Speaker
Description
The geodetic mass balance is a method to describe the development of glaciers or ice sheets without the need for modelling of precipitation and melting. The measurement is based on changes in surface elevation that occur over the entire area attributed to a single glacier or sector of an ice sheet. Most recent geodetic mass balance measurements on ice sheets have been performed with laser and radar altimeters. These sensors offer the advantage of a coarse, but complete coverage over the vast regions of ice sheets with a high number of repeating acquisitions. However, spaceborne altimeters have a worse performance in areas with steeper slopes at the margins of the ice sheet than over the flat interior.
The TanDEM-X mission (TDM) provides a different possibility to obtain temporally spaced elevation measurements. With the twin satellite constellation we are able to generate DEMs for a region of interest from bistatic InSAR acquisitions. Due to the swath width of approx. 30 km there are only a limited number of scenes available for each location. Especially for the flat ice sheet interior, the acquisitions from 2010/2011 and 2011/2012 are too close in time to capture surface elevation change (SEC) within the accuracy of the instrument. However, during the effort to create a global high resolution DEM with TDM, additional coverages of the margin of the Greenland Ice Sheet have been acquired in 2013/2014.
We combine the SEC measured with the SIRAL instrument on the Cryosat-2 (CS-2) radar altimeter together with DEM differencing of InSAR DEMs from TanDEM-X in the time period of winter 2010/11 to winter 2013/2014. The sensors complement each other in the way that TDM measurements are used at regions of interest at the margin of the Northeast Greenland Ice Sheet. The SAR mode of CS-2 is utilised for the remaining area. This ensures high resolution maps of SEC where dynamic processes are expected on the glacier termini. At the same time, a complete coverage of entire drainage basins or sectors can be achieved. Additionally, the signal penetration at the dry snow zone of the ice sheet interior is kept minimal by using retracking procedures during the CS-2 processing.
To identify the area over which SEC is integrated to calculate volume change, a method has been developed to generate individual drainage basins for outlet glaciers. This approach employs a modified watershed algorithm, that uses the TDM global DEM as source of slope information and takes ice surface flow directions into account for areas moving faster than 20 m/a.
