The Development of Drop Size Distributions in Light Rain

Abstract

A model of rain development based on the quasi-stochastic coalescence equation and including the sedimentation of drops has been used to study the formation of drop size distributions in conditions of weak updraft. Comparisons with “box model” results indicate that sedimentation effects are crucial in establishing the shapes of the distribution. Under realistic conditions of cloud droplet distribution with size, the raindrop size distributions as simulated by the model compare well with observations of orographic rain made in Hawaii. On the other hand, Doppler radar measurements of drop size distributions just below a bright band confirm that the Marshall-Palmer distribution results from processes affecting particles in the solid phase rather than from the interaction of raindrops.