Gravity Waves, Compensating Subsidence and Detrainment around Cumulus Clouds

Gravity waves play an important role in the redistribution of heat and moisture in a deep convecting cloud field. We explore this role in a two-dimensional numerical experiment on a simple moist convecting system consisting of an isolated long-lasting nonprecipitating cloud in a calm atmosphere with no surface forcing. The cloud develops a horizontally averaged density variation with height which is neutrally buoyant with respect to a moist adiabatic. The buoyancy difference between the cloud and the undisturbed sounding produces circulations that can be understood as spreading gravity waves which adjust the environmental buoyancy to be equal to the cloud buoyancy by compensating subsidence. Unlike the circulations inside clouds, this adjustment takes place without turbulent mixing. Hence, the “buoyancy adjustment time” T1 during which the environment comes into rough buoyant equilibrium with the clouds is much shorter than the “mixing time” T2 which it takes a tracer, initially concentrated at some level, to become homogenized through the layer in a field of clouds. Associated with this gravity wave response are circulations which cause the preferential detrainment of “cloud-processed” air at heights at which the buoyancy of clouds relative to their far environment is decreasing with height. We confirmed these ideas in less idealized numerical experiments in which different soundings, precipitation, and surface heat fluxes were included.