A Case Study of Severe Storm Development along a Dryline within a Synoptically Active Environment. Part II: Multiple Boundaries and Convective Initiation

Abstract

A dryline that occurred on 16 May 1991 within a synoptically active environment is examined in detail using research aircraft, radar, surface, satellite, and upper air observations. The work focuses on multiple boundaries in the dryline environment and initiation of tornadic storms in two along-line areas. Aircraft measurements in the boundary layer reveal that both the east?west extent of moisture gradients and the number of regions containing large moisture gradients vary in the along-dryline direction. Aircraft penetrations of thinlines observed in clear air return from radar reveal that all thinlines are associated with convergence and a moisture gradient, and that more distinct thinlines are associated with stronger convergence. However, significant moisture gradients are not always associated with either thinlines or convergent signatures. Convective clouds on this day formed at the dryline rather than significantly east of the dryline. The three thunderstorm cells that occurred in east-central Oklahoma developed along a 20-km section of the dryline south of a dryline bulge and within a 30-min period. The storms appear to have developed in this location owing to enhanced convergence resulting from backed winds in the moist air in response to lowered pressure in the warm air to the northwest. Aircraft measurements in the boundary layer and satellite-sensed surface temperature both indicate localized warming in this area to the northwest. Farther north there was a 70?100-km segment along the dryline where few convective clouds formed during the afternoon. This coincided with a swath of cooler boundary layer air that resulted from reduced surface heating over an area that received significant thunderstorm rainfall during the previous night. A severe thunderstorm complex that developed along the Kansas?Oklahoma border was initiated at the intersection of the dryline and a cloud line that extended into the dry air. An aircraft penetration of the cloud line about 12 km from its intersection with the dryline shows convergence and deepened low-level moisture at the cloud line. The cloud field that evolved into the cloud line over a period of several hours developed over the area that had received the heaviest rainfall during the previous night.