Microwave snowpack studies made in the Austrian Alps during the SIR-C/X-SAR experiment

Abstract

Surface-based microwave studies made in the Austrian Alps during the Spaceborne Imaging Radar-C/X-Band Synthetic Aperture Radar (SIR-C/XSAR) experiment in April 1994 are described, and are compared with the SAR data. The SAR test region included glaciers and unglaciated surfaces which were snow covered. Clear differences between these two surface types were observed with the SIR-C/X-SAR. In addition, the angular dependencies of the backscattering were useful in order to distinguish between the accumulation and the ablation areas of the glaciers, and the snow-covered areas showed characteristic differences depending on the presence or absence of layers containing wet snow. The surface-based studies were carried out at an alpine meadow in the valley Kaunertal at an altitude of 2160 m. The measurements included snow-physical observations supported by microwave dielectric probes, microwave radiometers at 21 and 35GHz and polarimetric scatterometers at 5.3 and 35 GHz. The dielectric measurements were used to retrieve the liquid-water profile of the snowpack. Three main layers were observed: a wet bottom layer as a remnant of a warm period in March, a thick crust (i.e., the refrozen part of the old pack), and a fresh and mostly dry snow layer on top. Except for the top layer, the temporal changes were small during the observation period from 8 to 12 April 1994. The radiometric measurements made at 21 and 35GHz showed pronounced temporal variations of the brightness temperature in the afternoons due to melt-and-refreeze cycles in the fresh snow. The liquid water in the top layer was monitored by a special radiometric method using a metal plate at the bottom of this layer. The radiometer and scatterometer measurements revealed the refrozen crust as the dominant scatterer, especially at 35 GHz. However, the wet bottom layer inhibited the detection of signals scattered at the snow-ground interface and therefore reduced the backscatter at 5.3 GHz. The comparison of the C-band SAR data with those of the C-band scatterometer shows higher returns in the SAR data for the considered test site. Explanations for the differences are discussed. Nevertheless, also the SAR backscatter data are clearly lower in the zones where wet snow was present than in areas either covered by dry snow or which were snow-free.