A Model for Cloud Droplet Growth by Condensation in an Inhomogeneous Medium

Abstract

A new model for droplet growth by condensation is proposed which can produce a broad droplet size spectrum as well as local fluctuations in droplet number concentration and liquid water content. The model depends neither on mixing nor on the type of condensation nuclei present. It is based on small fluctuations in relative humidity coupled with droplet sedimentation. During the early stages of condensation, small local variations in relative humidity can produce significant variations in droplet sizes. The resulting non-uniformities in droplet sedimentation rate create inhomogeneities in droplet number concentration, which, in turn, cause variations in vapor pressure and differences in droplet growth rates. Numerical experiments show that the initial conditions required for this model to produce a broad droplet size spectrum are sufficiently general to be of common occurrence. Although the model does not include any turbulence effects, it is argued that the process modeled does not require the complete absence of turbulence.