Ochs/Abstract Laboratory studies are being conducted on collisions between cloud-size and precipitation-size drops in free fall to obtain the collection efficiences critical to the formation and evolution of precipitation in warm-base clouds. Unique laboratory equipment are being employed for generating drops at terminal velocity and studying subsequent collisions. The research is focused on two problems: obtaining critical collision efficiencies needed to calculate the rates for 1) autoconversion and 2) self collection, processes that control the initiation of rain and spread of the raindrop distribution. For the autoconversion problem, an experiment is being used to measure the collection efficiency for cloud-size drops of minimal charge for comparison with previous laboratory data at moderate charge and with theoretical collision efficiencies for uncharged spheres. The influence of turbulence on the collision efficiency is also being studied. For the self- collection problem, several experiments are being employed to measure the collection efficiency for small precipitation-size drops as a function of temperature and ppessure. Other experiments are focused on extending the physical scaling parameter to predict collection efficiencies for a wide range of drop sizes. Numerical cloud models employed to evaluate the influence of the laboratory results on the formation and evolution of precipitation.
