Seasonal variations of the backscattering coefficient measured by radar altimeters over the Antarctic Ice Sheet

Abstract. Spaceborne radar altimeter is a valuable tool for observing the Antarctica Ice Sheet. The radar wave penetration into the snow provides information both on the surface and the subsurface of the snowpack due to its dependence on the snow properties. However this penetration also induces a negative bias on the estimated surface elevation. Empirical corrections of this space and time-varying bias are usually based on the backscattering coefficient variability. We investigate the spatial and seasonal variations of the backscattering coefficient at the S (3.2 GHz), Ku (13.6 GHz) and Ka (37 GHz) bands. We identified two clearly marked zones over the continent, one with the maximum of Ku band backscattering coefficient in the winter and another with the maximum in the summer. To explain this, we performed a sensitivity study of the backscattering coefficient at the S, Ku and Ka bands to surface snow density, snow temperature and snow grain size using an electromagnetic model. The results show that the seasonal cycle of the backscattering coefficient at the Ka band, is dominated by the volume echo and is mainly explained by snow temperature. In contrast, the cycle is dominated by the surface echo at the S band. At Ku band, which intermediate in terms of wavelength between S and Ka bands, the seasonal cycle is in the first zone dominated by the volume echo and by the surface echo in the second one. Such seasonal and spatial variations of the backscattering coefficient at different radar frequencies should be taken into account the for more precise estimation of the surface elevation changes.

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Geometric boundary conditions for modelling the velocity field of the Antarctic ice sheet

Annals of Glaciology ◽

10.1017/s0260305500013653 ◽

1996 ◽

Vol 23 ◽

pp. 364-373 ◽

Cited By ~ 32

Author(s):

Jonathan L. Bamber ◽

Philippe Huybrechts

Keyword(s):

Velocity Field ◽

Boundary Conditions ◽

Ice Sheet ◽

Surface Elevation ◽

Surface Velocity ◽

Ice Thickness ◽

Antarctic Ice Sheet ◽

Surface Slopes ◽

Geometric Boundary ◽

The Antarctic

This paper presents improved geometric boundary conditions (surface elevation and ice thickness) required as inputs to calculations of the surface-velocity field for the Antarctic ice sheet. A comparison of the two-dimensional horizontal velocity field obtained on the basis of conservation of mass (balance velocity) with the diagnostic velocity field calculated with an ice-sheet model (dynamic velocity) may yield information on shortcomings in the way the ice-sheet model describes the ice flow. Here, the surface-elevation grid is described in detail, as it has been generated specifically for such a study and represents a new standard in accuracy and resolution for calculating surface slopes. The digital-elevation model was generated on a 10 km grid size from over 20 000 000 height estimates obtained from eight 35 d repeat cycles of ERS-1 radar-altimeter data. For surface slopes less than 0.4°, the accuracy is better than 1.5 m. In areas of high surface slope (coastal and mountainous regions), the altimeter measurements have been supplemented with data taken from the Antarctic Digital Database. South of 81.5°, data from the SPRI folio map have been used. The ice-thickness grid was produced from a combination of a redigitization of the SPRI folio and the original radio-echo-sounding flight lines. For areas of grounded ice, the elevation of the bed was estimated from surface elevation and ice thickness. Significant differences (in excess of 25% of ice thickness) were obtained between an earlier digitization of the folio bed-elevation map and the data set derived here. Furthermore, a new value of 25.6 × 106 km3 was obtained for the total volume of the ice sheet and ice shelves, which is a reduction of 12% compared with the original estimate derived during the compilation of the SPRI folio. These differences will have an important influence on the results obtained by numerical ice-sheet models.

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Identifying areas of low-profile ice sheet and outcrop damming in the Antarctic ice sheet by ERS-1 satellite altimetry

Annals of Glaciology ◽

10.3189/1998aog27-1-1-6 ◽

1998 ◽

Vol 27 ◽

pp. 1-6 ◽

Cited By ~ 8

Author(s):

David G. Vaughan ◽

Jonathan L. Bamber

Keyword(s):

Satellite Altimetry ◽

Ice Sheet ◽

Surface Elevation ◽

Two Dimensional ◽

Ice Flow ◽

Antarctic Ice Sheet ◽

Continental Scale ◽

Low Profile ◽

Elevation Model ◽

The Antarctic

A digital elevation model (DEM) of the surface of the Antarctic ice sheet is compared with a simple two-dimensional ice-flow model to illuminate gross distortions (>500 m) of the ice-surface elevation. We use a DEM derived from ERS-1 satellite altimetry, airborne data and TWERLE balloon data. This is compared with an ice-sheet elevation model generated by applying theoretical surface elevations, calculated for two-dimensional ice flow, to isolines of distance from the grounding line (continentality). The model is scaled using only one parameter, to match the measured surface elevation at Dome Argus. The model is far from rigorous, violating continuity conditions, ignoring variations in surface mass balance and temperature, and assuming uniform basal conditions. However, the comparison of model and observed surface elevations is illuminating in terms of the behaviour of the ice sheet at a continental scale. Across the ice sheet the rms difference between modelled elevation and the DEM is around 300 m, but much of this results from isolated areas of much greater disagreement. We ascribe these gross differences to the effects of basal conditions. in four areas, the observed surface is more than 500 m higher than the modelled surface. Most of these are immediately upstream of substantial areas of rock outcrop and are caused by the damming effect of these mountain ranges. in nine areas, the measured surface is more than 500 m lower than predicted. Eight of these areas, in West Antarctica and the Lambert Glacier basin, are associated with suspected areas of basal sliding. The ninth is an area of 250 000 km2 in East Antarctica not previously noted as having unusual flow characteristics, but for which very few-data exist. We speculate that this area results from unusual basal conditions resulting in a low-profile ice sheet. A low-profile ice sheet of this size with in the East Antarctic ice sheet indicates that basal conditions are perhaps more variable than previously thought.

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Four decades of surface elevation change of the Antarctic Ice Sheet from multi-mission satellite altimetry

10.5194/tc-2018-49 ◽

2018 ◽

Cited By ~ 1

Author(s):

Ludwig Schröder ◽

Martin Horwath ◽

Reinhard Dietrich ◽

Veit Helm

Keyword(s):

Time Series ◽

Satellite Altimetry ◽

Ice Sheet ◽

Surface Elevation ◽

Elevation Change ◽

Antarctic Ice Sheet ◽

Mission Satellite ◽

Surface Elevation Change ◽

Precipitation Anomalies ◽

The Antarctic

Abstract. We developed an approach for a multi-mission satellite altimetry analysis over the Antarctic Ice Sheet which comprises Seasat, Geosat, ERS-1, ERS-2, Envisat, ICESat and CryoSat-2. In a first step we apply a consistent reprocessing of the radar alitmetry data which improves the measurement precision by up to 50 %. We then perform a joint repeat altimetry analysis of all missions. We estimate inter-mission offsets by approaches adapted to the temporal overlap or non-overlap and to the similarity or dissimilarity of involved altimetry techniques. Hence, we obtain monthly grids forming a combined surface elevation change time series. Owing to the early missions Seasat and Geosat, the time series span almost four decades from 07/1978 to 12/2017 over 25 % of the ice sheet area (coastal regions of East Antarctica and the Antarctic Peninsula). Since the launch of ERS-1 79 % of the ice sheet area is covered by observations. Over this area, we obtain a negative volume trend of −34 ± 5 km3 yr−1 for the more than 25-year period (04/1992–12/2017). These volume losses have significantly accelerated to a rate of −170 ± 11 km3 yr−1 for 2010–2017. Interannual variations significantly impact decadal volume rates which highlights the importance of the long-term time series. Our time series show a high coincidence with modeled cumulated precipitation anomalies and with satellite gravimetry. This supports the interpretation with respect to snowfall anomalies or dynamic thinning. Moreover, the correlation with cumulated precipitation anomalies back to the Seasat and Geosat periods highlights that the inter-mission offsets were successfully corrected and that the early missions add valuable information.

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Relationship of surface elevation changes of the Antarctic ice sheet with ice movement and subglacial water flow

Journal Ice and Snow ◽

10.15356/2076-6734-2013-4-5-12 ◽

2015 ◽

Vol 124 (4) ◽

pp. 5

Author(s):

V. M. Kotlyakov ◽

L. N. Vasiliev ◽

A. B. Kachalin ◽

M. Yu. Moskalevsky ◽

A. S. Tyuflin

Keyword(s):

Water Flow ◽

Ice Sheet ◽

Surface Elevation ◽

Antarctic Ice Sheet ◽

Elevation Changes ◽

Relationship Of ◽

The Antarctic

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Identifying areas of low-profile ice sheet and outcrop damming in the Antarctic ice sheet by ERS-1 satellite altimetry

Annals of Glaciology ◽

10.1017/s0260305500017146 ◽

1998 ◽

Vol 27 ◽

pp. 1-6 ◽

Cited By ~ 4

Author(s):

David G. Vaughan ◽

Jonathan L. Bamber

Keyword(s):

Satellite Altimetry ◽

Ice Sheet ◽

Surface Elevation ◽

Two Dimensional ◽

Ice Flow ◽

Antarctic Ice Sheet ◽

Continental Scale ◽

Low Profile ◽

Elevation Model ◽

The Antarctic

A digital elevation model (DEM) of the surface of the Antarctic ice sheet is compared with a simple two-dimensional ice-flow model to illuminate gross distortions (>500 m) of the ice-surface elevation. We use a DEM derived from ERS-1 satellite altimetry, airborne data and TWERLE balloon data. This is compared with an ice-sheet elevation model generated by applying theoretical surface elevations, calculated for two-dimensional ice flow, to isolines of distance from the grounding line (continentality). The model is scaled using only one parameter, to match the measured surface elevation at Dome Argus. The model is far from rigorous, violating continuity conditions, ignoring variations in surface mass balance and temperature, and assuming uniform basal conditions. However, the comparison of model and observed surface elevations is illuminating in terms of the behaviour of the ice sheet at a continental scale. Across the ice sheet the rms difference between modelled elevation and the DEM is around 300 m, but much of this results from isolated areas of much greater disagreement. We ascribe these gross differences to the effects of basal conditions. in four areas, the observed surface is more than 500 m higher than the modelled surface. Most of these are immediately upstream of substantial areas of rock outcrop and are caused by the damming effect of these mountain ranges. in nine areas, the measured surface is more than 500 m lower than predicted. Eight of these areas, in West Antarctica and the Lambert Glacier basin, are associated with suspected areas of basal sliding. The ninth is an area of 250 000 km2 in East Antarctica not previously noted as having unusual flow characteristics, but for which very few-data exist. We speculate that this area results from unusual basal conditions resulting in a low-profile ice sheet. A low-profile ice sheet of this size with in the East Antarctic ice sheet indicates that basal conditions are perhaps more variable than previously thought.

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Envisat and SARAL/AltiKa Observations of the Antarctic Ice Sheet: A Comparison Between the Ku-band and Ka-band

Marine Geodesy ◽

10.1080/01490419.2014.985347 ◽

2015 ◽

Vol 38 (sup1) ◽

pp. 510-521 ◽

Cited By ~ 19

Author(s):

Frédérique Rémy ◽

Thomas Flament ◽

Aurélie Michel ◽

Denis Blumstein

Keyword(s):

Ice Sheet ◽

Antarctic Ice Sheet ◽

Ka Band ◽

The Antarctic ◽

Ku Band

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Boundary conditions of an active West Antarctic subglacial lake: implications for storage of water beneath the ice sheet

The Cryosphere Discussions ◽

10.5194/tcd-7-2979-2013 ◽

2013 ◽

Vol 7 (3) ◽

pp. 2979-2999 ◽

Cited By ~ 1

Author(s):

M. J. Siegert ◽

N. Ross ◽

H. Corr ◽

B. Smith ◽

T. Jordan ◽

...

Keyword(s):

Ice Sheet ◽

Surface Elevation ◽

Altimeter Data ◽

Radar Altimeter ◽

Local Flow ◽

Antarctic Ice Sheet ◽

Subglacial Lake ◽

West Antarctic ◽

Ice Surface ◽

The Antarctic

Abstract. Repeat-pass IceSat altimetry has revealed 124 discrete surface height changes across the Antarctic Ice Sheet, interpreted to be caused by subglacial lake discharges (surface lowering) and inputs (surface uplift). Few of these active lakes have been confirmed by radio-echo sounding (RES) despite several attempts (notable exceptions are Lake Whillans and three in the Adventure Subglacial Trench). Here we present targeted RES and radar altimeter data from an "active lake" location within the upstream Institute Ice Stream, into which 0.12 km3 of water is calculated to have flowed between October 2003 and February 2008. We use a series of transects to establish an accurate appreciation of the influences of bed topography and ice-surface elevation on water storage potential. The location of surface height change is over the downslope flank of a distinct topographic hollow, where RES reveals no obvious evidence for deep (> 10 m) water. The regional hydropotential reveals a sink coincident with the surface change, however. Governed by the location of the hydrological sink, basal water will likely "drape" over existing topography in a manner dissimilar to subglacial lakes where flat strong specular RES reflections are measured. The inability of RES to detect the active lake means that more of the Antarctic ice sheet bed may contain stored water than is currently appreciated. Variation in ice surface elevation datasets leads to significant alteration in calculations of the local flow of basal water indicating the value of, and need for, high resolution RES datasets in both space and time to establish and characterise subglacial hydrological processes.

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Impact of radiation penetration on Antarctic surface melt and subsurface snow temperatures in RACMO2.3p3

10.5194/tc-2021-298 ◽

2021 ◽

Author(s):

Christiaan Timo van Dalum ◽

Willem Jan van de Berg ◽

Michiel Roland van den Broeke

Keyword(s):

Specific Surface ◽

Surface Area ◽

Specific Surface Area ◽

Ice Sheet ◽

Antarctic Ice Sheet ◽

Surface Mass ◽

Surface Melt ◽

The Antarctic ◽

The Impact ◽

Snow Temperature

Abstract. This study investigates the sensitivity of modeled surface melt and subsurface heating on the Antarctic ice sheet to a new spectral snow albedo and radiative transfer scheme in the Regional Atmospheric Climate Model (RACMO2), version 2.3p3 (Rp3). We tune Rp3 to observations by performing several sensitivity experiments and assess the impact on temperature and melt by changing one parameter at a time. When fully tuned, Rp3 compares well with in situ and remote sensing observations of surface mass and energy balance, melt, temperature, albedo and snow grain specific surface area. Furthermore, the introduction of subsurface heating in Rp3 significantly improves the snow temperature profile. Near surface snow temperature is especially sensitive to the prescribed fresh snow specific surface area and fresh dry snow metamorphism. These processes, together with the refreezing grain size and subsurface heating, are important for melt around the margins of the Antarctic ice sheet. Moreover, small changes in the albedo and the aforementioned processes can lead to an order of magnitude overestimation of melt, locally leading to runoff and a reduced surface mass balance.

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Four decades of Antarctic surface elevation changes from multi-mission satellite altimetry

The Cryosphere ◽

10.5194/tc-13-427-2019 ◽

2019 ◽

Vol 13 (2) ◽

pp. 427-449 ◽

Cited By ~ 18

Author(s):

Ludwig Schröder ◽

Martin Horwath ◽

Reinhard Dietrich ◽

Veit Helm ◽

Michiel R. van den Broeke ◽

...

Keyword(s):

Time Series ◽

Satellite Altimetry ◽

Average Rate ◽

Ice Sheet ◽

Surface Elevation ◽

Antarctic Ice Sheet ◽

Mission Satellite ◽

Elevation Changes ◽

Dronning Maud Land ◽

The Antarctic

Abstract. We developed a multi-mission satellite altimetry analysis over the Antarctic Ice Sheet which comprises Seasat, Geosat, ERS-1, ERS-2, Envisat, ICESat and CryoSat-2. After a consistent reprocessing and a stepwise calibration of the inter-mission offsets, we obtained monthly grids of multi-mission surface elevation change (SEC) with respect to the reference epoch 09/2010 (in the format of month/year) from 1978 to 2017. A validation with independent elevation changes from in situ and airborne observations as well as a comparison with a firn model proves that the different missions and observation modes have been successfully combined to a seamless multi-mission time series. For coastal East Antarctica, even Seasat and Geosat provide reliable information and, hence, allow for the analysis of four decades of elevation changes. The spatial and temporal resolution of our result allows for the identification of when and where significant changes in elevation occurred. These time series add detailed information to the evolution of surface elevation in such key regions as Pine Island Glacier, Totten Glacier, Dronning Maud Land or Lake Vostok. After applying a density mask, we calculated time series of mass changes and found that the Antarctic Ice Sheet north of 81.5∘ S was losing mass at an average rate of -85±16 Gt yr−1 between 1992 and 2017, which accelerated to -137±25 Gt yr−1 after 2010.

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