Circulations Associated with a Mature-to-Decaying Midlatitude Mesoscale Convective System. Part II: Upper-Level Features

Abstract

The vertical structure of a midtropospheric mesovortex that developed during the decay of a midlatitude mesoscale convective system over Kansas and Oklahoma on 23?24 June 1985 is documented. Surface, rawinsonde, wind profiler, and dual-Doppler data are used to define its structure. As has been observed in other midlatitude and tropical cases, the mesovortex occurred within the trailing stratiform precipitation region of a squall-line system. It was associated with a preexisting synoptic-scale short-wave trough, and as the circulation developed, it deformed the back edge of the stratiform precipitation region into a hooklike pattern. The ?100?200-km mesovortex was confined to the midtroposphere (3?8 km), with a maximum amplitude just above the 0°C level. The vortex axis sloped toward the northeast, but its orientation changed hour by hour over the 2-h period of dual-Doppler coverage. Overall, the mesovortex was warm core, although its thermal structure was complex and apparently significantly influenced at the analysis time by a descending rear-inflow jet entering the rear portion of the stratiform region. The warmest anomaly was found at low levels (near 850 mb) with a shallow cool anomaly in the midtroposphere near the 0°C level and a weak warm anomaly aloft. At 500 mb the warmest air was shifted to the north of the vortex center, where the atmosphere was also relatively dry, while at 400 mb (near the top of the mesovortex) the warmest air coincided with the vortex center. Although the mesovortex was sampled for only a brief portion of its lifetime, the observations suggest a close coupling between synoptic, mesoscale, and even microscale (cloud) processes in its formation. A vorticity budget based on the sounding data during the decaying stage of the storm indicates that convergence production of vorticity associated with a mesoscale updraft-downdraft couplet in the stratiform precipitation region was a critical factor in intensifying the circulation at that time. Vorticity production by tilting played a minor role during this period due to relatively weak environmental wind shear. The weak shear, however, likely contributed to the longevity of the mesovortex.