Gross Moist Stability Assessment during TOGA COARE: Amplification and Decay of Convection

Daily averaged TOGA COARE data are analyzed to test our hypothesis that convection is amplified (attenuated) via sub-critical (super-critical) gross moist stability which corresponds to a bottom-heavy (top-heavy) vertical velocity profile. Gross moist stability (GMS) is a quantity which represents efficiency of moist static energy (MSE) export by convection and associated large-scale circulations. It is hypothesized that a bottom-heavy vertical velocity profile with negative GMS imports MSE efficiently and enhances the convection, and in contrast that a top-heavy profile with positive GMS exports MSE from the atmospheric column, attenuating the convection. In order to test this hypothesis, precipitation changes and probabilities of precipitation increase are plotted as a function of the GMS. This analysis verifies that negative (positive) GMS corresponds to an increase (a decrease) in precipitation, which is consistent with the hypothesis. Furthermore, we find that values of the GMS are related to convective life-cycles, in which convection starts with a structure having high efficiency of MSE import (negative and large GMS), the MSE-import efficiency reduces as the convection develops (near neutral GMS), and eventually evolves into a structure which exports MSE from the convective system (positive GMS). In order to determine which factor regulates the efficiency of the MSE import/export, the effects of MSE advection, radiative heating and surface fluxes are examined. This investigation suggests that the efficiency of the MSE import/export is primarily regulated by variations of ii vertical velocity profiles, indicating that the convective amplification/attenuation is tightly connected with the variations of the shape of the vertical velocity profiles. A small modification of the definition of the GMS is introduced in order to investigate relationships between vertical atmospheric structures, the modified GMS, and intensity of the convection. Through the analysis, we find that a temperature inversion in the middle troposphere plays a crucial role in convective amplification. In the early stages of convection, the inversion layer in the middle troposphere behaves as a lid, preventing the convection from penetrating that layer. As a result, convection with a bottom-heavy shape cannot become a top-heavy shape, maintaining high efficiency of MSE import via low level convergence, which makes the convective system more favorable for further convection. Consequently, the convective system which stores enough MSE can evolve into intense convection.