Antarctic snow reflectance from ADEOS/POLDER measurements

The Antarctic ice sheet is very important in climatological studies. Its surface energy budget is dominated by radiation. Special attention has always been given to measurement of the snow albedo because it is in general highly variable except inland of the Antarctic Plateau where snow always remains well below freezing and metamorphism proceeds very slowly. Nevertheless, the albedo (integrated value of the hemispherical spectral reflectance values over of the solar spectrum [0.2/spl mu/m; 4.0 /spl mu/m]) is very high and ranges from 0.8 to 0.9 in summer, implying a 40% variation in absorbed shortwave radiation flux. A flat and smooth snow surface is not a lambertian reflector. The snow scatters the energy more likely in the forward direction. Snow albedo may be retrieved from bidirectional reflectance satellite measurements made at discrete wavelengths only if a snow bidirectional distribution of reflectance function (BRDF) model is known. Then, the model will give the spectral albedo values at many wavelengths within the solar spectrum. The albedo is then derived from the spectral integration of these values. The french remote-sensing instrument POLDER (POLarization and Directionality of the Earth's Reflectances) is very useful to derive precisely BRDFs because it acquires up to 14 sequences of any given Earth location at different viewing angles along a single path.