The Oklahoma Squall Line of 19 May 1977. Part II: Mechanisms for Maintenance of the Region of Strong Convection

Abstract

Mechanisms for maintenance of the strong convection along the leading edge of a broad squall line that occurred in Oklahoma on 19 May 1977 are investigated. The findings are based upon analysis of data from a surveillance radar, a surface mesonetwork, Doppler radars, proximity soundings and aircraft data, and upon the results of a two-dimensional, cloud-scale numerical simulation. The detailed results of the multiple Doppler analysis are contained in the Part I paper reporting results of research on this squall line. It is found that at a preferred location along the squall line, an area of intense convection is maintained over a long time period. A meso-? scale organized structure, which includes an area of low pressure near the southeast edge of the intense convection and an associated area of convergence extending to the east, promotes the formation of small showers in short line segments. These showers, due to their differing motion from elements within the main line, merge with the line to the north of the mesolow, resulting in maintenance of the strong area of convection. The observed meso-? structure on this day is believed to be made possible by a deep low-level layer of weak vertical wind shear and high water-vapor content. At other locations along the line, the numerical simulation indicates an unsteady behavior in the maintenance of squall line convection by gust frontal convergence. Perturbations in the vertical motion field are periodically initiated by either (i) enhanced convergence at the gust front resulting from diverging downdrafts at locations farther to the west, or (ii) Kelvin-Helmholtz instability produced at the gust front head. These perturbations move westward relative to the gust front above the low-level cold air and periodically invigorate the main region of updrafts located a few tens of kilometers west of the gust front. Low-level updrafts, forced by diverging surface outflow from weak downdrafts, occasionally interact with the translating perturbations to increase their amplitude. The existence of the westward-moving perturbations is tentatively substantiated by the presence of similar structures in the analyzed Doppler wind fields. Greater time resolution in Doppler data, in combination with more comprehensive surface and upper air data ahead of squall lines of this type, would aid in confirming the reported structures.