The effectiveness of spaceborne synthetic aperture radar for glacier monitoring
This work examines the effectiveness of spaceborne synthetic aperture radar (SAR) for investigating seasonally variable glaciological parameters, in particular its ability to discriminate glacier surface facies in order to estimate glacier mass balance. A multitemporal C-band SAR dataset of Nordenskiold Land, Spitsbergen, acquired by the ERS-1 satellite, is used for the analysis, which focuses on mountain glaciers rather than ice sheets. Validating field measurements of ice and snowpack parameters were obtained contemporaneously with two SAR images, prior to and during the ablation season. A general model for the annual backscatter cycle from a sub-polar glacier is derived from SAR data of three glacierised areas. This model reveals two seasonal reversals in the relative magnitude of backscatter from the ice and wet-snow facies, principally through a 10 dB change in the latter; these reversals mark the start and end of the ablation season. It is shown that a combination of winter and summer SAR imagery is necessary to estimate the equilibriumline altitude of a sub-polar glacier. Topographic distortion is the major limiting factor regarding the utilisation of SAR data for studying mountainous glaciers. Existing theoretical models of radar backscatter from snow and ice are validated for three scenarios: glacier ice, dry snow overlying glacier ice, and wet snow, using the in situ measurements. In addition, temporal variations of ice and snowpack parameters observed during the field campaigns are used to predict short-term seasonal changes in backscatter, and to corroborate the model of annual backscatter. ERS-1 SAR data are compared to NIR Landsat TM data in separate analyses of data information content and temporal resolution; the optical data are found to be better for both facies discrimination and obtaining synoptic glaciological information in mountainous regions. However, the Spitsbergen cloud cover is such that useful TM data may not necessarily be acquired in a given year; consequently SAR is the better sensor for obtaining guaranteed synoptic mass balance data for use in climate change studies, or for studying short-term events like glacier surges. These conclusions are shown to apply to the entire European Arctic sector except East Greenland, where the two sensors have similar temporal resolutions. Data from both sensors were integrated to provide an estimation of the synoptic mass balance of Nordenskiold Land for 1991/92; the results, which indicate an overall slightly negative mass balance, demonstrate that elevation is the principal factor governing glacier net mass balance in the region.