LES study of interactions between drizzle and stratocumulus cloud dynamics

Large ensembles of air parcel trajectories driven by the LES-generated velocity fields were analyzed focusing on effects of drizzle on flow circulation in stratocumulus topped boundary layer. It is demonstrated that air cycling in cloud depends on boundary layer stability which is in turn affected by cooling of evaporating drizzle. The results suggest a possible positive feedback mechanism between drizzle and decoupling, namely parcels with long time trajectories will 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.

Results also show significant inhomogeneity of in-cloud timescales, which leads to inhomogeneity in cloud microphysical parameters. The potential effects of in-cloud residence time spatial inhomogeneity on cloud microstructure are obvious and significant. Older parcels will contain larger droplets and previously processed CCN. Non-adiabatic mixing between old and new parcels provides new embryos for coagulation and accelerates drizzle formation. It is hypothesized that mixing of parcels with different histories, i.e., with drop size distributions at different stages of their evolution, may contribute to the drop spectrum broadening and enhance drizzle production.