RADAR GLACIER ZONES AND THEIR BOUNDARIES AS INDICATORS OF GLACIER MASS BALANCE AND CLIMATIC VARIABILITY

Abstract

The measurement of glaciers mass balance and the derivation of meteorologica l parameters by means of remote sensing techniques are of particular scientific interest for glaciological and climatological studies. Using synergistic approaches, SAR-data complement field studies in remote and inaccessible areas and provide information with a high temporal and spatial resolution. However, the operational application of the existing SAR-archives for monitoring purposes require a more detailed understanding of the spatial and temporal evolution of the observable radar glacier zones. Backscatter modelling based on snow pit data gathered on the Antarctic Peninsula is used to investigate typical radar returns from the upper and lower boundary lines of the frozen percolation zone. The lower one is a clearly identifiable limit between bare ice and highly metamorphosed, coarsegrained snow. As it is impossible to identify the year of origin of this snowpack, the term firn line is suggested. Only occasionally the late-summer firn line position coincides with the equilibrium line. In the absence of field measurements, firn line altitude can be regarded as a first approximation to the ELA, although it might lead to an overestimation of glaciers mass balance. The upper boundary, the dry snow line, is described as a temporarily and spatially very persistent feature in SARimagery. Its position is very stable in the time range of years to decades. Positional shifts of this line provide information about the occurrence of singular high temperature events leading to enhanced metamorphism of the snow and accumulation conditions in the highest areas of the glaciers. Additionally, model results confirm the assumption that a threshold of –8 dB is appropriate to delimit the upper boundary line of the frozen percolation radar zone. This facilitates direct monitoring of the dry snow line.