RETREAT: A new freely available data set of Sentinel-1 glacier velocities in regions outside the polar ice sheets

Climate induced glacier change has important implications for global sea level rise, freshwater availability and geomorphologic hazards. Changes in ice dynamics and mass flow can globally be observed by long- and short-term changes in ice surface velocity. Consistent and continuous data on glacier surface velocity are important inputs to time series analyses, numerical ice dynamic modelling and glacier mass balance calculations. Therefore, glacier surface velocities have been identified as an Essential Climate Variable (ECV) that should be monitored on a regular and global scale. Since 2014, repeat-pass Synthetic Aperture Radar (SAR) data, acquired by the Sentinel-1 constellation as part of ESA’s (European Space Agency) Copernicus program, enable global, near real time-like and fully automatic processing of glacier velocity fields at up to 6-day temporal resolution, independent of weather conditions, season and daylight.

We present a new near-global data set of Sentinel-1 glacier velocities that comprises continuously updated image pair velocity fields, as well as monthly and annually averaged velocity mosaics at 200 m spatial resolution, derived from applying intensity feature tracking on both archived and new acquisitions. The data set covers all major glaciated regions outside the polar ice sheets and is generated in an HPC (High Performance Computing) environment at the University of Erlangen-Nuremberg. By the beginning of January 2021, we processed more than 110.000 Sentinel-1 scenes, amounting to roughly 450 TB of data. The velocity products are freely accessible via an interactive web portal (http://retreat.geographie.uni-erlangen.de) that provides capabilities for download and simple online analyses. We give information on the procedures of data generation, as well as on how to access the data and demonstrate the capabilities of our products for velocity time series analyses at very high temporal resolution. We compare our data to velocity products generated from very high resolution TerraSAR-X SAR (Synthetic Aperture Radar) and Landsat-8 optical (ITS_LIVE, GoLIVE) data. For this comparison we selected Svalbard as an example region, as it includes glaciers of a broad variety of sizes, different velocitiy magnitudes and seasonal velocity patterns, as well as very fast flowing surging glaciers and almost featureless ice caps.