THE EVOLUTION OF THE 10-11 JUNE 1985 PRESTORM SQUALL LINE - INITIATION, DEVELOPMENT OF REAR INFLOW, AND DISSIPATION

Mesoscale analysis of surface observations and mesoscale modeling resu lts show that the 10-11 June squall line, contrary to prior studies, d id 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 Ice of the Rocky Mountains and a midle vel short-wave trough. Three additional mechanisms were active: a sout heastward-moving cold front formed the northern part of the line, conv ection 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 por tion of the line. Mesoscale model results show that the large-scale en vironment significantly influenced the mesoscale circulations associat ed with the squall line. The qualitative distribution of along-line ve locities within the squall line is attributed to the larger-scale circ ulations associated with the lee trough and midlevel baroclinic wave. Ambient rear-to-front (RTF) flow to the rear of the squall line, produ ced by the squall line's nearly perpendicular orientation to strong we sterly flow at upper levels, contributed to the exceptional strength o f the rear inflow in this storm. The mesoscale model results suggest t hat the effects of the line ends and the generation of horizontal buoy ancy gradients at the back edge of the system combined with this ambie nt RTF flow to concentrate the strongest convection and back-edge subl imative cooling along the central portion of the line, which then prod uced a core of maximum rear inflow with a horizontal scale of approxim ately 100-200 km. The formation of the rear-inflow core followed the o nset of strong sublimative cooling at the back edge of the storm and s uggests that the rear inflow maximum was significantly influenced by m icrophysical processes. In a sensitivity test, in which sublimative co oling 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 occurr ed over Missouri, ahead of the squall line, and caused the presquall B ow to veer from southeasterly to southwesterly, which decreased the lo w-level inflow and line-normal vertical wind shear. The reduction in l ow-level wind shear decreased the effectiveness of the cold pool in su staining deep convection along the gust front.