Environmental Conditions Associated with the Dallas Microburst Storm Determined from Satellite Soundings

Abstract

The thermodynamic structure of the troposphere in the vicinity of the microburst storm at Dallas-Ft. Worth Airport (DFW), Texas on 2 August 1985 is described. The analysis was based principally on a set of vertical soundings from the Visible and Infrared Spin Scan Radiometer (VISSR) Atmospheric Sounder (VAS) onboard the Geostationary Operational Environmental Satellite (GOES), valid about 1 h before the occurrence of peak surface winds. Convection in the DFW area developed in a gradient of stability on the west side of a tongue of low lifted index and high precipitable water. The lapse rates in the 850 mb-700 mb layer were large (8°?9°C km?1). Vertical profiles of VAS data showed that DFW was in a transition zone in which conditions became drier at all levels and slightly warmer near 500 mb to the south and southwest. The midlevel warming reduced the buoyant energy available above cloud base, thus acting as a capping mechanism for the unstable, northward- moving low-level air. The potential instability was released in the vicinity of DFW by low-level convergence, caused in part by an outflow boundary from earlier convection. The storm had characteristics of both the wet and dry types of microbursts based on current models. There was a large decrease with height in total static energy (inferred from equivalent potential temperatures) from the surface to 700 mb, resulting in a source of potentially cool air fairly close to the surface.