ESTIMATING THE GROSS MOIST STABILITY OF THE TROPICAL ATMOSPHERE

Recent theoretical studies have indicated that large-scale circulation in deep convective regions evolves subject to an overall static stabi lity-termed the gross moist stability-that takes into account both dry static stability and moist convective effects. The cross moist stabil ity has been explicitly defined for a continuously stratified atmosphe re under convective quasi-equilibrium constraints. A subsidiary quanti ty-the gross moisture stratification-measures the overall effectivenes s in producing precipitation subject to these quasi-equilibrium constr aints. These definitions are relevant in regions that experience deep convection sufficiently often: criteria based on climatological precip itation and maximum level of convection are used io define a domain of applicability. In this paper, 10-yr monthly mean rawinsonde data, and European Centre for Medium-Range Weather Forecasts (ECMWF) and Nation al Meteorological Center (NMC) analyses are used to estimate the magni tude and horizontal distribution of these two quantities in the Tropic s within the domain of applicability. The gross moist stability is fou nd to be positive bur much smaller than typical dry static stability v alues. Its magnitude varies modestly from 200 to 800 J kg(-1) and exhi bits relatively little dependence on sea surface temperature (SST). Th ese values correspond, for instance, to a phase speed change from 8 to 16 in s(-1) for the Madden-Julian oscillation, The gross moisture str atification is larger and exhibits strong dependence on SST, varying f rom 1500 to 3500 J kg(-1) between cold and warm SST regions. A high de gree of cancellation between effects of increasing low-level moisture and maximum level of convection, respectively. tends to keep the press moist stability values relatively constant. Differences among the ECM WF and NMC analysis products and the rawinsonde data affect the estima te, but there is qualitative agreement. It is encouraging that reasona bly robust estimates of a small, positive gross moist stability (as th e difference between larger dry static stability and gross moisture st ratification quantities) can be obtained. This helps justify use of sm all, constant moist phase speeds in some simple models of tropical. ci rculation, although it also points out inconsistencies in how such mod els neglect variations in the height of convection.