Kinematic and Precipitation Structure of the 10–11 June 1985 Squall Line

Abstract

High-frequency (90 min) rawinsonde data from a special mesoscale network (26 sites) have been combined with wind profiler, dense automated surface network data (80 stations spaced 50 km apart), and a series of high-resolution dual-Doppler radar analyses in a common framework attached to a moving squall-line system to form a comprehensive dataset describing the mature phase of the 10?11 June 1985 squall line observed during PRE-STORM. The dual-Doppler radar analyses covered a 200 ? 300 km2 area, from the leading edge of the convective line to the back edge of the trailing stratiform precipitation region, thus, providing high-resolution wind information over a very broad portion of the storm system. The comprehensive analysis is used to resolve several aspects of the trailing stratiform region that had remained unclear from previous studies. First, a difference in the horizontal scale was found between the mesoscale updraft, which at upper levels was on the scale of the trailing stratiform cloud, and the strong mesoscale downdraft, which at mid-to-lower levels was on the scale of the trailing stratiform precipitation. Second, the region of heaviest stratiform precipitation (the secondary band) was found to be immediately downwind of the most intense portions of the convective line, and the width of the trailing stratiform precipitation region was controlled by a combination of the wind velocity and microphysical fall-speed scales. Third, the radar reflectivity minimum observed at mid-to-lower levels in the region just behind the convective line was found to coincide with deep subsidence from mid-to-upper levels, which may have reduced the mass of the hydrometeors through sublimation and evaporation. However, precipitation trajectories computed from the comprehensive analysis indicate another contributing factor; namely, the source region of hydrometeors at low levels just behind the convective line was at a lower altitude than the source region of low-level hydrometeors in the heavy stratiform precipitation farther behind the convective line. Thus, even if all other factors had been the same, the hydrometeors in the heavy stratiform rain would have had more time to grow than those found in the region of the radar reflectivity minimum just behind the convective line. Moreover, hydrometeor detrainment may have been greater near cloud top than at lower levels.