Impacts of changing winter warm spells on snow ablation in the mountains of western North America

Abstract:

Increasing air temperature shifts snowfall to rainfall and produces earlier snow-melt, reducing peak snow water equivalent (SWE) with significant ecologic and economic impacts. Little is known about the effect of extreme temperature events (winter warm spells, WWS) on snow ablation, particularly during the snow accumulation season, and how it might change under a warmer climate. Our hypothesis is that ablation during WWS can be a significant portion of the total sublimation, and that it behaves differently than non-WWS ablation events, with unknown consequences in the future. Research goal is to quantify WWS ablation over the mountains of Western US and Southern Canada, understand the mechanisms and local conditions that drive its spatial variability, and quantify potential changes under a warmer climate.

This research uses simulations from the CONUS-I project, which is a 13-year and continental-scale simulation using the Weather Research and Forecasting (WRF) model in convection-permitting configuration at a 4-km grid spacing with the Noah-MP land-surface model. WRF is tested against observational gridded data showing that it realistically represents the magnitude of longitudinally increasing frequency of WWS in the Western U.S. The historical fraction of ablation during WWS to peak SWE is the largest in the Coastal Mountains, with the Sierra Nevada and the Pacific Northwest averaging 12% and 16% correspondingly. While in the Rocky Mountains is significantly lower at 4% on average. The mean ablation rate during WWS is also the largest in the Coastal Mountains at 9 mm/day, with a strong elevation control especially during rainfall events. This spatially heterogeneous pattern demonstrates a complex and varied response of snow ablation to WWS, likely controlled by local climate and topography. Future climate simulations using a pseudo global warming approach suggest an average increase of 1-8 °C in WWS magnitude and 43% (3.7 days) in frequency, with a significant impact on the magnitude of WWS ablation. These results are expected to be useful for water managers and policy makers in mountainous regions of North America.