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

The vorticity dynamics of a lower-tropospheric mesovortex within a mature mesoscale convective system (MCS) is investigated with observations from a special sounding network and with dual-Doppler radar measurements. The data show that the vorticity distribution and tendency were dictated by the mesoscale downdraft, which formed within the storm's trailing stratiform region, and not by the mesoscale updraft. At midlevels, preexisting vertical vorticity was amplified by convergence as environmental air overtook the storm from the rear and funneled into the evaporatively cooled mesoscale downdraft. Vertical vorticity within the stratiform region also increased by the twisting of horizontal vorticity 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.