Rain-on-snow response to a warmer Pyrenees

Abstract. Climate warming is changing the magnitude, timing, and spatial patterns of mountain snowpacks. A warmer atmosphere may also lead to precipitation phase shifts, with decreased snowfall fraction (Sf). The combination of Sf and snowpack decreases directly affects the frequency and intensity of rain-on-snow (ROS) events, a common cause of flash-flood events in snow dominated regions. In this work we examine the ROS patterns and sensitivity to temperature and precipitation change (delta-change) in the Pyrenees using a physical-based snow model forced with reanalysis climate data perturbed following 21^st century climate projections for this mountain range. ROS patterns are characteritzed by their frequency, rainfall quantity and snow ablation. The highest ROS fr for the baseline climate period (1980–2019) are found in South-West high-elevations sectors of the Pyrenees (17 days/year). Maximum ROS rain is detected in South-East mid-elevations areas (45 mm/day, autumn), whereas the highest ROS ablation is found in North-West high-elevations zones (−10 cm/day, summer). When air temperature is increased from 1 ºC to 4 ºC, ROS rain and frequency increase at a constant rate during winter and early spring for all elevation zones. For the rest of the seasons, non-linear responses of the ROS frequency and ablation to warming are found. Overall, ROS frequency decreases in the shoulders of the season across eastern low-elevated zones due to snow cover depletion. However, ROS increases in cold, high-elevated zones where long-lasting snow cover exists until late spring. Similarly, warming triggers fast ROS ablation (+10 % per ºC) during the coldest months of the season, high-elevations, and northern sectors where the deepest snow depths are found. On the contrary, slow, and non-changes in ROS ablation are expected for warm and marginal snowpacks. These results highlight the different ROS responses to warming across the mountain range, suggest similar ROS sensitivities in near mid-latitude zones, and will help anticipate future ROS impacts in hydrological, environmental, and socioeconomic mountain systems.