Systematic Analysis of Rain-on-Snow Events and Their Trends in the French Alps: Distinguishing All and Impactful Events across Mountain Ranges

Abstract. Rain-on-snow (ROS) events are a major hydrometeorological phenomenon in mountainous regions, as the combination of liquid precipitation and snowmelt enhances flood hazard. This study provides the first systematic analysis of ROS events in the French Alps at high temporal and spatial resolution, based on the S2M reanalysis (1958–2024) with an hourly time step. By combining meteorological data with an event database of torrential floods and landslides, ROS events are explicitly linked to observed impacts. Averaged over space, 5.5 ROS events occurred per year in the French Alps over 1958–2024, with a clear contrast between the more frequently affected north-western mountain ranges and the south-eastern ones. Seasonality strongly depends on elevation, with ROS events occurring mainly in winter at lower elevations and in spring at higher elevations. Impactful ROS events, associated with torrential floods or landslides, are characterized by high cumulative rainfall and long durations. Above 76 mm of cumulative rainfall, the probability that a ROS event is impactful exceeds 50 %, defining heavy-rainfall ROS events. Trend analyses reveal a marked decline in overall ROS occurrence (-22 %) between 1959–1988 and 1995–2024, primarily in spring and early summer due to a shortening of the snow season, while increases are observed in December and January as precipitation more frequently falls as rain. Heavy-rainfall ROS events show a more moderate decrease (-9 %), with declining trends in most southern mountain ranges but increases in several northern ones. They now predominantly occur in early winter at lower elevations, which are more densely populated, making them more likely to produce damaging impacts. The next step in this systematic approach would be to create a watershed-based subdivision of the French Alps to enable comprehensive hydrological analyses of ROS events.