SAR firn line detection and correlation to glacial mass balance; Svartisen Glacier, northern Norway

The glacial firn line of the Svartisen Glacier has been detected using ERS II SAR and ENVISAT ASAR data from 1995 and up to today. The firn line is detected by first correcting the image backscatter intensity for topographic and geometric contributions using the Muhleman backscattering model. Then we discriminate between firn and ice facies based on the backscatter intensity since frozen firn has a much higher backscatter than ice. Transects across different areas of the glacier were chosen based on requirements of smoothness of topography, precipitation zones, as well as the availability of field data for validation and comparison. From having quite stable conditions during the nineties we have observed a substantial retreat of the firn line over the last few years. The equilibrium line derived from field measurements show a similar trend as the firn line changes, but has a much larger year to year variability. This indicates that the firn line may be a better indicator of climate change than the equilibrium line due to the smaller variance