Parameterization of the Bulk Effects of Lateral and Cloud-Top Entrainment in Transient Shallow Cumulus Clouds

Abstract

A parameterization of shallow cumulus clouds for use in atmospheric general circulation models is proposed. The parameterization uses a bulk representation of an ensemble of transient clouds. Entrainment of environmental air occurs at the ascending top of the cumulus cloud and also at the lateral boundaries of the region below the top of the cloud. Complete detrainment of the air in the cloud occurs when the top of the cloud reaches its maximum height, chosen to be the level of neutral buoyancy. The parameterization is calibrated using results from the undisturbed period of the Barbados Oceanographic and Meteorological Experiment (BOMEX). Vertical profiles of in-cloud properties and mass fluxes obtained from large eddy simulations (LES) for the undisturbed BOMEX period are successfully reproduced by the parameterization. Good agreement is also found in comparisons with large-scale heat and moisture budgets diagnosed from observations during the same period of the experiment. However, this is achieved with a different choice for the parameters of the scheme.