LONG-TERM BEHAVIOR OF PRECIPITATING TROPICAL CLOUD SYSTEMS - A NUMERICAL STUDY

Results of a 24-day, two-dimensional integration of tropical cloud sys tems forced by large-scale ascent, surface fluxes and radiation in a t ypical sheared tropical environment are presented. A non-hydrostatic, cloud-resolving numerical model containing sophisticated microphysical parametrizations as well as turbulence, surface flux and short/long-w ave radiative representations was used. A predominant cloud-system hie rarchy was identified: fast westward-moving mesoscale convective syste ms, producing extensive cirrus anvils and a strong radiative effect; a nd slow-moving regions of enhanced precipitation, causing a significan t modification of the fast-moving cloud-system behaviour on the time-s cale of about one day. The experimental set-up was similar to that use d by Sui et al. The demonstrated episodic convective activity and the fundamental role of organized deep convection by and large agrees with their analysis. However, despite many similarities, the results for t he mean thermodynamic statistical equilibrium are dramatically differe nt: a warm and humid regime, as opposed to the cold and dry regime of Sui et al. High relative humidities and very high upper-tropospheric c irrus cloud amount led to a strong greenhouse effect with low outgoing long-wave radiation at the top of the atmosphere (120-150 W m(-2)). A t the same time, these clouds screened the ocean surface from the sola r radiation and caused very low solar radiation absorption at the surf ace (only about 30 W m(-2) when averaged over a diurnal cycle). The su rface net cloud forcing was very large, about -200 W m(-2) which is in accord with earlier findings of Ramanathan and Collins. The above num bers should be considered as an upper limit of effects of deep convect ion on radiative fluxes in the tropics because of the warm and moist r egime demonstrated in the experiment.