The Role of Ice Phase Microphysics in Aerosol Convection Interactions

The effect of aerosols on clouds has been studied fairly extensively for warm clouds, but less so for deep convective storms. The addition of the ice phase in these clouds and the more numerous microphysical processes that play a role in producing precipitation lead to a much more complex response of deep convective processes to the presence of enhanced aerosol concentrations. However, large amounts of precipitation can be produced in deep convective storms, making it especially important to understand the precipitation response to aerosols. Typically, studies investigating the response of clouds to increased aerosols have found that clouds forming in environments with more aerosols available to act as cloud condensation nuclei (ccn) have a less efficient warm rain process due to the narrower cloud droplet spectrum created by the nucleation of large numbers of cloud drops. However, the total precipitation produced by deep convective storms has sometimes been found to increase with aerosol concentration, counter to what is often found in warm clouds. This suggests that different mechanisms may be important, related to interactions with ice in these clouds. Possibly the amount of rain formed from melted graupel and hail is enhanced, or the storms undergo convective invigoration due to the larger latent heat released in the freezing of more supercooled droplets. The goal of this project is to investigate the effects that enhanced aerosol concentrations can have on deep convective storms. This will be accomplished using large-scale two-dimensional long-term simulations of the tropics that have been run in a radiative-convective equilibrium framework. These simulations only vary in the number of aerosols available to act as ccn, and offer a large sample of deep convective profiles to examine statistically. The model being used is the Regional Atmospheric Modeling System (RAMS), which allows for the ccn number concentrations to be prognosed based on background aerosol concentration and environmental conditions. Initial results from the RCE simulations show that convection that forms in an environment with more aerosols available to act as ccn does not show evidence of stronger updrafts, contrary to many studies that have found convective invigoration to be the case. Examining the microphysical budgeting terms available in the model will allow for increased understanding of which processes (i.e. freezing and melting of different species, riming, aggregation, etc.) contribute to this result. Findings from the large-domain RCE simulations will be compared with high resolution, three-dimensional single cloud simulations of deep convective storms in polluted environments. Results will be presented showing the relevant microphysical processes that are affected by changing aerosol concentrations, in an attempt to explain how the presence of the ice phase affects the convective cloud response to high aerosol concentrations.