Global patterns of rain-on-snow and its impacts on runoff from past to future projections
Maina & Kumar (2025)
What happens when rain falls on snow?
In cold regions, this “rain-on-snow” (ROS) phenomenon accelerates snowmelt, triggering floods, avalanches, and shifts in seasonal water supply. ROS is important for communities in mountain and high-latitude regions, yet it remains one of the least understood processes in hydrology.
Maina & Kumar (2025) present the first global overview of ROS, drawing on NASA’s Land Information System and model scenarios from CMIP6. They examine how ROS has evolved historically (1950–2014) and how it may change under a range of future projections extending to 2100.
Their findings show that:
- ROS will increasingly occur in high-latitude and high-altitude regions such as High Mountain Asia, the European Alps, and Northern Eurasia.
- Despite becoming more frequent, ROS will contribute less to water available for runoff because snowpack is reduced and more precipitation falls as rain.
- By century’s end, rainfall-driven floods are projected to overshadow those triggered by ROS, especially in regions like the western United States where snowpack is declining.
The study validates simulations against satellite and reanalysis products (IMERG, MERRA-2, ERA5-Land), underscoring the role of remote sensing in constraining and verifying large-scale hydrologic projections. As one of the 23 unsolved problems in hydrology, understanding ROS requires such integration of modeling and remote sensing to track how shifts in snow, rain, and runoff interact across regions.
Co-authors: Fadji Z. Maina & Sujay V. Kumar