Estimating the Gross Moist Stability of the Tropical Atmosphere*

Abstract

Recent theoretical studies have indicated that large-scale circulation in deep convective regions evolves subject to an overall static stability—termed the gross moist stability—that takes into account both dry static stability and moist convective effects. The gross moist stability has been explicitly defined for a continuously stratified atmosphere under convective quasi-equilibrium constraints. A subsidiary quantity—the gross moisture stratification—measures the overall effectiveness in producing precipitation subject to these quasi-equilibrium constraints. These definitions are relevant in regions that experience deep convection sufficiently often; criteria based on climatological precipitation and maximum level of convection are used to define a domain of applicability. In this paper, 10-yr monthly mean rawinsonde data, and European Centre for Medium-Range Weather Forecasts (ECMWF) and National Meteorological Center (NMC) analyses are used to estimate the magnitude and horizontal distribution of these two quantities in the Tropics within the domain of applicability. The gross moist stability is found to be positive but much smaller than typical dry static stability values. Its magnitude varies modestly from 200 to 800 J kg−1 and exhibits relatively little dependence on sea surface temperature (SST). These values correspond, for instance, to a phase speed change from 8 to 16 m s−1 for the Madden–Julian oscillation. The gross moisture stratification is larger and exhibits strong dependence on SST, varying from 1500 to 3500 J kg−1 between cold and warm SST regions. A high degree of cancellation between effects of increasing low-level moisture and maximum level of convection, respectively, tends to keep the gross moist stability values relatively constant. Differences among the ECMWF and NMC analysis products and the rawinsonde data affect the estimate, but there is qualitative agreement. It is encouraging that reasonably robust estimates of a small, positive gross moist stability (as the difference between larger dry static stability and gross moisture stratification quantities) can be obtained. This helps justify use of small, constant moist phase speeds in some simple models of tropical circulation, although it also points out inconsistencies in how such models neglect variations in the height of convection.