Understanding the Role of Riming in Deep Convection Through Variability in Collection Efficiencies and Aerosol Effects

In many mixed-phase cloud systems, the riming of cloud droplets is a key microphysical process in the growth of precipitation-sized hydrometeors. The relative importance of riming compared to other hydrometeor growth mechanisms depends on the type of cloud system and the mass, size, and number concentration of cloud droplets and ice species. In a perturbed climate state, the nature of deep convection may be altered through changes in the environmental conditions and aerosol concentrations. Such climate-induced variability can modify the dominant microphysical processes that generate precipitation through changes in hydrometeor size spectra. Such changes could either increase or decrease precipitation production and efficiency as well as impact associated morphology of clouds and convection. If the efficiency of the riming process is modified through these climate changes, it can alter the accumulated precipitation, precipitation intensity, spatial and temporal distribution of cloud and ice water, and the radiation budget through the modification of ice spectra and areal coverage of upper-level anvil clouds. The impacts of variability in aerosol concentration and collision efficiencies on the riming process in deep convection is explored through the use of high-resolution cloud resolving model simulations of a squall line that occurred May 20, 2011 over the U.S. central plains during the MC3E field project. Results thus far have demonstrated that changes in the riming rates strongly impact the intensity and partitioning of squall line precipitation between convective and stratiform, the lofting of condensate to upper levels, the vertical distribution of latent heating, and the area and optical thickness of stratiform anvil clouds. Analyses from direct tests of microphysical processes have revealed that a reduction in riming rates leads to less precipitation, greater amounts of lofted cloud water, and greater ice mass in cirrus anvils. Further, an increase in aerosol concentration to moderately high values tends to increase riming rates, which reduces lofted cloud water and leads to less anvil ice mass but greater anvil areal extent due to modified ice spectra.