Understanding Heavy Lake-Effect Snow: On Secondary Circulations within LLAP Bands Over Lake Ontario

One of the goals of the Ontario Winter Lake-effect Systems (OWLeS) field campaign was to understand more about the structure of long-lake-axis-parallel (LLAP) lake-effect snow bands and how they develop and intensify. These types of bands most frequently form over the eastern Great Lakes (i.e., Lakes Erie and Ontario) and are typically well organized and quite intense, often producing large snowfall rates and significant accumulations downwind over land. Within these bands, total accumulations of several feet in only a day or two are not uncommon, leading to hazardous travel conditions and other concerns. This presentation examines several LLAP bands that were observed over Lake Ontario during OWLeS from the perspective of the Wyoming Cloud Radar (WCR), mounted aboard the University of Wyoming King Air (UWKA) research aircraft. Characteristics related to the depth, vertical structure, and intensity of the bands are examined. In each case, dual-Doppler analysis using the two downward-pointing WCR beams reveals the presence of a secondary circulation composed of two counter-rotating cells on either side of the band. Longstanding conceptual models of lake-effect snow indeed recognize the existence of such circulations within deep lake-effect convection, which presumably play some role in organizing the convection and maintaining the updraft and low-level convergent flow. The results presented here constitute the first observations of the full two-dimensional cross-band vertical structure of such circulations. They are thought to be driven thermally, by the buoyancy over the lake and latent heating within the band, although other mechanisms including land breezes and surface friction may contribute to the onset of the low-level convergence. The dual-Doppler data also shows that the circulations tended to persist downwind (i.e., east) of the lake and become more organized, although not necessarily stronger, as the bands moved over land, away from the source of buoyancy. This is likely a consequence of the transition from surface-based convection (which mixes momentum) over the lake to a decoupled secondary circulation over land.