Parameterization of the Evaporation of Rainfall for Use in General Circulation Models

Abstract

A parameterization of evaporation losses below cloud base is presented for use in general circulation models to assist in quantification of water content in the hydrological cycle. The scheme is based on detailed model calculations of the evolution of raindrop spectra below cloud base and includes the processes of collision coalescence/breakup. Evaporation is expressed as a percentage decrease in the liquid water mixing ratio, and the parameterization is formulated as an algebraic equation in (i) the cloud-base values of the mixing ratio and the drop concentration, (ii) the fall distance, and (iii) the lapse rate of temperature in the subcloud environment. Results show that when compared to the detailed model calculations, good estimates of evaporation (usually within 20% and often within 10%) are obtained for a wide range of conditions. An analysis of the errors in evaporation calculations associated with errors in the parameterization variables is performed.