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Ian Joughin - One of the best experts on this subject based on the ideXlab platform.
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contribution to the glaciology of northern greenland from Satellite Radar interferometry
Journal of Geophysical Research, 2001Co-Authors: Eric Rignot, Ian Joughin, S Gogineni, William B. KrabillAbstract:Interferometric synthetic aperture Radar (InSAR) data from the ERS-1 and ERS-2 Satellites are used to measure the surface velocity, topography, and grounding line position of the major outlet glaciers in the northern sector of the Greenland ice sheet. The mass output of the glaciers at and above the grounding line is determined and compared with the mass input. We find that the grounding line output is approximately in balance with the input, except for the three largest glaciers for which the mass loss is 4±3 km3 ice year-1 or 11±8% of the mass input. Along the coast we detect a systematic retreat of the grounding lines between 1992 and 1996 with InSAR, which implies that the outlet glaciers are thinning. The inferred coastal thinning is too large to be explained by a few warm summers. Glacier thinning must be of dynamic origin, that is, caused by spatial and temporal changes in ice velocity. Iceberg production from the glaciers is uncharacteristically low. It accounts for only 8% of the ice discharge to the ocean. About 55% of the ice is lost through basal melting (5-8 m ice year-1 on average) from the underside of the floating glacier tongues that are in contact with warm ocean waters. Mass losses are highest in the first 10 km of floating ice, where ice reaches the greatest depths and basal melting is 3 times larger than on average. Only a small increase in basal melting would suffice to disintegrate the floating glacier tongues. Copyright 2001 by the American Geophysical Union.
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measurement of ice sheet topography using Satellite Radar interferometry
Journal of Glaciology, 1996Co-Authors: Ian Joughin, R. Kwok, Mark Fahnestock, Dale P Winebrenner, William B. KrabillAbstract:Detailed digital elevation models (DEMs) do not exist for much of the Greenland and Antarctic ice sheets. Radar altimetry is at present the primary, in many cases the only, source of topographic data over the ice sheets, but the horizontal resolution of such data is coarse. Satellite-Radar interferometry uses the phase difference between pairs of synthetic aperture Radar (SAR) images to measure both ice-sheet topography and surface displacement. We have applied this technique using ERS-1 SAR data to make detailed (i.e. 80 m horizontal resolution) maps of surface topography in a 100 km by 300 km strip in West Greenland. extending northward from just above Jakobshavns Isbrae. Comparison with a 76 km long line of airborne laser-altimeter data shows that we have achieved a relative accuracy of 2.5m along the profile. These observations provide a detailed view of dynamically supported topography near the margin of an ice sheet. In the final section we compare our estimate of topography with phase contours due to motion, and confirm our earlier analysis concerning vertical ice-sheet motion and complexity in ERS-1 SAR interferograms.
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estimation of ice sheet motion using Satellite Radar interferometry method and error analysis with application to humboldt glacier greenland
Journal of Glaciology, 1996Co-Authors: Ian Joughin, R. Kwok, Mark FahnestockAbstract:Satellite Radar interferometry provides glaciologists with an important new tool for determining the motion and topography of large ice sheets. We examine the sources of error in interferometrically derived ice-motion measurements, including those errors due to inaccurate estimates of the interferometric baseline. Several simulations are used to assess baseline accuracy in terms of tie-point error and the number and distribution of tie points. These results give insight into how best to select tie points, and also demonstrate the level of accuracy that can be achieved. Examination of two representative cases likely to occur in mapping ice-sheet motion leads to the conclusion that with adequate tie-point information ice velocity can be measured accurately to within a few meters per year. A method to correct horizontal velocity estimates for the effect of vertical displacement using surface slopes is also developed. Finally, we estimate the single-component velocity field for an area on Humboldt Glacier, northern Greenland, using interferograms formed from ERS-I SAR images. We estimate that these velocity measurements are accurate to within 2.3 m year -1 .
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observations of ice sheet motion in greenland using Satellite Radar interferometry
Geophysical Research Letters, 1995Co-Authors: Ian Joughin, Dale P Winebrenner, Mark FahnestockAbstract:We present interferometric observations of ice-sheet motion in western Greenland based on pairs of ERS-1 synthetic aperture Radar (SAR) images. These observations provide the first detailed regional view of ice motion associated with dynamically supported topography near the margin of an ice sheet. The interferograms of this area are much more complicated than other interferograms of ice sheets presented to date. We devote the largest part of this paper to explaining the source of the complexity in these interferograms. A synthetic interferogram based on a simple model helps to illustrate the effects of different components of the ice velocity field in interferometric data and suggests a method for estimating the large-scale ice velocity field from such data.
Mark Fahnestock - One of the best experts on this subject based on the ideXlab platform.
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measurement of ice sheet topography using Satellite Radar interferometry
Journal of Glaciology, 1996Co-Authors: Ian Joughin, R. Kwok, Mark Fahnestock, Dale P Winebrenner, William B. KrabillAbstract:Detailed digital elevation models (DEMs) do not exist for much of the Greenland and Antarctic ice sheets. Radar altimetry is at present the primary, in many cases the only, source of topographic data over the ice sheets, but the horizontal resolution of such data is coarse. Satellite-Radar interferometry uses the phase difference between pairs of synthetic aperture Radar (SAR) images to measure both ice-sheet topography and surface displacement. We have applied this technique using ERS-1 SAR data to make detailed (i.e. 80 m horizontal resolution) maps of surface topography in a 100 km by 300 km strip in West Greenland. extending northward from just above Jakobshavns Isbrae. Comparison with a 76 km long line of airborne laser-altimeter data shows that we have achieved a relative accuracy of 2.5m along the profile. These observations provide a detailed view of dynamically supported topography near the margin of an ice sheet. In the final section we compare our estimate of topography with phase contours due to motion, and confirm our earlier analysis concerning vertical ice-sheet motion and complexity in ERS-1 SAR interferograms.
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estimation of ice sheet motion using Satellite Radar interferometry method and error analysis with application to humboldt glacier greenland
Journal of Glaciology, 1996Co-Authors: Ian Joughin, R. Kwok, Mark FahnestockAbstract:Satellite Radar interferometry provides glaciologists with an important new tool for determining the motion and topography of large ice sheets. We examine the sources of error in interferometrically derived ice-motion measurements, including those errors due to inaccurate estimates of the interferometric baseline. Several simulations are used to assess baseline accuracy in terms of tie-point error and the number and distribution of tie points. These results give insight into how best to select tie points, and also demonstrate the level of accuracy that can be achieved. Examination of two representative cases likely to occur in mapping ice-sheet motion leads to the conclusion that with adequate tie-point information ice velocity can be measured accurately to within a few meters per year. A method to correct horizontal velocity estimates for the effect of vertical displacement using surface slopes is also developed. Finally, we estimate the single-component velocity field for an area on Humboldt Glacier, northern Greenland, using interferograms formed from ERS-I SAR images. We estimate that these velocity measurements are accurate to within 2.3 m year -1 .
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observations of ice sheet motion in greenland using Satellite Radar interferometry
Geophysical Research Letters, 1995Co-Authors: Ian Joughin, Dale P Winebrenner, Mark FahnestockAbstract:We present interferometric observations of ice-sheet motion in western Greenland based on pairs of ERS-1 synthetic aperture Radar (SAR) images. These observations provide the first detailed regional view of ice motion associated with dynamically supported topography near the margin of an ice sheet. The interferograms of this area are much more complicated than other interferograms of ice sheets presented to date. We devote the largest part of this paper to explaining the source of the complexity in these interferograms. A synthetic interferogram based on a simple model helps to illustrate the effects of different components of the ice velocity field in interferometric data and suggests a method for estimating the large-scale ice velocity field from such data.
Eric Rignot - One of the best experts on this subject based on the ideXlab platform.
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antarctic grounding line mapping from differential Satellite Radar interferometry
Geophysical Research Letters, 2011Co-Authors: Eric Rignot, J Mouginot, B ScheuchlAbstract:[1] The delineation of an ice sheet grounding line, i.e., the transition boundary where ice detaches from the bed and becomes afloat in the ocean, is critical to ice sheet mass budget calculations, numerical modeling of ice sheet dynamics, ice-ocean interactions, oceanic tides, and subglacial environments. Here, we present 15 years of comprehensive, high-resolution mapping of grounding lines in Antarctica using differential Satellite synthetic-aperture Radar interferometry (DInSAR) data from the Earth Remote Sensing Satellites 1–2 (ERS-1/2), RadarSAT-1 and 2, and the Advanced Land Observing System (ALOS) PALSAR for years 1994 to 2009. DInSAR directly measures the vertical motion of floating ice shelves in response to tidal oceanic forcing with millimeter precision, at a sample spacing better than 50 m, simultaneously over areas several 100 km wide; in contrast with earlier methods that detect abrupt changes in surface slope in Satellite visible imagery or altimetry data. On stagnant and slow-moving areas, we find that breaks in surface slope are reliable indicators of grounding lines; but on most fast-moving glaciers and ice streams, our DInSAR results reveal that prior mappings have positioning errors ranging from a few km to over 100 km. A better agreement is found with ICESat's data, also based on measurements of vertical motion, but with a detection noise one order of magnitude larger than with DInSAR. Overall, the DInSAR mapping of Antarctic grounding lines completely redefines the coastline of Antarctica.
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changes in the velocity structure of the greenland ice sheet
Science, 2006Co-Authors: Eric Rignot, P KanagaratnamAbstract:Using Satellite Radar interferometry observations of Greenland, we detected widespread glacier acceleration below 66° north between 1996 and 2000, which rapidly expanded to 70° north in 2005. Accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year. As more glaciers accelerate farther north, the contribution of Greenland to sea-level rise will continue to increase.
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warm ocean is eroding west antarctic ice sheet
Geophysical Research Letters, 2004Co-Authors: Andrew Shepherd, D J Wingham, Eric RignotAbstract:[1] Satellite Radar measurements show that ice shelves in Pine Island Bay have thinned by up to 5.5 m yr−1 over the past decade. The pattern of shelf thinning mirrors that of their grounded tributaries - the Pine Island, Thwaites and Smith glaciers - and ocean currents on average 0.5°C warmer than freezing appear to be the source. The synchronised imbalance of the inland glaciers is the result of reduced lateral and basal tractions at their termini, and the drawdown of grounded ice shows that Antarctica is more sensitive to changing climates than was previously considered.
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rapid bottom melting widespread near antarctic ice sheet grounding lines
Science, 2002Co-Authors: Eric Rignot, Stanley S JacobsAbstract:As continental ice from Antarctica reaches the grounding line and begins to float, its underside melts into the ocean. Results obtained with Satellite Radar interferometry reveal that bottom melt rates experienced by large outlet glaciers near their grounding lines are far higher than generally assumed. The melting rate is positively correlated with thermal forcing, increasing by 1 meter per year for each 0.1°C rise in ocean temperature. Where deep water has direct access to grounding lines, glaciers and ice shelves are vulnerable to ongoing increases in ocean temperature.
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contribution to the glaciology of northern greenland from Satellite Radar interferometry
Journal of Geophysical Research, 2001Co-Authors: Eric Rignot, Ian Joughin, S Gogineni, William B. KrabillAbstract:Interferometric synthetic aperture Radar (InSAR) data from the ERS-1 and ERS-2 Satellites are used to measure the surface velocity, topography, and grounding line position of the major outlet glaciers in the northern sector of the Greenland ice sheet. The mass output of the glaciers at and above the grounding line is determined and compared with the mass input. We find that the grounding line output is approximately in balance with the input, except for the three largest glaciers for which the mass loss is 4±3 km3 ice year-1 or 11±8% of the mass input. Along the coast we detect a systematic retreat of the grounding lines between 1992 and 1996 with InSAR, which implies that the outlet glaciers are thinning. The inferred coastal thinning is too large to be explained by a few warm summers. Glacier thinning must be of dynamic origin, that is, caused by spatial and temporal changes in ice velocity. Iceberg production from the glaciers is uncharacteristically low. It accounts for only 8% of the ice discharge to the ocean. About 55% of the ice is lost through basal melting (5-8 m ice year-1 on average) from the underside of the floating glacier tongues that are in contact with warm ocean waters. Mass losses are highest in the first 10 km of floating ice, where ice reaches the greatest depths and basal melting is 3 times larger than on average. Only a small increase in basal melting would suffice to disintegrate the floating glacier tongues. Copyright 2001 by the American Geophysical Union.
Jeffrey K Ridley - One of the best experts on this subject based on the ideXlab platform.
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determining basal ice sheet conditions in the dome c region of east antarctica using Satellite Radar altimetry and airborne radio echo sounding
Journal of Glaciology, 1998Co-Authors: Martin J. Siegert, Jeffrey K RidleyAbstract:Large subglacial lakes manifest themselves as flat regions on the ice surface. ERS-1 Satellite Radar altimetry of the Dome C region of East Antarctica was analyzed to correlate unusually flat areas on the ice surface with known locations of subglacial lakes identified from airborne radio-echo sounding (RES) data. The mean length of subglacial lakes which have an expression in the ice-sheet surface was ~8.3 km, whilst those that did not exhibit a surface morphological manifestation had a mean length of ~3.3 km. Thus, lakes up to about 4 km in length arc unlikely to be detected from Satellite Radar altimetry of the ice surface. Given that the spacing of radio-echo flight tracks within the SPRI-NSF-TUD Antarctic database is 50-100 km in many areas, a number of subglacial lakes probably lie undetected beneath the ice sheet. RES information from two large, flat surface regions within Dome C, and a further flat area located at 80° S, 127° E, indicates the absence of subglacial lakes beneath the ice-surface features. However, these areas are characterised by relatively strong radio-echo returns which may indicate the presence of water-saturated basal sediments. We suggest that (1) blankets of water-saturated basal sediments may cause similar surface morphological features to those produced by subglacial lakes; and (2) misidentification of subglacial lakes from Satellite altimeter observations of the ice-sheet surface is possible without the support of RES information relating to the ice-sheet base. Furthermore, our study indicates a lack of subglacial lake signals from RES data over relatively thick regions of East Antarctica such as the Adventure Subglacial Trough. We conclude that subglacial water produced in such regions may be transported by a basal hydrological system, driven by overburden pressure, to less thick regions of the ice sheet where subglacial lakes have been identified.
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determining basal ice sheet conditions in the dome c region of east antarctica using Satellite Radar altimetry and airborne radio echo sounding
Journal of Glaciology, 1998Co-Authors: Martin J. Siegert, Jeffrey K RidleyAbstract:Large subglacial lakes manifest themselves as flat regions on the ice surface. ERS-1 Satellite Radar altimetry of the Dome C region of East Antarctica was analyzed to correlate unusually flat areas on the ice surface with known locations of subglacial lakes identified from airborne radio-echo sounding (RES) data. The mean length of subglacial lakes which have an expression in the ice-sheet surface was ∼8.3 km, whilst those that did not exhibit a surface morphological manifestation had a mean length of ∼3.3 km. Thus, lakes up to about 4 km in length are unlikely to be detected from Satellite Radar altimetry of the ice surface. Given that the spacing of radio-echo flight tracks within the SPRI-NSF-TUD Antarctic database is 50-100 km in many areas, a number ofsubglacial lakes probably lie undetected beneath the ice sheet. RES information from two large, flat surface regions within Dome C, and a further flat area located at 80° S, 127° E, indicates the absence ofsubglacial lakes beneath the ice-surface features. However, these areas are characterised by relatively strong radio-echo returns which may indicate the presence of water-saturated basal sediments. We suggest that (1) blankets of water-saturated basal sediments may cause similar surface morphological features to those produced by subglacial lakes; and (2) misidentification ofsubglacial lakes from Satellite altimeter observations of the ice-sheet surface is possible without the support of RES information relating to the ice-sheet base. Furthermore, our study indicates a lack of sub-glacial lake signals from RES data over relatively thick regions of East Antarctica such as the Adventure Subglacial Trough. We conclude that subglacial water produced in such regions may be transported by a basal hydrological system, driven by overburden pressure, to less thick regions of the ice sheet where subglacial lakes have been identified.
C H Davis - One of the best experts on this subject based on the ideXlab platform.
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snowfall driven growth in east antarctic ice sheet mitigates recent sea level rise
Science, 2005Co-Authors: C H Davis, Joseph R Mcconnell, M M Frey, Edward HannaAbstract:Satellite Radar altimetry measurements indicate that the East Antarctic ice-sheet interior north of 81.6°S increased in mass by 45 ± 7 billion metric tons per year from 1992 to 2003. Comparisons with contemporaneous meteorological model snowfall estimates suggest that the gain in mass was associated with increased precipitation. A gain of this magnitude is enough to slow sea-level rise by 0.12 ± 0.02 millimeters per year.
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elevation change of the antarctic ice sheet 1995 2000 from ers 2 Satellite Radar altimetry
IEEE Transactions on Geoscience and Remote Sensing, 2004Co-Authors: C H Davis, A C FergusonAbstract:We analyzed Antarctic ice-sheet elevation change (dH/dt) from 1995 to 2000 using 123 million elevation change measurements from European Remote Sensing 2 ice-mode Satellite Radar altimeter data covering an area of about 7.2 million km/sup 2/. Almost all drainage basins in east Antarctica had average dH/dt values within /spl plusmn/3.0 cm/year, whereas drainage basins in west Antarctica had substantial spatial variability with average dH/dt values ranging between -11 to +12 cm/year. The east Antarctic ice sheet had a five-year trend of 1/spl plusmn/0.6 cm/year, where 13 out of the 14 basins had either a positive trend or a trend that was not significantly different than zero. The west Antarctic ice sheet had a five-year trend of -3.6/spl plusmn/1.0 cm/year due largely to strong negative trends of around 10 cm/year for basins in Marie Byrd Land along the Pacific sector of the Antarctic coast. The continent as a whole had a five-year dH/dt trend of 0.4/spl plusmn/0.4 cm/year. Finally, time series constructed for the Pine Island, Thwaites, De Vicq, and Land glaciers in west Antarctic showed five-year dH/dt trends from -26 to -135 cm/year that were significantly more negative than the average dH/dt trends in their respective basins. The strongly negative dH/dt values for these coastal glacier outlets are consistent with recently reported results indicating increased basal melting at these glaciers' grounding lines caused by ocean thermal forcing.
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improved elevation change measurement of the southern greenland ice sheet from Satellite Radar altimetry
IEEE Transactions on Geoscience and Remote Sensing, 2000Co-Authors: C H Davis, Craig A Kluever, Bruce J Haines, C Perez, Y T YoonAbstract:A new analysis of Seasat and Geosat Satellite Radar altimeter measurements over the Greenland ice sheet was performed to determine surface elevation change. The new analysis includes twice as many measurements and has 50% greater spatial coverage than the authors' previous study. In addition, a precise global ocean reference network created from four years of Topex/Poseidon altimeter data is used to obtain improved estimates of altimeter orbit errors and measurement system biases. The results show that the average elevation change of the southern Greenland ice sheet above 2000 m from 1978 to 1988 is not significantly different than zero. This contradicts earlier and even more recent studies that reported positive ice sheet growth rates and suggested increased precipitation due to a warmer polar climate.