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
$+2.8 \pm 0.2\ \hbox{cm/year}$
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
$\pm$
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
$-7.0 \pm 1.0\ \hbox{cm/year}$
, and the surface elevation decrease that started from 2000 has continued.