Evolution of vertical drafts and cloud-to-ground lightning within the convective region of a mesoscale convective complex

Abstract

The evolution of the area-averaged vertical velocity within the objectively defined convective region of the 4 June 1985 PRE-STORM (Preliminary Regional Experiment for Stormscale Operational and Research Meteorology-Central Phase) mesoscale convective complex (MCC) was examined over a 100 minute period during the storm's mature stage and compared to that of a squall line. The area-averaged vertical velocity profiles from twenty dual-Doppler analyses indicated significant differences in convective intensity both spatially and temporally. The peak magnitudes of the mid-to-upper level area-averaged vertical velocity were initially comparable to those from a squall line but became much weaker. The distribution of vertical velocities at mid-to-upper levels was consistent with weakening convection as the percentage of weak updrafts and downdrafts grew at the expense of stronger vertical motions. Typically the non-squall MCC had a much larger percentage of strong downdrafts than did the squall line. Horizontal and vertical cross-sections of both vertical velocity and reflectivity confirmed that the MCC was evolving toward a state with fewer and less intense updraft cores while cross-sections of storm relative flow revealed a descending rear-to-front inflow at later analysis times which reached the surface near locations of active convection. The relationship of cloud-to-ground (CG) lightning to the kinematic structure of the MCC was also studied. For mesoscale regions there was a 10-20 minute time lag between the peak convective intensity and the peak CG flash rate. A similar delay was found on the convective scale for some convective cells, however, other apparently similar cells had little CG lightning throughout their evolution. An analysis of the vertical profiles of the vertical velocity, reflectivity and their gradients above CG lightning suggested a preferred vertical structure of these fields for CG lightning. CG lightning occurred beneath greater than average vertical velocities at mid-to-upper levels and greater than average vertical velocity gradients and reflectivity at all levels. CG lightning also occurred beneath greater than average reflectivity gradients at mid and upper-levels but beneath smaller than average reflectivity gradients at low-levels due to its avoidance of the highest reflectivity gradient regions.