Conclusion

Fusing Sentinel-1 SAR with Sentinel-2 optical observations and thermal data delivers a robust, cloud-resilient view of alpine snow that neither sensor alone can provide. The Hohe Tauern time series over 2018–2025 confirms a systematically warming accumulation season, a downslope shift of the Dry Snow domain, and an earlier, more extensive wet-snow phase in the mid-elevation belt — an important signal for downstream water resources. Sentinel-2 achieved AUC = 0.98 for snow discrimination, and the PCA-based fusion enabled effective wet/dry separation using Sentinel-1 backscatter thresholds. Precipitation emerges as the master variable controlling wet snow distribution in the dominant 2000–3000 m band, while temperature controls dry-snow extent at lower elevations. The Google Earth Engine workflow developed here is transferable to other alpine catchments.