Impacts of climate change on winter flood mechanisms: Spatial variability, trends, and bivariate frequency of rain-on-snow and soil frost

Rain-on-snow (RoS) events are projected to become more frequent and affect more regions around the globe with the increase in temperatures and shifting of precipitation patterns due to climate change. However, to date, little research has addressed the coincidence of RoS and frost for the historical and future climatic conditions that can amplify surface flooding. The goal of this study was therefore to assess the impacts of climate change on the frequency, magnitude, and trends of RoS events co-occurring with soil frost at three sites with different cold climates (sub-arctic, continental, and arctic tundra). To that end, an ensemble of four regional climate models (RCMs) was used to estimate the potential changes in RoS during frost for a historical period (1950-2005) and three future climate scenarios representing low, medium, and high emissions scenarios (RCP 2.6, 4.5, and 8.5) for the period of 2006-2100. Field observations were used to evaluate the spatial variability and trends in hydrometeorological variables for the observed period dating back to 1949 and to calibrate and validate the Simultaneous Heat and Water model (SHAW) and the Accumulation and Ablation model (SNOW-17), which provided long-term soil temperature, frost, and snow depth data. RoS volume during frost was projected to decrease by the end of the century but remains an important (if not the largest) contributor to large winter runoff events at all study sites. The reduction in RoS volumes during frost was most pronounced in the two coastal regions in the south and north of Iceland, especially for the high emissions scenario (similar to 0.28 mm/year), while the inland northeastern site experienced the least changes for all emissions scenarios. Frost depth was projected to decrease at the three study sites, but at a higher rate at the coastal northern site than in the other two regions, and to mostly disappear in the south study site for the high-emissions scenario. High-latitude coastal regions are expected to transition in the coming decades to become more temperate and experience frequent RoS events more than inland regions, while RoS contribution to runoff in the presence of soil frost might become less relevant at low latitudes by the end of the century.