Laboratory Studies of the Evolution of Precipitation

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

Laboratory studies of the collisions of drops in free fall are conducted to provide the information needed to model the formation and evolution of precipitation in clouds. The experiments enable the measurement of collision outcomes over a wide range of drop size, atmospheric pressure, and temperature. The drop sizes employed are relevant to three processes: (1) accretion of cloud droplets by larger drops, a process that transfers cloud water to rain; (2) self collection between small raindrops, a mechanism thought to be responsible for the rapid formation of large raindrops; (3) breakup of raindrops caused by collisions, a process that increases the precipitation efficiency by multiplying the number of drops that can grow by accretion. Results of the experiments are described by formulas and tables of collision efficiencies, temporary coalescence probabilities, and the sizes and numbers of satellite drops produced by breakup, over a wide range of drop size, temperature, and pressure. The significance of the new data for precipitation initiation and evolution is evaluated using a microphysical cloud model.