The Role of an Apparent Mesoscale Frontogenetic Circulation in Squall Line Initiation

Abstract

A line of severe thunderstorms is observed in Satellite imagery to develop explosively from a narrow line of shallow convection at the most rapidly intensifying part of a surface cold front. Concurrent evaporation of the leading edge of a large area of stratus and stratocumulus clouds behind the front results in the appearance of a mesoscale clear zone adjoining the line convection feature. The clear zone enlarges to its maximum width of 65 km less than an hour prior to the genesis of the frontal squall line. These observations suggest the possibility that a transverse circulation about the front generated the line convection and clear zone (in the upward and downward branches of the circulation, respectively), and ultimately the squall line. Analysis of the synoptic surface data indicates the likely presence of a thermally direct frontogenetic circulation at the leading edge of the clear zone. The implied frontogenetic process exhibits a rapid e-folding time of ?3 h, corresponding to the development time of the clear zone. The transverse circulation implied by the observations cannot be explained solely on the basis of geostrophic deformation acting upon the cross-frontal horizontal temperature gradient field, since the observed circulation is characterized by spatial and temporal scales much smaller than those predicted by semigeostrophic theory. The observed scales can be explained by considering a superposition of the cross-frontal variation in surface sensible heat flux upon the deformation field. The resulting transverse circulation is shown to be capable of producing vertical motions strong enough to generate the clear zone and squall line. The possible relevance of other mesoscale processes as explanations for these satellite-observed features is also examined.