The Evolution of the 10–11 June 1985 PRE-STORM Squall Line: Initiation, Development of Rear Inflow, and Dissipation

Abstract

Mesoscale analysis of surface observations and mesoscale modeling results show that the 10?11 June squall line, contrary to prior studies, did not form entirely ahead of a cold front. The primary environmental features leading to the initiation and organization of the squall line were a low-level trough in the lee of the Rocky Mountains and a midlevel short-wave trough. Three additional mechanisms were active: a southeastward-moving cold front formed the northern part of the line, convection along the edge of cold air from prior convection over Oklahoma and Kansas formed the central part of the line, and convection forced by convective outflow near the lee trough axis formed the southern portion of the line. Mesoscale model results show that the large-scale environment significantly influenced the mesoscale circulations associated with the squall line. The qualitative distribution of along-line velocities within the squall line is attributed to the larger-scale circulations associated with the lee trough and midlevel baroclinic wave. Ambient rear-to-front (RTF) flow to the rear of the squall line, produced by the squall line?s nearly perpendicular orientation to strong westerly flow at upper levels, contributed to the exceptional strength of the rear inflow in this storm. The mesoscale model results suggest that the effects of the line ends and the generation of horizontal buoyancy gradients at the back edge of the system combined with this ambient RTF flow to concentrate the strongest convection and back-edge sublimative cooling along the central portion of the line, which then produced a core of maximum rear inflow with a horizontal scale of approximately 100?200 km. The formation of the rear-inflow core followed the onset of strong sublimative cooling at the back edge of the storm and suggests that the rear inflow maximum was significantly influenced by microphysical processes. In a sensitivity test, in which sublimative cooling was turned off midway through the simulation, the core of strong rear inflow failed to form and the squall line rapidly weakened. The evolution of the low-level mesoscale to synoptic-scale pressure field contributed to the dissipation of the squall line. Cyclogenesis occurred over Missouri, ahead of the squall line, and caused the presquall flow to veer from southeasterly to southwesterly, which decreased the low-level inflow and line-normal vertical wind shear. The reduction in low-level wind shear decreased the effectiveness of the cold pool in sustaining deep convection along the gust front.