Misra, V., & Beljaars, A. (2008). Integration of a prognostic sea surface skin temperature scheme into weather and climate models. Journal of Geophysical Research D: Atmospheres, 113(21).More infoAbstract: A prognostic skin sea surface temperature (SST) algorithm is included in the Community Atmosphere Model (CAM3.1). With this algorithm, the model produces a mean diurnal cycle of 0.5 K primarily in tropical and subtropical oceans of the summer hemisphere with isolated areas of >2 K. The diurnal cycle in air temperature is substantially increased over regions with large skin SST diurnal cycles, and the enhanced diurnal cycle agrees better with the TAO buoy data over the equatorial Pacific. There is also a significant effect on the diurnal cycle in precipitation particularly in the increased hourly maximum precipitation over the Bay of Bengal and the western Pacific in boreal summer. These changes to the diurnal cycle in SST affect the seasonal climatologies in the model with improved seasonal mean precipitation mainly in the tropics following similar patterns to the hourly maximum precipitation rates. The near-surface flow across the Indian subcontinent is also increased during the summer monsoon resulting in higher seasonal precipitation over the Bay of Bengal. Copyright 2008 by the American Geophysical Union.
- Misra, V., Marx, L., Brunke, M., & Zeng, X. (2008). The equatorial pacific cold tongue bias in a coupled climate model. Journal of Climate, 21(22), 5852-5869.More infoAbstract: A set of multidecadal coupled ocean-atmosphere model integrations are conducted with different time steps for coupling between the atmosphere and the ocean. It is shown that the mean state of the equatorial Pacific does not change in a statistically significant manner when the coupling interval between the atmospheric general circulation model (AGCM) and the ocean general circulation model (OGCM) is changed from 1 day to 2 or even 3 days. It is argued that because the coarse resolution of the AGCM precludes resolving realistic "weather" events, changing the coupling interval from 1 day to 2 or 3 days has very little impact on the mean coupled climate. On the other hand, reducing the coupling interval to 3 h had a much stronger impact on the mean state of the equatorial Pacific and the concomitant general circulation. A novel experiment that incorporates a (pseudo) interaction of the atmosphere with SST at every time step of the AGCM was also conducted. In this unique coupled model experiment, the AGCM at every time step mutually interacts with the skin SST. This skin SST is anchored to the bulk SST, which is updated from the OGCM once a day. Both of these experiments reduced the cold tongue bias moderately over the equatorial Pacific Ocean with a corresponding reduction in the easterly wind stress bias relative to the control integration. It is stressed from the results of these model experiments that the impact of high-frequency air-sea coupling is significant on the cold tongue bias. The interannual variation of the equatorial Pacific was less sensitive to the coupling time step between the AGCM and the OGCM. Increasing (reducing) the coupling interval of the air-sea interaction had the effect of weakening (marginally strengthening) the interannual variations of the equatorial Pacific Ocean. It is argued that the low-frequency response of the upper ocean, including the cold tongue bias, is modulated by the atmospheric stochastic forcing on the coupled ocean-atmosphere system. This effect of the atmospheric stochastic forcing is affected by the frequency of the air-sea coupling and is found to be stronger than the rectification effect of the diurnal variations of the air-sea interaction on the low frequency. This may be a result of a limitation in the coupled model used in this study in which the OGCM has an inadequate vertical resolution in the mixed layer to sustain diurnal variations in the upper ocean. © 2008 American Meteorological Society.
- Wang, Z., & Zeng, X. (2008). Snow albedo's dependence on solar zenith angle from in situ and MODIS data. Atmos. Oceanic Sci. Lett., 1, 45-50.
- Yang, F., Mitchell, K., Hou, Y., Dai, Y., Zeng, X., Wang, Z., & Liang, X. (2008). Dependence of land surface albedo on solar zenith angle: Observations and model parameterization. Journal of Applied Meteorology and Climatology, 47(11), 2963-2982.More infoAbstract: This study examines the dependence of surface albedo on solar zenith angle (SZA) over snow-free land surfaces using the intensive observations of surface shortwave fluxes made by the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program and the National Oceanic and Atmospheric Administration Surface Radiation Budget Network (SURFRAD) in 1997-2005. Results are used to evaluate the National Centers for Environmental Prediction (NCEP) Global Forecast Systems (GFS) parameterization and several new parameterizations derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) products. The influence of clouds on surface albedo and the albedo difference between morning and afternoon observations are also investigated. A new approach is taken to partition the observed upward flux so that the direct-beam and diffuse albedos can be separately computed. The study focused first on the ARM Southern Great Plains Central Facility site. It is found that the diffuse albedo prescribed in the NCEP GFS matched closely with the observations. The direct-beam albedo parameterized in the GFS is largely underestimated at all SZAs. The parameterizations derived from the MODIS product underestimated the direct-beam albedo at large SZAs and slightly overestimated it at small SZAs. Similar results are obtained from the analyses of observations at other stations. It is also found that the morning and afternoon dependencies of direct-beam albedo on SZA differ among the stations. Attempts are made to improve numerical model algorithms that parameterize the direct-beam albedo as a product of the direct-beam albedo at SZA = 60° (or the diffuse albedo), which varies with surface type or geographical location and/or season, and a function that depends only on SZA. A method is presented for computing the direct-beam albedos over these snow-free land points without referring to a particular land-cover classification scheme, which often differs from model to model. © 2008 American Meteorological Society.
- Zeng, X., Zeng, X., & Barlage, M. (2008). Growing temperate shrubs over arid and semiarid regions in the Community Land Model-Dynamic Global Vegetation Model. Global Biogeochemical Cycles, 22(3).More infoAbstract: Arid and semiarid regions represent a large fraction of global land, but most of the existing dynamic global vegetation models (DGVMs) do not include shrubs or do not effectively distinguish shrubs from grasses, and hence cannot realistically reproduce the ecosystem formation and variability there. A shrub submodel is developed here for the Community Land Model-DGVM (CLM-DGVM), and the major revisions include (1) explicit consideration of shrubs' drought tolerance in the photosynthesis computation; (2) use of appropriate phenology type and morphology parameters for shrubs; (3) consistent treatment of fractional vegetation coverage; (4) development of tree/grass/shrub hierarchy for light competition; and (5) improvement of the allocation scheme to avoid unrealistic behaviors. Preliminary global offline CLM-DGVM simulations for 400 years show that, with the shrub submodel, the simulated global distribution of temperate shrubs agrees with Moderate Resolution Imaging Spectroradiometer (MODIS) data. The simulated shrub coverage reaches its peak around annual precipitation (Pann) of 300 mm, the grass coverage reaches its peak over a broad range of Pann(from 400 to 1100 mm), and the tree coverage reaches its peak for Pann = 1500 mm or higher, all in good agreement with MODIS data. Copyright 2008 by the American Geophysical Union.
- Wang, Z., Zeng, X., & Barlage, M. (2007). Moderate resolution imaging spectroradiometer bidirectional reflectance distribution function-based albedo parameterization for weather and climate models. Journal of Geophysical Research D: Atmospheres, 112(2).More infoAbstract: The albedo of land surfaces is an important parameter for surface energy balance and is dependent on the solar zenith angle and the three-dimensional structure of the canopies. The Moderate Resolution Imaging Spectroradiometer (MODIS) bidirectional reflectance distribution function (BRDF) algorithm is reformulated to derive a new two-parameter scheme for the computation of land surface albedo and its solar zenith angle (SZA) dependence for use in weather and climate models as well as in the remote sensing retrieval of surface solar fluxes. In this formulation the season- and pixel- dependent black-sky albedo at 60° SZA can be directly prescribed using the MODIS BRDF data while the two parameters are taken as a function of vegetation type only. Comparison of this formulation with those used in weather, climate, and data assimilation models (at NCAR, NCEP, and NASA) reveals the deficiencies in the treatment of SZA dependence and white-sky albedo in these models. Similarly, comparison of this formulation with those computed using the remotely sensed solar flux data from three groups (International Satellite Cloud Climatology Project (ISCCP)-FD, NASA's Clouds and the Earth's Radiant Energy System Tropical Rainfall Measuring Mission (CERES/ TRMM), and University of Maryland) shows that surface albedos from these data sets are at times unrealistic, and suggestions are made regarding how to better treat the surface albedo and its SZA dependence in the retrieval of surface and atmospheric solar fluxes. Copyright 2007 by the American Geophysical Union.
- Zeng, X., & Wang, A. (2007). Consistent parameterization of roughness length and displacement height for sparse and dense canopies in land models. Journal of Hydrometeorology, 8(4), 730-737.More infoAbstract: While progress has been made in the treatment of turbulence below, within, and above canopy in land models, not much attention has been paid to the convergence of canopy roughness length and displacement height to bare soil values as the above-ground biomass, or the sum of leaf and stem area indices, becomes zero. Preliminary formulations have been developed to ensure this convergence for the Community Land Model version 3 (CLM3) and are found to significantly improve the wintertime simulation of sensible heat flux (SH) compared with observational data over the Cabauw site in the Netherlands. The simulation of latent heat flux (LH) is also moderately improved. For global offline CLM3 simulations, the new formulations change SH by more than 5 W m-2 over many regions, while the change of LH is less than 1 W m-2 over most of the regions. © 2007 American Meteorological Society.
- Zeng, X., & Zeng, X. (2007). Transition and pattern diversity in arid and semiarid grassland: A modeling study. Journal of Geophysical Research G: Biogeosciences, 112(4).More infoAbstract: Abrupt transitions between large-scale grassland and desert in arid and semiarid regions have been observed in nature and reproduced by modeling studies. Observations also show the existence of nonuniform fine-scale vegetation patterns along the transition zone. This paper attempts to better understand these observations from two very different spatial scales. By explicitly introducing horizontal interaction terms into our previous dynamical grassland model, vegetation patterns with high diversities are found in the transition zone, and the system possesses an infinite number of equilibrium states in response to a given climatic forcing. The transition can be elucidated in two ways. In terms of the vegetation formations, the ecosystem undergoes the transition from uniform grassland to regular and irregular vegetation patterns, and then to pure desert as the moisture index (i.e., the ratio of precipitation over potential evaporation) decreases. In terms of biomass, the transition from grassland to desert goes through a narrow range of moisture index under which grassland is most fragile, as indicated by erratic vegetation patterns and large variation of average biomass. The existence of this range, however, has not been reported in previous modeling studies, and still needs to be validated using observational data. Copyright 2007 by the American Geophysical Union.
- Brunke, M. A., Zhou, M., Zeng, X., & Andreas, E. L. (2006). An intercomparison of bulk aerodynamic algorithms used over sea ice with data from the Surface Heat Budget for the Arctic Ocean (SHEBA) experiment. Journal of Geophysical Research C: Oceans, 111(9).More infoAbstract: The presence of sea ice fundamentally changes the energy and momentum exchange between the ocean and the atmosphere in the Arctic. Thus an accurate representation of the surface turbulent fluxes in climate models is a necessity. An intercomparison of bulk aerodynamic algorithms that calculate surface turbulent fluxes in four climate and numerical weather prediction models is undertaken using data from the Surface Heat Budget of the Arctic Ocean (SHEBA) field experiment, which occurred on the ice in the Beaufort and Chukchi seas north of Alaska from October 1997 to October 1998. Algorithm deficiencies include the consistently higher wind stresses produced by the Arctic Regional Climate System Model (ARCSYM) algorithm; the lower sensible heat fluxes under stable conditions by the algorithms in ARCSYM, the National Center for Environmental Prediction's Global Forecasting System model, and the European Centre for Medium-Range Weather Forecasts (ECMWF) model; and the lower wind stresses by the National Center for Atmospheric Research's Community Climate System Model (CCSM) algorithm under stable conditions. Unlike the constants used in most of the four model algorithms, the roughness lengths for momentum can be fitted by an exponential function with parameters that account for the seasonality in the roughness length. The roughness lengths for heat, Zot, can be considered a constant (e.g., that used in CCSM, 0.5 mm), similar to what was found by Andreas et al. (2004). When these roughness lengths were implemented into the CCSM and ECMWF algorithms, they produced slightly better wind stresses and sensible heat fluxes most of the time. Copyright 2006 by the American Geophysical Union.
- Decker, M., & Zeng, X. (2006). An empirical formulation of soil ice fraction based on in situ observations. Geophysical Research Letters, 33(5).More infoAbstract: In situ data from around the globe are used to evaluate the parameterization of the fraction of soil ice over total moisture (fi) as used in four weather and climate models. Based on these evaluations and data analysis, an overall realistic (empirical) formulation is developed to compute fi as a function of soil temperature and the ratio of total moisture over its saturated value. This new formulation is found to reduce the winter ground temperature by up to 3 K over high latitudes in the offline Community Land Model (CLM3) simulation. Copyright 2006 by the American Geophysical Union.
- Dickinson, R. E., Oleson, K. W., Bonan, G., Hoffman, F., Thornton, P., Vertenstein, M., Yang, Z., & Zeng, X. (2006). The Community Land Model and its climate statistics as a component of the Community Climate System Model. Journal of Climate, 19(11), 2302-2324.More infoAbstract: Several multidecadal simulations have been carried out with the new version of the Community Climate System Model (CCSM). This paper reports an analysis of the land component of these simulations. Global annual averages over land appear to be within the uncertainty of observational datasets, but the seasonal cycle over land of temperature and precipitation appears to be too weak. These departures from observations appear to be primarily a consequence of deficiencies in the simulation of the atmospheric model rather than of the land processes. High latitudes of northern winter are biased sufficiently warm to have a significant impact on the simulated value of global land temperature. The precipitation is approximately doubled from what it should be at some locations, and the snowpack and spring runoff are also excessive. The winter precipitation over Tibet is larger than observed. About two-thirds of this precipitation is sublimated during the winter, but what remains still produces a snowpack that is very large compared to that observed with correspondingly excessive spring runoff. A large cold anomaly over the Sahara Desert and Sahel also appears to be a consequence of a large anomaly in downward longwave radiation; low column water vapor appears to be most responsible. The modeled precipitation over the Amazon basin is low compared to that observed, the soil becomes too dry, and the temperature is too warm during the dry season. © 2006 American Meteorological Society.
- Kursinski, A. L., & Zeng, X. (2006). Areal estimation of intensity and frequency of summertime precipitation over a midlatitude region. Geophysical Research Letters, 33(22).More infoAbstract: Spatially averaged precipitation characteristics derived from observations are needed to evaluate climate model generated precipitation intensity and frequency. In this study, observations from gauges and radar datasets are used to address three issues that are related to the areal estimation of intensity and frequency of summertime precipitation over the state of Ohio, USA. First, spatial averages of intensity and frequency from point precipitation are found to be very sensitive to the averaging method (i.e., spatial averaging followed by temporal averaging versus temporal followed by spatial averaging), particularly at lower precipitation thresholds. Second, approximately 30 gauges are found to be necessary to construct average intensity for a typical climate model grid, which is much higher than the gauge number required for average precipitation amount. Finally, the fractional area receiving rainfall is found to have a fairly linear relation with the precipitation amount averaged over the grid, but the slope differs between the gauge and radar data. Copyright 2006 by the American Geophysical Union.
- Miller, J., Barlage, M., Zeng, X., Wei, H., Mitchell, K., & Tarpley, D. (2006). Sensitivity of the NCEP/Noah land surface model to the MODIS green vegetation fraction data set. Geophysical Research Letters, 33(13).More infoAbstract: Land surface processes are strongly controlled by vegetation cover. Current land surface models represent vegetation as a combination of leaf area index (LAI) and green vegetation fraction (GVF) parameters. The purpose of the study is to examine the impact of a spatially and temporally detailed Moderate Resolution Imaging Spectroradiometer (MODIS)-based GVF on surface processes in the NCEP Noah land surface model. The largest differences between the GVF data set currently used by the Noah model and the new MODIS GVF data set occur in winter and for tree-dominated vegetation classes. The greatest impact of the new GVF data on the surface energy and water balance is seen during the summer, when the transpiration is increased by more than 10 W/m2 on average for most vegetation types and the July averaged daily transpiration rate is increased by up to 50 W/m2 for evergreen needleleaf sites. Copyright 2006 by the American Geophysical Union.
- Wang, A., Zeng, X., Shen, S. S., Zeng, Q., & Dickinson, R. E. (2006). Time scales of land surface hydrology. Journal of Hydrometeorology, 7(5), 868-879.More infoAbstract: This paper intends to investigate the time scales of land surface hydrology and enhance the understanding the hydrological cycle between the atmosphere, vegetation, and soil. A three-layer model for land surface hydrology is developed to study the temporal variation and vertical structure of water reservoirs in the vegetation-soil system in response to precipitation forcing. The model is an extension of the existing one-layer bucket model. A new time scale is derived, and it better represents the response time scale of soil moisture in the root zone than the previously derived inherent time scale (i.e., the ratio of the field capacity to the potential evaporation). It is found that different water reservoirs of the vegetation-soil system have different time scales. Precipitation forcing is mainly concentrated on short time scales with small low-frequency components, but it can cause long time-scale disturbances in the soil moisture of root zone. This time scale increases with soil depth, but it can be reduced significantly under wetter conditions. Although the time scale of total water content in the vertical column in the three-layer model is similar to that of the one-layer bucket model, the time scale of evapotranspiration is very different. This suggests the need to consider the vertical structure in land surface hydrology reservoirs and in climate study. © 2006 American Meteorological Society.
- Zeng, X., Wang, A., Zeng, Q., Dickinson, R. E., Zeng, X., & Shen, S. S. (2006). Intermediately complex models for the hydrological interactions in the atmosphere-vegetation-soil system. Advances in Atmospheric Sciences, 23(1), 127-140.More infoAbstract: This paper investigates the hydrological interactions in the atmosphere-evegetation-soil system by using the bucket model and several new simplified intermediately complex models. The results of mathematical analysis and numerical simulations show that these models, despite their simplicity, can very clearly reveal the essential features of the rather complex hydrological system of atmosphere-ecosystem-soil. For given atmospheric variables, these models clearly demonstrate multiple timescales, the "red shift" of response spectra, multi-equilibria and limit cycles, bifurcation, abrupt change, self-organization, recovery, "desertification", and chaos. Most of these agree with observations. Especially, the weakening of "shading effect" of living canopy and the wilted biomass might be a major mechanism leading to the desertification in a relatively short period due to overgrazing, and the desertification in a relatively long period or in climate of change might be due to both Charney's mechanism and the shading effect. These ideas could be validated with further numerical simulations. In the paper, some methods for improving the estimation of timescales in the soil water evolution responding to the forcing are also proposed.
- Zhou, M., Li, S., Chen, Z., Zhang, Z., & Zeng, X. (2006). Characteristics of the atmospheric surface layer and heat budget over the Arctic ice in summer. Chinese Journal of Geophysics (Acta Geophysica Sinica), 49(2), 353-359.More infoAbstract: Observational data of August 19-24, 1999 during the first Chinese arctic research exploration were analyzed and the vertical distribution of meteorological variables in the surface layer and the heat budget problem over ice in the Arctic region in summer were discussed. The results show that the humidity of surface layer is very high relative to the ice surface, and is often close to its saturated value. The ice temperature has a significant diurnal variation. The ice temperature gradient near the surface is relatively large at night, and is very small during daytime. For the top 40cm of ice, the ice temperature sometimes reaches a maximum value at 20cm depth. The heat budget analysis shows that the heat loss during night caused by the radiation cooling effect is compensated by heat transferred from the inner ice layer to the ice surface. In daytime, the inner ice heat comes mainly from penetrating shortwave radiation, and heat exchange between the ice surface and inner ice is very weak. The ice/snow thaw heat in daytime can not be neglected.
- Zhou, M., Zeng, X., Brunke, M., Zhang, Z., & Fairall, C. (2006). An analysis of statistical characteristics of stratus and stratocumulus over eastern Pacific. Geophysical Research Letters, 33(2).More infoAbstract: Observational measurements of cloud base, top, thickness, fraction, and liquid water path (LWP) are analyzed to study the macro- and micro-physical properties of stratus and stratocumulus over the eastern Pacific. It is found that the increase in liquid water content with height in cloud is significantly less than the adiabatic rate. The variations of cloud base and top both contribute to the variation of cloud thickness that approximately fits a normal distribution better than either cloud base or top. Because the distribution of LWP depends on the intervals of cloud fraction (or other environmental variables, for example, relative humidity), the determination of cloud fraction from the mean LWP alone would introduce large uncertainties particularly when LWP is large. Copyright 2006 by the American Geophysical Union.
- Barlage, M., Zeng, X., Wei, H., & Mitchell, K. E. (2005). A global 0.05° maximum albedo dataset of snow-covered land based on MODIS observations. Geophysical Research Letters, 32(17), 1-5.More infoAbstract: A new global 0.05° maximum albedo for snow-covered land is developed from BRDF/Albedo, reflectance and land cover measured from the MODIS sensor on board the Terra and Aqua satellites. The dataset is similar to previous maximum snow albedo datasets, but is available at higher resolution. The dataset displays: (1) high albedo at very high latitudes with tundra and open shrub land cover; (2) a local minimum (0.35) in zonally-averaged albedo at 56-60°N due to boreal forest cover; and (3) a local maximum (0.70) in zonally-averaged albedo at 42-45°N due to crop and grassland. The dataset is tested in the Noah land model at 0.125° resolution as used in the North American Land Data Assimilation System. Compared with the original data, the new dataset increases the spatial heterogeneity in the Noah model during winter and produces surface energy component differences of 10 W/m2 during the snowmelt period. Copyright 2005 by the American Geophysical Union.
- Er, L. u., & Zeng, X. (2005). Understanding different precipitation seasonality regimes from water vapor and temperature fields: Case studies. Geophysical Research Letters, 32(22), 1-5.More infoAbstract: Different precipitation seasonality regimes, although produced by different surface and atmospheric conditions, can be understood from the basic atmospheric fields. By comparing the change of water vapor from winter to summer with the change of temperature using the NARR reanalysis, three seasonality cases over the United States and Mexico are analyzed. In the western coast of the U.S., the change of temperature from winter to summer is much greater than the change of water vapor. So, relative to summer, the coldness of the winter air is much more significant than the dryness, which makes the winter have a large saturation extent and thus precipitation. In contrast, over South Mexico, the much more significant moistness of the summer air than its warmness is important to the summer monsoon precipitation. In the southeastern U.S. where precipitation occurs throughout the year, the changes of water vapor and temperature are roughly equivalent. Copyright 2005 by the American Geophysical Union.
- Wang, Z., Barlage, M., Zeng, X., Dickinson, R. E., & Schaaf, C. B. (2005). The solar zenith angle dependence of desert albedo. Geophysical Research Letters, 32(5), 1-4.More infoAbstract: Most land models assume that the bare soil albedo is a function of soil color and moisture but independent of solar zenith angle (SZA). However, analyses of the Moderate Resolution Imaging Spectroradiometer (MODIS) Bidirectional Reflectance Distribution Function (BRDF) and albedo data over thirty desert locations indicate that bare soil albedo does vary with SZA. This is further confirmed using the in situ data. In particular, bare soil albedo normalized by its value at 60° SZA can be adequately represented by a one-parameter formulation (1 + C)/(1 + 2C * cos(SZA)) or a two-parameter formulation (1 + B1 * f1(SZA) + B2 * f2(SZA)). Using the MODIS and in situ data, the empirical parameters C, B1, and B2 are taken as 0.15, 0.346 and 0.063. The SZA dependence of soil albedo is also found to significantly affect the modeling of land surface energy balance over a desert site. Copyright 2005 by the American Geophysical Union.
- Zeng, X., & Beljaars, A. (2005). A prognostic scheme of sea surface skin temperature for modeling and data assimilation. Geophysical Research Letters, 32(14), 1-4.More infoAbstract: A prognostic scheme is derived for the computation of sea surface skin temperature in weather forecasting, four-dimensional data assimilation, and ocean-atmosphere coupled modeling. This scheme is then tested using the in situ data over tropical and midlatitude oceans. By implementing this scheme into the ECMWF model, the diurnal variation of sea surface temperature as measured by the geostationary satellite can also be reproduced. Copyright 2005 by the American Geophysical Union.
- Zeng, X., Dickinson, R. E., Barlage, M., Dai, Y., Wang, G., & Oleson, K. (2005). Treatment of undercanopy turbulence in land models. Journal of Climate, 18(23), 5086-5094.More infoAbstract: In arid and semiarid regions most of the solar radiation penetrates through the canopy and reaches the ground, and hence the turbulent exchange coefficient under canopy Cs becomes important. The use of a constant Cs that is only appropriate for thick canopies is found to be primarily responsible for the excessive warm bias of around 10 K in monthly mean ground temperature over these regions in version 2 of the Community Climate System Model (CCSM2). New Cs formulations are developed for the consistent treatment of undercanopy turbulence for both thick and thin canopies in land models, and provide a preliminary solution of this problem. © 2005 American Meteorological Society.
- Zeng, X., Wang, A., Zhao, G., S., S., Zeng, X., & Zeng, Q. (2005). Ecological dynamic model of grassland and its practical verification. Science in China, Series C: Life Sciences, 48(1), 41-48.More infoPMID: 15844356;Abstract: Based on the physico-biophysical considerations, mathematical analysis and some approximate formulations generally adopted in meteorology and ecology, an ecological dynamic model of grassland is developed. The model consists of three interactive variables, i.e. the biomass of living grass, the biomass of wilted grass, and the soil wetness. The major biophysical processes are represented in parameterization formulas, and the model parameters can be determined inversely by using the observational climatological and ecological data. Some major parameters are adjusted by this method to fit the data (although incomplete) in the Inner Mongolia grassland, and other secondary parameters are estimated through sensitivity studies. The model results are well agreed with reality, e.g., (i) the maintenance of grassland requires a minimum amount of annual precipitation (approximately 300 mm); (ii) there is a significant relationship between the annual precipitation and the biomass of living grass; and (iii) the overgrazing will eventually result in desertification. A specific emphasis is put on the shading effect of the wilted grass accumulated on the soil surface. It effectively reduces the soil surface temperature and the evaporation, hence benefits the maintenance of grassland and the reduction of water loss in the soil. Copyright by Science in China Press 2005.
- Zeng, X., Zeng, X., Shen, S. S., Dickinson, R. E., & Zeng, Q. (2005). Vegetation-soil water interaction within a dynamical ecosystem model of grassland in semi-arid areas. Tellus, Series B: Chemical and Physical Meteorology, 57(3), 189-202.More infoAbstract: A dynamical ecosystem model with three variables, living biomass, wilted biomass and available soil wetness, is developed to examine the vegetation-soil water interaction in semi-arid areas. The governing equations are based on the mass conservation law. The physical and biophysical processes are formulated with the parameters estimated from observational data. Both numerical results and qualitative analysis of the model as well as observational data indicate that the maintenance of a grassland requires a minimum precipitation (or equivalently, a minimum moisture index), and the grassland and desert ecosystem can coexist when precipitation is within a range above this threshold. Sensitivity studies show that these numerical results are robust with respect to model parameters and the transformation functions. It is also found that the wilted vegetation plays a very important role in shaping the transition between grassland and desert. By using the theories of an attractor basin and multiple equilibrium states, the conditions for grassland maintenance and the strategy of grazing are also analysed. Copyright © Blackwell Munksgaard, 2005.
- Barlage, M., & Zeng, X. (2004). Impact of observed vegetation root distribution on seasonal global simulations of land surface processes. Journal of Geophysical Research D: Atmospheres, 109(9), D09101 1-14.More infoAbstract: Using a global root distribution derived from observations, results from June to August ensemble simulations are presented. The new root distribution shifts the location of roots in the soil in most regions of the world. Root relocation depends on land use type with some roots located shallower (e.g., grasslands) and others deeper (e.g., tropical forests). Comparison of the boreal summer results of 1988 and 1993 for a control simulation and simulation with the new root distribution produces, in several regions of the world, statistically significant differences of up to 40 W/m2 in the components of the surface energy budget. Analysis of the eastern and western United States shows statistically significant changes of over 1 K in surface air temperature and over 25 W/m2 in surface energy components for both seasonal averages and diurnal cycles. Comparison with observations shows that the new root distribution improves the surface air temperature simulation, especially in 1993, but any precipitation improvement is statistically insignificant. Copyright 2004 by the American Geophysical Union.
- Barlage, M., & Zeng, X. (2004). The effects of observed fractional vegetation cover on the land surface climatology of the community land model. Journal of Hydrometeorology, 5(5), 823-830.More infoAbstract: Accurate modeling of surface processes requires a specification of the amount of land covered by vegetation. The National Center for Atmospheric Research Community Land Model (CLM2) does not realistically represent sparsely vegetated regions because of a lack of bare soil in the model. In this study, the existing CLM2 surface dataset is replaced by a global 1-km fractional vegetation cover dataset. This results in a doubling of global bare soil fraction in the model. It also significantly affects the fractional coverages of shrub, grass, and crop compared with only minor changes to trees. Regional changes occur most greatly in Australia, with an increase of over 0.4 in bare soil fraction. The western United States, southern South America, and southern Africa show fractional increases of more than 0.2. Simulations of CLM2 coupled with the Community Atmosphere Model (CAM2) show several regions with statistically significant decreases of up to 2 K in 2-m air temperature and up to 10 K in ground temperature, which reduces the high temperature bias in arid and semiarid regions in the model. In Australia, the vegetation changes result in an increase in net downward longwave radiation, which is balanced by an increase of latent and sensible heat fluxes and a decrease of absorbed solar radiation. © 2004 American Meteorological Society.
- Chase, T. N., Pielke Sr., R. A., Herman, B., & Zeng, X. (2004). Likelihood of rapidly increasing surface temperatures unaccompanied by strong warming in the free troposphere. Climate Research, 25(3), 185-190.More infoAbstract: Recent model simulations of the effects of increasing greenhouse gases combined with other anthropogenic effects predicted larger rates of warming in the mid and upper troposphere than near the Earth's surface. In multiple model comparisons we find that accelerated upper-level warming is simulated in all models for the greenhouse-gas/direct-aerosol forcing representative of 1979-2000. However, in a test of model predictive skill, a comparison with observations shows no warming of the free troposphere over this period. We assessed the likelihood that such a disparity between model projection and observations could be generated by forcing uncertainties or chance model fluctuations, by comparing all possible 22 yr temperature trends in a series of climate simulations. We find that it is extremely unlikely for near-surface air temperatures (surface temperatures) to increase at the magnitude observed since 1979 without a larger warming in the mid-troposphere. Warming of the surface relative to the mid-troposphere was also more likely in control simulations than under anthropogenic forcing. Because errors in the vertical temperature structure would be expected to create errors in water-vapor feedback, cloud cover and moisture content, these results suggest the need for great caution when applying the simulations to future climate predictions and to impact assessments. © Inter-Research 2004.
- Curry, J. A., Bentamy, A., Bourassa, M. A., Bourras, D., Bradley, E. F., Brunke, M., Castro, S., Chou, S. H., Clayson, C. A., Emery, W. J., Eymard, L., Fairall, C. W., Kubota, M., Lin, B., Perrie, W., Reeder, R. A., Renfrew, I. A., Rossow, W. B., Schulz, J., , Smith, S. R., et al. (2004). SEAFLUX. Bulletin of the American Meteorological Society, 85(3), 409-424.More infoAbstract: The SEAFLUX strategy to assess satellite flux products, and to improve the accuracy and resolution of satellite-derived sensible and latent heat fluxes, is described. Analysis shows that at present, zonal and monthly averaged flux values have significant uncertainties, based on the comparison of climatologies determined from ships, numerical weather prediction analyses, and satellite products.
- Tian, Y., Dickinson, R. E., Zhou, L., Zeng, X., Dai, Y., Myneni, R. B., Knyazikhin, Y., Zhang, X., Friedl, M., Yu, H., Wu, W., & Shaikh, M. (2004). Comparison of seasonal and spatial variations of leaf area index and fraction of absorbed photosynthetically active radiation from Moderate Resolution Imaging Spectroradiometer (MODIS) and Common Land Model. Journal of Geophysical Research D: Atmospheres, 109(1), D01103 1-14.More infoAbstract: This paper compares by land cover type seasonal and spatial variations of MODIS leaf area index (LAI) and fraction of photosynthetically active radiation (0.4-0.7 μm) absorbed by vegetation (FPAR) from 2.5 years with those from the Common Land Model (CLM) and investigates possible reasons for notable differences. The FPAR value is mainly determined by LAI in MODIS and both LAI and stem area index (SAI) in CLM. On average, the model underestimates FPAR in the Southern Hemisphere and overestimates FPAR over most areas in the Northern Hemisphere compared to MODIS observations during all seasons except northern middle latitude summer. Such overestimation is most significant in winter over northern high latitudes. The MODIS LAI is generally consistent with the model during the snow-free periods but may be underestimated in the presence of snow, especially for evergreen trees. The positive FPAR bias is mainly attributed to CLM SAI of deciduous canopy and higher LAI than MODIS for evergreen canopy as well. The negative FPAR bias results from several factors, including differences in LAI and soil albedo between CLM and MODIS or limitations of the geometric optics scheme used in the model. Therefore the MODIS algorithm needs to better represent the winter LAI retrievals, while the model needs to better quantify LAI and SAI. Since stems will not have the same single-scattering albedo as green leaves, it may be inappropriate for the model to treat LAI and SAI the same in the FPAR and albedo parameterizations. If so, the role of SAI in these parameterizations needs reformulation. Copyright 2004 by the American Geophysical Union.
- Tsukernik, M., Chase, T. N., Serreze, M. C., Barry, R. G., Pielke Sr., R., Herman, B., & Zeng, X. (2004). On the regulation of minimum mid-tropospheric temperatures in the Arctic. Geophysical Research Letters, 31(6), L06112 1-4.More infoAbstract: Observations indicate a minimum mid-tropospheric Arctic winter temperature of about -45°C at 500 hPa. This minimum temperature coincides with that predicted for moist adiabatic ascent over a sea surface near its salinity-adjusted freezing point. NCAR/NCEP Reanalysis data show that convective heating maxima averaged over the 50-70°N latitude band coincide both in longitude and altitude with total horizontal energy flux maxima entering the Arctic, indicating the significance of convection over open water on the winter Arctic energy budget. NCAR CCM single column model experiments simulating convective warming of a cold airmass moving over open water and radiative cooling as it moves again over cold land/sea ice support the hypothesis that the -45°C threshold can be maintained for 10-14 days after convective warming occurs. We speculate on the implications of this regulatory mechanism on surface temperatures. Copyright 2004 by the American Geophysical Union.
- Wang, Z., Zeng, X., Barlage, M., Dickinson, R. E., Gao, F., & Schaaf, C. B. (2004). Using MODIS BRDF and albedo data to evaluate global model land surface albedo. Journal of Hydrometeorology, 5(1), 3-14.More infoAbstract: The land surface albedo in the NCAR Community Climate System Model (CCSM2) is calculated based on a two-stream approximation, which does not include the effect of three-dimensional vegetation structure on radiative transfer. The model albedo (including monthly averaged albedo, direct albedo at local noon, and the solar zenith angle dependence of albedo) is evaluated using the Moderate Resolution Imaging Spectroradiometer (MODIS) Bidirectional Reflectance Distribution Function (BRDF) and albedo data acquired during July 2001-July 2002. The model monthly averaged albedos in February and July are close to the MODIS white-sky albedos (within 0.02 or statistically insignificant) over about 40% of the global land between 60°S and 70°N. However, CCSM2 significantly underestimates albedo by 0.05 or more over deserts (e.g., the Sahara Desert) and some semiarid regions (e.g., parts of Australia). The difference between the model direct albedo at local noon and the MODIS black-sky albedo for the near-infrared (NIR) band (with wavelength > 0.7 μm) is larger than the difference for the visible band (with wavelength < 0.7 μm) for most snow-free regions. For eleven model grid cells with different dominant plant functional types, the model diffuse NIR albedo is higher by 0.05 or more than the MODIS white-sky albedo in five of these cells. Direct albedos from the model and MODIS (as computed using the BRDF parameters) increase with solar zenith angles, but model albedo increases faster than the MODIS data. These analyses and the MODIS BRDF and albedo data provide a starting point toward developing a BRDF-based treatment of radiative transfer through a canopy for land surface models that can realistically simulate the mean albedo and the solar zenith angle dependence of albedo. © 2004 American Meteorological Society.
- Zeng, X., & Er, L. u. (2004). Globally unified monsoon onset and retreat indexes. Journal of Climate, 17(11), 2241-2248.More infoAbstract: Different criteria have been used in the past to define the monsoon onset and retreat over different monsoon regions and even over different parts of the same monsoon region. Here an objective criterion is proposed to define, for the first time, globally unified summer monsoon onset (or retreat) dates using a single meteorological variable (i.e., the global daily 1× 1° normalized precipitable water data) with the threshold value being the Golden Ratio (0.618). Results are found to be consistent with those determined using long-term rainfall data over most monsoon regions. The precipitable water data have also been used to refine the definition of monsoon regions on a grid-cell-by-cell basis. The objective definitions of these basic monsoon characteristics would provide one of the necessary foundations for global monsoon research. They, along with the onset/retreat data over a 10-yr period (1988-97), would also facilitate the diagnostics and validation of global monsoon simulations. © 2004 American Meteorological Society.
- Zeng, X., Brunke, M. A., Zhou, M., Fairall, C., Bond, N. A., & Lenschow, D. H. (2004). Marine atmospheric boundary layer height over the Eastern Pacific: Data analysis and model evaluation. Journal of Climate, 17(21), 4159-4170.More infoAbstract: The atmospheric boundary layer (ABL) height (h) is a crucial parameter for the treatment of the ABL in weather and climate models. About 1000 soundings from 11 cruises between 1995 and 2001 over the eastern Pacific have been analyzed to document the large meridional, zonal, seasonal, and interannual variations of h. In particular, its latitudinal distribution in August has three minima: Near the equator, in the intertropical convergence zone (ITCZ), and over the subtropical stratus/stratocumulus region near the west coast of California and Mexico. The seasonal peak of h in the ITCZ zone (between 5.6° and 11.2°N) occurs in the spring (February or April), while it occurs in August between the equator and 5.6°N. Comparison of these data with the 10-yr monthly output of the Community Climate System Model (CCSM2) reveals that overall the model underestimates h, particularly north of 20°N in August and September. Directly applying the radiosonde data to the CCSM2 formulation for computing h shows that, at the original vertical resolution (with the lowest five layers below 2.1 km), the CCSM2 formulation would significantly underestimate h. In particular, the correlation coefficient between the computed and observed h values is only 0.06 for cloudy cases. If the model resolution were doubled below 2.1 km, however, the performance of the model formulation would be significantly improved with a correlation coefficient of 0.78 for cloudy cases. © and 2004 American Meteorological Society.
- Zeng, X., Shen, S. S., Zeng, X., & Dickinson, R. E. (2004). Multiple equilibrium states and the abrupt transitions in a dynamical system of soil water interacting with vegetation. Geophysical Research Letters, 31(5), L05501 1-5.More infoAbstract: In semi-arid areas, multiple equilibrium states of an ecosystem (e.g., grassland and desert) are found to coexist, and the transition from grassland to desert is often abrupt at the boundary. A simple ecosystem model is developed to provide the biophysical explanation of this phenomenon. The model has three variables: living biomass, wilted biomass, and soil wetness. The moisture index, which is the ratio of the annual precipitation to potential evaporation, is the only external climate driving force, and the key mechanism is the vegetation-soil interaction. It is found that the maintenance of a grassland requires a minimum moisture index, and the abrupt transition occurs when the moisture index is around this critical value. These results are robust within a wide range for most model parameters, suggesting that the model may be applicable to other temperate grasslands. The characteristics of the wilted biomass also strongly influence the ecosystem's dynamics. Copyright 2004 by the American Geophysical Union.
- Brunke, M. A., Fairall, C. W., Zeng, X., Eymard, L., & Curry, J. A. (2003). Which bulk aerodynamic algorithms are least problematic in computing ocean surface turbulent fluxes?. Journal of Climate, 16(4), 619-635.More infoAbstract: Bulk aerodynamic algorithms are needed to compute ocean surface turbulent fluxes in weather forecasting and climate models and in the development of global surface flux datasets. Twelve such algorithms are evaluated and ranked using direct turbulent flux measurements determined from covariance and inertial-dissipation methods from 12 ship cruises over the tropical and midlatitude oceans (from about 5°S to 60°N . The four least problematic of these 12 algorithms based upon the overall ranking for this data include the Coupled Ocean-Atmosphere Response Experiment (COARE) version 3.0 and The University of Arizona (UA) schemes as well as those used at the European Centre for Medium-Range Weather Forecasts (ECMWF) and the National Aeronautics and Space Administration (NASA) Data Assimilation Office for version 1 of the Goddard Earth Observing System reanalysis (GEOS-1). Furthermore, the four most problematic of these algorithms are also identified along with possible explanations. The overall ranking is not substantially affected by the use of the average of covariance and inertial-dissipation flux measurements or by taking into consideration measurement uncertainties. The differences between computed and observed fluxes are further evaluated as a function of near-surface wind speed and sea surface temperature to understand the rankings. Finally, several unresolved issues in terms of measurement and algorithm uncertainties are raised.
- Dai, Y., Zeng, X., Dickinson, R. E., Baker, I., Bonan, G. B., Bosilovich, M. G., Denning, A. S., Dirmeyer, P. A., Houser, P. R., Niu, G., Oleson, K. W., Schlosser, C. A., & Yang, Z. (2003). The common land model. Bulletin of the American Meteorological Society, 84(8), 1013-1023.More infoAbstract: Scientists from several institutions and with different research backgrounds have worked together to develop a prototype modular land model for weather forecasting and climate studies. The resultant model, called Common Land Model (CLM), is now available for public use and further development. The model includes three elements, including, the core single-column soil-snow-vegetation biophysical code, the land boundary data, and the scaling procedures within a climate model required to interface atmospheric model grid-square inputs to land single-column processes.
- Dickinson, R. E., Wang, G., Zeng, X., & Zeng, Q. (2003). How does the partitioning of evapotranspiration and runoff between different processes affect the variability and predictability of soil moisture and precipitation?. Advances in Atmospheric Sciences, 20(3), 475-478.More infoAbstract: Water stored as part of the land surface is lost to evapotranspiration and runoff on different time scales, and the partitioning between these time scales is important for modeling soil water in a climate model. Different time scales are imposed on evapotranspiration primarily because it is derived from different reservoirs with different storage capacities, from the very rapid evaporation of canopy stores to the slow removal by transpiration of rooting zone soil moisture. Runoff likewise ranges in time scale from rapid surface terms to the slower base-flow. The longest time scale losses of water determine the slow variation of soil moisture and hence the longer time scale effects of soil moisture on precipitation. This paper shows with a simple analysis how shifting the partitioning of evapotranspiration between the different reservoirs affects the variability of soil moisture and precipitation. In particular, it is concluded that a shift to shorter time scale reservoirs shifts the variance of precipitation from that which is potentially predictable to unpredictable.
- Pan, X., Zeng, X., Zhang, J., Shi, Q., Qing, H. e., Chao, Q., & Chao, J. (2003). Preliminary study on the relationship between temporal and spatial evolution of ecological landscape pattern and climate change in Xinjiang, China. Proceedings of SPIE - The International Society for Optical Engineering, 4890(1), 58-73.More infoAbstract: Fractional vegetation cover data (FVC) based on the annual maximum NOAA/AVHRR NDVI (Advanced Very High Resolution Radiometer normalized difference vegetation index) data and the ground meteorological data have been used to analyze the relationship between the temporal and spatial evolution of ecological landscape pattern and climate change in Xinjiang autonomous region, China from 1982 to 1993. It is found that during this period, the averaged oasis cropland or low-damp forest-shrub meadow area occupies 7.50% of the whole region, desert dune/barren desert 25.70%; temperate semi-arbor desert 21.70%; temperate shrub and semi-shrub desert 8.36%; alpine subalpine desert steppe 7.71%; temperate gramineous and ruderal steppe 7.63%; mountain forest steppe 2.13%; high-cold barren 8.93%; high-cold low semi-shrub desert 9.08%; water/ice 1.26%. Compared with the 30-year (1961-1990) climatology, the 12-year averaged annual surface air temperature and precipitation over the whole region increase by 0.28 °C and 23.6 mm respectively. The maximum increase of precipitation does not occur over the mountain region; instead it occurs over the region with a concentrated distribution of oases, suggesting the positive role of oases in adjusting regional climate. The area of temporal steppe also increases with the increase of precipitation and temperature. Together, these results demonstrate that, during the 12-year period, the condition of vegetation cover has been improved and large-scale desertification does not occur in Xinjiang.
- Zeng, X., Rao, P., DeFries, R. S., & Hansen, M. C. (2003). Interannual variability and decadal trend of global fractional vegetation cover from 1982 to 2000. Journal of Applied Meteorology, 42(10), 1525-1530.More infoAbstract: Fractional vegetation cover (FVC) is one of the most important variables in land surface modeling and also provides a continuous field to complement discrete land cover classification. A global 8-km FVC dataset for 1982-2000 is derived using the NOAA-NASA land Pathfinder normalized difference vegetation index data. The confidence in the dataset is provided by the insensitivity of the algorithm to the data resolution (between 1 and 8 km), the good agreement of the results with the field survey data over Germany, the consistency of the results with previous observational studies over the savannas in North Africa and the forests in Bolivia, and the robustness of the algorithm, as demonstrated by the small interannual variability of FVC over areas where anthropogenic land cover change is expected to be small, based on the 30-m Landsat data analysis. Significant interannual variability is found over shrubland, savanna, and grassland; both positive and negative trends exist over different areas of the same region in many parts of the world. In particular, the trend analysis pinpoints areas with statistically significant trends (i.e., "hotspots") for further study using higher-resolution satellite data and field-survey data.
- Zhou, L., Dickinson, R. E., Tian, Y., Zeng, X., Dai, Y., Yang, Z. -., Schaaf, C. B., Gao, F., Jin, Y., Strahler, A., Myneni, R. B., Yu, H., Wu, W., & Shaikh, M. (2003). Comparison of seasonal and spatial variations of albedos from Moderate-Resolution Imaging Spectroradiometer (MODIS) and Common Land Model. Journal of Geophysical Research D: Atmospheres, 108(15), ACL 15-1 - ACL 15-20.More infoAbstract: This paper compares seasonal and spatial variations of Moderate Resolution Imaging Spectroradiometer (MODIS) albedos with those from the Common Land Model (CLM) by land cover type. MODIS albedo data in the year 2001 were used to determine seasonal, spatial, and land cover dependence at 1 km resolution and to investigate the biases in CLM. Albedo dependence on vegetation type is smaller than that on snow and soil. Snow causes the largest temporal and spatial variations, especially in the visible band (0.3-0.7 μm). CLM has visible albedos that are lower by up to 0.4-0.5 in winter over northern high latitudes but are globally higher by 0.02-0.04 in summer over most vegetation, mainly due to its overestimated leaf and stem area index in winter and slightly higher prescribed canopy albedos in summer, respectively. MODIS and CLM differ considerably in soil albedo over desert and semidesert regions, especially in the near-infrared band (0.7-5.0 μm), with the largest low bias of about 0.1 in the Sahara. Adjustments of the prescribed albedos in CLM based on MODIS observations could reduce such biases. Therefore the model should better represent leaf and stem area index, vegetation albedo in the presence of snow, and soil albedo. Copyright 2003 by the American Geophysical Union.
- Bonan, G. B., Oleson, K. W., Vertenstein, M., Levis, S., Zeng, X., Dai, Y., Dickinson, R. E., & Yang, Z. (2002). The land surface climatology of the community land model coupled to the NCAR community climate model. Journal of Climate, 15(22), 3123-3149.More infoAbstract: The land surface parameterization used with the community climate model (CCM3) and the climate system model (CSM 1), the National Center for Atmospheric Research land surface model (NCAR LSM 1), has been modified as part of the development of the next version of these climate models. This new model is known as the community land model (CLM2). In CLM2, the surface is represented by five primary subgrid land cover types (glacier, lake, wetland, urban, vegetated) in each grid cell. The vegetated portion of a grid cell is further divided into patches of up to 4 of 16 plant functional types, each with its own leaf and stem area index and canopy height. The relative area of each subgrid unit, the plant functional type, and leaf area index are obtained from 1-km satellite data. The soil texture dataset allows vertical profiles of sand and clay. Most of the physical parameterizations in the model were also updated. Major model differences include: 10 layers for soil temperature and soil water with explicit treatment of liquid water and ice; a multilayer snowpack; runoff based on the TOPMODEL concept; new formulation of ground and vegetation fluxes: and vertical root profiles from a global synthesis of ecological studies. Simulations with CCM3 show significant improvements in surface air temperature, snow cover, and runoff for CLM2 compared to LSM1. CLM2 generally warms surface air temperature in all seasons compared to LSM1, reducing or eliminating many cold biases. Annual precipitation over land is reduced from 2.35 mm day-1 in LSM1 to 2.14 mm day-1 in CLM2. The hydrologic cycle is also different. Transpiration and ground evaporation are reduced. Leaves and stems evaporate more intercepted water annually in CLM2 than LSM1. Global runoff from land increases from 0.75 mm day-1 in LSM1 to 0.84 mm day-1 in CLM2. The annual cycle of runoff is greatly improved in CLM2, especially in arctic and boreal regions where the model has low runoff in cold seasons when the soil is frozen and high runoff during the snowmelt season. Most of the differences between CLM2 and LSM1 are attributed to particular parameterizations rather than to different surface datasets. Important processes include: multilayer snow, frozen water, interception, soil water limitation to latent heat, and higher aerodynamic resistances to heat exchange from ground.
- Brunke, M. A., Zeng, X., & Anderson, S. (2002). Uncertainties in sea surface turbulent flux algorithms and data sets. Journal of Geophysical Research C: Oceans, 107(10), 5-1.More infoAbstract: An intercomparison of eight bulk sea surface turbulent flux algorithms used in data set generation as well as weather and climate prediction is performed for the tropical Pacific and midlatitude Atlantic. The results show some significant differences in fluxes due to differences in the way the algorithms consider wave spectrum, convective gustiness, and salinity as well as the way the algorithms parameterize roughness lengths and turbulent exchange coefficients. For instance, for sea surface temperature between 27.75°C and 28.25°C, the maximum differences in monthly latent heat flux and wind stress among algorithms over the tropical Pacific are about 23 W m-2 (or 16% relative to the algorithm-averaged flux) and 0.013 N m-2 (or 19% relative to the algorithm-averaged value) respectively. Evaluation of these algorithms using 270 hourly samples of observed turbulent flux data over the midlatitude Pacific shows that algorithms are largely consistent with observations. However, some algorithms show significant deviations from observations under certain conditions (e.g., weak wind conditions). Insights from the above intercomparison are then used to evaluate ocean surface turbulent fluxes from two global data sets, one is derived from satellite remote sensing while the other is a reanalysis. Over two buoy sites in the eastern and western tropical Pacific, the mean heat flux differences between the two data sets are primarily caused by differences in bulk variables (e.g., wind speed) rather than by differences in bulk algorithms. However, bulk algorithm differences could contribute up to 17 W m-2 to the long-term averaged latent heat flux over the tropical Pacific if different algorithms were used. This suggests that both bulk algorithms and the deviation of environmental variables need to be further improved in order to produce ocean surface turbulent fluxes with an accuracy of 10 W m-2.
- Buermann, W., Wang, Y., Dong, J., Zhou, L., Zeng, X., Dickinson, R. E., Potter, C. S., & Myneni, R. B. (2002). Analysis of a multiyear global vegetation leaf area index data set. Journal of Geophysical Research D: Atmospheres, 107(22), 14-1-14-16.More infoAbstract: The analysis of a global data set of monthly leaf area index (LAI), derived from satellite observations of normalized difference vegetation index (NDVI) for the period July 1981 to September 1994, is discussed in this paper. Validation of this retroactive, coarse resolution (8 km) global multiyear data set is a challenging task because repetitive ground measurements from all representative vegetation types are not available. Therefore the magnitudes and interannual variations in the derived LAI fields were assessed as follows. First, the use of a NDVI-based algorithm, as opposed to a more physically based approach, is estimated to result in relative errors in LAI of about 10-20%, which is comparable to the mean uncertainty of AVHRR NDVI data. Second, the satellite LAI values compared reasonably well to ground measurements from three field campaigns. Third, comparison with an existing multiyear LAI data set showed qualitative agreement with regards to interannual variability, although the LAI values of the earlier data were consistently larger than those derived here. Fourth, interannual variations in LAI were evaluated through correlations with climate data sets, e.g., sea surface temperatures and precipitation in tropical semiarid regions known for ENSO impacts, temperature dependence of vegetation growth, and therefore LAI, in the northern latitudes. The general consistency between these independent data sets imbues confidence in the LAI data set, at least for use in large-scale modeling studies. Finally, improvements in near-surface climate simulation are documented in a companion article when satellite LAI values were used in a global climate model. The data set is available to the community via our Web server (http://cybele.bu.edu). Copyright 2002 by the American Geophysical Union.
- Chase, T. N., Herman, B., Pielke Sr., R. A., Zeng, X., & Leuthold, M. (2002). A proposed mechanism for the regulation of minimum midtropospheric temperatures in the Arctic. Journal of Geophysical Research D: Atmospheres, 107(14), 2-1-2-7.More infoAbstract: This paper documents an observed lower extreme of midtropospheric (500 mbar) temperatures in the Arctic of approximately -45°C during the winter season in several data sets. Each data set shows that the coldest air masses in the Arctic reach -45°C during the fall months but seldom get much colder even into late winter despite a continued net radiative loss. We demonstrate that midtropospheric temperatures are significantly skewed toward warmer temperatures, indicating a regulatory mechanism at work. We further provide evidence that minimum Arctic midtropospheric temperatures are regulated by moist convective processes and that minimum 500 mbar temperatures are controlled to a large extent by high-latitude sea surface temperatures. The temperature -45°C is the expected 500 mbar temperature in an atmosphere regulated by moist adiabatic ascent from a surface temperature of l°-2° below 0°C, the approximate freezing point of seawater. This implies that Arctic air masses are regularly in contact with unfrozen seawater to the south, an easily verified observation. Climate model simulations of the effects of increased greenhouse gasses hypothesize that high, northern latitude regions should warm at a faster rate than the globe as a whole, a hypothesis which does not appear to have strong observational support. We discuss the implications of this result for the accelerated Arctic warming hypothesis. Copyright 2002 by the American Geophysical Union.
- Jin, Y., Schaaf, C. B., Gao, F., Xiaowen, L. i., Strahler, A. H., Zeng, X., & Dickinson, R. E. (2002). How does snow impact the albedo of vegetated land surfaces as analyzed with MODIS data?. Geophysical Research Letters, 29(10), 12-1 - 12-4.More infoAbstract: Albedo derived from MODIS observations is found to be very stable during November 2000-January 2001. We analyze shortwave albedo under snow and snow-free conditions by IGBP land cover types. Snow changes the spectral property of the surface reflectivity and causes high heterogeneity in the surface albedo between and within land types. The mean black sky (or direct beam) albedo at local solar noon for snow-covered forests is less than 0.30 in the shortwave (0.3-5.0 μm), but it reaches 0.57 for snow-covered grassland and barren. Although we are unable to further separate within-class albedos with fractional tree cover, we find that the normalized difference snow index (NDSI) is highly correlated with surface albedo and hence can be taken as a measure of snow, soil and canopy fraction.
- Tsvetsinskaya, E. A., Schaaf, C. B., Gao, F., Strahler, A. H., Dickinson, R. E., Zeng, X., & Lucht, W. (2002). Relating MODIS-derived surface albedo to soils and rock types over Northern Africa and the Arabian peninsula. Geophysical Research Letters, 29(9), 67-1 - 67-4.More infoAbstract: We use the MODerate resolution Imaging Spectroradiometer (MODIS) aboard the Terra spacecraft to derive surface albedo for the arid areas of Northern Africa and the Arabian peninsula. Albedo in seven MODIS spectral bands for land and three broad bands (for shortwave, near infrared, and visible portions of the spectrum) is produced. Surface albedo is derived from MODIS observations during a sixteen-day period and is analyzed at 1 km spatial resolution. MODIS data show considerable spatial variability of surface albedo in the study region that is related to soil and geological characteristics of the surface. For example, solar shortwave white-sky albedo varies by a factor of about 2.5 from the darkest volcanic terrains to the brightest sand sheets. Vegetation contribution to surface reflectance is essentially negligible since we only considered pixels with under ten percent fractional canopy cover. Few, if any, coupled land- atmosphere global or regional models capture this observed spatial variability in surface reflectance or albedo. Here we suggest a scheme that relates soil groups (based on the United Nations Food and Agriculture Organization, FAO, soil classification) and rock types (based on the United States Geological Survey, USGS, geological maps) to MODIS derived surface albedo statistics. This approach is a first step towards the incorporation of the observed spatial variability in surface reflective properties into climate models.
- Zeng, X., Shajkh, M., Dai, Y., Dickinson, R. E., & Myneni, R. (2002). Coupling of the Common Land Model to the NCAR Community Climate Model. Journal of Climate, 15(14), 1832-1854.More infoAbstract: The Common Land Model (CLM), which results from a 3-yr joint effort among seven land modeling groups, has been coupled with the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM3). Two 15-yr simulations of CCM3 coupled with CLM and the NCAR Land Surface Model (LSM), respectively, are used to document the relative impact of CLM versus LSM on land surface climate. It is found that CLM significantly reduces the summer cold bias of surface air temperature in LSM, which is associated with higher sensible heat fluxes and lower latent heat fluxes in CLM, and the winter warm bias over seasonally snow-covered regions, especially in Eurasia. CLM also significantly improves the simulation of the annual cycle of runoff in LSM. In addition, CLM simulates the snow mass better than LSM during the snow accumulation stage. These improvements are primarily caused by the improved parameterizations in runoff, snow, and other processes (e.g., turbulence) in CLM. The new land boundary data (e.g., leaf-area index, fractional vegetation cover, albedo) also contribute to the improvement in surface air temperature simulation over some regions. Overall, CLM has little impact on precipitation and surface net radiative fluxes.
- Zeng, X., Zhang, Q., Johnson, D., & Tao, W. -. (2002). Parameterization of wind gustiness for the computation of ocean surface fluxes at different spatial scales. Monthly Weather Review, 130(8), 2125-2133.More infoAbstract: Analysis of the Goddard cloud-ensemble (GCE) model output forced by observational data over the tropical western Pacific and eastern tropical North Atlantic has shown that ocean surface latent and sensible heat fluxes averaged in a typical global-model grid box are reproduced well using bulk algorithms with grid-box-average scalar wind speed but could be significantly underestimated under weak wind conditions using average vector wind speed. This is consistent with previous observational and modeling studies. The difference between scalar and vector wind speeds represents the subgrid wind variability (or wind gustiness) that is contributed by boundary layer large eddies, convective precipitation, and cloudiness. Based on the GCE data analysis for a case over the tropical western Pacific, a simple parameterization for wind gustiness has been developed that considers the above three factors. This scheme is found to fit well the GCE data for two other cases over the tropical western Pacific and eastern tropical North Atlantic. Its fit is also much better than that of the traditional approach that considers the contribution to wind gustiness by boundary layer large eddies alone. A simple formulation has also been developed to account for the dependence of the author's parameterization on spatial scales (or model grid size). Together, the preliminary parameterization and formulation can be easily implemented into weather and climate models with various horizontal resolution.
- Buermann, W., Dong, J., Zeng, X., Myneni, R. B., & Dickinson, R. E. (2001). Evaluation of the utility of satellite-based vegetation leaf area index data for climate simulations. Journal of Climate, 14(17), 3536-3550.More infoAbstract: In this study the utility of satellite-based leaf area index (LAI) data in improving the simulation of near-surface climate with the NCAR Community Climate Model, version 3 (CCM3), GCM is evaluated. The use of mean LAI values, obtained from the Advanced Very High Resolution Radiometer Pathfinder data for the 1980s, leads to notable warming and decreased precipitation over large parts of the Northern Hemisphere lands during the boreal summer. Such warming and decreased rainfall reduces discrepancies between the simulated and observed near-surface temperature and precipitation fields. The impact of interannual vegetation extremes observed served during the 1980s on near-surface climate is also investigated by utilizing the maximum and minimum LAI values from the 10-yr LAI record. Surface energy budget analysis indicates that the dominant impact of interannual LAI variations is modification of the partitioning of net radiant energy between latent and sensible heat fluxes brought about through changes in the proportion of energy absorbed by the vegetation canopy and the underlying ground and not from surface albedo changes. The enhanced latent heat activity in the greener scenario leads to an annual cooling of the earth land surface of about 0.3°C, accompanied by an increase in precipitation of 0.04 mm day-1. The tropical evergreen forests and temperature grasslands contribute most to this cooling and increased rainfall. These results illustrate the importance and utility of satellite-based vegetation LAI data in simulations of near-surface climate variability.
- Wei, X., Hahmann, A. N., Dickinson, R. E., Yang, Z., Zeng, X., Schaudt, K. J., Schaaf, C. B., & Strugnell, N. (2001). Comparison of albedos computed by land surface models and evaluation against remotely sensed data. Journal of Geophysical Research D: Atmospheres, 106(D18), 20687-20702.More infoAbstract: The albedos of two land surface models, the Biosphere-Atmosphere Transfer Scheme (BATS) and the NCAR Land Surface Model (LSM), are compared with remotely sensed data and each other. The model albedos differ primarily because of their assumptions about and model differences in soil moisture content, soil color, snow albedo, shading of snow by canopy, and prescribed parameters for each land cover type. Global albedo maps for February and July 1995, developed from the advanced very high resolution radiometer (AVHRR) data, are used to evaluate model albedos. The models display a high bias as compared to the remotely sensed values in desert and semidesert regions. Over North Africa, LSM, whose albedos were previously tuned to data from the Earth Radiation Budget Experiment (ERBE), has the highest albedos. Elsewhere, and overall, BATS has the highest bias for desert and semidesert regions. Both models demonstrate a high bias over regions of winter snow, where the AVHRR data are expected to have a negative bias. LSM has especially high winter albedos, apparently because of intercepted snow increasing its canopy albedo. Copyright 2001 by the American Geophysical Union.
- Zeng, X. (2001). Global vegetation root distribution for land modeling. JOURNAL OF HYDROMETEOROLOGY, 2(5), 525-530.More infoVegetation root distribution is one of the factors that determine the overall water holding capacity of the land surface and the relative rates of water extraction from different soil layers for vegetation transpiration. Despite its importance, significantly different root distributions are used by different land surface models. Using a comprehensive global field survey dataset, vegetation root distribution (including rooting depth) has been developed here for three of the most widely used land cover classifications [i.e., the Biosphere-Atmosphere Transfer Scheme (BATS), International Geosphere-Biosphere Program (IGBP), and version 2 of the Simple Biosphere Model (SiB2)] for direct use by any land model with any number of soil layers.
- Zhang, Q., Zeng, X., Dickinson, R., Johnson, D., & Tao, W. (2001). Progress in the parameterization of mesoscale enhancement of ocean surface fluxes in large-scale models. Chinese J. Oceanogr., 23, 132-142.
- Lenschow, D. H., Zhou, M., Zeng, X., Chen, L., & Xiangde, X. u. (2000). Measurements of fine-scale structure at the top of marine stratocumulus. Boundary-Layer Meteorology, 97(2), 331-357.More infoAbstract: During the Dynamics and Chemistry of the Marine Stratocumulus (DYCOMS) experiment in July-August 1985, the NCAR Electra aircraft flew a series of flight legs just at the top of the marine stratocumulus cloud decks that cap the mixed layer off the coast of southern California. Because of the corrugated structure of the cloud-top, the aircraft, which was flown at a nearly constant level and adjusted only to maintain its altitude at the average cloud-top height, was alternately within and above the clouds - roughly half the time in each domain. These legs were used to examine the structure of the cloud-top by compositing the segments on either side of the cloud/clear-air interface, which was identified by the transitions of liquid water measured by the Forward Scattering Spectrometer Probe (either increasing or decreasing) through a threshold of 0.04 x 10-3 kg m-3. An equivalent vertical distance (EVD) from the cloud-top was obtained from the horizontal flight legs by estimating the average slope of the cloud-top from the cloud-top radiation temperature. The results show that a near discontinuity occurs in variables across cloud top over an EVD of 0.3 m, but that above this, the air has already been modified by boundary-layer air. Thus, cloud-top is not the limit of mixing of boundary-layer air. This mixing may extend to tens of metres or more. The bulk Richardson number in the vicinity of cloud-top increases from near zero within the cloud to about 1.2 at an EVD of 3-6 m above cloud. Fluctuations of the three velocity components within cloud are nearly equal; above cloud the vertical component structure function is about half the horizontal components. The scalar structure functions are about an order of magnitude higher above cloud than in cloud. The structure parameters of temperature and humidity measured just below cloud-top agree reasonably well with predicted values based on a previously-developed model for the clear convective boundary layer. Above cloud, the scalar structure parameters are much larger, but their interpretation is questionable, since this region does not contain isotropic turbulence.
- Zeng, X., Dickinson, R. E., Walker, A., Shaikh, M., Defries, R. S., & Jiaguo, Q. i. (2000). Derivation and evaluation of global 1-km fractional vegetation cover data for land modeling. Journal of Applied Meteorology, 39(6), 826-839.More infoAbstract: Fractional vegetation cover (σv) is needed in the modeling of the land-atmosphere exchanges of momentum, energy, water, and trace gases. From global 1-km, 10-day composite Advanced Very High Resolution Radiometer normalized difference vegetation index (NDVI) data from April 1992 to March 1993, global 1-km σv is derived based on the annual maximum NDVI value for each pixel in comparison with the NDVI value that corresponds to 100% vegetation cover for each International Geosphere-Biosphere Program land cover type. This dataset is pixel dependent but season independent, with the seasonal variation of vegetation greenness in a pixel accounted for by the leaf area index. The authors' algorithm is found to be insensitive to the use of a specific land cover classification. In comparison with an independent dataset derived by DeFries et al. by using a more sophisticated statistical approach, the current dataset has a similar spatial distribution but systematically smaller σv (particularly over shrublands and barren land cover). It also gives σv values that overall are consistent with those derived from higher-resolution aircraft and satellite data over Arizona and field-survey data over Germany.
- Zhao, M., & Zeng, X. (2000). Some problems in the computation of ocean surface moisture and heat fluxes in atmospheric models. Chinese J. Meteor., 58, 340-346.
- Zeng, X. (1999). The relationship among precipitation, cloud-top temperature, and precipitable water over the tropics. Journal of Climate, 12(8 PART 2), 2503-2514.More infoAbstract: The relationship of monthly precipitation P to precipitable water w and cloud-top temperature as represented by the Geostationary Operational Environmental Satellite (GOES) Precipitation Index (GPI) is obtained over tropical land, coast, and ocean: P = exp[a1(w - a2)] GPI, where coefficients a1 and a2 are determined using one year of the Global Precipitation Climatology Project (GPCP) monthly rain gauge data and then independently tested using four other years of gauge data. This algorithm, over land, gives more accurate precipitation estimates than are obtained using the cloud-top temperature alone (i.e., GPI) and is as accurate as the state-of-the-art multisatellite algorithm (MS) from GPCP. Over coastal and oceanic regions, this algorithm has a smaller bias in precipitation estimation than GPI but has the same correlation coefficient with gauge data as GPI. Compared with MS, it has a much smaller bias but larger mean absolute deviation. Evaluation using the Pacific atoll-island gauge data also shows that this algorithm can reproduce well the observed meridional distribution of precipitation across the ITCZ and SPCZ near the date line. This algorithm is then used to produce a five-year (January 1988-December 1992) 2.5°X 2.5°integrated dataset of precipitation and precipitable water between 40°N and 40°S for climate model evaluation. The small bias of this algorithm (particularly over ocean) also suggests that it would be a good data source for precipitation merging algorithms.
- Zeng, X., Zhao, M., Dickinson, R. E., & Yanping, H. e. (1999). A multiyear hourly sea surface skin temperature data set derived from the TOGA TAO bulk temperature and wind speed over the tropical Pacific. Journal of Geophysical Research C: Oceans, 104(C1), 1525-1536.More infoAbstract: A theoretical relationship is derived to estimate the sea surface skin temperature from near-surface wind speed and the diurnal variation of sea surface bulk (or bucket) temperature. Coefficients in the relation are determined using the R/V Franklin data during the Tropical Ocean-Global Atmosphere (TOGA) Coupled Ocean-Atmosphere Response Experiment (COARE). In contrast to previous methods, surface energy flux data are not explicitly required but rather are implied by the temporal variation of bulk temperature. A multiyear hourly skin temperature data set is obtained using data of bulk temperature at 1-m depth and wind speed from the TOGA Tropical Atmosphere-Ocean (TAO) moored buoys spanning the tropical Pacific Ocean from 95°W in the eastern Pacific to 137°E in the western Pacific between 9°N and 8°S. The diurnal amplitude of skin temperature reaches its maximum of about 2.8 K for daily averaged wind speed between 1-2 m s -1 and skin temperature between 20°-21°C and decreases with greater wind speeds. The most frequent amplitude is about 0.5 K, the average amplitude is 0.65 K, and the accumulated frequency for amplitudes greater than 1 K is 10% within the parameter space of daily averaged wind speed between 1 and 15 m s -1 and daily averaged skin temperature between 18° and 34°C. Copyright 1999 by the American Geophysical Union.
- Zhao, M., & Zeng, X. (1999). A diagnostic study of the relationship between surface fluxes and environmental variables over the tropical western Pacific. J. Tropical Meteor., 5, 214-224.
- Chen, M., Zeng, X., & Dickinson, R. E. (1998). Adjustment of GCM precipitation intensity over the United States. Journal of Applied Meteorology, 37(9), 876-887.More infoAbstract: A regression equation is developed to adjust the simulated monthly averaged intensity of hourly precipitation over the continental United States using air temperature at the first model level (about 80 m above ground) simulated by a revised version of NCAR's Community Climate Model version 2 coupled with the Biosphere-Atmosphere Transfer Scheme. The adjusted precipitation intensity is in much closer agreement with the locally observed precipitation intensity, both in pattern and magnitude, than is the simulated intensity produced by climate models The multiresponse randomised block permutation analysis shows that the authors' adjustment method is statistically significant The equation is robust with respect to seasonal and interannual variations, as demonstrated by applying it to independent months of different years. The relationship between precipitation intensity and near-surface air temperature likely reflects a linkage among precipitation, precipitable water, and vertical velocity. It can be applied to estimate fractional precipitation coverage with temporal and spatial variability over the continental United States, and to help statistically construct a time series of local precipitation events both for the present climate and for the studies of climate change.
- Pielke Sr., R. A., Avissar, R., Raupach, M., Dolman, A. J., Zeng, X., & Denning, A. S. (1998). Interactions between the atmosphere and terrestrial ecosystems: influence on weather and climate. Global Change Biology, 4(5), 461-475.More infoAbstract: This paper overviews the short-term (biophysical) and long-term (out to around 100 year timescales; biogeochemical and biogeographical) influences of the land surface on weather and climate. From our review of the literature, the evidence is convincing that terrestrial ecosystem dynamics on these timescales significantly influence atmospheric processes. In studies of past and possible future climate change, terrestrial ecosystem dynamics are as important as changes in atmospheric dynamics and composition, ocean circulation, ice sheet extent, and orbit perturbations.
- Zeng, X., & Dickinson, R. E. (1998). Effect of surface sublayer on surface skin temperature and fluxes. Journal of Climate, 11(4), 537-550.More infoAbstract: The surface sublayer is the layer of air adjacent to the surface where the transfer of momentum and heat by molecular motion becomes important. Equations are derived to incorporate this surface sublayer (or the variable ratio of the roughness length for momentum over that for heat, Zo/Zoh) over bare soil into a commonly used formulation for aerodynamic transfer coefficients. Along with the consideration of the laminar layer around vegetation leaves in the Biosphere-Atmosphere transfer Scheme (BATS), these equations provide a consistent approach for the computation of surface fluxes over bare soil or vegetated surface. Qualitative and quantitative analysis show that the surface sublayer tends to substantially increase the surface skin temperature for a given sensible heat flux and decrease the heat flux for a given surface versus air temperature difference. Using a climate model output as the atmospheric forcing data for BATS over a semiarid region, it is also found that the surface sublayer significantly increases the monthly and July-averaged hourly surface skin temperature and decreases surface sensible heat and net radiation fluxes. Comparison with limited observations of Zo/Zoh also suggests that the same (or different) exchange coefficients should be used over bare soil and vegetated portions in a grid box for dense canopies (eg grassland or forest [or sparse canopies (eg semiarid regions)].
- Zeng, X., & Dickinson, R. E. (1998). Impact of diurnally-varying skin temperature on surface fluxes over the tropical Pacific. Geophysical Research Letters, 25(9), 1411-1414.More infoAbstract: Multi-year hourly data of air temperature, wind speed, and humidity from the TOGA TAO moored buoys over the tropical Pacific along with our derived hourly sea surface skin temperature data are analyzed to show that there are substantial diurnal variations of monthly averaged surface fluxes of latent heat, sensible heat, and momentum (eg, one-third of the cases show monthly averaged latent heat diurnal amplitudes greater than 20 Wm-2). Daily or monthly average surface temperatures cannot provide such flux variations, suggesting that numerical modeling may require the inclusion of the diurnal variation of surface skin temperature.
- Zeng, X., Dai, Y., Dickinson, R. E., & Shaikh, M. (1998). The role of root distribution for climate simulation over land. Geophysical Research Letters, 25(24), 4533-4536.More infoAbstract: A comprehensive global root database is used to derive vertical root distribution and rooting depth for various vegetation categories in one of the most widely-used land models; i.e., the Biosphere-Atmosphere Transfer Scheme (BATS). Using a variety of observational datasets, observed root distribution is found to significantly improves the offline simulation of surface water and energy balance. Global climate modeling further demonstrates that observed root distribution primarily affects latent heat flux and soil wetness over tropical and midlatitude land, respectively.
- Zeng, X., Zhao, M., & Dickinson, R. E. (1998). Intercomparison of bulk aerodynamic algorithms for the computation of sea surface fluxes using TOGA COARE and TAO data. Journal of Climate, 11(10), 2628-2644.More infoAbstract: A bulk aerodynamic algorithm is developed for all stability conditions for the computation of ocean surface fluxes. It provides roughness lengths of wind, humidity, and temperature for a wind speed range from 0 to 18 m s-1: z(o) = 0.013u(*)2/g + 0.11ν/u(*) and ln(z(o)/z(ot)) = ln(z(o)/z(oq)) = 2.67Re(*)(1/4) - 2.57 as derived using the Tropical Oceans Global Atmosphere (TOGA) Coupled Ocean-Atmosphere Response Experiment (COARE) data constrained by other observations under high wind conditions. Using the TOGA COARE ship data and the multiyear hourly TOGA Tropical Atmosphere-Ocean moored buoy data, intercomparison of six different algorithms, which are widely used in research, operational forecasting, and data reanalysis, shows that algorithms differ significantly in heat and momentum fluxes under both very weak and very strong wind conditions, but agree with each other under moderate wind conditions. Algorithms agree better for wind stress than for heat fluxes. Based on past observations, probable deficiencies in roughness lengths (or neutral exchange coefficients) of some of the algorithms are identified along with possible solutions, and significant issues (particularly the trend of the neutral exchange coefficient for heat with wind speed under strong wind conditions) are raised for future experiments. The vapor pressure reduction of 2% over saline seawater has a significant impact on the computation of surface latent heat flux under strong wind conditions and should be considered in any bulk aerodynamic algorithm.
- Pielke, R. A., Zeng, X., Lee, T. J., & Dalu, G. A. (1997). Mesoscale fluxes over heterogeneous flat landscapes for use in larger scale models. Journal of Hydrology, 190(3-4), 317-336.More infoAbstract: We have developed a framework to parameterize mesoscale fluxes of heat, moisture, and momentum, in a cloud-free environment, which result from heterogeneous heating of flat land surfaces. The importance and parameterizability of these mesoscale fluxes is demonstrated using the mathematical concept of predictability. This methodology is used to estimate the relative importance of mesoscale, as contrasted with turbulent fluxes, in the Konza Prairie of Kansas during the FIFE field experiment.
- Chen, M., Dickinson, R. E., Zeng, X., & Hahmann, A. N. (1996). Comparison of precipitation observed over the continental United States to that simulated by a climate model. Journal of Climate, 9(9), 2233-2249.More infoAbstract: This study compares monthly average frequency, intensity, and amount of hourly precipitation simulated by National Center for Atmospheric Research Community Climate Model version 2/Biosphere-Atmosphere Transfer Scheme to the smoothed estimates of those observed locally over the continental United States. The observed monthly intensities vary from less than 1 to 5 mm h-1, with lowest values in the winter in northern midcontinent and highest around the Gulf Coast in summer. Model intensities are on the average 3-4 times less when drizzle is excluded and an order of magnitude less when drizzle is included. As might be anticipated, relative frequencies are much too high and intensities much too low in the model fields, compared to those observed. The spatial pattern of these quantities and the total precipitation are more realistic. The study also compares extreme events and diurnal and seasonal variations and finds that, in general, the simulated precipitation has larger spatial variability, larger diurnal variation, and longer maximum continuous wet and dry periods than those observed. These discrepancies are largely due to the different nature of model (area averaged) precipitation and observed (local) precipitation. Therefore, the use of GCM output to directly represent local values of intensities and frequencies may lead to large errors in the coupling of the land surface to the atmosphere above it.
- Dalu, G. A., Pielke, R. A., Baldi, M., & Zeng, X. (1996). Heat and momentum fluxes induced by thermal inhomogeneities with and without large-scale flow. Journal of the Atmospheric Sciences, 53(22), 3286-3302.More infoAbstract: The authors present an analytical evaluation of the vertical heat and momentum fluxes associated with mesoscale flow generated by periodic and isolated thermal inhomogeneities within the convective boundary layer (CBL). The influence of larger-scale wind flow is also included. The results show that, with little or no synoptic wind, the vertical velocity is in phase with the diabatic temperature perturbations and that the mesoscale heat flux is positive and of the same order as the diabatic heat flux within the CBL. Above the CBL, the heat flux is negative and penetrates into the free atmosphere through a depth comparable to the depth of the CBL. In the presence of synoptic flow, the mesoscale perturbation is in the form of propagating waves that penetrate deeply into the free atmosphere. As a result, there is a net downward flux of momentum, which is dissipated within the CBL by turbulence. Furthermore, mixing with the environment of the air particles displaced by the waves results in a net negative mesoscale heat flux, which contributes to the weakening of the stability of the free atmosphere. Strong synoptic advection can significantly weaken the horizontal temperature gradients in the CBL, thereby weakening the intensity of the mesoscale flow. Turbulent diffusion also weakens the temperature gradients and the intensity of the mesoscale flow at large wavenumbers when the wavelength is comparable to the CBL depth. Finally, when the synoptic wind is very strong, the mesoscale perturbation is very weak and vertically trapped.
- Zeng, N., Dickinson, R. E., & Zeng, X. (1996). Climatic impact of Amazon deforestation - A mechanistic model study. Journal of Climate, 9(4), 859-883.More infoAbstract: Recent general circulation model (GCM) experiments suggest a drastic change in the regional climate, especially the hydrological cycle, after hypothesized Amazon basinwide deforestation. To facilitate the theoretical understanding of such a change, we develop an intermediate-level model for tropical climatology, including atmosphere-land-ocean interaction. The model consists of linearized steady-state primitive equations with simplified thermodynamics. A simple hydrological cycle is also included. Special attention has been paid to land-surface processes. In comparison with previous simple modeling work on tropical climatology or anomalies, the present model is more sophisticated in predicting, with little input, most of the important meteorological variables; nevertheless, it is computationally simple. It generally better simulates tropical climatology and the ENSO anomaly than do many of the previous simple models. The climatic impact of Amazon deforestation is studied in the context of this model. Model results show a much weakened Atlantic Walker-Hadley circulation as a result of the existence of a strong positive feedback loop in the atmospheric circulation system and the hydrological cycle. The regional climate is highly sensitive to albedo change and sensitive to evapotranspiration change. The pure dynamical effect of surface roughness length on convergence is small, but the surface flow anomaly displays intriguing features. Analysis of the thermodynamic equation reveals that the balance between convective heating, adiabatic cooling, and radiation largely determines the deforestation response. Studies of the consequences of hypothetical continuous deforestation suggest that the replacement of forest by desert may be able to sustain a dry climate. Scaling analysis motivated by our modeling efforts also helps to interpret the common results of many GCM simulations. When a simple mixed-layer ocean model is coupled with the atmospheric model, the results suggest a 1°C decrease in SST gradient across the equatorial Atlantic Ocean in response to Amazon deforestation. The magnitude depends on the coupling strength.
- ZENG, X., & PIELKE, R. (1995). FURTHER STUDY ON THE PREDICTABILITY OF LANDSCAPE-INDUCED ATMOSPHERIC FLOW. JOURNAL OF THE ATMOSPHERIC SCIENCES, 52(10), 1680-1698.More infoThe effect of the synoptic-scale wind on the predictability of landscape-induced atmospheric flow as a function of surface characteristics is studied by means of over 200 two-dimensional numerical simulations. The predictability measure is defined based on the domain-averaged signal to noise ratio. The decomposition of the signal into two different parts is also used to further understand the effect of the synoptic-scale wind on the mesoscale atmospheric predictability. It is emphasized that different predictability measures should be used for different purposes, and different predictability results can be obtained based on different measures.
- ZENG, X., & PIELKE, R. (1995). LANDSCAPE-INDUCED ATMOSPHERIC FLOW AND ITS PARAMETERIZATION IN LARGE-SCALE NUMERICAL-MODELS. JOURNAL OF CLIMATE, 8(5), 1156-1177.More infoExtensive numerical simulations are performed to demonstrate that the landscape-induced mesoscale sensible heat, moisture, and momentum fluxes associated with spatially heterogeneous convective boundary layers can be larger than, and have a different vertical structure from, the turbulent fluxes for a typical general circulation model grid box. The mesoscale fluxes above the convective boundary layer may also provide a natural mechanism for the strong coupling between the boundary layer and the free atmosphere above. The parameterizability of these mesoscale and turbulent fluxes is demonstrated based on their predictability.
- Zeng, X. (1995). : Stochastic aspects in estimating the probability of producing good products by a system. The Earth Observer, 7(6), 23-24.
- PIELKE, R., & ZENG, X. (1994). LONG-TERM VARIABILITY OF CLIMATE. JOURNAL OF THE ATMOSPHERIC SCIENCES.
- Pielke, R. A., Schimel, D. S., Lee, T. J., Kittel, T. G., & Zeng, X. (1993). Atmosphere-terrestrial ecosystem interactions: implications for coupled modeling. Ecological Modelling, 67(1), 5-18.More infoAbstract: This paper overviews the important interactions between ecosystem and atmospheric dynamics. Examples of suggested procedures to model these interrelationships are introduced. The concept of equilibrium versus non-equilibrium non-linear dynamics in the context of chaos is discussed. The paper concludes with a description of a parameterization of vegetation effects on atmospheric structure, including a model sensitivity simulation of the importance of land surface processes on this structure. © 1993.
- ZENG, X., & PIELKE, R. (1993). ERROR-GROWTH DYNAMICS AND PREDICTABILITY OF SURFACE THERMALLY-INDUCED ATMOSPHERIC FLOW. JOURNAL OF THE ATMOSPHERIC SCIENCES, 50(17), 2817-2844.More infoUsing the CSU Regional Atmospheric Modeling System (RAMS) in its nonhydrostatic and compressible configuration, over 200 two-dimensional simulations with DELTAx = 2 km and DELTAx = 100 m are performed to study in detail the initial adjustment process and the error-growth dynamics of surface thermally induced circulations including the sensitivity to initial conditions (i.e., the traditional predictability), boundary conditions, and model parameters, and to study the predictability as a function of the size of surface heat patches under a calm mean wind. It is found that the error growth (at least at the stage when the surface forcing is strong) is not sensitive to the characteristics of the initial perturbations. The numerical smoothing has a strong impact on the initial adjustment process and on the error-growth dynamics. The predictability is variable dependent. The mesoscale flow is insensitive to lateral and top boundary conditions. Among the conclusions regarding the influence of the boundary-layer structures and model parameters on the predictability and flow structures, it is found that the vertical velocity field is strongly affected by the mean wind, and the flow structures are quite sensitive to the initial soil water content.
- ZENG, X., PIELKE, R., & EYKHOLT, R. (1993). CHAOS THEORY AND ITS APPLICATIONS TO THE ATMOSPHERE. Bull. Amer. Meteor., 631-644.More infoA brief overview of chaos theory is presented, including bifurcations, routes to turbulence, and methods for characterizing chaos. The paper divides chaos applications in atmospheric sciences into three categories: new ideas and insights inspired by chaos, analysis of observational data, and analysis of output from numerical models. Based on the review of chaos theory and the classification of chaos applications, suggestions for future work are given.
- Zeng, X., & Pielke, R. A. (1993). What does a low-dimensional weather attractor mean?. Physics Letters A, 175(5), 299-304.More infoAbstract: Low-dimensional attractors are obtained reliably for a surface thermally-induced atmospheric flow. Different physical processes corresponding with these low dimensions are discussed, and a mathematical mapping of the atmospheric flow patterns from these physical processes is determined. © 1993.
- Zeng, X., Pielke, R. A., & Eykholt, R. (1992). Estimating the fractal dimension and the predictability of the atmosphere. Journal of the Atmospheric Sciences, 49(8), 649-659.More infoAbstract: The fractal dimension, Lyapunov-exponent spectrum, Kolmogorov entropy, and predictability are analyzed for chaotic attractors in the atmosphere by analyzing the time series of daily surface temperature and pressure over several regions of the United States and the North Atlantic Ocean with different climatic signal-to-noise ratios. Though the total number of data points (from about 13 800 to about 36 500) is larger than those used in previous studies, it is still too small to obtain a reliable estimate of the Grassberger-Procaccia correlation dimension because of the limitations discussed by Ruelle. However, it can be shown that this dimension is greater than 8. Also, it is pointed out the most, if not all, of the previous estimates of low fractal dimensions in the atmosphere are spurious. -from Authors
- Zeng, X., Pielke, R., & Eykholt, R. (1992). Extracting Lyapunov exponents from short time series of low precision. Mod. Phys. Lett., 6B, 55-75.
- Zeng, X., Eykholt, R., & Pielke, R. A. (1991). Estimating the Lyapunov-exponent spectrum from short time series of low precision. Physical Review Letters, 66(25), 3229-3232.More infoAbstract: We propose a new method to compute Lyapunov exponents from limited experimental data. The method is tested on a variety of known model systems, and it is found that the algorithm can be used to obtain a reasonable Lyapunov-exponent spectrum from only 5000 data points with a precision of 10-1 or 10-2 in three- or four-dimensional phase space, or 10000 data points in five-dimensional phase space. We also apply our algorithm to the daily-averaged data of surface temperature observed at two locations in the United States to quantitatively evaluate atmospheric predictability.
- Zeng, X., Pielke, R. A., & Eykholt, R. (1990). Chaos in daisyworld. Tellus, Series B, 42 B(4), 309-318.More infoAbstract: A simple model, referred to as daisyworld, was later developed to illustrate the Gaia concept. In the current paper, this daisyworld model is used to study the interaction between biota and their environment in more detail. It is found that periodic, and even chaotic, states can exist when the parameter controlling the feedback between biota and environmental temperature is changed. The existence of periodic and chaotic solutions is verified by their power spectra, fractal dimensions, and Lyapunov exponents. These results show that stable climatic conditions are not always maintained in daisyworld, despite the presence of daisies which supply the required feedback. -from Authors
- Pielke, R., & Zeng, X. (1989). Influence on severe storm development of irrigated land. Natl. Wea. Dig., 14, 16-17.
- Zeng, X., Zhao, M., & Li, X. (1989). Three-dimensional numerical simulations of the interaction between urban heat island circulation and land breeze under sloping terrains. Chinese J. Atmos. Sci., 13, 358-366.
- Zeng, X., Zhao, M., & Miao, M. (1987). A study of wind profiles below 140 M under stable conditions. Chinese J. Atmos. Sci., 11, 183-191.
- Zhao, M., Zeng, X., & Wang, Y. (1986). The computation of the wind speed in the tower layer under near-neutral atmosphere. Chinese J. Meteor. Sci., 7-16.
- Shuttleworth, J., Zreda, M., Zeng, X., Zweck, C., & Ferre, T. (2010, Jul.). The COsmic-ray Soil Moisture Observing System (COSMOS): a non-invasive, intermediate scale soil moisture measurement network. In British Hydrological Society Third International Symposium: "Role of Hydrology in Managing Consequences of a Changing Global Environment",, 757-763.More infoNewcastle University, Newcastle, United Kingdom, 19-23 July 2010. ISBN: 1 903741 17 3
- Zeng, Q., Zeng, X., Wang, A., Dickinson, R., Zeng, X., & Shen, S. (2005, May). Models and numerical simulations of atmosphere-vegetaion-soil interactions and ecosystem dynamics. In Computational Physics: Proceedings of the Joint Conference of ICCP6 and CCP 2003, 98-109.More infoX.G. Zhao, S. Jiang, and X.J. Yu, Eds.
- Guo, B., Zeng, X., Stepanov, M., & Johnson, G. (2019, October 24). A numerical scheme for saturated flows based on convex optimization. Department of Hydrology and Atmospheric Sciences weekly Colloquium. University of Arizona.More infoA talk (by Misha Stepanov) at local (UofA) seminar, namely Colloquium at HAS. Presented the current state of the joint project on new numerical scheme for soil moisture evolution.
- Zeng, X. (2018, Spring). 10 Invited Presentations from January to December 2018 (detailed omitted here). Various conferences and institutions.
- Zeng, X. (2018, Spring). 10+ Contributed Presentations from January to December 2018 (detailed omitted here). Various Conferences.
- Clark, M. P., Fan, Y., Lawrence, D. M., Adam, J. C., Bolster, D., Gochis, D. J., Hooper, R. P., Kumar, M., Leung, L. R., Mackay, D. S., Maxwell, R. M., Shen, C., Swenson, S. C., & Zeng, X. (2015. Improving the representation of hydrologic processes in Earth System Models(pp 5929-5956).
- Zeng, X. (2004. Review of two-volume books “Large-Scale Atmosphere-Ocean Dynamics I and II”(pp 282-284).More infoBull. Amer. Meteor. Soc., 85
- Smith, R., Carbone, R., & Zeng, X. (2013, Jun.). Final Report of The Lower Atmospheric Observing Facilities Workshop - Meeting the Challenges of Climate System Science (on 18-19 June 2012, Boulder, Colorado, USA).More info11 June 2013. 44 pp (submitted to NSF AGS).
- Snow, J., Zeng, X., Klein, P., Shepherd, M., Sarnat, S., & Stanley, Sr., E. (2012, Jun.). NRC Report on “Urban Meteorology: Forecasting, Monitoring, and Meeting End Users' Needs”.More infoNational Academies Press, Washington, D.C., 190 pp.
- Dabberdt, W., Carbone, R., Chen, S., Forbes, G., Foufoula-Georgiou, E., Morss, R., Snow, J., & Zeng, X. (2010, May). NRC Report on “When Weather Matters: Science and Service to Meet Critical Societal Needs”.More infoNational Academies Press, Washington, D.C., 146 pp.
- Oleson, K., other coauthors, t., & Zeng, X. (2010, Sep.). Technical Description of version 4.0 of the Community Land Model (CLM).More infoNCAR Technical Note (NCAR/TN-478+STR), pp. 257.
- including, A. C., & Zeng, X. (2010, Mar.). The 2009 AMS member Survey – A summary of findings and response to members.More infoBull. Amer. Meteor. Soc., 91, 657-673.
- Asner, G., other coauthors, t., & Zeng, X. (2007, Oct.). Final Report of the 2007 NASA Earth Science Senior Review.More info36 pages, submitted to the NASA Earth Science Division
- Robinson, D., other coauthors, t., & Zeng, X. (2007, Jun.). NRC Report on Principles and Guidelines for Archiving Environmental and Geospatial Data at NOAA.More infoNational Academies Press, Washington, D.C., 116 pp.
- Zeng, X., Zeng, X., Shen, S., & Dickinson, R. (2005, MAY 6). Reply to comment by Dekker and Rietkerk on "Multiple equilibrium states and the abrupt transitions in a dynamical system of soil water interacting with vegetation''. GEOPHYSICAL RESEARCH LETTERS.
- Oleson, K., other coauthors, t., & Zeng, X. (2004, Sep.). Technical Description of the Community Land Model (CLM).More infoNCAR Technical Note (NCAR/TN-461+STR), pp. 174.
- Overpeck, J., other coauthors, s., & Zeng, X. (2004, May). Earth Sciences and Environmental Programs at the University of Arizona: A Plan for Focused Excellence.More info32 pages, submitted to the University of Arizona Provost
- Herman, B., Zeng, X., Chase, T., & Pielke, R. (2002, MAY). More heat over greenhouse gases. PHYSICS TODAY.
- ZENG, ., PIELKE, R., & EYKHOLT, R. (1992, JUL). CHAOS IN DAISYWORLD - REPLY. TELLUS SERIES B-CHEMICAL AND PHYSICAL METEOROLOGY.