A Mesovortex within a Near-Equatorial Mesoscale Convective System during TOGA COARE

Abstract

Airborne Doppler radar observations are used to investigate the internal structure of a midlevel mesovortex that developed within the rear part of the stratiform precipitation region of a mature-to-decaying mesoscale convective system. This system, composed of several convective elements, occurred on 13 December 1992, on the eastern side of the intensive flux array of the Tropical Ocean Global Atmosphere Coupled Ocean?Atmosphere Response Experiment, near the equator (2°S). The mesovortex contained several features that were previously observed in other tropical and midlatitude systems. Radar observations show that the closed wind circulation at midlevels had a horizontal dimension of ?150 km and was coincident with a marked rear inflow at the central rear of the stratiform region where a notch pattern clearly identified the rear dry air intrusion. It was associated with positive vertical vorticity (anticyclonic for the Southern Hemisphere) and could be classified as a cooling-induced vortex. Maximum positive vorticity was concentrated along the converging interface between this flow and the in-cloud rearward flow. The mesoscale downdraft air was the primary source of the rearward outflow observed at low levels. The vorticity budget performed within the mesovortex reveals the tilting of the horizontal component of vorticity to be the prominent dynamical process that influenced the vortex development, although a part of the vortex amplification at mid-to-high levels was due to stretching. At lower levels, tilting tended to inhibit the mesovortex, by converting horizontal vorticity to negative vertical vorticity. A close examination of the system-induced vertical wind shear through mesoscale momentum transports reveals an evident correlation with the tilting mechanism. Overall, the vorticity changes by tilting and stretching were negatively correlated with the vertical and horizontal advection, respectively. The vertical advection redistribution was accomplished through mean vertical motion, which is found to transport positive vorticity from the higher part of the mesovortex down to the low levels.