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2018WCRPspon col July2018 01 1

HRott- Satellite techniques for observing global glacier mass balance


There is still considerable uncertainty about the global mean mass balance of glaciers and the regional pattern. This impairs the assessment of recent contributions to sea level rise from mountain glaciers and ice caps, and affects the estimates for future decrease in glacier volume for which large regional differences are expected. High resolution satellite sensors (radar interferometry – InSAR and optical stereo) are able to deliver spatially detailed observations of surface topography with near global access. These sensors offer excellent opportunities for reducing the uncertainty on mass balance of glaciers and ice caps world wide. The geodetic mass balance technique is applied, converting volume changes into net mass balance numbers. The retrieval and intercomparison of surface topography on glaciers from different sensor types requires compensation of various effects on the observed signal. Proper assessment of measurement uncertainty is also an issue which has not always been taken into account properly for the published data.

Main uncertainties in determining the net mass balance over a certain time interval with the geodetic method are due to (1) uncertainties and/or systematic errors in the topographic data; (2) uncertainties in converting volume changes into mass changes. For point (1) in case of radar interferometry (InSAR) a potential error source arises from radar penetration which varies with radar wavelength and with the propagation and scattering conditions of the snow and ice volume. On the other hand InSAR offers advantages over optical stereo sensors in regions of low optical contrast (bright snow surfaces), and the SAR acquisitions are not affected by clouds so that observations in regular time intervals are possible. Uncertainties related to point (2) are relevant if the vertical density profiles of snow and firn change.

In the presentation main issues of the techniques for retrieving glacier volume changes were reviewed. Particular attention was paid to the German TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurements) satellite of DLR which was launched in June 2010. Results of the analysis of glacier volume changes from InSAR and optical sensors are compared and the accuracy is assed in comparison with airborne and in situ data sources. The results confirm the high potential of TanDEM-X for regular and precise surveys of glacier surface topography, a potential which has so far been exploited only marginally due to focus on the acquisition of a global DEM.

Presented by Helmut Rott, Institut of Meteorology and Geophysics, University of Innsbruck, at the 9th CliC Scientific Steering Group meeting in Potsdam, Germany 2013.

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