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PhD Defense

Department of Atmospheric Science

Thurs., Sept. 20, 3:10 pm, EN6085

Understanding Intense Lake-Effect Snowstorms Produced by Lake Ontario

Philip Bergmaier

University of Wyoming


Areas east of Lake Erie and Lake Ontario are occasionally clobbered by massive snowfall from intense lake-effect snowstorms. Such storms are rarely observed downwind of the other Great Lakes, even though they are equally affected by cold-air outbreaks producing equally large surface sensible and latent heat fluxes. The heavy snowfall is all the more puzzling since lake-effect systems are quite shallow and typically develop in a post-frontal, subsident environment. Predicting lake-effect snowfall remains quite challenging. Forecasters have long known that the locations and amounts of lake-effect snowfall are quite sensitive to the direction of the prevailing wind, which determines the ability of long-lake-axis-parallel (LLAP) systems and bands to form. Here, we demonstrate that a thermally-direct cross-band secondary circulation plays a significant role in the development and downwind maintenance of LLAP bands

This conclusion was reached upon examining the cross-band vertical structure of several strong LLAP bands using high-resolution airborne single- and dual-Doppler radar measurements from the Wyoming Cloud Radar (WCR), mounted aboard the University of Wyoming King Air (UWKA) in the 2013–14 Ontario Winter Lake-effect Systems (OWLeS) field campaign. These observations provide a unique perspective of the precipitation structure and 2D wind field in the vertical plane across the band. The observed secondary circulations—generally characterized by low-level convergence, a convective main updraft, and divergent flow aloft—tended to strengthen and deepen downwind over the lake and only gradually weaken as they moved onshore. High-resolution numerical simulations beautifully capture the strength and depth of the secondary circulations, and provide the broader dynamically-consistent context. They show that the secondary circulation and resulting LLAP band develops upstream along a shallow, baroclinic land-breeze front originating along a bulge in Lake Ontario’s southern shoreline. As the band extends downstream and lake-induced warming weakens the low-level baroclinicity, buoyancy—enhanced by latent heating within the updraft region—becomes the dominant forcing mechanism. This transition helps explain the band intensification and the deeper, stronger, and more symmetric secondary circulation typically observed near the downwind shoreline.

In the cases with strong westerly winds, deep and intense LLAP bands with well-organized secondary circulations formed, penetrating well inland and producing heavy snowfall. In the weaker wind case, the secondary circulation was still present, and in fact stronger, but the LLAP band was shallower and weaker, rapidly dissipating as it moved onshore and producing only light snowfall closer to the shoreline. Under suitable winds, we find that this secondary circulation can also form within even shallower bands over lakes two orders of magnitude smaller than Lake Ontario, i.e., the New York Finger Lakes. 

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