Temporal change in the extinction coefficient of snow on the Greenland ice sheet from an analysis of Seasat and Geosat altimeter data

The extinction coefficient of snow k_e along the central portion of the Greenland ice sheet is mapped using data from the Seasat (1978) and Geosat (1985-1989) altimeters. The extinction coefficient is obtained by fitting altimeter waveforms with a surface/volume scattering model. The authors find that in the lower latitudes the Seasat and Geosat extinction coefficients are very nearly the same, while in a specific higher latitude region of the ice sheet the Seasat k_e values exceed the Geosat values by over 100%. By analyzing 18 months of the Geosat data, the author quantified the variability inherent in the extinction coefficient measurements. The results show that the observed temporal variation in the extinction coefficient from 1978 to 1985 is three times larger than the measured variability. This indicates that the average grain size of the near surface snow in this region may have decreased during the time span between the two altimeter datasets. The temporal change in extinction coefficient found in this study demonstrates the important contributions that time-series analysis of satellite datasets can make to the study of the polar ice sheets. In addition, these results have important implications for the study of long-term elevation change over the ice sheets using altimeter data. The author´s study demonstrates that significant biases could be introduced into ice-sheet elevation change estimates because of temporal variations in the surface conditions of the ice sheet. Future investigations of ice-sheet mass balance using altimetry data should be aware of this possibility