A Parameterization of the Evaporation of Rainfall

Abstract

In this study we develop theoretical expressions for the rainfall rate, P(z), and the total evaporation rate from cloud base to a level z below cloud base, E(z). The resultant parameterization for the total evaporation is given by E(z) = CPα(0)?(z), where P(0) is the rainfall rate at cloud base, α and C are undetermined parameters, and ?(z) is a function of z. The latter is determined for the cases of constant relative humidity and constant absolute humidity below cloud base which correspond approximately to stratiform and cumuloform clouds, respectively. The parameters α and C are determined from radar observations of the rain failing from continental convective cells in central South Africa. The resultant values are α = 0.606 and C = 2.63 ? 10?2 for P(0) and E(z) in millimeters per hour. Subsequent analyses of other radar observations utilizing the method developed in this study are needed to obtain corresponding evaporation parameterizations for cumuloform precipitation in other climatic regimes and for stratiform precipitation.