Radar/model comparison of precipitation initiation in tropical cumulus clouds

The onset of precipitation is commonly explained by the hypotheses that the existence of very large wettable or soluble nuclei can almost immediately begin to collect cloud droplets when carried through cloud base. Model calculations are carried out to test the hypotheses. Concentrations of cloud droplets produced in the model are compared with those actually measured during the Small Cumulus Microphysics Study (SCMS) and the computed model radar reflectivities are compared with radar observations prior to the onset of precipitation based on Rayleigh scattering. Initial results suggest that ultragiant CCN are a viable explanation for the onset of precipitation in SCMS clouds.