Seasonal surface melt onset and firn freeze-up across the central Wrangell and St. Elias Mountains

Abstract. High-elevation alpine firn is increasingly influenced by surface melt and meltwater retention, yet the spatial extent and timing of these processes remain poorly quantified. Here we present spatially distributed estimates of seasonal surface melt onset and firn freeze-up across the central Wrangell and St. Elias Mountains using time series C-band Synthetic Aperture Radar data from the Sentinel-1 mission, 2015–2024. Melt onset and freeze-up are identified from characteristic changes in backscatter associated with the presence of liquid water in snow and firn. Seasonal melt is detected across nearly all elevations in the range. Melt onset broadly tracks the seasonal rise of the 0 °C isotherm up to ∼3,000 m a.s.l., while freeze-up shows pronounced delays relative to subfreezing air temperatures at mid-elevations, indicating widespread meltwater retention within the firn. Combining freeze-up timing, air temperature, and elevation, we classify firn water-retention regimes and find that dry firn is confined to the highest elevations, covering only 3 % of our area of interest. These results highlight the influence of meltwater on firn evolution in the Wrangell/St. Elias Mountains and demonstrate the utility of SAR for monitoring alpine glacier melt dynamics in data-sparse regions.