A Nonclassical Cold Front Observed during COPS-91: Frontal Structure and the Process of Severe Storm Initiation

Abstract

This case study addresses the issue of gravity current and bore development at surface cold fronts, and the role of these phenomena in the generation of severe frontal convection. The event investigated occurred on 27 April 1991 during the Cooperative Oklahoma Profiler Studies 1991 field project. The development of a bore from a gravity current?like structure along a cold front, the subsequent propagation of the bore ahead of the front on a low-level inversion, and the process of severe thunderstorm development along the front are revealed by a dense network of remote sensing and other special observations. Evidence for the gravity current and bore is strengthened by comparisons made between the synthesized observations and theory. The bore developed after a nocturnal inversion, which acted as a waveguide, had become established. The bore and gravity current were both evident as ?fine lines? in the radar reflectivity displays. A microscale envelope of enhanced water vapor with an embedded roll cloud, a strong vertical circulation, and a low-level microscale?jetlet? were associated with the bore. A pronounced ?feeder flow? was present behind the gravity current, in association with a second vertical circulation, which was more elevated than the one associated with the bore. The jetlet provided an efficient wave-trapping mechanism for the bore, due to the combined effects of wind curvature on the Scorer parameter profile and mass convergence enhancement by the low-level shear. Effects of the bore and gravity current passage on the atmosphere were assessed by applying parcel displacement profiles derived from wind profiler analysis to an observed prebore sounding, and then to a computed postbore sounding. These calculations suggest that the strong bore-induced lifting was insufficient to trigger the storms; rather, it was the dual lifting provided by the bore and the gravity current that made it possible for low-level parcels to reach their level of free convection. These results confirm other recent findings that indicate that even though bores generated by gravity currents can produce strong lifting, this may be insufficient to trigger deep convection whenever the lifting is confined to too shallow a layer and/or is of insufficient duration.