MODIFICATION OF SURFACE FLUXES BY ATMOSPHERIC CONVECTION IN THE TOGA COARE REGION

The mesoscale variability of surface heat fluxes induced by atmospheri c convection is studied by using 3D cloud explicit simulations and sur face observations. Two convective cases observed during the Coupled Oc ean-Atmosphere Response Experiment are simulated (26 November 1992 and 17 February 1993) corresponding to different ambient surface wind con ditions, namely, light and moderate winds. Numerical results in the fi rst case are successfully compared to surface observations. Local enha ncements of two times for the latent heat flux and three times more fo r the sensible one are produced in the rainy area. Intense wind gusts generated by convective outflow are found mainly responsible for these increases. For the second case, the simulated surface fluxes are foun d to vary greatly, although they are structured in response to an orga nized convective system. At the domain scale (90 km x 90 km) correspon ding to a general circulation model (GCM) grid box, it is shown that c onvective activity significantly enhances the averaged surface heat fl uxes. This effect is important since the preconvective wind is weak. T o compute these surface fluxes with a bulk formula using fields define d on the domain scale, special attention must be given to the determin ation of the mean wind speed. In GCMs, gusts generated by downdrafts a re subgrid scale and are hence unresolved. This study suggests that fl ux enhancement due to clouds may be parameterized in GCMs by extending to deep convection the gustiness correction previously proposed for f ree convection by other authors. Analysis of both model simulations an d observed time series suggest that once convection increases above a rather small threshold value, gustiness saturates at about 3 m s(-1), whereas surface air humidity varies only sightly. These are the major elements of a proposed new parameterization of evaporation from the tr opical ocean.