Structure and Evolution of a Severe Squall Line over Oklahoma

Abstract

A squall line on 2 May 1979 developed in Oklahoma in proximity to a synoptic-scale cold front. This line is analyzed during its growth and mature periods using radar, satellite, sounding and surface data. Some of the cells produced hail, and many of the cell tops reached 16 km. However, there were no reports of tornadoes. Three main topics are addressed in the paper: 1) examination of squall line and cell propagation mechanisms; 2) the three-dimensional structure of the squall line and individual cells during the mature period and 3) mass and moisture fluxes and precipitation efficiency. Comparison is made between the 2 May case and other tropical and Midwest squall line cases. The 2 May case does not exhibit a ?trailing stratiform? anvil during the period mechanism requiring veering environmental wind shear in the lowest levels; and 2) mechanism where the cell motion is eventually governed by the moisture convergence and lifting provided the convergence line. The motion of the squall line (defined by centroids of cells along the line) follows closely that of a convergence line found to be associated with a synoptic scale cold front. Initially, cells move along the low- to midlevel shear vector, which is directed ?45° clockwise from the line orientation; then the cells turn to the right (nearly normal to the line). It is postulated that two mechanisms are responsible for this rightward turn of the cells: 1) mechanism requiring veering environmental wind shear in the lowest levels; and 2) mechanism where the cell motion is eventually governed by the moisture convergence and lifting provided the convergence line. Triple Doppler analysis of a cell along the line indicates maximum updrafts of ?35 m s?1, and strongest downdrafts at middle to upper levels located between cells along the line. The structure of the squall line is somewhat different from that in the case presented by Newton and other documented squall line studies in that there are not well-organized downdrafts on the rear side at low to midlevels. In addition, low-to midlevel inflow on the rear side of the squall line is apparently absent. Man and moisture fluxes computed from sounding and radar data indicate magnitudes comparable to previous squall line cases. However, the precipitation efficiency of the squall line is estimated to fall in the range 25?40%, which is somewhat lower than other reported values (>50%). The low precipitation efficiency is suggested to be due in part to large moisture losses at upper levels.