The Pan-European Yearly Snow Melt-Off Day Derived from Optical and Microwave Radiometer Data

Abstract

We describe the methodology for deriving yearly pixel-wise snow melt-off day maps from optical data-based FSC (Fractional Snow Cover) without conducting any interpolation for cloud-obscured pixels or otherwise missing data. The Copernicus CryoLand Pan-European FSC time series for 2001–2016 re-gridded to 0.1 ° serves as input for the production of 16 years of melt-off day maps for Europe. These maps are compared with passive microwave radiometer (MWR) melt retrievals. These independent datasets are evaluated against melt-off day derived from in situ snow depth (SD) time series observed at European weather stations. Our results show that the melt-off day derived from optical springtime FSC time series provides the best correlation with the snow melt-off day as indicated by in situ data. The obtained bias is 0.9 days, and RMSE is 13.1 days. For 85 % of the analyzed cases the differences are between $\pm 10$ days. Across Europe the MWR-based detection of melt-off day is less accurate, as the applied method performs the best for areas with sustained seasonal snow cover. Based on the time series 1980-2016 for MWR-based melt-off day, separately for boreal forests and tundra, we also found a clear trend towards earlier snow clearance: a decrease of melt-off day by as much as ~5 days per decade in boreal forests was observed.