Convective variability associated with a mesoscale vortex in a midlatitude squall line system

Abstract

The relationship between the kinematic structure of the convective line and the mesoscale stortn-relative flow associated with an embedded mesovortex in the trailing stratiform region of the 28 May 1985 squall line system is examined using Doppler radar data collected during the Preliminary Regional Experiment for Stonnscale Operational and Research Meteorology-Centml Phase (PRE-STORM). Ten dual-Doppler analyses of the kinematic and reflectivity fields are constructed for roughly a 50-minute period over the storrif s mature stage. Reflectivity and flow fields exhibit significant variability along the convective line. Large, somewhat isolated reflectivity cores, elongated in the direction of storm propagation, were located in the southern and central portions of the storm. In contrast, the northern part of the convective line was characterized by smaller, more closely spaced reflectivity cores which were organized perpendicularly to the storm propagation vector. Deepest reflectivity cores and strongest vertical drafts were consistently found on the southern flank of the system. The southern end of the convective line expanded during the analysis period while the convective intensity of the northern end of the line continuously decreased. A well organized cyclonic mesovortex was found at midlevel in the stratiform cloud trailing the north-central portion of the leading convective line. The variability in the structure of the convective cers along the convective line appeared to be related to the interaction between the mesoscale low-level outflow from this vortex and the environmental low-level inflow. The outflow was opposite the environmental inflow in the southern and central portions of the storm. In contrast, the outflow was directed nearly parallel to the inflow along the northem portion of the storm system. This led to variation in the depth and direction of propagation of the convective downdraft outflow such that there was greater low-level convergence and enhanced lifting in the southern and central portions of the convective line compared to the northem part. Hence, the variability in convective structure appears to have resulted from a scale interaction between the storm-induced relative flow and the environmental winds.