Applications of ERS satellite radar altimetry in the Lambert Glacier-Amery Ice Shelf system, East Antarctica

The Lambert Glacier-Amery Ice Shelf system is a major component of the East Antarctic ice sheet. Few accurate elevation data exist for the system, and many of its important glaciological parameters remain uncertain, including the mass budget and location of the grounding zone. This study uses European Remote Sensing satellite (ERS) radar altimeter waveform data to expand knowledge of the system's structure and surface properties. Surface elevations are derived by retracking ERS-1/2 altimeter waveforms, making corrections for slope-induced error and tides, and are validated against insitu Global Positioning System (GPS) observations. Elevation differences, at the intersections of the ERS-1 ground tracks with the GPS survey, have a mean of 0.0 ± 0.1 m and RMS of 1.7 m, and are spatially correlated with topographic variations. Two Digital Elevation Models (DEMs) are constructed from the calibrated altimeter heights: one on a 1-km grid for the Amery Ice Shelf only (AIS-DEM), which exhibits unprecedented resolution, and another on a 5-km grid for the entire basin (LAS-DEM). The altimeter DEMs allow new insight into the glaciology of the system. The AIS-DEM is combined with measured ice thicknesses and a simple density model to derive a 'hydrostatic height anomaly' term (i.e., the excess height above that required for buoyancy). Where this term is close to zero indicates floating ice; this occurs over most of the ice shelf and up the trunk of the Lambert Glacier. Significantly positive values of the term occur around the, margins of the floating ice, identifying the location of the grounding zone. A region in the north-west of the shelf, where the ice is known to be afloat, displays anomalous values of the hydrostatic height anomaly term. This is an area of marine ice accretion, where the ice thickness sounder only detected the upper boundary of the accreted layer. These results reveal that the Amery Ice Shelf extends much further south than previously reported, and that the marine ice is up to 200 m thick, oriented along the ice flow direction and restricted to the west of the shelf. The LAS-DBM is used to define flowlines for the grounded ice within the drainage basin, where the average surface slope determines ice flow direction. These are combined with accumulation distributions to derive balance fluxes for the system. Comparison with flux measurements from a traverse program suggests that the interior of the system has a slightly positive mass budget. The DEMs also provide reference surfaces against which to monitor any future change. Near-surface snow parameters are estimated from the ERS waveform data using a surface arfd volume scattering model in the retracking. These indicate that surface scattering dominates the altimeter return power over most of the ice shelf, whilst volume scattering predominates elsewhere. Standing water is detected on the ice shelf by the occurrence of specular returns. This melt-water flowed along surface troughs observed in the AIS-DEM. Backscatter observations along a 3-day repeat track of ERS-1 showed variations in meltstream onset, extent and duration between the 1991-92 and 1993-94 summers. This study, using derived high-precision altimeter products, has quantified many parameters of the Lambert-Amery system that were previously unknown, and provided a reference against which to monitor any future change in the system. The new findings are consistent with observed in situ data and sub-ice shelf model results, and provide credence for the long-term use of altimetric data for ice sheet studies