Characteristics of heat and moisture budgets of a mesoscale convective system observed during TOGA‐COARE

Abstract

AbstractThe large‐scale heat (Q1) and moisture (Q 2) budgets of a tropical convective system occurring in the warm sea surface region of the western Pacific Ocean are analysed over a 36 h period. The system was observed on 26 and 27 November 1992, during the Tropical Ocean/Global Atmosphere (TOGA) Coupled Ocean‐Atmosphere Response Experiment (COARE), and 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 clouds 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 budgets and the associated precipitation rates over the two contiguous regions containing different convective activity.Globally, the vertical distributions of the heat source and moisture sink over each domain are found to be in qualitative agreement with those of other regions of the western Pacific. The heating profiles show vertical variations closely consistent with the shape of the observed mean vertical‐velocity profiles, suggesting the importance of latent heating. The mature stage provides a heating peak at the upper level in the 450–550 hPa layer, and a marked double peak drying structure at low (800–850 hPa) and middle (400–600 hPa) levels. The series of Q1 and Q2 profiles obtained every 6 hours over the two domains indicates the progressive increase of stratiform cloud which is associated with an elevation of the heating‐peak level, while the upper drying peak intensifies and the lower drying peak is decreasing. Advection contributes to these changes. Differences are observed between the profiles over both regions; these can be explained by reduced convective activity over the IFA and also by the presence of advected anvil clouds from the OSA, which lead to heating and drying over the IFA half as large as over the OSA. Similarly, the associated maxima are located at lower levels. An interesting feature is the existence of a secondary heating maximum in the high troposphere near 200–300 hPa, which can be explained as a contribution from the advected high‐level stratiform clouds.The budget‐derived rainfall rates compare well with those deduced from surface and satellite data, but to a lesser degree with model‐forecast results. This comparison highlights the importance of data density in the behaviour of the large‐scale model in representing convection in equatorial regions, as already identified for the present TOGA‐COARE. Although vertical advection of moisture globally balances the quantity of water that condenses and precipitates, the contribution of horizontal moisture advection can be an important component of the moisture budget, to counterbalance the storage of moisture in a column.