Projected Rain-on-Snow Events in a sub-Arctic River Basin under Climate Change

Record-breaking floods have been induced by rain-on-snow (ROS) events as well as sudden snowmelt in mountainous regions. Besides, during ROS events in cold regions, liquid precipitation can percolate through the snowpack, freeze and form ice layers, causing hardship for wintertime ungulate foraging. Therefore, it is critical to study the characteristics (frequency, intensity) of ROS events and the corresponding hydrological (average daily snowmelt, runoff conditions) and ecological impacts in a changing climate. In this study, ROS days have been classified into minor and major based on minimum 3mm and 10mm of rain on minimum 5mm of snow water equivalent (SWE), with the latter leading to more severe hydrological and ecological impacts. By defining a set of 7 indices, we describe the seasonal future minor and major ROS days using the projections of 7 multivariate bias corrected (MBCn) global climate models (GCM) members of the 5th phase of the Coupled Model Intercomparison Project (CMIP5) (under the medium (4.5) and high (8.5) Representative Concentration Pathways (RCPs)) under 1.5, 2, 3, and 4 C global warming levels. The downscaled GCM forced output fluxes from the high-resolution Variable Infiltration Capacity (VIC) hydrologic model over the sub-Arctic Liard River Basin (LRB) are used to investigate the hydrologic response to minor and major ROS days. In terms of frequency, projections indicate that October December (OND) and January March (JFM) are the most susceptible seasons to minor and major ROS occurrences, respectively. Although infrequent minor and major ROS days are projected for April June (AMJ), the ROS intensity is higher in this period, especially over high elevations. Further, the implications of major ROS days are significant in JFM, as they can generate runoff and thaw snow respectively 10-times and 1.5-times greater than that of minor ROS days. Our findings assert that global warming is changing the precipitation regime from snow to rain, as well as early shifting of the peak annual streamflow. These changes facilitate favorable climatic conditions for minor and major ROS days to occur in winter, while offsetting them in summer over low elevation areas. Projecting future ROS characteristics enables evaluating this natural hazards impacts on LRBs infrastructure, communities, and ecosystem.