CHARACTERISTICS OF HEAT AND MOISTURE BUDGETS OF A MESOSCALE CONVECTIVE SYSTEM OBSERVED DURING TOGA-COARE

The large-scale heat (Q(1)) and moisture (Q(2)) budgets of a tropical convective system occurring in the warm sea surface region of the west ern Pacific Ocean are analysed over a 36 h period. The system was obse rved on 26 and 27 November 1992, during the Tropical Ocean/Global Atmo sphere (TOGA) Coupled Ocean-Atmosphere Response Experiment (COARE), an d propagated from the north-eastern part of the Outer Sounding Array ( OSA) toward the Intensive Flux Array (IFA) following a south-westward direction. It was composed of an extensive shield of high cirrus cloud s over the OSA, which advected into the IFA. Rawinsonde data collected during these two days, at 6 h intervals, and also surface, satellite and operational forecast-model data are used to investigate the budget s and the associated precipitation rates over the two contiguous regio ns containing different convective activity. Globally, the vertical di stributions of the heat source and moisture sink over each domain are found to be in qualitative agreement with those of other regions of th e western Pacific. The heating profiles show vertical variations close ly consistent with the shape of the observed mean vertical-velocity pr ofiles, suggesting the importance of latent heating. The mature stage provides a heating peak at the upper level in the 450-550 hPa layer, a nd a marked double peak drying structure at low (800-850 hPa) and midd le (400-600 hPa) levels. The series of Q(1) and Q(2) profiles obtained every 6 hours over the two domains indicates the progressive increase of stratiform cloud which is associated with an elevation of the heat ing-peak level, while the upper drying peak intensifies and the lower drying peak is decreasing. Advection contributes to these changes. Dif ferences are observed between the profiles over both regions; these ca n be explained by reduced convective activity over the IFA and also by the presence of advected anvil clouds from the OSA, which lead to hea ting and drying over the IFA half as large as over the OSA. Similarly, the associated maxima are located at lower levels. An interesting fea ture is the existence of a secondary heating maximum in the high tropo sphere near 200-300 hPa, which can be explained as a contribution from the advected high-level stratiform clouds. The budget-derived rainfal l rates compare well with those deduced from surface and satellite dat a, but to a lesser degree with model-forecast results. This comparison highlights the importance of data density in the behaviour of the lar ge-scale model in representing convection in equatorial regions, as al ready identified for the present TOGA-COARE. Although vertical advecti on of moisture globally balances the quantity of water that condenses and precipitates, the contribution of horizontal moisture advection ca n be an important component of the moisture budget, to counterbalance the storage of moisture in a column.