USING A MULTI-YEAR DATA ARCHIVE OF ERS SAR IMAGERY FOR THE MONITORING OF FIRN LINE POSITIONS AND ABLATION PATTERNS ON THE KING GEORGE ISLAND ICE CAP (ANTARCTICA)

Multi-year SAR data is used to study the seasonal dynamic of the snow melt patterns on the ice cap of King George Island, Antarctica. The snow cover of the entire ice cap becomes wet during summer months, however, in the highest elevations frequent refreeze and melt cycles are observed. Bare ice areas form in the lowest elevations of the ice cap. These findings are in good agreements with energy balance studies and results of a ground penetrating radar survey. Backscatter values in intermediate elevations show a marked rise at the end of the ablation season. This fact is attributed to an increasing surface roughness on a wet metamorphosed snow cover at the end of the ablation season. The influence of thresholds on the determination of the extent of the bare ice radar zone in summer and winter imagery as well as in ascending and descending orbit is investigated. As a consequence, the border of the bare ice radar zone have been mapped using thresholds of –14 dB in summer and –6 dB in winter imagery. It could be shown, that thresholds of –12 dB and –8 dB, respectively, do not change significantly the extent of this radar glacier zone. Firn line altitudes were mapped for several consecutive years. A low inter-annual variation of this line with elevations of about 200 m a.s. l. was observed. The maximum altitude of the firn line was recorded in the mass balance year 1996/97with about 250 m a.s. l. Comparing these values to equilibrium line positions of previous measurements indicate an upward shift in the equilibrium line altitude since the 1970’s. INTRODUCTION The Antarctic Peninsula and the adjacent South Shetland Islands have been identified as a region with a significant warming trend (e.g. King 1994, Smith et al. 1996, Skvarca et al. 1998). On the northern Antarctic Peninsula and the South Shetland Islands, snow and ice melt are important mass balance parameters of the small glaciers and ice caps (Bintanja 1995, Braun and Schneider 2000, Braun et al. 2001-a). Moreover, melt water has been identified as a probable source for the destabilisation and spectacular disintegration of the ice shelves in that region (Hulbe 1997). On King George Island itself, extended glacier retreat has been observed by several authors (e.g. Braun and Goßmann in press, Park et al. 1998, Simões et al. 1998, Wunderle 1996) and attributed to changes in glacier mass balance. Therefore, more detailed spatially distributed information on glacier mass balance parameters such as ablation patterns and firn line positions are required, a) as input and verification parameters for glacier melt models, b) to improve the monitoring of short-term glacier mass balance changes, c) to increase the accuracy of long-term predictions of glacier mass balance changes. Due to the operation of the German Antarctic Receiving Station at the Chilean base O’Higgins, it is now possible to use a multi-year data record of ERS-1/2 SAR imagery for glaciological studies on the Antarctic Peninsula. This time series, starting in 1991, enables the monitoring of the seasonal evolution and inter-annual variation of ablation patterns on the Antarctic ice cap of King George Island as already demonstrated for single years by Braun et al. (2000) and Wunderle (1996). Proceedings of EARSeL-SIG-Workshop Land Ice and Snow, Dresden/FRG, June 16 – 17, 2000 EARSeL eProceedings No. 1 282 In this study, we utilize ERS SAR data from King George Island to complement glacier melt modelling based on micro-meteorological ground observations, to observe inter-annual variations of the snow cover dynamics and firn line positions on the ice cap and to support the interpretation of a comprehensive ground penetrating radar survey (Pfender 1999). STUDY SITE With about 1250 km2, King George Island is the largest of the South Shetland Islands. 93 % of the island are ice covered. The islands major ice cap ranges between sea level and about 700 m a.s. l. (Figure 1). More than 30 percent of the island is located in an elevation range below 250 m a.s. l. Low gradient slopes prevail on the north-western part of King George Island, whereas steep fjordlike inlets intersect the smooth surface morphology of the ice cap on the southern side. A detailed outline of the island’s topography can be found in Simões et al. (1999) and Braun et al. (2001-b). Due to its location in the southern hemisphere west-wind zone, King George Island is subject to a highly maritime climate. The mean annual air temperature (1944-1999) at Bellingshausen Station is – 2.9 °C. Positive air temperatures are recorded throughout all summer months at lower elevations. Snow melt events also occur frequently during winter (Rachlewicz 1997). Generally, advection of warm humid air masses from northerly directions lead to the highest snowmelt rates (Braun et al. 2001-a). Figure 1: Sketch map of King George Island. Locations of the areas of interest (AOIs) are indicated with numbers As a consequence of the smooth surface morphology, relief-induced distortions in the SAR imagery are small on the north-western part of the ice cap. Furthermore, changes in the size of the ablation area using satellite remote sensing are more easily detected on moderately inclined surfaces than on steep slopes (Bindschadler 1998). Therefore, the present study will focus on this area of King George Island. Proceedings of EARSeL-SIG-Workshop Land Ice and Snow, Dresden/FRG, June 16 – 17, 2000 EARSeL eProceedings No. 1 283 DATABASE AND DATA PROCESSING For the present investigation, 40 ERS-1/2 scenes were analysed using data from both ascending and descending orbits. The imagery covers the time period between July 1992 and November 1999 (Figure 2). Data preparation was realized using processing chains in the ESA SAR Toolbox. From all images backscatter values were calculated using the algorithm based on Laur et al. (1998). In this procedure a compensation for losses during the analogue-digital conversion and a correction for the replica power variation was included. Subsequently, the images were co-registered using a master image from 18 February 1998 for the descending and an image from 15 July 1997 for the ascending orbit. 0.0 3.0 6.0 9.0 12.0 15.0 18.0 Number of ERS-1/2 SAR images 1991/92