The Significance of Thermodynamic Forcing by Cumulus Convection in a General Circulation Model

Abstract

To assess the effects of cumulus convection on the general circulation of the atmosphere, a medium-resolution, spectral general circulation model was integrated twice for 40 simulated days from identical initial conditions, with and without a version of a cumulus parameterization scheme developed by Kuo. The cumulus parameterization scheme allows cumuli to interact with the large-scale flow by condensation and cumulus flux convergences of entropy and moisture; cumulus friction is not included. Cumulus convection warms the upper troposphere and slightly cools the lower tropical troposphere; additional cooling occurs in the lower troposphere of the winter-hemisphere baroclinic zone. Cumulus convection also dries the lower troposphere, especially in the tropics and summer hemisphere, and weakens the Hadley cells. The zonal wind field responds geostrophically to cumulus-induced temperature changes. Condensation and cumulus vertical-flux convergence are both important in determining the interaction between cumuli and the large-scale flow. Cumulus convection influences the general circulation both directly through heating and moistening and also indirectly by inducing changes in the mean meridonal circulation. Such cumulus convection does not appear to alter substantially the heat balance which maintains the time-mean, zonally-averaged temperature field, and the changes which do occur in the temperature balance are predominantly dynamic.