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

Abstract

ABSTRACT. We present an analysis of many European Remote-sensing Satellite (ERS) synthetic aperture radar (SAR) images of Vatnajo« kull, Iceland, by comparing them with Advanced Very High Resolution Radiometer (AVHRR) images, massbalance 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 Vatnajo« kull 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 backscatter between melting snow and firn, and for most years no differences in reflectance either. ForVatnajo« kull, 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 Vatnajo« kull 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 .

The use of remote-sensing data for mass-balance studies at Mýrdalsjökull ice cap, Iceland

“…Bright areas within the otherwise dark wet-snow zone are also visible on other summer images, but there they do not disturb the course of the TSL-FL. De Ruyter de Wildt and Oerlemans (2003) found similar bright backscatter areas within the wetsnow zone on 5 out of 43 ERS-1/-2 SAR summer images of Vatnajö kull ice cap, located northeast of Mýrdalsjökull (see Fig. 1).…”

“…Nevertheless, the temporal and spatial evolution of the TSL, which varies each year, also contains information about the mass balance of a glacier. De Ruyter de Wildt and Oerlemans (2003) explain that the TSL or FL on a glacier separates the area of high albedo (snow/firn) and relatively low albedo (ice) which strongly determines the amount of surface melt and hence the mass balance. In addition, the evolution of the TSL reflects the accumulation history of the preceding winter.…”

“…Since the boundary between wet snow and bare ice on radar summer images is either the TSL or the FL, it is abbreviated to TSL-FL (first used by de Ruyter de Wildt and Oerlemans, 2003) in this study. Furthermore, the term 'radar glacier zones', introduced by Forster and others (1996) and also used by other authors (e.g.…”

“…Thus, the annual evolution of the transient snowline (TSL) and the firn line (FL), if exposed in summer, can be monitored (e.g. Rott and Mätzler, 1987;Kö nig and others, 2001;de Ruyter de Wildt and Oerlemans, 2003). Most of the preceding work relating radar backscatter to glacier surface and near-surface characteristics analyzed 0 backscatter images (dB values) in order to separate the glacier into different zones.…”

“…A few studies utilized a combination of optical and radar imagery for glacier investigations (e.g. Hall and others, 2000;de Ruyter de Wildt and Oerlemans, 2003).…”