Raindrop size distribution and evolution

Collision-induced breakup of raindrops is assumed to be the main factor controlling the temporal evolution of raindrop size distributions (RSDs). Owing to this mechanism, fragment drops are produced, the size distributions of which have been determined from laboratory measurements of pairs of colliding drops conducted 30 years ago. From these measurements, different representations and parameterizations have been derived. When the earliest parameterizations, based on the results of laboratory collisions of raindrop pairs falling at their terminal velocities, were implemented in numerical models, stationary distributions with three peaks in number concentrations were realized. Subsequent studies, using a more physical basis for the parameterized relations or using relations based on computational fluid dynamics models, predicted distributions with two peaks from the opposing effects of breakup and coalescence or coagulation. Despite observations of peaks in RSDs in a variety of locations, there has been little systematic evidence for the occurrence of peaks at the specific diameters predicted by the modeling studies. Because factors other than collision-induced breakup, such as evaporation, size sorting, spontaneous breakup, updrafts, and mixing of rain shafts, also influence RSDs, peaks at specific diameters would not be expected to occur consistently. However, given sufficiently long averaging of observations acquired in heavy rain, some evidence of these peaks might be expected. Thus, there is the need for more observations under conditions of heavy rainfall and for studies to process heavy rain rate data acquired at a variety of locations in a consistent manner.