Mesoscale Vortices over the Great Lakes in Wintertime

Abstract

The occasional occurrence of wintertime mesoscale lake vortices is documented. The vortices are readily discernible in satellite imagery, in which they take one of three forms: a miniature comma cloud, a swirl of cloud bands (resembling a miniature tropical storm) or a swirl of cloud streets. Despite their impressive appearance in satellite imagery, these vortices are usually relatively mild in comparison with other lake-effect storms and produce only gusty winds and brief snow squalls as they move onshore. The vortices are accompanied by a slightly lowered surface pressure and a weak cyclonic low-level wind circulation. Fourteen vortices were detected over the Great Lakes in the years 1978?82; they occurred under conditions of relatively weak surface pressure gradient, with a ridge of high pressure usually found over or west of the region. Convergence was generally detected in the surface winds prior to vortex development, apparently related to land breeze circulations. Comparisons are made between the conditions favoring the occurrence of shoreline-parallel cloud bands and lake vortices. Comparisons are also made between lake vortices and polar vortices and i.e., mesoscale vortices occurring in polar airstreams over oceans.