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 sheet s are analyzed using an algorithm based upon a combined surface- and v olume-scattering model. The results demonstrate that sub-surface volum e-scattering occurs over major parts of the ice sheets. Quantitative e stimates of geographic variations in the near-surface ice-sheet proper ties are derived by retracking individual altimeter wave forms. The de rived surface properties correlate with elevation, latitude and microw ave brightness-temperature data. Specifically, the extinction coeffici ent 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 Wi lkes Land in East Antarctica where the elevation increases from 2550 t o 3150m. Analysis of a passive-microwave data over East Antarctica sho ws that the brightness temperature increases with elevation as the ext inction 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 a nd decrease the emitted energy (brightness temperature) from greater s now 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, w hereas no melt events are observed for latitudes above 69-degrees-N ov er 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 Gre enland ice sheet compared to East Antarctica, where surface temperatur es are never high enough to cause surface melting.