| Summary: | The synthetic-aperture radar-interferometry technique is used to detect the migration of the limit of tidal flexing, or hinge line, of the floating ice tongue of Petermann Gletscher, a major outlet glacier of north Greenland. The hinge line is detected automatically from differential interferograms using a model-fitting technique based on an elastic-beam theory. The statistical noise of the model fit is less than 3 mm, and the hinge line is mapped with a precision of 30 m. Following automatic registration of multidate image data to a precision of 5 m, hinge-line migration is subsequently detected with a precision of 40 m in the horizontal plane across the glacier width. The results show that the hinge line of Petermann Gletscher migrates back and forth with tide by ± 70 m, in excellent agreement with the migration calculated from ocean tides predicted by a tidal model combined with the glacier surface and basal slope measured by an ice-sounding radar. Superimposed on the short-term hinge-line migration due to tide, we detect a hinge-line retreat of 270 m in 3.87 years which varies across the glacier width by ± 120 m. The retreat suggests glacier thinning at a rate of 78 ± 35 cm ice a-1. Coincidentally, an analysis of ice-volume fluxes indicates that the hinge-line ice flux of Petermann Gletscher exceeds its balance flux by 0.88±1 km3ice a-1, which in turn implies glacier thinning at 83 ± 95 cm ice a-1in the glacier lower reaches. Both methods therefore suggest that Petermann Gletscher is currently losing mass to the ocean.
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