North American rain-on-snow ablation climatology

Rain-on-snow ablation events carry a relatively high risk for rapid snowmelt and runoff due to the combination of liquid precipitation and generally high turbulent fluxes into the snowpack. Determining the variability in rain-on-snow ablation is critical in describing local hydroclimate. This study uses a gridded observational snow dataset to examine spatiotemporal variations in North American rain-on-snow ablation over a 50 yr period. Here we show rain-on-snow ablation represents approximately 33% of all ablation events in the eastern third of the continent, compared to 10 events observed per year on average. A central band of enhanced event frequencies propagated meridionally during the calendar year, most prominently in the eastern half of the continent. Seasonal (September to August) event frequency from 1960-2009 significantly decreased by approximately 50% across much of northern Quebec and in the southern Appalachians, while it significantly increased in portions of British Columbia and southeastern Quebec. Interannual variations in event frequency were primarily forced by variations in seasonal-scale snowfall and snow depth, and only moderately associated with variations in air temperatures.