Evolution and Structure of the 6–7 May 1985 Mesoscale Convective System and Associated Vortex

Abstract

Observations collected during the Oklahoma?Kansas PRE-STORM experiment are used to document the evolution and structure of the mesoscale convective system (MCS) that occurred on 6?7 May 1985. The storm began when a short squall line developed in an area of preexisting thunderstorm activity. Thunderstorm updrafts along the squall line lifted warm, moist air with its southerly momentum to the upper troposphere. A broad region of convective outflow and a meso?-scale updraft region with a mean vertical velocity in excess of ?15 ? 10?3 mb s?1 were created. A stratiform rain area with an embedded mesovortex formed behind the squall line. The vortex resided beneath the deepest upper-level outflow. The mesovortex altered the wind field and consequently became the principal organizational feature within the MCS. A descending current from the storm's rear that, depending on location, extended from 1 km to the upper troposphere was intensified and focused by the vortex. The descending rear inflow had a peak vertical velocity of 10 ? 10?3 mb s?1 and concentrated into a jet that passed to the south of the vortex. The intruding flow caused the precipitation and cloud fields to develop comma-like shapes and determined the distribution of kinematic parameters within the MCS. Mesovortex vertical vorticity was a maximum (25 ? 10?5 s?1) at middle-storm levels where environmental air converged into the mesoscale downdraft but was also strong at lower levels where the mesoscale downdraft dominated. Stretching of preexisting vorticity seems the primary amplification mechanism at middle levels. Tilting of horizontal vorticity generated by baroclinicity in the rear inflow is given as an explanation for the low-level vorticity.