A Mechanism of Tropical Convection Inferred from Observed Variability in the Moist Static Energy Budget

Temporal variability in the moist static energy (MSE) budget is studied with measurements from a coordination of different satellites including the Tropical Rainfall Measuring Mission (TRMM) and Afternoon Train (A-Train) platforms. A composite time series before and after the development of moist convection is obtained from the observations to delineate the evolution of MSE and moisture convergences and, in their combination, gross moist stability (GMS). A newly developed algorithm is then applied so large-scale vertical motion is estimated from energy budget constraints through a vertical mode decomposition into first and second baroclinic modes and a background-shallow mode. Findings from the present analysis are summarized as follows into a possible mechanism of tropical convection. A gradual destabilization is brought by the MSE convergence intrinsic to the positive second baroclinic mode (congestus mode) that increasingly counteracts a weak MSE divergence in the background state. GMS is driven to the verge of zero as the first baroclinic mode begins to intensify, accelerating the growth of vigorous large-scale updraft and deep convection. It is when the convective burst peaks, however, that the positive second mode switches to the negative mode (stratiform mode) and introduces an abrupt rise of MSE divergence that likely discourages a further maintenance of deep convection. The first mode quickly dissipates and GMS increases away from zero, eventually leaving behind the background-shallow mode alone. A notable caveat to this scenario is that GMS serves as a more reliable metric when defined with radiative cooling rate included to offset MSE convergence.