Séminaire

A variety of cutting-edge instruments aboard low earth orbiting (LEO) satellites offer a powerful tool to seamlessly observe the whole planet, but are limited in temporal sampling in that the infrequency of satellite overpasses (i.e., twice daily at best) hinders a continuous monitoring of short-term weather variability. In this talk, it is first demonstrated that the sub-daily to daily variability in air temperature and moisture before and after convection develops can be reconstructed as a composite temporal sequence of Aqua AIRS soundings sorted with respect to the timing of precipitation detection by TRMM PR. This composite time series is of great utility for studying tropical convection and its large-scale environment. For instance, the evolution of moisture convergence and moist static energy (MSE) convergence in the course of convective development is derived so the energy budget equations are satisfied under observational constraints. Even large-scale vertical motion may be estimated from the energy budget constraints through vertical-mode decomposition into first and second baroclinic modes and a background shallow mode.

The findings from the present analysis are indicative of a possible mechanism of organized convective systems. A gradual destabilization is brought about by the MSE convergence intrinsic to the positive second baroclinic mode (congestus mode) that increasingly counteracts a weak MSE divergence in the background state. Gross moist stability (GMS) is driven to nearly zero as the first baroclinic mode begins to intensify, accelerating the growth of vigorous large-scale updrafts and deep convection. As the convective burst peaks, the positive second mode switches to the negative mode (stratiform mode) and introduces an abrupt rise in MSE divergence that likely discourages further maintenance of deep convection. The first mode quickly dissipates and GMS increases away from zero, eventually returning to the background shallow-mode state. A notable caveat to this scenario is that GMS serves as a more reliable metric when defined with a radiative heating rate included to offset MSE convergence.

This approach is also useful in search of the key elements separating the dynamic and quiescent phases characterizing the tropical atmosphere, with the composite analysis being performed individually for isolated cumuli as well as organized convective systems. It is found that precipitation is fed primarily by free-tropospheric (FT) moisture convergence as organized systems develop, while FT moisture remains weakly diverging over time when only isolated cumuli prevail. It is suggested from an additional thermodynamic consideration that organized systems develop under a self-sustaining growth of large-scale updraft in the dynamic phase as delineated above, while the quiescent phase is maintained by stable, shallow circulation accompanying isolated cumuli.

MP Lefebvre 0144272799