The Role of Synoptic-Scale Circulation in Great Lakes Basin Snow Cover Ablation

Synoptic-scale atmospheric conditions play a critical role in determining the frequency and intensity of snow cover ablation events. Using a synoptic weather type 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 1960-2009. An ablation event is considered an inter-diurnal decrease in areal-weighted snow depth 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 individual case studies of the snow-surface energy fluxes derived from reanalysis are presented to understand the diversity of meteorological influences on regional snow cover ablation. Trends in synoptic type frequency and relationships to major teleconnections are also assessed seeking greater understanding to the causes of ablation variability. Results indicate a variety of synoptic weather conditions lead to snow ablation in the Great Lakes basin. The ten most common synoptic weather types result in approximately 65% of the 392 ablation events detected from 1960-2009. Collectively, snow cover ablation in the Great Lakes basin most commonly occurs when there is advection of warm and moist air into the region leading to high latent and sensible heat fluxes. Typically this advected air originates over the Gulf of Mexico. Ablation also frequently occurs during rain-on-snow events and in instances of high-pressure overhead leading to clear skies and high levels of incoming shortwave radiation reaching the snowpack. The variability of a number of the most prominent ablation-causing synoptic weather types can be partially explained by the relative phases of major teleconnections such as the Pacific/North American Pattern.