Numerical simulation of tropical cumulus congestus during TOGA COARE

Abstract

Recent observational studies of tropical deep convection typically include some mention of cumulus congestus, a third mode of tropical convection, in addition to shallow trade cumulus and deep convection. This study analyzes congestus behavior in a multiday cloud-resolving model simulation based on the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) field campaign. Simulation results exhibit a pronounced congestus cloud mode, present during both suppressed and active phases of the intraseasonal oscillation (ISO), with a unique signature consistent with cloudy-air detrainment near the 0°C isotherm. Congestus clouds in the simulation contribute 34% of the total precipitation during a 10 day transition period from suppressed to active phases, a number which corresponds well with previous estimates of the congestus contribution to precipitation. Domain-mean profiles and statistics from conditionally sampled buoyant cloud cores are compared with similar quantities from a recent model intercomparison of RICO trade cumulus. In many respects, cumulus congestus act like overgrown trade cumulus clouds. Both cloud types demonstrate multiple cloud fraction maxima associated with cloud base and detrainment layers. Profiles of buoyancy flux and vertical velocity variance suggest that the buoyancy production of turbulence behaves similarly in both cloud types. The greater precipitation production in the simulated congestus clouds nearly balances the surface latent heat flux, and thus the congestus contribution to moistening the atmosphere is limited. The computational configuration is a compromise between providing both sufficient resolution to represent shallow cumulus and sufficient domain size to handle broader, deep convective clouds.