Mass Balances of the Greenland and Antarctic Ice Sheets from Satellite Radar Altimetry
Jay Zwally(1) , Anita Brenner(2) , Helen Cornejo(3) , and Donghui Yi(3)
NASA Goddard Space Flight Center,
Greenbelt, MD 20771,
(2) Science Systems & Applications, Inc., 10210 Greenbelt Rd, Lanham, MD 20706, United States
(3) SGT, Inc., 7701 Greenbelt Road,, Greenbelt, MD 20770 , United States
The Earth’s ice sheets and ice shelves are of special interest because of the uncertainty in their current mass balance, the direct effect of changes in ice volume on sea level change, and the potential for significant ice changes with climate warming. Since the 1980's, measurement of surface elevation changes (dH/dt) by satellite altimeters and airborne laser surveys has been pursued as a means of directly measuring changes in ice volume and therefore the mass balance. Ice sheet surface elevations have been measured by radar altimeters on Geos-3, Seasat, Geosat, Geosat follow-on, ERS-1 and 2, ENVISAT, and CryoSat. The characteristics of the snow-ice surface that are different from ocean surfaces and significantly affect the elevation measurements include: irregular topography (mean-slope and undulations), variability of surface reflectivity and transmissivity, and variability of sub-surface backscatter. For analysis of elevation data from a single-mission or nearly-identical missions (e.g. ERS-1 and 2), these characteristics are sufficiently well understood to enable construction of reliable time-series of elevation changes H(t). This includes the application of empirically-determined corrections for the dependence of the measured height on the back-scattered signal strength (spatially and temporally variant) and inter-mode and inter-satellite height biases (spatially variant), even thought the physical causes of the dependence and biases are not well understood. The most reliable time-series have been constructed for the approximately 11 years of altimeter data from ERS-1 and 2, and new time-series are being constructed from ENVISAT data. However, the effects of the characteristics of the snow-ice surface on the radar signal and the elevation measurement are similar to or are interactive with instrument and satellite properties such as antenna-beam width, range resolution, microwave frequency, tracking parameters, and orbital height. Therefore, most if not all attempts to derive elevation changes by comparing elevations obtained from different missions (other than ERS-1 and 2) have not been successful. Separate H(t) time-series constructed from ENVISAT data and from ERS-1 and 2 data are evaluated for continuity in elevation change properties between the successive missions.