THE DEVELOPMENT OF DROP SIZE DISTRIBUTIONS IN LIGHT RAIN

A model of rain development based on the quasi-stochastic coalescence equation and including the sedimentation of drops has been used to stu dy the formation of drop size distributions in conditions of weak updr aft. Comparisons with ''box model'' results indicate that sedimentatio n effects are crucial in establishing the shapes of the distribution. Under realistic conditions of cloud droplet distribution with size, th e raindrop size distributions as simulated by the model compare well w ith observations of orographic rain made in Hawaii. On the other hand, Doppler radar measurements of drop size distributions just below a br ight band confirm that the Marshall-Palmer distribution results from p rocesses affecting particles in the solid phase rather than from the i nteraction of raindrops.