Wet snow avalanche preconditions from Sentinel-1 multi-track composites

Information about snowpack at high spatio-temporal resolution is important for the timely identification of conditions favouring snow avalanche release. However, such information is often available only at specific instrumented locations. Spaceborne synthetic aperture radar (SAR) sensors can facilitate the acquisition of such information over large areas and in remote and challenging terrain. In this work, we evaluate the use of European Space Agency's (ESA) Copernicus Sentinel-1 (S1) SAR multi-track composites to monitor snowpack wetness evolution. We focus on a study area of 400 km2 around Davos, Switzerland, where comprehensive in-situ information are available for validation of remotely sensed snowpack conditions. We found statistically relevant anticorrelation between S1 SAR backscatter decrease in both polarisations and increase in modelled liquid water content (VV: −0.42, VH: −0.38) and modelled runoff (VV: −0.44, VH: −0.49). We calculate a wet snow ratio (0 referring to dry, 1 to fully wet snowpack conditions) relying on dual-polarisation S1 backscatter time series. By comparing our indicator against the SAR Wet Snow (SWS) products, openly available from the Copernicus Land Monitoring Service, we found clear benefit in terms of spatial performance. We also compare our wet snow ratio time series to a unique catalogue of snow avalanche, aiming to identify conditions that may precede an increase in wet snow avalanche activity. We found a clear transition from dry snow avalanche dominated to wet snow avalanche dominated conditions when the S1 derived wet snow ratio reaches values of 0.17, while at 0.35 only wet snow avalanche were reported. Our results suggest that, despite current limitations in spatial and temporal resolution, S1 multi-track composites may assist wide area evaluation of snowpack wetness conditions, and provide additional indicatots on the initiation of wet snow avalanche release. The continued operation of the S1 mission, together with the growing availability of additional spaceborne SAR platforms, will enable increasingly accurate characterisation of snowpack conditions related to snow avalanche release. As climate warming drives a projected shift towards a higher proportion of wet relative to dry snow avalanche activity, such capabilities will become increasingly critical for operational hazard assessment in alpine environments.