Ch. Davis et Hj. Zwally, GEOGRAPHIC AND SEASONAL-VARIATIONS IN THE SURFACE-PROPERTIES OF THE ICE SHEETS BY SATELLITE-RADAR ALTIMETRY, Journal of Glaciology, 39(133), 1993, pp. 687-697
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.