Shear-Parallel Moist Convection over the Tropical Ocean: A Case Study from 18 February 1993 TOGA COARE

Abstract

This paper describes a newly observed form of shear-parallel rainbands that were observed on 18 February 1993 in Tropical Ocean Global Atmosphere Coupled Ocean?Atmosphere Response Experiment using the NCAR ELDORA airborne Doppler radar. Interesting features of these rainbands included their rapidly changing and highly cellular nature, their complicated and layered kinematic structure, and the effects of the rainbands on the tropical environment. In their lower levels, the rainbands consisted of east?west elongated clusters of small convective cells, aligned with the z = 0?5 km shear vector. Strong low-level convergence, inflow from the north, and enhanced westerly winds extended from the surface up to 5 km. Much of this air was vented out of the storm at midlevels, exiting to the north in the 0°C stable layer at z = 5?7 km. This midlevel outflow fed an area of widespread, stratiform precipitation lying to the north of the rainband. The cold pool outflow emanating from this area of precipitation then fed southward, back into the north side of the rainband. This recycling of air spelled the eventual demise of this stationary rainband. Within the layer below 0°C, the westerly winds were strongly enhanced above ambient levels; this enhancement appears related to the combined effects of the low-level convergence, the release of latent heat in the rainbands, and the capping effects of the 0°C stable layer. Over a 2-h period, the rainband was observed to remain fixed in location, expanding in north?south extent, and then to decay. Subsequently, a second rainband formed about 50 km to the south of rainband 1 and grew to exhibit features similar to rainband 1. The initial development of rainband 3 was later observed, about 50 km farther south of rainband 2. Radar and aircraft in situ data are used to investigate the structure and discrete propagation of these rainbands and to relate the speed of propagation to cold pool flow. The radar data also measure the increase in the westerlies and relate the observed increases in westerlies to the passage of the rainbands. These observations are in confirmatory agreement with the hypothesis, by others, that the origin of tropical westerly wind bursts is related to mesoscale convective processes in the tropical atmosphere.