Altimeter data from the Geosat Exact Repeat Mission (ERM), which extended from fall 1986 through fall 1989, and Seasat, which extended from midsummer through early fall 1978, were employed to measure mean changes in elevation on lower Lambert Glacier, the floating Amery Ice Shelf, and the adjacent East Antarctic Ice Sheet. This region, which is approximately south of India, is bounded by Lat. 68_ to 72_ S, Long. 66_ to 75_ E. Lambert Glacier, which is the largest ice stream in East Antarctica, is about 90 km wide in its lower reaches by 400 km long. It discharges ice from an accumulation area of about 900,000 km2.
Mean changes in elevation were measured on a same-season basis between the three-month Seasat time frame—austral late-winter 1978—and the same time frame during each of the Geosat ERM years, to minimize the effects of seasonal changes in penetration depth. The altimeter-derived height measurements from both satellites, which were retracked at NASA Goddard Space Flight Center (GSFC), were referenced to Goddard Earth Model (GEM) T2 orbits. The elevation changes were measured via the method of orbit crossover analysis, using altimetry with random noise levels reduced by low-pass filtering, and also using unfiltered altimetry. Apparent orbit bias between the two satellites, combined with systematic vertical error from other sources, was estimated using crossover differences from a region of austral late-winter sea ice seaward of the calving front of Amery Ice Shelf. The apparent Seasat-Geosat ERM orbit bias was found to be negligible in the case of the low-pass filtered altimeter data (-3 + 120 mm), but more significant in the case of the unfiltered data (80 ± 130 mm). (Positive bias implies the Geosat surface was found to be systematically higher.) The crossover results obtained using unfiltered altimetry were adjusted for this apparent orbit bias. The effects of crossover locations where the altimeters may have been measuring to two different points on the surface (perhaps the crests of two different undulations) were suppressed by using only crossover differences with absolute value _ 5m. Error ranges for the results were computed using variogram methods to estimate the spatially-varying noise levels in the data, which were then propagated through the crossover computations.
On lower Lambert Glacier throughout an area including the grounding zone, extending from Lat. 72.1_ to 70.4_ S (about 190 km in the along-flow direction), the mean rate of increase of the surface height was found to be 30 + 10 mm/yr during the decade from 1978 to 1987-'89, using low-pass filtered altimetry. The mean rate of increase was found to be 40 + 50 mm/yr using unfiltered altimetry. On the floating Amery Ice Shelf, the mean rate of increase of the surface height, measured using low-pass filtered altimetry, was found to be 30 + 5 mm/yr during the same time period. The mean rate of increase was found to be 40 + 20 mm/yr using unfiltered altimetry.
Throughout the entire 309 km east-west by 444 km north-south (137,200 km2) region, including the lower Lambert Glacier-Amery Ice Shelf system and the more steeply-sloping grounded ice sheet on both sides, the surface elevations were found to be increasing at 40 + 3 mm/yr during the same time period, using low-pass filtered altimetry. The mean rate of increase was found to be 60 + 20 mm/yr, using unfiltered altimetry.
The results suggest that the East Antarctic Ice Sheet was characterized by a positive mass balance is this region during the decade from 1978 to 1987-'89.
NASA Grant NAGW-2614: Scientific personnel; C.S. Lingle, L.-H. Lee, and V.A. Voronina; H.J. Zwally (NASA Goddard Space Flight Center) and T.C. Seiss (Hughes STX Corporation)