Ice and snow
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2017-06-07 09:00 - 2017-06-07 10:40
Chairs: Rott, Helmut - Hogg, Anna
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Paper 18 - Session title: Ice and snow
09:20 Interferometric mapping of ice motion and grounding lines with Sentinel-1a/b and other data
Rignot, Eric (1,2); Mouginot, Jeremie (1); Scheuchl, Bernd (1) 1: UC Irvine, United States of America; 2: Jet Propulsion Laboratory, Pasadena, CA.
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The largest uncertainty in future sea level rise arises from the potential rapid melting of the ice sheets in Greenland and Antarctica. Here, we employ Sentinel-1a/1b and other InSAR data to map ice motion in the polar regions and the line of grounding line of the glaciers since ERS-1 in the 1990s. The results are employed in combination with ice thickness data to build time series of ice discharge around the ice sheet periphery and detect temporal changes which have an impact on the total ice sheet mass balance. Detecting of grounding line migration is also conducted to signal areas of change and in particular of ice thinning due to ice flow acceleration. The results are employed to reconstruct and monitor ice sheet mass balance by comparing ice discharge with interior accumulation of snowfall from regional atmospheric climate models since the 1970s in Antarctica and 1950s in Greenland, and to detect ungrounding of major glaciers since the 1990s in both ice sheets. We will discuss recent advances enabled by data acquisitions from Sentinel-1a/b along Greenland and Antarctica, with highlights on major areas of change in East Antarctica and north Greenland. This work is funded by a grant from NASA.
[Authors] [ Overview programme] [ Keywords]
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Paper 116 - Session title: Ice and snow
09:00 Continuous monitoring of ice motion and discharge of Antarctic and Greenland outlet glaciers by Sentinel-1A and 1B
Nagler, Thomas; Hetzenecker, Markus; Wuite, Jan; Rott, Helmut ENVEO IT GmbH, Austria
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The Sentinel-1A and 1B satellite constellation offers excellent opportunities for operational monitoring of the Earth’s surface, including ice sheets and their outlet glaciers. The Interferometric Wide Swath (IWS) mode of Sentinel-1 is the standard operation mode over land surfaces and inland ice. Applying Terrain Observation by Progressive Scans (TOPS) acquisition technology, it provides spatial resolution of about 3 m and 22 m in slant range and azimuth, respectively, with a swath width of 250 km. With these powerful imaging capabilities, in combination with a coordinated acquisition strategy, the Sentinel-1A and 1B constellation has become the main source for regular and comprehensive monitoring of ice motion over Antarctica and Greenland.
The Sentinel-1 acquisition planning for Greenland includes an annual ice sheet wide campaign with 4 to 6 repeat acquisitions for each track. During winter 2016/2017 the third ice sheet wide Sentinel-1 campaign is planned, which is the first campaign including both, Sentinel-1A and 1B, satellites providing 6 days repeat pass observations. We investigate the radar signal coherence over 6 and 12 days time periods, required for InSAR and coherent offset tracking applications. For generation of the ice sheet wide velocity map we apply an iterative offset tracking algorithm using coherent and incoherent image cross-correlation. The full spectrum of flow velocities is mapped by using variable sizes of the matching windows. We present ice sheet wide Sentinel-1 velocity maps of Greenland, with 250 m pixel spacing for 2015, 2016 and 2017, and will highlight the improvements in ice velocity monitoring of the dual satellite constellation especially for fast moving glaciers and inland regions with high accumulation rates. The Sentinel-1 ice velocities agree very well with velocities derived from high resolution TerraSAR-X Stripmap mode data which are available for several outlet glaciers. The first Sentinel-1A ice sheet wide acquisition campaign for Antarctica (with polar gap) took place from May to October 2015, complemented by additional acquisitions in 2016 used for gap filling. About 45 tracks, each with mostly 4 consecutive repeat observations, were collected and processed to produce ice velocity maps by applying the same offset tracking procedure as for Greenland. We show the new Antarctic ice sheet wide velocity map (at 200 m pixel spacing) derived from Sentinel-1A and 1B data, validated with GPS measurements and velocity maps from high resolution satellite data.
We use the almost 3 years of Sentinel-1 acquisitions to report on short term variations (weekly to monthly intervals) of ice speed of outlet glaciers in Greenland and Antarctica, complemented by less frequent observations by other SAR sensors (PALSAR, TerraSAR-X, TanDEM-X, ERS) before the Sentinel-1 era. The improved signal coherence over 6 days repeat also enables mapping of the grounding line by InSAR, applying a combination of interferometric processing and offset tracking. Derived grounding lines and velocity fields are shown for selected outlet glaciers in Antarctica and Greenland, as well as time series of ice discharge based on ice thickness data from airborne RES campaigns and ice velocities across flux gates.
Presentation
[Authors] [ Overview programme] [ Keywords]
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Paper 224 - Session title: Ice and snow
10:00 Antarctic Ice Sheet Grounding line migration monitoring using COSMO-SkyMed very short repeat-time SAR Interferometry.
Milillo, Pietro (1); Rignot, Eric (1); Mouginot, Jeremie (2); Scheuchl, Bernd (2); Morlighem, Mathieu (2); Li, Xin (2); Salzer, Jacqueline (3) 1: NASA Jet Propulsion Laboratory, United States of America; 2: University of California Irvine, United States of America; 3: 4GFZ German Research Centre for Geosciences, Physics of Earthquakes and Volcanoes, Germany
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We employ data from the second generation of SAR systems e.g. the Italian COSMO-SkyMed constellation and the German TanDEM-X formation to monitor the characteristics of grounding line migration using short repeat-time interferometry and accurate InSAR DEM in the Amundsen Sea Embayment (ASE), West Antarctica. The ASE is a marine-based ice sheet with a retrograde bed containing enough ice to raise global sea level by 120 cm. Several studies have inferred the mechanical properties of portions of ASE using observationally constrained numerical models, but these studies offer only temporal snapshots of basal mechanics owing to a dearth of observational time series. Prior attempts of grounding lines mapping have been limited because few space-borne SAR missions offer the short-term repeat pass capability required to map the differential vertical displacement of floating ice at tidal frequencies with sufficient detail to resolve grounding line boundaries in areas of fast ice deformation. Using 1-day CSK repeat pass data and TanDEM-X DEMs, we collected frequent, high-resolution grounding line measurements of Pine Island (PIG), Thwaites, Kohler and Smith glaciers spanning 2015-2016. We compare the results with ERS data spanning 1996-2011, and Sentinel-1a 2014-2015 data. We observe an ongoing, rapid 2km/yr grounding line retreat on Smith, 0.5 km/yr retreat on Pope, ongoing 1 km/yr retreat on Thwaites and PIG and a slight re-advance on Kohler since 2011. On PIG, the data reveal rapid subsidence km along the glacier flanks, significantly more than in 1996/2000. We do not observe similar patterns on the other glaciers.
[Authors] [ Overview programme] [ Keywords]
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Paper 359 - Session title: Ice and snow
10:20 InSAR Acquisition Strategies for Antarctica
Scheuchl, Bernd (1); Mouginot, Jeremie (1); Rignot, Eric (1,2) 1: University of California, Irvine, United States of America; 2: JPL
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Ice sheets are acknowledged by the World Meteorological Organization (WMO) and the United Nations Framework Convention on Climate Change (UNFCCC) as an Essential Climate Variable (ECV) needed to make significant progress in the generation of global climate products and derived information. Spaceborne Synthetic Aperture Radar (SAR) data prove an extremely useful source to provide relevant information on ice sheets. Specifically, ice velocity, grounding line, and ice front location can be extracted.
SAR acquisition campaigns in Antarctica started in the late 1990s, however, the first continent-wide InSAR coverage of the continent was not achieved until the collaboration of several international space agencies as contribution to the International Polar Year 2007-8.
Due to strong support by ESA and open access to data, the Sentinel-1 mission represents a fundamental change in the way we are monitoring ice sheets going forward. Through coordination by the Polar Space Task Group (PSTG), other international SAR missions augment the Sentinel-1 acquisitions to maximize the scientific value of the data. Several new SAR missions are coming online soon and are expected to provide a contribution to ice sheet science provided that data are being collected.
Using selected examples around Antarctica, we will evaluate current acquisition strategies and discuss how future missions can contribute.
Presentation
[Authors] [ Overview programme] [ Keywords]
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Paper 434 - Session title: Ice and snow
09:40 Grounding Line Derivation Over Antarctic Ice Sheet From Sentinel-1, TerraSAR-X and ERS-1/2
Chowdhury, Tanvir Ahmed (1); Floricioiu, Dana (1); Wuite, Jan (2); Nagler, Thomas (2) 1: Remote Sensing Technology Institute, German Aerospace Center (DLR IMF), Oberpfaffenhofen, Germany; 2: ENVEO - Environmental Earth Observation, Innsbruck, Austria
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The grounding line is the transition between the grounded ice sheet and the floating ice shelf. Its position depends on the ice thickness, the sub-glaciological topography and the ocean tide level. The delineation of an ice sheet grounding line is critical to ice sheet mass budget calculations, ice sheet dynamics modeling and the understanding of ice-ocean interactions.
The grounding line location (GLL) is difficult to identify in situ. Within ESA's Antarctic Ice Sheet (AIS) project which is carried out within the framework of Climate Change Initiative (CCI) programme, we use Synthetic Aperture Radar (SAR) data from different satellites e.g. TerraSAR-X, Sentinel-1, ERS-1/2 to map GLL for major ice streams and outlet glaciers in Antarctica. A minimum of three subsequent repeat pass acquisitions (at different tidal conditions) are considered and two independent interferograms can be combined in order to remove the common horizontal velocity component. The remaining vertical component is due to tidal motion. The repeat interval of the SAR acquisitions must not be too long otherwise temporal decorrelation prohibits the characteristic fringe belt pattern required for mapping. In the AIS_cci project the upper limit of flexure is taken as a very good approximation of the actual grounding line and is provided as GLL product.
The AIS_cci GLL product internally may contain many separate grounding line items. Each item has geometric information (location) along with attributes (metadata). It is obvious that, if already one parameter changes (e.g. the satellite track, the sensor or date/time), this grounding line segment cannot be connected to the others but must be a separate item with respect to time and the conditions (mainly ocean tides) under which it was acquired. The metadata includes information about model based ocean tide level and air pressure at satellite acquisition times for meaningful and interpretable comparison of GLLs. None of the existing GLL's database of Antarctica contains this information.
GLL products are provided to the users in three formats: kml for quick visualization or inspection in Google Earth, ERSI shapefile for GIS users and further geospatial analyses and WKT format for the users who prefer plain text. A product user guide explaining the data formats and parameters is also available for download from ENVEO's (http://cryoportal.enveo.at) web portal.
Besides product generation AIS_cci aims at comparisons between recent GLLs with those derived from ERS-1/2 tandem data with the same methodology in order to detect possible migrations of the grounding line. Moreover, for validation purposes AIS_cci GLLs are also compared with currently existing GLL database by means of spatial approach.
The areas of investigations have been focused on ice streams at the interior of the Antarctic Ice Sheet where highly coherent interferometric data sets are expected. On fast moving glaciers at the margins of the continent mapping the GLL is limited by the phase decorrelation over longer periods due to high deformation rates. With the launch of Sentinel-1b the temporal baseline of the constellation is shortened to 6 days which will improve the robustness of the DInSAR procedure for mapping grounding lines along the margins of the continent.
Presentation
[Authors] [ Overview programme] [ Keywords]