A Numerical Model of Cumulus Dynamics and Microphysics

Abstract

A one-dimensional, time-dependent numerical model of cumulus convection is presented. The model considers the processes of horizontal mixing, evaporation, precipitation generation and freezing, as well as the standard thermodynamic and dynamic processes in isolated cumuli. The initial calculations show that: 1) the vertical velocity and liquid water content undergo coupled damped oscillations in time; 2) the rate at which precipitation-sized drops are formed and grow by collection are not sensitive parameters in the amount of rain which falls from the clouds; 3) the amount of liquid water in the form of cloud droplets that must be present in the cloud before a few large drops can be developed is a crucial parameter in determining the amount of rain which falls from the clouds (the higher the threshold cloud liquid water content, the lower the ultimate rainfall amount); and 4) the freezing time and precise ice-nucleation temperature affect the amount of rain that falls from the cloud in a way often very different from the way they affect the cloud-top height. Trial runs with the model agree well with observations in the two cases tried.