Sylvia Sullivan

Interests

Research

atmospheric ice nucleation and microphysics, dynamics and microphysics of mesoscale convective storms, atmospheric and climate modeling, benchtop microphysical experiments

Teaching

aerosol physics, cloud and precipitation physics, (geophysical) fluid dynamics, numerical methods

Courses

Scholarly Contributions

Books

• Sullivan, S., & Hoose, C. (2023). Clouds and Their Climatic Impacts: Radiation, Circulation, and Precipitation. wiley. doi:10.1002/9781119700357
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  Clouds and Their Climatic Impacts: Clouds are an influential and complex element of Earth’s climate system. They evolve rapidly in time and exist over small spatial scales, but also affect global radiative balance and large-scale circulations. With more powerful models and extensive observations now at our disposal, the climate impact of clouds is receiving ever more research attention. Clouds and Their Climatic Impacts: Radiation, Circulation, and Precipitation presents an overview of our current understanding on various types of clouds and cloud systems and their multifaceted role in the radiative budget, circulation patterns, and rainfall. Volume highlights include: • Interactions of aerosol with both liquid and ice clouds • Surface and atmospheric cloud radiative feedbacks and effects • Arctic, extratropical, and tropical clouds • Cloud-circulation coupling at global, meso, and micro scales • Precipitation efficiency, phase, and measurements • The role of machine learning in understanding clouds and climate The American Geophysical Union promotes discovery in Earth and space science for the benefit of humanity. Its publications disseminate scientific knowledge and provide resources for researchers, students, and professionals.

Chapters

• Chakraborty, S., Sullivan, S., & Feng, Z. (2023). An Overview of Mesoscale Convective Systems: Global Climatology, Satellite Observations, and Modeling Strategies. In Geophysical Monograph Series(pp 195--221). Wiley.
• Sullivan, S., & Hoose, C. (2023). Science of cloud and climate science: An analysis of the literature over the past 50 years. In Geophysical Monograph Series.

Journals/Publications

• Makgoale, T. E., & Sullivan, S. C. (2025). Characterization and Comparison of Simulated Precipitation Efficiency From Global Storm-Resolving Models Over the Asian Monsoon Region. Journal of Geophysical Research: Atmospheres, 130(Issue 15). doi:10.1029/2025jd044228
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  This study evaluates the simulated tropical precipitation production from six global storm-resolving models participating in the DYnamics of the Atmospheric general circulation Modeled on Nonhydrostatic Domains (DYAMOND) intercomparison project. We first assess how well the DYAMOND models reproduce precipitation intensity ((Formula presented.)) relative to observations of the Integrated MultisatellitE Retrievals for Global Precipitation Measurement, as well as cloud water paths (CWP) relative to the ERA5 reanalysis. Both (Formula presented.) and CWP differences vary greatly between the models. We then look at the ratio of these fields in a recently defined index of precipitation efficiency (PE) ((Formula presented.)), which is straightforward both to calculate from 2D output and to compare to satellite observations. Moderate rainfall events have greater spatial variability in and lower values of (Formula presented.) than intense events for all models. Oceanic (Formula presented.) also tends to be greater than or equal to continental values in the models. We then phase partition (Formula presented.) using the ice water path ((Formula presented.)) and the liquid water path ((Formula presented.)) to understand which phases drive these behaviors in (Formula presented.). While the mean value and land-ocean contrast of (Formula presented.) correlate well with (Formula presented.), its spread and intermodel variability during intense events are driven by (Formula presented.). Lastly, we consider the sensitivity of (Formula presented.) to averaging timescales, as well as its variability over timescales in power spectra, and find that the models tend to underestimate the variability of (Formula presented.) at higher frequencies. These analyses indicate that explicit deep convection is insufficient to generate consistent (Formula presented.) and that both cloud microphysics and subdaily precipitation phenomena like mesoscale convective systems contribute to persistent intermodel variations in (Formula presented.).
• Sepulveda Araya, E. I., Sullivan, S. C., & Voigt, A. (2025). Ice crystal complexity leads to weaker ice cloud radiative heating in idealized single-column simulations. Atmospheric Chemistry and Physics, 25(Issue 15). doi:10.5194/acp-25-8943-2025
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  Ice clouds play an important role in the atmospheric radiation budget, both by reflecting shortwave radiation and by absorbing or emitting longwave radiation. These effects can modulate the cloud radiative heating (CRH) rate, which in turn influences circulation and precipitation. Ice cloud radiative properties depend on the size, shape (or habit), and complexity, including surface roughness or hollowness, of in-cloud ice crystals. To better predict ice cloud radiative effects, there has been a continuous effort to account for more ice crystal habits and complexity in current radiative transfer calculations. Here, we conduct a series of idealized single-column radiative transfer calculations to study how ice CRH responds to including ice crystal complexity. We evaluate four ice optical schemes for a range of ice cloud formation temperatures or altitudes, geometrical depths, ice water paths (IWPs), and ice crystal effective radii. In addition, we present a heating rate sensitivity matrix as a condensed visualization of the CRH response across a broad parameter space. We find that including ice complexity in cold thin clouds with high IWPs can diminish the net in-cloud heating and cloud-top cooling by 2.5 and 15 K d−1, respectively. Furthermore, while temperature-based schemes behave similarly to other schemes at warmer temperatures, they predict net CRH at the cloud bottom more than 10 K d−1 higher than size-dependent schemes at the coldest temperatures. Either weakening of CRH by ice complexity or strengthening by temperature-dependent schemes can alter anvil cloud lifetime and evolution, as well as large-scale atmospheric circulation.
• Sullivan, S. C., Vautravers, P., Beucler, T., Makgoale, T., & Yin, J. (2025). Moisture–Precipitation Couplings for Mesoscale Convective Systems in Tracking Data and Idealized Simulations. Journal of the Atmospheric Sciences, 82(Issue 9). doi:10.1175/jas-d-24-0174.1
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  An increase in extreme precipitation has been well established in the transition from disorganized to organized convection, but studies conflict about how precipitation changes with the degree of clustering in organized convection. Mesoscale convective systems (MCSs) are one form of organized convection, and we examine here how precipitation intensities and various moisture–precipitation couplings change with MCS morphology, in both a multidecade tracking dataset and idealized radiative–convective equilibrium (RCE) simulations. Both in general and for a given column saturation fraction (CSF), mean and extreme precipitation robustly increase for larger MCSs in the tracking dataset but show limited changes or decrease for larger MCSs in the RCE simulation output. In an attempt to explain this discrepancy, we examine other moisture–precipitation relationships within the two datasets, including how insufficient moisture suppresses precipitation, how saturation deficit generates convective available potential energy (CAPE), how CAPE generates ascent, and how much condensate the systems contain. In both datasets, reduced CAPE production at MCS margins and higher stability from a warmer upper troposphere reduce vertical velocities within the larger MCSs. In both datasets, larger MCSs also contain less condensate, especially above 500 hPa. We then explain how sampling and model biases could contribute to the different responses of precipitation to MCS extent and suggest that a “dual role” of vertical velocity}in determining both condensate formation and sedimentation}should be considered in future studies of precipitation intensity from mesoscale clusters. SIGNIFICANCE STATEMENT: Storms that organize over hundreds of kilometers are called mesoscale convective systems, and they can cause flooding with their intense rainfall. Here, we look at how rainfall from these storms changes with the size of the storm in both observational and modeling data. The two datasets predict different trends in rainfall with storm size, and we explore why. In particular, we examine how environmental dryness may promote atmospheric instability, how that instability converts to atmospheric ascent, and how that ascent forms condensed liquid droplets or ice crystals within the storm. These behaviors are consistent across the two datasets, so we turn to different sampling in space and time to expand the differing trends in precipitation with storm size.
• Gasparini, B., Sullivan, S. C., Sokol, A. B., Kärcher, B., Jensen, E., & Hartmann, D. L. (2023). Opinion: Tropical cirrus - from micro-scale processes to climate-scale impacts. Atmospheric Chemistry and Physics, 23(Issue 24). doi:10.5194/acp-23-15413-2023
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  Tropical cirrus clouds, i.e., any type of ice cloud with tops above 400hPa, play a critical role in the climate system and are a major source of uncertainty in our understanding of global warming. Tropical cirrus clouds involve processes spanning a wide range of spatial and temporal scales, from ice microphysics on cloud scales to mesoscale convective organization and planetary wave dynamics. This complexity makes tropical cirrus clouds notoriously difficult to model and has left many important questions stubbornly unanswered. At the same time, their multi-scale nature makes them well-positioned to benefit from the rise of global, high-resolution simulations of Earth's atmosphere and a growing abundance of remotely sensed and in situ observations. Rapid progress on our understanding of tropical cirrus requires coordinated efforts to take advantage of these modern computational and observational abilities. In this opinion paper, we review recent progress in cirrus studies, highlight important unanswered questions, and discuss promising paths forward. Significant progress has been made in understanding the life cycle of convectively generated "anvil"cirrus and the response of their macrophysical properties to large-scale controls. On the other hand, much work remains to be done to fully understand how small-scale anvil processes and the climatological anvil radiative effect will respond to global warming. Thin, in situ formed cirrus clouds are now known to be closely tied to the thermal structure and humidity of the tropical tropopause layer, but microphysical uncertainties prevent a full understanding of this link, as well as the precise amount of water vapor entering the stratosphere. Model representation of ice-nucleating particles, water vapor supersaturation, and ice depositional growth continue to pose great challenges to cirrus modeling. We believe that major advances in the understanding of tropical cirrus can be made through a combination of cross-tool synthesis and cross-scale studies conducted by cross-disciplinary research teams.
• Gu, L., Yin, J., Gentine, P., Wang, H. M., Slater, L. J., Sullivan, S. C., Chen, J., Zscheischler, J., & Guo, S. (2023). Large anomalies in future extreme precipitation sensitivity driven by atmospheric dynamics. Nature Communications, 14(Issue 1). doi:10.1038/s41467-023-39039-7
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  Increasing atmospheric moisture content is expected to intensify precipitation extremes under climate warming. However, extreme precipitation sensitivity (EPS) to temperature is complicated by the presence of reduced or hook-shaped scaling, and the underlying physical mechanisms remain unclear. Here, by using atmospheric reanalysis and climate model projections, we propose a physical decomposition of EPS into thermodynamic and dynamic components (i.e., the effects of atmospheric moisture and vertical ascent velocity) at a global scale in both historical and future climates. Unlike previous expectations, we find that thermodynamics do not always contribute to precipitation intensification, with the lapse rate effect and the pressure component partly offsetting positive EPS. Large anomalies in future EPS projections (with lower and upper quartiles of −1.9%/°C and 8.0%/°C) are caused by changes in updraft strength (i.e., the dynamic component), with a contrast of positive anomalies over oceans and negative anomalies over land areas. These findings reveal counteracting effects of atmospheric thermodynamics and dynamics on EPS, and underscore the importance of understanding precipitation extremes by decomposing thermodynamic effects into more detailed terms.
• Sullivan, S., Keshtgar, B., Albern, N., Bala, E., Braun, C., Choudhary, A., Hörner, J., Lentink, H., Papavasileiou, G., & Voigt, A. (2023). How does cloud-radiative heating over the North Atlantic change with grid spacing, convective parameterization, and microphysics scheme in ICON version 2.1.00?. Geoscientific Model Development, 16(Issue 12). doi:10.5194/gmd-16-3535-2023
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  Cloud-radiative heating (CRH) within the atmosphere and its changes with warming affect the large-scale atmospheric winds in a myriad of ways, such that reliable predictions and projections of circulation require reliable calculations of CRH. In order to assess the sensitivities of upper-tropospheric midlatitude CRH to model settings, we perform a series of simulations with the ICOsahedral Nonhydrostatic Model (ICON) over the North Atlantic using six different grid spacings, parameterized and explicit convection, and one- versus two-moment cloud microphysics. While sensitivity to grid spacing is limited, CRH profiles change dramatically with microphysics and convection schemes. These dependencies are interpreted via decomposition into cloud classes and examination of cloud properties and cloud-controlling factors within these different classes. We trace the model dependencies back to differences in the mass mixing ratios and number concentrations of cloud ice and snow, as well as vertical velocities. Which frozen species are radiatively active and the broadening of the vertical velocity distribution with explicit convection turn out to be crucial factors in altering the modeled CRH profiles.
• Krämer, M., Miltenberger, A., Rolf, C., Sullivan, S., & Voigt, A. (2022). A Lagrangian Perspective of Microphysical Impact on Ice Cloud Evolution and Radiative Heating. Journal of Advances in Modeling Earth Systems, 14(11). doi:10.1029/2022ms003226
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  We generate trajectories in storm-resolving simulations in order to quantify the effect of ice microphysics on tropical upper-tropospheric cloud-radiative heating. The pressure and flow field tracked along the trajectories are used to run different ice microphysical schemes, both one- and two-moment formulations within the Icosahedral Non-hydrostatic Model model and a separate offline box microphysics model (CLaMS-Ice). This computational approach allows us to isolate purely microphysical differences in a variant of “microphysical piggybacking;” feedbacks of microphysics onto pressure and the flow field, for example, via latent heating, are suppressed. Despite these constraints, we find about a 5-fold difference in median cloud ice mass mixing ratios (qi) and ice crystal number (Ni) between the microphysical schemes and very distinct qi distributions versus temperature and relative humidity with respect to ice along the trajectories. After investigating microphysical formulations for nucleation, depositional growth, and sedimentation, we propose three cirrus lifecycles: a weak source-strong sink lifecycle whose longwave and shortwave heating are smallest due to short lifetime and low optical depth, a strong source-weak sink lifecycle whose longwave and shortwave heating are largest due to long lifetime and high optical depth, and a strong source-strong sink lifecycle with intermediate radiative properties.
• Bacer, S., Sullivan, S. C., Sourdeval, O., Tost, H., Lelieveld, J., & Pozzer, A. (2021). Cold cloud microphysical process rates in a global chemistry-climate model. Atmospheric Chemistry and Physics, 21(Issue 3). doi:10.5194/acp-21-1485-2021
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  Microphysical processes in cold clouds which act as sources or sinks of hydrometeors below 0 ◦C control the ice crystal number concentrations (ICNCs) and in turn the cloud radiative effects. Estimating the relative importance of the cold cloud microphysical process rates is of fundamental importance to underpin the development of cloud parameterizations for weather, atmospheric chemistry, and climate models and to compare the output with observations at different temporal resolutions. This study quantifies and investigates the ICNC rates of cold cloud microphysical processes by means of the chemistry-climate model EMAC (ECHAM/MESSy Atmospheric Chemistry) and defines the hierarchy of sources and sinks of ice crystals. Both microphysical process rates, such as ice nucleation, aggregation, and secondary ice production, and unphysical correction terms are presented. Model ICNCs are also compared against a satellite climatology. We found that model ICNCs are in overall agreement with satellite observations in terms of spatial distribution, although the values are overestimated, especially around high mountains. The analysis of ice crystal rates is carried out both at global and at regional scales. We found that globally the freezing of cloud droplets and convective detrainment over tropical land masses are the dominant sources of ice crystals, while aggregation and accretion act as the largest sinks. In general, all processes are characterized by highly skewed distributions. Moreover, the influence of (a) different ice nucleation parameterizations and (b) a future global warming scenario on the rates has been analysed in two sensitivity studies. In the first, we found that the application of different parameterizations for ice nucleation changes the hierarchy of ice crystal sources only slightly. In the second, all microphysical processes follow an upward shift in altitude and an increase by up to 10 % in the upper troposphere towards the end of the 21st century.
• Sullivan, S. C., & Voigt, A. (2021). Ice microphysical processes exert a strong control on the simulated radiative energy budget in the tropics. Communications Earth and Environment, 2(Issue 1). doi:10.1038/s43247-021-00206-7
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  Simulations of the global climate system at storm-resolving resolutions of 2 km are now becoming feasible and show promising realism in clouds and precipitation. However, shortcomings in their representation of microscale processes, like the interaction of cloud droplets and ice crystals with radiation, can still restrict their utility. Here, we illustrate how changes to the ice microphysics scheme dramatically alter both the vertical profile of cloud-radiative heating and top-of-atmosphere outgoing longwave radiation (terrestrial infrared cooling) in storm-resolving simulations over the Asian monsoon region. Poorly-constrained parameters in the ice nucleation scheme, overactive conversion of ice to snow, and inconsistent treatment of ice crystal effective radius between microphysics and radiation alter cloud-radiative heating by a factor of four and domain-mean infrared cooling by 30 W m−2. Vertical resolution, on the other hand, has a very limited impact. Even in state-of-the-art models then, uncertainties in microscale cloud properties exert a strong control on the radiative budget that propagates to both atmospheric circulation and regional climate. These uncertainties need to be reduced to realize the full potential of storm-resolving models.
• Yin, J., Guo, S., Gentine, P., Sullivan, S. C., Gu, L., He, S., Chen, J., & Liu, P. (2021). Does the Hook Structure Constrain Future Flood Intensification Under Anthropogenic Climate Warming?. Water Resources Research, 57(Issue 2). doi:10.1029/2020wr028491
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  Atmospheric moisture holding capacity increases with temperature by about 7% per °C according to the Clausius-Clapeyron relationship. Thermodynamically then, precipitation intensity should exponentially intensify and thus worsen flood conditions as the climate warms. However, regional and global analyses often report a nonmonotonic (hook) scaling of precipitation and runoff, in which extremes strengthen with rising temperature up to a maximum or peak point (Tpp) and decline thereafter. The underlying cause of this hook structure is not yet well-understood, and whether it may shift and/or regulate storm runoff extremes under anthropogenic climate warming remains unknown. Here, we examine temperature scaling of precipitation and storm runoff extremes under different climate conditions using observations and large ensemble hydroclimatic simulations over mainland China. In situ observations suggest a spatially homogeneous, negative response of relative humidity to warming climates over 34.6% of the land area, and the remaining hook-dominated regions usually show a colder Tpp than that of precipitation or storm runoff extremes. The precipitation and streamflow series over mainland China's catchments throughout the 21st century are projected by a model cascade chain under a high-end emission scenario (RCP 8.5), which involves 31 CMIP5 climate models, 11 CMIP6 climate members, a daily bias correction method, and four lumped conceptual hydrological models. The CMIP5 ensemble projects that the hook structures shift toward warmer temperature bins, resulting in 10%–30% increases in storm runoff extremes over mainland China, while the CMIP6 ensemble projects more severe flood conditions in future warming climates.
• Gu, L., Chen, J., Yin, J., C Sullivan, S., Wang, H. M., Guo, S., Zhang, L., & Kim, J. S. (2020). Projected increases in magnitude and socioeconomic exposure of global droughts in 1.5 and 2 °C warmer climates. Hydrology and Earth System Sciences, 24(Issue 1). doi:10.5194/hess-24-451-2020
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  The Paris Agreement sets a long-term temperature goal to hold global warming to well below 2.0 ĝC and strives to limit it to 1.5 ĝC above preindustrial levels. Droughts with either intense severity or a long persistence could both lead to substantial impacts such as infrastructure failure and ecosystem vulnerability, and they are projected to occur more frequently and trigger intensified socioeconomic consequences with global warming. However, existing assessments targeting global droughts under 1.5 and 2.0 ĝC warming levels usually neglect the multifaceted nature of droughts and might underestimate potential risks. This study, within a bivariate framework, quantifies the change in global drought conditions and corresponding socioeconomic exposures for additional 1.5 and 2.0 ĝC warming trajectories. The drought characteristics are identified using the Standardized Precipitation Evapotranspiration Index (SPEI) combined with the run theory, with the climate scenarios projected by 13 Coupled Model Inter-comparison Project Phase 5 (CMIP5) global climate models (GCMs) under three representative concentration pathways (RCP 2.6, RCP4.5 and RCP8.5). The copula functions and the most likely realization are incorporated to model the joint distribution of drought severity and duration, and changes in the bivariate return period with global warming are evaluated. Finally, the drought exposures of populations and regional gross domestic product (GDP) under different shared socioeconomic pathways (SSPs) are investigated globally. The results show that within the bivariate framework, the historical 50-year droughts may double across 58 % of global landmasses in a 1.5 ĝC warmer world, while when the warming climbs up to 2.0 ĝC, an additional 9 % of world landmasses would be exposed to such catastrophic drought deteriorations. More than 75 (73) countries' populations (GDP) will be completely affected by increasing drought risks under the 1.5 ĝC warming, while an extra 0.5 ĝC warming will further lead to an additional 17 countries suffering from a nearly unbearable situation. Our results demonstrate that limiting global warming to 1.5 ĝC, compared with 2 ĝC warming, can perceptibly mitigate the drought impacts over major regions of the world.
• Schiro, K. A., Sullivan, S. C., Kuo, Y. H., Su, H., Gentine, P., Elsaesser, G. S., Jiang, J. H., & Neelin, J. D. (2020). Environmental controls on tropical mesoscale convective system precipitation intensity. Journal of the Atmospheric Sciences, 77(Issue 12). doi:10.1175/jas-d-20-0111.1
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  Using multiple independent satellite and reanalysis datasets, we compare relationships between mesoscale convective system (MCS) precipitation intensity Pmax, environmental moisture, large-scale vertical velocity, and system radius among tropical continental and oceanic regions. A sharp, nonlinear relationship between column water vapor and Pmax emerges, consistent with nonlinear increases in estimated plume buoyancy. MCS Pmax increases sharply with increasing boundary layer and lower free tropospheric (LFT) moisture, with the highest Pmax values originating from MCSs in environments exhibiting a peak in LFT moisture near 750 hPa. MCS Pmax exhibits strikingly similar behavior as a function of water vapor among tropical land and ocean regions. Yet, while the moisture–Pmax relationship depends strongly on mean tropospheric temperature, it does not depend on sea surface temperature over ocean or surface air temperature over land. Other Pmax-dependent factors include system radius, the number of convective cores, and the large-scale vertical velocity. Larger systems typically contain wider convective cores and higher Pmax, consistent with increased protection from dilution due to dry air entrainment and reduced reevaporation of precipitation. In addition, stronger large-scale ascent generally supports greater precipitation production. Last, temporal lead–lag analysis suggests that anomalous moisture in the lower–middle troposphere favors convective organization over most regions. Overall, these statistics provide a physical basis for understanding environmental factors controlling heavy precipitation events in the tropics, providing metrics for model diagnosis and guiding physical intuition regarding expected changes to precipitation extremes with anthropogenic warming.
• Sotiropoulou, G., Sullivan, S., Savre, J., Lloyd, G., Lachlan-Cope, T., Ekman, A. M., & Nenes, A. (2020). The impact of secondary ice production on Arctic stratocumulus. Atmospheric Chemistry and Physics, 20(Issue 3). doi:10.5194/acp-20-1301-2020
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  In situ measurements of Arctic clouds frequently show that ice crystal number concentrations (ICNCs) are much higher than the number of available ice-nucleating particles (INPs), suggesting that secondary ice production (SIP) may be active. Here we use a Lagrangian parcel model (LPM) and a large-eddy simulation (LES) to investigate the impact of three SIP mechanisms (rime splintering, break-up from ice-ice collisions and drop shattering) on a summer Arctic stratocumulus case observed during the Aerosol-Cloud Coupling And Climate Interactions in the Arctic (ACCACIA) campaign. Primary ice alone cannot explain the observed ICNCs, and drop shattering is ineffective in the examined conditions. Only the combination of both rime splintering (RS) and collisional break-up (BR) can explain the observed ICNCs, since both of these mechanisms are weak when activated alone. In contrast to RS, BR is currently not represented in large-scale models; however our results indicate that this may also be a critical ice-multiplication mechanism. In general, low sensitivity of the ICNCs to the assumed INP, to the cloud condensation nuclei (CCN) conditions and also to the choice of BR parameterization is found. Finally, we show that a simplified treatment of SIP, using a LPM constrained by a LES and/or observations, provides a realistic yet computationally efficient way to study SIP effects on clouds. This method can eventually serve as a way to parameterize SIP processes in large-scale models.
• Sullivan, S. C., Schiro, K. A., Yin, J., & Gentine, P. (2020). Changes in Tropical Precipitation Intensity With El Niño Warming. Geophysical Research Letters, 47(Issue 14). doi:10.1029/2020gl087663
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  Mesoscale convection generates the majority of extreme precipitation in tropical regions. Changes to these precipitation intensities, P, with long-term modes of climate variability have been hard to assess because they are not well represented in current climate models. Here we stratify a satellite climatology of convective systems by El Niño phase and cloud top temperature. We find that gains (losses) in high precipitation intensity ((Formula presented.) 10 mm hr−1) are largest for the deepest (least deep) systems during El Niño relative to La Niña. The surface temperature and wind changes that define El Niño manifest as surface flux changes but are not sufficient to explain these (Formula presented.) trends. We explore also the dynamical component of precipitation generation with a vertical momentum budget. Midtropospheric drying in the vicinity of the deepest systems boosts instability and ascent rates during El Niño, while the strengthened large-scale ascent minimizes the drag force on their updrafts.
• Sullivan, S. C., Schiro, K. A., Stubenrauch, C., & Gentine, P. (2019). The Response of Tropical Organized Convection to El Niño Warming. Journal of Geophysical Research: Atmospheres, 124(Issue 15). doi:10.1029/2019jd031026
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  Convective organization has a large impact on precipitation and feeds back on larger-scale circulations in the tropics. The degree of this convective organization changes with modes of climate variability like the El Niño–Southern Oscillation (ENSO), but because organization is not represented in current climate models, a quantitative assessment of these shifts has not been possible. Here, we construct multidecade satellite climatologies of occurrence of tropical convective organization and its properties and assess changes with ENSO phase. The occurrence of organized deep convection becomes more concentrated, increasing threefold in the eastern and central Pacific during El Niño and decreasing twofold outside of these regions. Both horizontal extent of the cold cloud shield and convective depth increase in regions of positive sea surface temperature anomaly (SSTa); however, the regions of greatest convective deepening are those of large-scale ascent, rather than those of warmest SSTa. Extent decreases with SSTa at a rate of about 20 km/K, while the SSTa dependence of depth is only about 0.2 K/K. We introduce two values to describe convective changes with ENSO more succinctly: (1) an information entropy metric to quantify the clustering of convective system occurrences and (2) a growth metric to quantify deepening relative to spreading over the system lifetime. Finally, with collocated precipitation data, we see that rainfall attributable to convective organization jumps up to 5% with warming. Rain intensity and amount increase for a given system size during El Niño, but a given rain amount may actually fall with higher intensity during La Niña.
• Bacer, S., Sullivan, S. C., Karydis, V. A., Barahona, D., Krämer, M., Nenes, A., Tost, H., Tsimpidi, A. P., Lelieveld, J., & Pozzer, A. (2018). Implementation of a comprehensive ice crystal formation parameterization for cirrus and mixed-phase clouds in the EMAC model (based on MESSy 2.53). Geoscientific Model Development, 11(Issue 10). doi:10.5194/gmd-11-4021-2018
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  A comprehensive ice nucleation parameterization has been implemented in the global chemistry-climate model EMAC to improve the representation of ice crystal number concentrations (ICNCs). The parameterization of Barahona and Nenes (2009, hereafter BN09) allows for the treatment of ice nucleation taking into account the competition for water vapour between homogeneous and heterogeneous nucleation in cirrus clouds. Furthermore, the influence of chemically heterogeneous, polydisperse aerosols is considered by applying one of the multiple ice nucleating particle parameterizations which are included in BN09 to compute the heterogeneously formed ice crystals. BN09 has been modified in order to consider the pre-existing ice crystal effect and implemented to operate both in the cirrus and in the mixed-phase regimes. Compared to the standard EMAC parameterizations, BN09 produces fewer ice crystals in the upper troposphere but higher ICNCs in the middle troposphere, especially in the Northern Hemisphere where ice nucleating mineral dust particles are relatively abundant. Overall, ICNCs agree well with the observations, especially in cold cirrus clouds (at temperatures below 205 K), although they are underestimated between 200 and 220 K. As BN09 takes into account processes which were previously neglected by the standard version of the model, it is recommended for future EMAC simulations.
• Sullivan, S., Barthlott, C., Crosier, J., Zhukov, I., Nenes, A., & Hoose, C. (2018). The effect of secondary ice production parameterization on the simulation of a cold frontal rainband. Atmospheric Chemistry and Physics, 18(22). doi:10.5194/acp-18-16461-2018
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  Secondary ice production via processes like rime splintering, frozen droplet shattering, and breakup upon ice hydrometeor collision have been proposed to explain discrepancies between in-cloud ice crystal and ice-nucleating particle numbers. To understand the impact of this additional ice crystal generation on surface precipitation, we present one of the first studies to implement frozen droplet shattering and ice-ice collisional breakup parameterizations in a mesoscale model. We simulate a cold frontal rainband from the Aerosol Properties, PRocesses, And InfluenceS on the Earth's Climate campaign and investigate the impact of the new parameterizations on the simulated ice crystal number concentrations (ICNC) and precipitation. Near the convective regions of the rainband, contributions to ICNC can be as large from secondary production as from primary nucleation, but ICNCs greater than 50 Lg-1 remain underestimated by the model. The addition of the secondary production parameterizations also clearly intensifies the differences in both accumulated precipitation and precipitation rate between the convective towers and non-convective gap regions. We suggest, then, that secondary ice production parameterizations be included in large-scale models on the basis of large hydrometeor concentration and convective activity criteria.
• Yin, J., Gentine, P., Zhou, S., Sullivan, S. C., Wang, R., Zhang, Y., & Guo, S. (2018). Large increase in global storm runoff extremes driven by climate and anthropogenic changes. Nature Communications, 9(Issue 1). doi:10.1038/s41467-018-06765-2
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  Weather extremes have widespread harmful impacts on ecosystems and human communities with more deaths and economic losses from flash floods than any other severe weather-related hazards. Flash floods attributed to storm runoff extremes are projected to become more frequent and damaging globally due to a warming climate and anthropogenic changes, but previous studies have not examined the response of these storm runoff extremes to naturally and anthropogenically driven changes in surface temperature and atmospheric moisture content. Here we show that storm runoff extremes increase in most regions at rates higher than suggested by Clausius-Clapeyron scaling, which are systematically close to or exceed those of precipitation extremes over most regions of the globe, accompanied by large spatial and decadal variability. These results suggest that current projected response of storm runoff extremes to climate and anthropogenic changes may be underestimated, posing large threats for ecosystem and community resilience under future warming conditions.
• Sullivan, S. C., Hoose, C., & Nenes, A. (2017). Investigating the contribution of secondary ice production to in-cloud ice crystal numbers. Journal of Geophysical Research: Atmospheres, 122(Issue 17). doi:10.1002/2017jd026546
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  In-cloud measurements of ice crystal number concentration can be orders of magnitude higher than the precloud ice nucleating particle number concentration. This disparity may be explained with secondary ice production processes. Several such processes have been proposed, but their relative importance and even the exact physics are not well known. In this work, a six-hydrometeor-class parcel model is developed to investigate the ice crystal number enhancement, both its bounds and its value for different cloud states, from rime splintering and breakup upon graupel-graupel collision. The model also includes ice aggregation and droplet coalescence, ice hydrometeor nonsphericity, and a time delay formulation for hydrometeor growth. Conditions to maximize the breakup contribution, as well as the effects of nonsphericity and turbulence, are discussed. We find that the largest enhancement of ice crystal number occurs for “intermediate” conditions, characterized by moderate updrafts and activation and nucleation rates. In this case, vertical motion is strong enough, and new hydrometeor formation limited enough, to sustain supersaturation as hydrometeors grow to larger sizes. After these larger hydrometeors form at sufficient number concentrations, the ice crystal number can be enhanced by a factor of 104 in some cases relative to the number generated by primary ice nucleation alone. Excluding ice hydrometeor nonsphericity limits secondary production significantly, and the parcel updraft can modulate it by about an order of magnitude.
• Sullivan, S. C., Lee, D., Oreopoulos, L., & Nenesa, A. (2016). Role of updraft velocity in temporal variability of global cloud hydrometeor number. Proceedings of the National Academy of Sciences of the United States of America, 113(Issue 21). doi:10.1073/pnas.1514039113
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  Understanding how dynamical and aerosol inputs affect the temporal variability of hydrometeor formation in climate models will help to explain sources of model diversity in cloud forcing, to provide robust comparisons with data, and, ultimately, to reduce the uncertainty in estimates of the aerosol indirect effect. This variability attribution can be done at various spatial and temporal resolutions with metrics derived from online adjoint sensitivities of droplet and crystal number to relevant inputs. Such metrics are defined and calculated from simulations using the NASA Goddard Earth Observing System Model, Version 5 (GEOS-5) and the National Center for Atmospheric Research Community Atmosphere Model Version 5.1 (CAM5.1). Input updraft velocity fluctuations can explain as much as 48% of temporal variability in output ice crystal number and 61% in droplet number in GEOS-5 and up to 89% of temporal variability in output ice crystal number in CAM5.1. In both models, this vertical velocity attribution depends strongly on altitude. Despite its importance for hydrometeor formation, simulated vertical velocity distributions are rarely evaluated against observations due to the sparsity of relevant data. Coordinated effort by the atmospheric community to develop more consistent, observationally based updraft treatments will help to close this knowledge gap.
• Sullivan, S. C., Morales Betancourt, R., Barahona, D., & Nenes, A. (2016). Understanding cirrus ice crystal number variability for different heterogeneous ice nucleation spectra. Atmospheric Chemistry and Physics, 16(Issue 4). doi:10.5194/acp-16-2611-2016
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  Along with minimizing parameter uncertainty, understanding the cause of temporal and spatial variability of the nucleated ice crystal number, Ni, is key to improving the representation of cirrus clouds in climate models. To this end, sensitivities of Ni to input variables like aerosol number and diameter provide valuable information about nucleation regime and efficiency for a given model formulation. Here we use the adjoint model of the adjoint of a cirrus formation parameterization (Barahona and Nenes, 2009b) to understand Ni variability for various ice-nucleating particle (INP) spectra. Inputs are generated with the Community Atmosphere Model version 5, and simulations are done with a theoretically derived spectrum, an empirical lab-based spectrum and two field-based empirical spectra that differ in the nucleation threshold for black carbon particles and in the active site density for dust. The magnitude and sign of Ni sensitivity to insoluble aerosol number can be directly linked to nucleation regime and efficiency of various INP. The lab-based spectrum calculates much higher INP efficiencies than field-based ones, which reveals a disparity in aerosol surface properties. Ni sensitivity to temperature tends to be low, due to the compensating effects of temperature on INP spectrum parameters; this low temperature sensitivity regime has been experimentally reported before but never deconstructed as done here.
• Sheyko, B. A., Sullivan, S. C., Morales, R., Capps, S. L., Barahona, D., Shi, X., Liu, X., & Nenes, A. (2015). Quantifying sensitivities of ice crystal number and sources of ice crystal number variability in CAM 5.1 using the adjoint of a physically based cirrus formation parameterization. Journal of Geophysical Research, 120(Issue 7). doi:10.1002/2014jd022457
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  We present the adjoint of a cirrus formation parameterization that computes the sensitivity of ice crystal number concentration to updraft velocity, aerosol, and ice deposition coefficient. The adjoint is driven by simulations from the National Center for Atmospheric Research Community Atmosphere Model version 5.1 CAM 5.1 to understand the sensitivity of formed ice crystal number concentration to 13 variables and quantify which contribute to its variability. Sensitivities of formed ice crystal number concentration to updraft velocity, sulfate number, and is sufficient but sulfate number concentration is low, indicating a sulfate-limited regime. Outside of the tropics, competition between homogeneous and heterogeneous nucleation may shift annually averaged sensitivities to higher magnitudes, when infrequent strong updrafts shift crystal production away from purely heterogeneous nucleation. Outside the tropics, updraft velocity is responsible for approximately 52.70% of the ice crystal number variability. In the tropics, sulfate number concentration and updraft jointly control variability in formed crystal number concentration. Insoluble aerosol species play a secondary, but still important, role in influencing the variability in crystal concentrations, with coarse-mode dust being the largest contributor at nearly 50% in certain regions. On a global scale, more than 95% of the temporal variability in crystal number concentration can be described by temperature, updraft velocity, sulfate number, and coarse-mode dust number concentration.