Implicit Versus Explicit Convective Heating in Numerical Weather Prediction Models

Abstract The ability of several explicit formulations of convective heating to predict the precipitation associated with a mesoscale convective complex was compared to that of a cumulus parameterization on a ½ deg latitude-longitude mesh. In the explicit approaches, prediction equations were present for both water vapor and cloud water, or vapor alone. The simplest explicit approach, for which any condensed water was assumed to fall immediately as rain, produced localized excessive rainfall. This explicit heating instability arose as a result of the requirements of saturation prior to rainfall, which delayed condensation and allowed excessive convective instability to build, and neglect of fluxes, which prevented the instability from being released in a realistic manner. These results, combined with those of previous investigators, indicate that the simplest form of explicit heating is prone to instability and unsuitable for mesoscale models. Instability problems were significantly reduced by the inclusio...