Midlevel Cyclonic Vortices Generated by Mesoseale Convective Systems

Abstract

Despite the large number of convective systems that occur over the central United States every year, there are typically only a few well-defined, midlevel vortices apparent in satellite imagery after the overlying anvil debris from some convective complexes has dissipated or advected away. A climatology of mesoscale convectively generated vortex (MCV) events for 1981-1988 is presented and the synoptic setting in which the circulation becomes apparent is discussed. Proximity sounding data from numerous cases are used to examine features of the kinematic and thermodynamic setting of MCVs at various lifelycle stages defined by satellite imagery. Features ofthe large-scale environment that appear conducive to the formation and longevity of MCVS include weak flow, weak vertical shear, weak background relative vorticity, and intense horizontal and vertical moisture gradients. The rapid mesovortex generation observed can be explained by the stretching term of the vorticity equation. Most MCVs emerge from MCC-type (i.e., circular) systems, but of the cases noted (24 events over the central United States between 1981-1988) only half originated in systems that met Maddox's stringent MCC size and duration criteria. Furthermore, since several MCVs emerged from small and relatively short-lived convective systems, the background synoptic environment, in addition to the magnitude of latent heating, may provide important controlling factors in determining which MCSs will lead to documentable MCVS. The majority of MCVs (i.e., 80%) were first observed at latitudes south of 40°N. Since many convective syftms occur at latitudes north of 40°N, the paucity of MCVs in northern latitudes is not the result of a lack of convective systems.