Quantification of North American Rain-on-Snow Ablation Frequency Variability

Abstract:

Snow cover ablation is an important contributor to hydrologic resources, however large and rapid events pose a substantial hazard, particularly with the presence of concurrent rainfall. Rainfall on a snowpack, or “rain-on-snow” can greatly increase the potential for flooding, with liquid precipitation and melted snow potentially both running off the land surface, affecting life and property. This study uses a gridded dataset of North American snow, temperatures, and precipitation to examine the spatial and temporal variations of rain-on-snow ablation from 1960-2009. Here we show rain-on-snow ablation frequency is highest in the Pacific Coast Ranges and in northeastern North American, with the presence of a seasonally-propagating latitudinal band of enhanced frequencies forced by seasonal variations in snow depth and meteorological conditions suitable for ablation. Rain-on-snow represents approximately 23% of ablation events across the continent, but percentages are highly spatially heterogeneous, and values exceed 70% along much of the Pacific coast and are below 10% in the continent's interior. Event frequencies are significantly decreasing by as much as 60% across the southern Appalachians and in large portions of the Canadian Shield, primarily driven by decreases in snowfall and snow depth, and increases in surface air temperature, particularly daily minimums. Determining the variability and forcing mechanisms of rain-on-snow ablation is critical for describing local hydroclimate.