Great Lakes Basin Snow-Cover Ablation and Synoptic-Scale Circulation

Synoptic-scale atmospheric conditions play a critical role in determining the frequency and intensity of snow-cover-ablation events. Using a synoptic weather-classification technique, distinct regional circulation patterns influencing the Great Lakes basin of North America are identified and examined in conjunction with daily snow-ablation events from 1960 to 2009. An ablation event is considered in this study to be an interdiurnal decrease in areal-weighted average snow depth of greater than 2.54 cm in magnitude over the entire Great Lakes basin. General meteorological characteristics associated with ablation-causing synoptic types are examined, and three individual case studies from prominent synoptic types are presented to understand the diversity of meteorological influences on regional snow ablation. Results indicate that a variety of synoptic weather conditions lead to snow ablation in the Great Lakes basin. The 10 most common synoptic types accounted for 66% of the 349 ablation events detected from 1960 to 2009. Snow ablation in the Great Lakes basin most commonly occurs when there is advection of warm and moist air into the region to provide the sensible and latent heat fluxes that are needed for melt, but ablation frequently occurs during rain-on-snow events and in instances of high pressure overhead. Ablation magnitude is highest during rain-on-snow synoptic types, and the interannual frequency of these types significantly decreased by 37% over 1960-2009. Conversely, the frequency of high-pressure-overhead synoptic types significantly increased by more than 30% from 1960 to 2009. Such changes may influence the hydrologic impact of these synoptic types on ablation over time.