Numerical Experiments on the Relation Between Microphysics and Dynamics in Cumulus Convection

Abstract

An existing numerical model of cumulus growth, treating condensation but not precipitation, is modified by the incorporation of a parameterized treatment of liquid phase microphysics. This modification improves the realism of the results in several important respects; among them are maximum height of cloud growth, maximum liquid content, amount and distribution of temperature departure, cloud shape, and occurrence and strength of subcloud downdraft. We found that one of the most important controlling features is the rate of evaporation of droplets. In particular, the introduction of a class of large particles with relatively slow evaporation rate produces a smaller temperature deficit at the cloud summit, hence more vigorous cloud growth. In this model, the upper and lower parts of the cloud are, to a large extent, decoupled dynamically, the development of a strong subcloud downdraft by evaporation of precipitation having little effect on the ultimate extent of cloud growth.