The impacts of changing Winter Warm Spells on snow ablation over Western North America

Winter Warm Spells (WWS) are defined as a winter period (December to February, DJF) of at least 3 consecutive days with daily maximum temperature anomaly above the 90th percentile (using a moving-average of 15 days between 2001 and 2013). We calculate WWS for every 4-km grid cell within an atmospheric model over western North America to characterize WWS and analyze snow ablation and their changes in a warmer climate.

We find that days with ablation during WWS represent a small fraction of winter days (0.6 days), however, 49% of total winter ablation (33.4 mm/DJF) occurs during WWS. Greater extreme ablation rates (99th percentile) occur 18% more frequently during WWS than during non-WWS days. Ablation rates during WWS in humid regions are larger (9 mm d-1) than in dry regions (7 mm d-1) in a warmer climate, which can be explained by differences in the energy balance and the snowpack's cold content.