Simulation of precipitating shallow convective cloud systems using LES with explicit microphysics

Shallow cumulus convection is one of the most prevalent cloud types on the planet and typically is dominated by warm rain processes. One of the objectives of the Rain in Cumulus over the Ocean (RICO) field project was to characterize and explain the parameters of cumulus precipitation on ambient conditions in general, and the rapid onset of precipitation in shallow tropical clouds, in particular.

Investigating these clouds necessitates simultaneously a fine grid spacing and a large domain; these are particularly computationally intensive constraints when employing size-resolved microphysics. To this end, we have expended significant effort to rewrite the CIMMS LES model to run on distributed parallel computing architectures based on the SAM dynamical core developed at CSU by M. Khairoutdinov.

Results for a simulation of precipitating trade cumulus from the RICO field project indicate significant sensitivity of the cloud thermodynamic and microphysical parameters to ambient aerosol load. Lower values of CCN are associated with enhanced precipitation and deeper, more energetic cloud updraft cores. The more polluted cases tend to produce enhanced cloud coverage in the detrainment region. The rain intensity, rain fractional cover and spatial distribution are no less sensitive to ambient thermodynamic conditions.