MESOSCALE DOWNDRAFT INFLUENCES ON VERTICAL VORTICITY IN A MATURE MESOSCALE CONVECTIVE SYSTEM

The vorticity dynamics of a lower-tropospheric mesovortex within a mat ure mesoscale convective system (MCS) is investigated with observation s from a special sounding network and with dual-Doppler radar measurem ents. The data show that the vorticity distribution and tendency were dictated by the mesoscale downdraft, which formed within the storm's t railing stratiform region, and not by the mesoscale updraft. At midlev els, preexisting vertical vorticity was amplified by convergence as en vironmental air overtook the storm from the rear and funneled into the evaporatively cooled mesoscale downdraft. Vertical vorticity within t he stratiform region also increased by the twisting of horizontal vort icity associated with a backing wind. The horizontal vorticity vector pointed opposite the velocity vector and tipped into positive vertical vorticity when the flow encountered the mesoscale downdraft. At lower -middle storm levels the twisted vorticity was subsequently amplified by convergence along with preexisting vertical vorticity. Below 2 km, vorticity decreased largely in response to horizontal divergence.