Partitioning Tropical Heat and Moisture Budgets into Cumulus and Mesoscale Components: Implications for Cumulus Parameterization

Abstract

Results from recent heat and moisture budget studies of tropical mesoscale convective systems have been used to partition the total heating in tropical disturbances into cumulus and mesoscale components The mesoscale component refer to that part of tropical cloud systems which contains mesoscale anvil circulations, viz., a mesoscale updraft in an upper-tropospheric cloud shield extending from near the 0°C level to near the 0°C level to near the tropopause and a mesoscale downdraft in the lower troposphere. The cumulus component, which is determined as a residual, consists of contributions from cumulus elements of all sizes, ranging from deep cumulonimbus to shallow cumulus; however, deep cumulus effects predominate in the tropical disturbances studied here. The method of partitioning requires an estimate of the fraction of f of the total rainfall in tropical disturbances produced by mesoscale anvil systems. The total apparent heat source Q1, and moisture sink Q2 of Yanai and others and the mesoscale anvil Q1, and Q2 profiles of Johnson and Young form the basis for the partitioning. For realistic estimates of ƒ, the total heating, which has a peak near 450 mb (6 km), is found to be a consequence of two distinctly different circulation features 1) the mesoscale anvil, which has a heating peak near 330 mb (8 km) and a cooling peak below new 700 mb (3 km) and 2) the cumulus, which produces a heating peak centered near 600 mb (4 km). The partitioning of the apparent moisture sink Q2 produces qualitatively similar results. The mesoscale anvils give a drying peak in the upper troposphere near 350 mb (8 km) and a moistening peak (through evaporation) near 800 mb (2 km). However, the effect of the cumulus in this case (which dry the lower troposphere through removal of water vapor by net condensation) are such that the cumulus drying has a peak somewhat lower in the troposphere (near 750 mb or 2.5 km). Thus, the double-peak structure in Q2 often seen in tropical budget composite studies is a consequence of the combined, but vertically-separated drying effects of two distinct convective phenomena: mesoscale anvils and deep cumulus. The results of this study have implications for cumulus parameterization schemes in general, but particularly for those that assign vertical distributions to the convective heating. It has been shown that the cumulus and mesoscale heating distributions are considerably different. Schemes that use an assigned vertical distribution of convective heating chosen to match those obtained from large-scale tropical budget studies should consider carefully the different contributions to total convective heating by the separate cumulus and mesoscale components. Possible errors may result if the proportion of cumulus versus mesoscale-produced rainfall in the region of model application is different from that in the region where the assigned distribution was derived. The results of this study suggest that cumulus parameterization schemes that permit vertical heating distributions to evolve in a realistic way during the course of model integrations are preferred, at least on a physical basis, over those that prescribe the distributions.