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.