Detection and Prediction of Warm Season Midtropospheric Vortices by the Rapid Update Cycle

Abstract

The authors perform a statistical and dynamical analysis of midtropospheric mesoscale vortices captured by analyses from the Rapid Update Cycle, version 2 (RUC-2), during the period 1 May to 31 August 1999. A total of 203 vortices meeting conditions of an automated algorithm were found. Of these, 86 were observed to form within organized convection and were termed mesoscale convective vortices (MCVs). MCVs were favored over a broad area from eastern Colorado and western Nebraska to the Mississippi River valley, essentially collocated with the loci of organized convection. The remaining 117 vortices (termed dry vortices) clustered in the immediate lee of the Rocky Mountains and over the southeastern United States. Vortices arising from convection had considerably greater intensity and longevity than dry vortices. They were roughly five times more likely to be involved with the triggering of new convection. A relationship was found between intensity and longevity such that there appears to be a maximum vortex lifetime that can be estimated from its maximum intensity. Vortices arising from convection had markedly greater humidity and water vapor mixing ratio underneath their centers compared to dry vortices, consistent with many dry vortices having a topographic origin and MCVs arising from organized convection. Parameters such as horizontal scale, background vertical wind shear, and horizontal deformation were not systematically related to intensity or longevity. Prediction of mesoscale vortices by the RUC-2 was examined for a subsample of all cases. In general, the RUC-2 was able to predict the evolution of vortices once analyzed, but had virtually no skill at predicting (12 h in advance) the formation of the vortices. However, forecasts of organized convection should still benefit from accurate predictions of long-lived vortex tracks.