SAR-derived flow velocity and its link to glacier surface elevation change and mass balance

Modern remote sensing techniques, such as Synthetic Aperture Radar (SAR), can measure the direction and intensity of glacier flow. Yet the question remains as to what these measurements reveal about glaciers' adjustment to the warming climate. Here, we present a technique that addresses this question by linking the SAR-derived velocity measurements with the glacier elevation change and the specific mass balance (i.e. mass balance per unit area). The technique computes the speckle offset tracking results from the north, east and vertical flow displacement time series, with the vertical component further split into a Surface Parallel Flow (SPF) advection component due to the motion along a glacier surface slope and a non-Surface Parallel Flow (nSPF). The latter links the glacier surface elevation change with the specific mass balance and strain rates. We apply this technique to ascending and descending Sentinel-1 data to derive the four-dimensional flow displacement time series for glaciers in southeast Alaska during 2016–2019. Time series extracted for a few characteristic regions demonstrate remarkable temporal variability in flow velocities. The seasonal signal observed in the nSPF component is modeled using the Positive Degree Day model. This method can be used for computing either mass balance or glacier surface elevation change if one of these two parameters is known from external observations.