Variations in orographic precipitation enhancement during lake-effect storms over the Tug Hill Plateau: Observations from OWLeS IOP2

Orography downstream of large bodies of water, such as the Great Lakes of North America, often enhances lake-effect snowfall, which can be intense, extremely localized, and challenging to predict. Here we examine orographic effects during lake-effect storms over the Tug Hill Plateau, which gradually rises ~500 m above the eastern shore of Lake Ontario and receives nearly twice the annual snowfall as the surrounding lowlands. Our analysis uses data collected during the Ontario Winter Lake-effect Systems (OWLeS) field program, including S-band surveillance radar scans and data from a transect of profiling K-band radars and snow measurement stations that extended inland from the eastern shoreline of Lake Ontario to the upper Tug Hill Plateau. We specifically examine how the morphology and three-dimensional structure of lake-effect convection during OWLeS IOP2 modulates the orographic enhancement of precipitation over and around the Tug Hill Plateau.

IOP2 produced over 100 cm of snow on the western slope of the Tug Hill Plateau in 24 h. During the event significant vacillations in lake-effect morphology were observed. Organized long-lake-axis parallel (LLAP) bands centered over the transect produced the highest precipitation rates and accumulations at both lowland and upland sites, but also the lowest orographic ratios (i.e., the ratio of upland to lowland liquid precipitation equivalent). Rimed crystals and graupel were frequently observed at both sites during LLAP periods. Broad coverage periods, featuring less-organized convective coverage made up of distinct individual cells, produced lower precipitation rates at both sites but higher orographic ratios as cellular convection produced over the lake transitioned to a more stratiform mode with greater coverage and more persistent precipitation over the Tug Hill Plateau. Mixed mode periods, which consisted of broad coverage concomitant with an organized LLAP band, also exhibited broadening and a convective-to-stratiform transition. During these periods there were low orographic ratios between the upland and lowland site when the band was centered over the transect and high orographic ratios when the band was to the north or south of the transect.

Climatologically, organized bands account for 10% of lake-effect hours over Lake Ontario with broad coverage and mixed mode making up the majority of the remainder. Accordingly, this study suggests that the Tug Hill Plateau precipitation maximum may be largely due to the broadening of precipitation coverage and frequency over the uplands rather than the invigoration of LLAP bands over the Tug Hill Plateau.