Unraveling the complexities of rain-on-snow events in High Mountain Asia

Rain-on-snow (ROS) events, defined as rainfall on a snow-covered landscape, pose a significant flood risk. While climate change is altering precipitation patterns and snow cover in High Mountain Asia (HMA), our understanding of ROS patterns and their underlying mechanisms remains limited. This study comprehensively analyzed the spatiotemporal distribution, trends, and potential flood risks associated with ROS (snow water equivalent (SWE) ≥ 1 mm and rainfall ≥ 1 mm) and ROS flood (SWE ≥ 10 mm and rainfall ≥ 10 mm) events in HMA using SWE and ERA5-Land rainfall data. Approximately 37.1% of HMAs have had at least one ROS event during hydrological years from 1979 to 2018 and their frequency increased with elevation. The dominant seasons for ROS production were winter at low altitudes, spring at mid-altitudes, and summer at high altitudes. Trend analysis revealed that lower elevations showed a decline in ROS events attributable to reduced SWE; whereas, higher elevations experienced an increase owing to a shift from snowfall to rainfall. ROS_flood events significantly accelerated snowmelt (1.27-times faster than non-ROS events, respectively). ROS_flood events, though low-frequent, accounted for 2.52% and 1.55% of the total regional rainfall and snowmelt, respectively, with the main flood risk concentrated in mid-altitude regions around 3.0‒4.5 km. This research provides a scientific basis for water resource management, flood prediction, and adaptation in HMA.