GEOGRAPHIC AND SEASONAL-VARIATIONS IN THE SURFACE-PROPERTIES OF THE ICE SHEETS BY SATELLITE-RADAR ALTIMETRY

Geosat-altimeter wave forms from the Greenland and Antarctic ice sheets are analyzed using an algorithm based upon a combined surface- and volume-scattering model. The results demonstrate that sub-surface volume-scattering occurs over major parts of the ice sheets. Quantitative estimates of geographic variations in the near-surface ice-sheet properties are derived by retracking individual altimeter wave forms. The derived surface properties correlate with elevation, latitude and microwave brightness-temperature data. Specifically, the extinction coefficient of snow obtained by this method varies from 0.48 to 0. 13 m-1 over the latitudes from 65-degrees to 72-degrees-N on the central part of the Greenland ice sheet and from 0.20 to 0.10 m-1 over a section of Wilkes Land in East Antarctica where the elevation increases from 2550 to 3150m. Analysis of a passive-microwave data over East Antarctica shows that the brightness temperature increases with elevation as the extinction coefficient decreases. Larger snow grain-sizes occur at lower elevations of the ice sheet because of higher mean annual temperatures. The larger grain-sizes increase the extinction coefficient of snow and decrease the emitted energy (brightness temperature) from greater snow depths. The passive-microwave data are also used to determine the average number of melt d year-1 (1979-87) for the central part of the Greenland ice sheet. For latitudes from 65-degrees to 68.5-degrees-N, the average number of melt days decreases from 3.5 to 0.25 d year-1, whereas no melt events are observed for latitudes above 69-degrees-N over the 8 year period. Snow subjected to alternate melting and freezing has enhanced grain-sizes compared to that of dry snow. This accounts for the larger values and larger spatial variations of k(e) on the Greenland ice sheet compared to East Antarctica, where surface temperatures are never high enough to cause surface melting.