Mechanisms Contributing to the Tug Hill Lake-Effect Precipitation Maximum during OWLeS IOP2b

Lake-effect storms frequently produce a precipitation maximum over the modest Tug Hill Plateau (hereafter Tug Hill), which rises 500 m above the eastern shore of Lake Ontario. Here we use Weather Research and Forecasting model simulations to examine the mechanisms responsible for the Tug Hill precipitation maximum observed during IOP2b of the Ontario Winter Lake-effect Systems (OWLeS) field program. We find that while the Tug Hill precipitation maximum was produced in part by orographic effects, an additional contributor was a thermally induced mesoscale airmass boundary east of Lake Ontario that formed between warmer, lake-modified westerly flow and cooler southwesterly flow that did not traverse the lake. Localized ascent upstream of and along this boundary and the resultant patterns of hydrometeor advection and fallout contributed to an inland precipitation maximum even in simulations in which Tug Hill was removed. The presence of Tug Hill did, however, intensify the region of maximum ascent along the airmass boundary just upstream of the windward slope of Tug Hill and produce moderate low-level ascent along its convex windward slopes, which increased rates of deposition and accretion. Additionally, because the upper reaches of Tug Hill extended above cloud base, the effects of sub-cloud sublimation in the cooler and drier layer of near-surface southwesterly inflow were reduced over the high terrain. Together, these factors enhanced the precipitation gradient between the surrounding lowlands and Tug Hill and broadened the region of snowfall. These findings address several remaining gaps in understanding about the mechanisms producing the Tug Hill precipitation maximum and document the existence of an airmass boundary that influenced the distribution of precipitation east of Lake Ontario.