LES microphysical study of interactions between cloud dynamics and drizzle

Although basic processes affecting warm cloud microstructure and rain initiation are well known, the interactions between microphysical processes, cloud dynamics, and turbulence, as well as their effect on drizzle, are still poorly understood. These interactions were studied using the CIMMS LES stratocumulus cloud model with explicit microphysics and the Trajectory Ensemble Model driven by the LES derived velocity fields. For the stratocumulus cloud case the results suggest a possible positive feedback mechanism between drizzle and decoupling, namely parcels with long time trajectories may favor enhanced drizzle growth, which, in turn, will lead to stronger evaporation below cloud base followed by a stronger increase in stability of the subcloud layer and stronger decoupling; all resulting in more air parcel cycling in cloud and more drizzle which may eventually lead to stratocumulus cloud breakup. The effect of air parcel cycling may be no less important in cumulus clouds where coagulation processing enhanced by air cycling may significantly transform the CCN nucleation properties. The latter effect is evaluated using exact solutions to the coagulation equation.