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The Cryosphere An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 21 Jan 2019

Research article | 21 Jan 2019

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal The Cryosphere (TC).

Multisensor validation of tidewater glacier flow fields derived from SAR intensity tracking

Christoph Rohner, David Small, Daniel Henke, Martin P. Lüthi, and Andreas Vieli Christoph Rohner et al.
  • Department of Geography, University of Zurich, 8057 Zurich, Switzerland

Abstract. Following the general warming trend in Greenland, an increase in calving rates, retreat and ice flow has been observed at ocean-terminating outlet glaciers. These changes contribute substantially to the current mass loss of the Greenland Ice Sheet. In order to constrain models of ice dynamics as well as estimates of mass change, detailed knowledge of geometry and ice-flow are needed, in particular on the rapidly changing tongues of ocean-terminating outlet glaciers. In this study, we validate velocity estimates and spatial patterns close to the calving terminus of such an outlet derived from an iterative offset tracking method based on SAR intensity data with a collection of three independent reference measurements of glacier flow. These reference data sets are comprised of measurements from differential GPS, a Terrestrial Radar Interferometer (TRI) and repeated UAV surveys. Our approach for the SAR-velocity processing aims achieving at high spatial and temporal resolution in order to best resolve the steep velocity gradients in the terminus area and to exploit the 12 day repeat interval of the single-satellite Sentinel-1A sensor. Results from images of the medium-sized ocean terminating outlet glacier Eqip Sermia acquired by Sentinel-1A and RADARSAT-2 exhibit a mean difference of 8.7 % when compared to the corresponding GPS measurements. An areal comparison of our SAR velocity-fields with independently generated velocity maps from TRI and UAV showed a good agreement in magnitude and spatial patterns, with mean differences smaller than 0.7 md−1. In comparison with existing operational velocity products, our SAR-derived velocities showed a strongly improved spatial velocity pattern near the margins and calving front. There 10 % to 20 % higher surface ice velocities are produced, which has substantial implications on ice fluxes and on mass budget estimates of ice sheets. Further, we showed that offset tracking from SAR intensity data at a high spatio-temporal resolution is a valid method to derive glacier flow fields for fast-flowing glacier termini of outlet glaciers and, given the repeat interval of 12 days of the Sentinel-1A sensor (6 days with Sentinel-1B), has the potential to be applied operationally in a quasi-continuous mode.

Christoph Rohner et al.
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Christoph Rohner et al.
Christoph Rohner et al.
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Publications Copernicus
Short summary
The recent increase in ice flow and calving rates of ocean-terminating glaciers contributes substantially to the mass loss of the Greenland ice sheet. Using in situ reference observations, we validate the satellite-based method of iterative offset tracking of Sentinel-1A data for deriving flow-speeds. Our investigations highlight the importance of spatial resolution near the fast-flowing calving front, resulting in significantly higher ice velocities compared to large scale operational products.
The recent increase in ice flow and calving rates of ocean-terminating glaciers contributes...