Group Head (Aug 2014-Aug 2025):

Sonia Lasher-Trapp, Blue Waters Professor
Dept. of Atmospheric Sciences
217-244-4250
slasher@illinois.edu

Entrainment on the edge of the rotating updraft on a simulated supercell thunderstorm.  Blue shading is entrainment (air moving into the core) and red shading is detrainment (air moving out of the core).  For more details, see Lasher-Trapp et al. (2021): Entrainment in a Simulated Supercell Thunderstorm. Part I: The Evolution of Different Entrainment Mechanisms and Their Dilutive Effects, J. Atmos. Sci., 78, 2725-2740.

NEWS

LATEST PUBLICATIONS

Ross, T. I. D., and S. Lasher-Trapp, 2025: Investigating the relative roles of INPs and CCN in a simulated thunderstorm using a new immersion freezing algorithm. J. Geog. Res. Atmos., 130, e2024JD042592, https://doi.org/10.1029/2024JD042592

Abstract:  Microphysical processes in deep convective clouds are sensitive to the number concentrations of cloud condensation nuclei (CCN) and ice nucleating particles (INPs), but the effects of INPs are less studied. Modeling studies investigating the effects of INPs and/or CCN on deep convection typically retain a volumedependent raindrop freezing relation. The resulting neglect of aerosol accumulation in raindrops via drop collisions has likely produced unrealistic storm responses to INPs in past studies. To address this deficiency, a new immersion freezing algorithm was developed and embedded in a bulk microphysics scheme that freezes both cloud drops and raindrops using the same immersion freezing INP (IF‐INP) activity spectrum based on measurements. Multiple idealized simulations of a single case of deep convection observed during the Clouds, Aerosols, and Complex Terrain Interactions (CACTI) field campaign were conducted, with microphysical differences produced by independently altering IF‐INP temperature dependencies and CCN number concentrations from their observed values. Surface precipitation in all simulations resulted almost exclusively from riming graupel that melted upon descending to the surface. Rainfall and cold pools were substantially and systematically weakened with increased CCN due to decreased graupel riming rates but were relatively insensitive to variations in the magnitude and slope of IF‐INP spectra due to compensating depletion of supercooled liquid water. These compensating processes were a consequence of the accumulation of IF‐INPs in raindrops, encouraging caution in studying IF‐INP effects upon thunderstorms using traditional volumedependent drop freezing relationships.

 

Sari, F. P., and S. Lasher-Trapp, 2025: Hailstorm events over a maritime tropical region: Storm environments and characteristics. J. Geog. Res. Atmos., 130, e2024JD042718, https://doi.org/10.1029/2024JD042718

Abstract:  In recent years, the Maritime Tropics (mT) have reported an increase in hail events, including five occurrences over Surabaya, Indonesia. Past studies of mT hailstorms have been limited to individual case studies. A more comprehensive study is needed to improve understanding of hailstorms in this region, and may also provide new insights into the requirements for hailstorms worldwide. This study uses simulations of five recent hailstorms created with the Weather Research and Forecasting Model to evaluate the pre‐storm environments, and five additional simulations help differentiate these environments from those associated with storms that lacked hailfall. Compared to other regions experiencing hailstorms, the mT environments exhibit moderate CAPE, much less deep‐layer shear, and much higher low‐level specific humidity. These environmental conditions typically result in ordinary single‐cell, pulse‐type storms. The introduction of a new variable, NetCAPE (the net value of CAPE after accounting for precipitation loading and any CIN at lower levels), demonstrates potential in distinguishing hail from no‐hail events in the mT. The median NetCAPE for the five hail events is over 40% greater within various depths of the hail growth zone, and when combined with lower near‐surface relative humidity and melting level heights, differentiates most hail from no‐hail cases.

 

GROUP OVERVIEW

Our group uses numerical modeling with observational analysis to investigate research problems associated with the development of clouds and precipitation. Our successes in the last decade, often with other collaborators, include demonstrating when giant aerosol particles are (or are not) important in warm rain formation, how the productivity of the warm rain process may change in a future warmer climate, the importance of variability resulting from entrainment and mixing upon accelerating or preventing warm rain formation, the influence of a strong warm rain process upon ice production in oceanic cumuli, the behavior of clouds as shedding thermals that thus entrain air through their leading edges, and entrainment into supercell thunderstorms. We have published multiple articles in peer-reviewed journals and regularly present our work at the AMS Cloud Physics Conference and the International Conference on Clouds and Precipitation.

We have also contributed to the development of tools for visualization of ground-based and airborne radar data and high-resolution numerical simulations of clouds, evaluated the performance of aircraft-mounted cloud microphysical probes, and tested microphysical parameterizations in larger-scale cloud models. Finally, we have contributed to science education through studies on improving undergraduate understanding of the nature of science, and the development and evaluation of research-based laboratories for undergraduates in atmospheric science.

 

WORK IN PROGRESS

We continue to shift our emphasis toward microphysical processes in deep convection.  Current projects include using very high-resolution simulations to investigate aerosol effects (both CCN and ice-nucleating particles) upon convective outflows and cold pools, studying the possible effects of climate change on hail storms and wind storms, and studying hail storms in Indonesia and Switzerland.