Circulations Associated with a Mature-to-Decaying Midlatitude Mesoscale Convective System. Part I: Surface Features—Heat Bursts and Mesolow Development

This study examines surface features associated with a mature-to-dissipating midlatitude mesoscale convective system that occurred on 23–24 June 1985 during the Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central. The primary data sources include a 400 × 500 km surface mesonetwork on a 50 km grid, rawinsonde observations from 12 supplementary sites in Kansas and Oklahoma and radar measurements from conventional as well as dual-Doppler networks. The mesoscale convective system under investigation developed in an environment with weak vertical shear and had a lifetime of 9–12 h. It consisted in its mature stage of a southward-moving arc-shaped line of deep convective cells with a trailing stratiform precipitation region to the north. Thirty-three percent of the surface rain in the portion of the mesonetwork experiencing storm passage was from the stratiform region. An intense mesoscale downdraft developed beneath the stratiform cloud with a strong mesohigh at the surface. A wake low was positioned just to the rear of the trailing stratiform region. Local “heat bursts” were observed within the wake low. These phenomena am tentatively attributed to downbursts (which develop in a nearly dry-adiabatic environment created by the mesoscale downdraft) that penetrate a shallow, stable layer near the ground. During the final dissipation of the stratiform precipitation (in a matter of 2 h), the surface mesohigh transformed into a mesolow. Observations suggest that at least part of this transformation process can be explained as a collapsing cold pool or spreading density current. This mechanism may also have contributed to the observed development or intensification of a midlevel mesovortex as the storm dissipated. Following the decay of the mesoscale convective system during the nighttime hours, new deep convection broke out in the region of the remnant midlevel circulation the next morning.