Satellite retrieval of mass balance: comparing SAR images with albedo images and in situ mass-balance observations

We present an analysis of many European Remote-sensing Satellite (ERS) synthetic aperture radar (SAR) images of Vatnajökull, Iceland, by comparing them with Advanced Very High Resolution Radiometer (AVHRR) images, mass-balance observations and modelled firn stratigraphy. Summer SAR and AVHRR images both detect the surface firn line (or the transient snow-line when it lies below the firn line) as a distinct boundary.Winter SAR images of Vatnajökull generally display the late-summer surface firn line, and often also a transition at a higher elevation that possibly reflects a subsurface continuation of the firn line. On some winter images the firn line is not adequately detected, possibly due to a shallow firn pack. We found no differences in back-scatter between melting snow and firn, and for most years no differences in reflectance either. For Vatnajökull, equilibrium-line altitude is therefore not a particularly useful estimator of the mean specific mass balance ( B m ). Unlike albedo images, SAR images do not display interannual variations of the signal within the accumulation area that are clearly related to B m . Hence, SAR images of Vatnajökull contain less information about B m than albedo images. However, for some drainage basins, mean transient snow-line/firn-line altitude during the melting season can be used to infer B m .