https://doi.org/10.1029/2023WR034492

Scaff, L., Krogh, S. A., Musselman, K., Harpold, A., Li, Y., Lillo-Saavedra, M., Oyarzún, R., Rasmussen, R. (2024) The Impacts of Changing Winter Warm Spells on Snow Ablation Over Western North America, John Wiley & Sons, Ltd, Water Resources Research, Vol 60, Iss 5, e2023WR034492, 0043-1397, https://doi.org/10.1029/2023WR034492

An increase in winter air temperature can amplify snowmelt and sublimation in mountain regions with implications to water resources and ecological systems. 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. We find that warmer (0.8°C), longer (1.8 days) and more frequent (3.7 more events) WWS increase total winter ablation (on average 109% or 18 mm/DJF) in a warmer climate. Winter melt during WWS in warm and humid places is expected to increase about 3 times more than in the cold and dry region. This study provides a comprehensive description of WWS and their impact on snowpack dynamics, which is relevant to reservoir operations and water security.

Keywords: Winter Warm Spells, snowapack ablation, convection-permitting simulation, Western North American mountains, warmer climate