ABSTRACT

This project studies the fundamental dynamics through which tropical convection, meaning the overturning motion found in tropical cumulus clouds and associated with intense precipitation, interacts with the larger-scale horizontal atmospheric circulation in which it is embedded. The work focuses on the evolution of convection within African easterly waves (AEWs), tropical waves disturbances with wavelengths of 2,000 to 2,500km which originate over West Africa and propagate westward over the northern tropical Atlantic. AEWs are of particular interest as they play a key role in the formation of tropical storms and hurricanes.

The work is based on preliminary findings suggesting a chain of meteorological relationships that can lead to the development of heavy rain. One is that cyclonic vorticity (a measure of the strength of cyclonic rotation or shear in the horizontal circulation) at a mid-tropospheric level (say 600mb) can produce warmer temperatures above and colder temperatures below that level, reducing moist convective instability around the level. This reduction in mid-tropospheric instability leads to a bottom-heavy mass flux profile in the cumulus clouds, in which most of the ascending motions within the clouds occur at low to middle levels of the troposphere. The bottom-heavy mass flux profile in turn leads to a reduction in the efficiency of convection for exporting static energy from the convecting region, which requires stronger convection to maintain the energy export. Meanwhile, lower instability leads to moistening of the convecting region through a process referred to as moisture quasi-equilibrium (a result derived in part from research under AGS-1148594), so that both enhanced moisture and more vigorous overturning motions contribute to intensification of rainfall. Preliminary work suggesting these relationships was performed using field campaign data from T-PARC/TCS-08 (Thorpex Pacific Asian Regional Campaign/Tropical Cyclone Motion experiment) and PREDICT (Pre-Depression Investigation of Cloud Systems in the Tropics). Work performed here further tests and explores the proposed chain of causality from mid-level vorticity to convective precipitation, and also considers mechanisms through which more vigorous convection in turn affects the mid-level vorticity and large-scale circulation. The work is conducted through both examination of observational data, including data from field campaigns, satellites, and reanalysis products, and the construction of simple models representing the hypothesized relationships.

As noted above, processes leading to the development of convective precipitation in AEWs are of practical as well as scientific interest given that they can lead to landfalling Atlantic hurricanes. More specifically, the work focuses on the early stages of tropical storm genesis, which is among the most difficult challenges in tropical cyclone forecasting. The project also fosters international scientific collaboration, as it involves work with a collaborators in Chile and Croatia, and the work is performed at a Hispanic-serving institution. In addition, the project provides support and training for a graduate student and an undergraduate summer intern, thereby providing for the future workforce in this research area.

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