Moist Convective Instability, Rainfall Rate, and Gross Moist Stability

Our studies of deep convection over warm tropical oceans show, unsurprisingly, that the convective mean rainfall rate is higher when environmental moisture is greater. However, these results also demonstrate, counter-intuitively, that weaker moist convective instability promotes higher rainfall rates. In this analysis, environmental moisture is defined by the saturation fraction or column relative humidity, i.e., the precipitable water divided by the saturated precipitable water. Moist convective instability is characterized as minus the vertical gradient of saturated moist entropy (or saturated equivalent potential temperature) at low to middle levels, or more specifically, by an instability index, defined as the mean saturated moist entropy in the 1-3 km layer minus that in the 5-7 km layer. This index is related to, but somewhat different from the convective available potential energy. These results were first predicted in cloud-resolving model calculations using weak temperature gradient lateral boundary conditions. They were later confirmed by observations made in tropical cyclone precursor systems in two projects, TPARC/TCS-08 and PREDICT. Environments with low instability index typically exhibit strong cyclonic vorticity in the middle troposphere due to the requirements of thermal wind balance. Low instability index convection also exhibits low gross moist stability, making it a potent actor in many types of tropical disturbances. Incorporation of these results into models of tropical disturbances should therefore result in better understanding and predictability for such systems.