ANALYSIS AND PARAMETERIZATION OF THE COMBINED COALESCENCE, BREAKUP, AND EVAPORATION PROCESSES

A parameterization of raindrop coalescence and breakup has been extend ed to include evaporation. The parameterization is developed through a nalysis of accurate numerical solutions of the coalescence/breakup/eva poration equation. Modeled drop size distributions are found to evolve first toward a trimodal form characteristic of the equilibrium distri bution that occurs when only collisional processes are at work. With s ustained evaporation, the trimodality disappears and a unimodal-type d rop size distribution emerges. The results imply that the trimodal for m occurs when collisional processes are dominant but that a unimodal d istribution prevails as the water mass is reduced. The mass reduction causes collisions to become infrequent and allows evaporation to deple te the small-sized raindrop population. When subjected to continued ev aporation, the coalescence/breakup equilibrium itself undergoes a tran sition from trimodal to unimodal form, and it is this evolving form to ward which all other drop size distributions converge. In the transiti on, the liquid water content decreases exponentially with a time const ant of 300 S-1 s, where S is the saturation deficit: furthermore, the shape of the evaporating distribution is determined by the ratio of th e liquid water content to the saturation deficit. The parameterization procedure makes use of the analysis results in order to describe syst em behavior.