Bulk Thermodynamic Effects and Properties of Small Tropical Cumuli

Abstract

Separate diagnostic models of shallow and inversion-penetrating, trade wind cumulus clouds are combined with large-scale BOMEX heat and moisture budgets to obtain the thermodynamic effects and properties of the shallow clouds. The deeper trade wind cumulus effects are obtained using a modified version of Nitta's (1975) spectral diagnostic method, while a bulk model based on the bubble theory of convection is used for the shallow clouds. Results show that the buoyancy of the small cumuli is an order of magnitude less than the buoyancy of the deeper clouds, and suggest that their effects on the temperature stratification are small compared with radiative cooling and subsidence warning due to deep clouds. Also, the results show that the primary role of the small cumuli is to support the growth of deeper clouds through the transport of water vapour from the subcloud layer into the lower cloud layer. The deduced shallow cloud properties are found to be consistent with observations and Sommeria's (1976) numerical simulation of populations of small tropical cumuli. The implications of these results for cumulus parameterization schemes am discussed.