Kirill Khvorostovsky

Merging and Analysis of Elevation Time Series Over Greenland Ice Sheet From Satellite Radar Altimetry

Spatial-temporal variability and changes of Greenland ice sheet elevation from 1992 to 2008 are analyzed from merged ERS-1, ERS-2, and Envisat satellite radar altimeter data. A methodology for determining intersatellite biases was developed and applied in order to merge measurements from these different satellites and to create continuous and consistent time series. Intersatellite biases of elevation and backscatter coefficient have shown to be significantly affected by the bias between measurements in ascending and descending orbits. Adjustment of elevation time series for its dependence on backscatter coefficient and other waveform parameters performed in this paper substantially reduced the amplitude of elevation seasonal variations and locally corrected elevation change-rate estimates by up to several centimeters per year. It was found that the correction depends not only on the variations in the waveform parameters but also on the temporal variations of the correlation gradients, which represent the sensitivity of the elevation change to the change in the waveform parameters. An elevation change rate of +2.8 ±0.2 cm/year from 1992 to 2008 over 76% of the Greenland ice sheet area was found. Increases in surface elevation from 1995 observed over the high-elevation regions of Greenland were followed by an elevation decrease from 2006. For the whole period of 1992-2008, the elevation increase is 4.0 ± 0.2 cm/year over 87% of the area above 1500 m. In contrast, over 38% of the low-elevation areas below 1500 m, the rate of elevation change is -7.0 ±1.0 cm/year, and the surface elevation decrease that started from 2000 has continued.

ESA ice sheet CCI: derivation of the optimal method for surface elevation change detection of the Greenland ice sheet – round robin results

For more than two decades, radar altimetry missions have provided continuous elevation estimates of the Greenland ice sheet (GrIS). Here, we propose a method for using such data to estimate ice-sheet-wide surface elevation changes (SECs). The final data set will be based on observations acquired from the European Space Agency’s Environmental Satellite (ENVISAT), European Remote Sensing (ERS)-1 and -2, CryoSat-2, and, in the longer term, Sentinel-3 satellites. In order to find the best-performing method, an intercomparison exercise has been carried out in which the scientific community was asked to provide their best SEC estimates as well as feedback sheets describing the applied method. Due to the hitherto few radar-based SEC analyses as well as the higher accuracy of laser data, the participants were asked to use either ENVISAT radar or ICESat (Ice, Cloud, and land Elevation Satellite) laser altimetry over the Jakobshavn Isbræ drainage basin. The submissions were validated against airborne laser-scanner data, and intercomparisons were carried out to analyse the potential of the applied methods and to find whether the two altimeters were capable of resolving the same signal. The analyses found great potential of the applied repeat-track and cross-over techniques, and, for the first time over Greenland, that repeat-track analyses from radar altimetry agreed well with laser data. Since topography-related errors can be neglected in cross-over analyses, it is expected that the most accurate, ice-sheet-wide SEC estimates are obtained by combining the cross-over and repeat-track techniques. It is thus possible to exploit the high accuracy of the former and the large spatial data coverage of the latter. Based on CryoSat’s different operation modes, and the increased spatial and temporal data coverage, this shows good potential for a future inclusion of CryoSat-2 and Sentinel-3 data to continuously obtain accurate SEC estimates both in the interior and margin ice sheet.