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

1996 ◽  
Vol 23 ◽  
pp. 364-373 ◽  
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.

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

1996 ◽  
Vol 23 ◽  
pp. 364-373 ◽  
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.

Developing an emulator to calculate present temperature field in the Antarctic Ice Sheet

2021 ◽  
Author(s):  
Catherine Ritz ◽  
Christophe Dumas ◽  
Marion Leduc-Leballeur ◽  
Giovanni Macelloni ◽  
Ghislain Picard ◽  
...  
Keyword(s):  
Temperature Field ◽  
Ice Sheet ◽  
Surface Velocity ◽  
Computational Time ◽  
Ice Thickness ◽  
Ice Flow ◽  
Antarctic Ice Sheet ◽  
Computationally Expensive ◽  
The Antarctic ◽  
Geothermal Flux

<p><span>Ice temperature within the ice is a crucial characteristic to understand the Antarctic ice sheet evolution because temperature is coupled to ice flow. Since temperature is only measured at few locations in deep boreholes, we only rely on numerical modelling to assess ice sheet-wide temperature. However, the design of such models leads to a number of challenges. One important difficulty is that the temperature field strongly depends on the geothermal flux which is still poorly known (see White paper by Burton-Johnson and others,2020 </span><span></span><span>). Another point is that up to now there is no fully suitable model, especially for inverse approaches: i</span><span>)</span><span> analytical solutions are only valid in slowly flowing regions; ii</span><span>)</span><span> models solving only the heat equation by prescribing geometry and ice flow do not take into account the past changes in ice thickness and ice flow and </span><span>do not couple </span><span>ice flow and temperature. Conversely, 3D thermomechanical models that simulate the evolution of the ice sheet take into account all the relevant processes but they are too computationally expensive to be used in inverse approaches. Moreover, they do not provide a perfect fit between observed and simulated geometry </span><span>(ice thickness, surface elevation) </span><span>for the present-day ice sheets </span><span>and this affects the simulated temperature field</span><span>.</span></p><p><span>GRISLI (Quiquet et al. 2018), belongs to this family of thermomechanically coupled ice sheet models An emulator, based on deep neural network (DNN), has been developed in order to speed-up the simulation of present-day ice temperature. We use GRISLI outputs that come from 4 simulations, each covers 900000 years (8 glacial-interglacial cycles) to get rid of the initial configuration influence. The simulations differ by the geothermal flux map used as boundary condition. Finally a database is built where each ice column for each simulation is a sample used to train the DNN. For each sample, the input layer (precursor) is a vector of the present-day characteristics: ice thickness, surface temperature, geothermal flux, accumulation rate, surface velocity and surface slope. The predicted output (output layer) is the vertical profile of temperature. In the training, the weights of the network are optimized by comparison with the GRISLI temperature. </span></p><p><span>The first results are very encouraging with a RMSE of ~ 0.6 °C (calculated from the difference between the emulated temperatures and GRISLI temperatures over all the samples and all the depths). Once trained, the computational time of GRISLI-DNN for generating temperature field of whole Antarctica (16000 columns) is about 20 s.</span></p><p><span>The first application (in the framework of the ESA project 4D-Antarctica, see Leduc-Leballeur<span> presentation in this session</span>) will be to use this emulator associated with SMOS satellite observations to infer the 3D temperature field and improve our knowledge of geothermal flux. Indeed, it has been shown that SMOS data, coupled with glaciological and electromagnetic models, give an indication of temperature in the upper 1000 m of the ice sheet. Our emulator could also be used for initialization of computationally expensive ice sheet models.</span></p>

scholarly journals Are longitudinal ice-surface structures on the Antarctic Ice Sheet indicators of long-term ice-flow configuration?

2014 ◽  
Vol 2 (2) ◽  
pp. 911-933 ◽  
Author(s):  
N. F. Glasser ◽  
S. J. A. Jennings ◽  
M. J. Hambrey ◽  
B. Hubbard
Keyword(s):  
Flow Line ◽  
Ice Sheet ◽  
Surface Structures ◽  
Surface Velocity ◽  
Ice Flow ◽  
Antarctic Ice Sheet ◽  
Ice Surface ◽  
Ice Stream ◽  
The Antarctic

Abstract. Continent-wide mapping of longitudinal ice-surface structures on the Antarctic Ice Sheet reveals that they originate in the interior of the ice sheet and are arranged in arborescent networks fed by multiple tributaries. Longitudinal ice-surface structures can be traced continuously down-ice for distances of up to 1200 km. They are co-located with fast-flowing glaciers and ice streams that are dominated by basal sliding rates above tens of m yr-1 and are strongly guided by subglacial topography. Longitudinal ice-surface structures dominate regions of converging flow, where ice flow is subject to non-coaxial strain and simple shear. Associating these structures with the AIS' surface velocity field reveals (i) ice residence times of ~ 2500 to 18 500 years, and (ii) undeformed flow-line sets for all major flow units analysed except the Kamb Ice Stream and the Institute and Möller Ice Stream areas. Although it is unclear how long it takes for these features to form and decay, we infer that the major ice-flow and ice-velocity configuration of the ice sheet may have remained largely unchanged for several thousand years, and possibly even since the end of the last glacial cycle. This conclusion has implications for our understanding of the long-term landscape evolution of Antarctica, including large-scale patterns of glacial erosion and deposition.

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

2018 ◽  
Vol 12 (5) ◽  
pp. 1767-1778 ◽  
Author(s):  
Fifi Ibrahime Adodo ◽  
Frédérique Remy ◽  
Ghislain Picard
Keyword(s):  
Seasonal Variations ◽  
Seasonal Cycle ◽  
Ice Sheet ◽  
Surface Elevation ◽  
Backscattering Coefficient ◽  
Antarctic Ice Sheet ◽  
Radar Wave ◽  
The Antarctic ◽  
Snow Temperature ◽  
Ku Band

Abstract. Spaceborne radar altimeters are a valuable tool for observing the Antarctic Ice Sheet. The radar wave interaction with the snow provides information on both the surface and the subsurface of the snowpack due to its dependence on the snow properties. However, the penetration of the radar wave within the snowpack 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 ∼ 9.4 cm), Ku (13.6 GHz ∼ 2.3 cm) and Ka (37 GHz ∼ 0.8 cm) bands. We identified that the backscattering coefficient at Ku band reaches a maximum in winter in part of the continent (Region 1) and in the summer in the remaining (Region 2), while the evolution at other frequencies is relatively uniform over the whole continent. To explain this contrasting behavior between frequencies and between regions, we studied the sensitivity of the backscattering coefficient at three frequencies to several parameters (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 Ka frequency is dominated by the volume echo and is mainly driven by snow temperature evolution everywhere. In contrast, at S band, the cycle is dominated by the surface echo. At Ku band, the seasonal cycle is dominated by the volume echo in Region 1 and by the surface echo in Region 2. This investigation provides new information on the seasonal dynamics of the Antarctic Ice Sheet surface and provides new clues to build more accurate corrections of the radar altimeter surface elevation signal in the future.

PISM paleo simulations of the Antarctic Ice Sheet over the last two glacial cycles

2020 ◽  
Author(s):  
Torsten Albrecht ◽  
Ricarda Winkelmann ◽  
Anders Levermann
Keyword(s):  
Boundary Conditions ◽  
Sea Level ◽  
Ice Sheet ◽  
Sea Level Changes ◽  
Glacial Cycles ◽  
Antarctic Ice Sheet ◽  
History Of ◽  
Global Sea Level ◽  
Glacial Time ◽  
The Antarctic

<p>Simulations of the glacial-interglacial history of the Antarctic Ice Sheet provide insights into dynamic threshold behavior and estimates of the ice sheet's contributions to global sea-level changes, for the past, present and future. However, boundary conditions are weakly constrained, in particular at the interface of the ice-sheet and the bedrock. We use the Parallel Ice Sheet Model (PISM) to investigate the dynamic effects of different choices of input data and of various parameterizations on the sea-level relevant ice volume. We evaluate the model's transient sensitivity to corresponding parameter choices and to different boundary conditions over the last two glacial cycles and provide estimates of involved uncertainties. We also present isolated and combined effects of climate and sea-level forcing on glacial time scales. </p>

scholarly journals The Dynamics of the Antarctic Ice Sheet

1989 ◽  
Vol 12 ◽  
pp. 16-22 ◽  
Author(s):  
W.F. Budd ◽  
D. Jenssen
Keyword(s):  
Heat Flux ◽  
Surface Temperature ◽  
Accumulation Rate ◽  
Three Dimensional ◽  
Ice Sheet ◽  
Surface Velocity ◽  
Geothermal Heat ◽  
Antarctic Ice Sheet ◽  
The Past ◽  
The Antarctic

A three-dimensional dynamic, thermodynamic ice-sheet model has been developed to simulate the past, present, and future behaviour of the Antarctic ice sheet. The present ice velocities depend on the deep ice temperatures which in turn depend on the past changes of the ice sheet, including surface temperature, accumulation rate, and ice thickness. The basal temperatures are also strongly dependent on the geothermal heat flux. The model has therefore been used to study the effect on the basal temperatures, of changes to the geothermal heat flux, as well as the past changes of surface temperature and accumulation rate based on results obtained from the Vostok deep ice core. The model is also used to compute the distribution of surface velocity required to balance the present accumulation rate and the dynamics velocity based on the stress, temperature, and flow properties of ice, for the internal deformation, plus a component due to ice sliding. These velocities are compared to observed surface velocities in East Antarctica to assess the state of balance and the performance of the dynamics formulation.

scholarly journals Uncertainties in mass balance estimation of the Antarctic Ice Sheet using the input and output method

2021 ◽  
Author(s):  
Yijing Lin ◽  
Yan Liu ◽  
Zhitong Yu ◽  
Xiao Cheng ◽  
Qiang Shen ◽  
...  
Keyword(s):  
Mass Balance ◽  
Scale Effect ◽  
Ice Sheet ◽  
Ice Thickness ◽  
Clear Understanding ◽  
Antarctic Ice Sheet ◽  
Grounding Line ◽  
Ice Velocity ◽  
The Antarctic ◽  
The Impact

Abstract. The input-output method (IOM) is one of the most popular methods of estimating the ice sheet mass balance (MB), with a significant advantage in presenting the dynamics response of ice to climate change. Assessing the uncertainties of the MB estimation using the IOM is crucial to gaining a clear understanding of the Antarctic ice-sheet mass budget. Here, we introduce a framework for assessing the uncertainties in the MB estimation due to the methodological differences in the IOM, the impact of the parameterization and scale effect on the modeled surface mass balance (SMB, input), and the impact of the uncertainties of ice thickness, ice velocity, and grounding line data on ice discharge (D, output). For the assessment of the D’s uncertainty, we present D at a fine scale. Compared with the goal of determining the Antarctic MB within an uncertainty of 15 Gt yr−1, we found that the different strategies employed in the methods cause considerable uncertainties in the annual MB estimation. The uncertainty of the RACMO2.3 SMB caused by its parameterization can reach 20.4 Gt yr−1, while that due to the scale effect is up to 216.7 Gt yr−1. The observation precisions of the MEaSUREs InSAR-based velocity (1–17 m yr−1), the airborne radio-echo sounder thickness (±100 m), and the MEaSUREs InSAR-based grounding line (±100 m) contribute uncertainties of 17.1 Gt yr−1, 10.5 ± 2.7 Gt yr−1 and 8.0~27.8 Gt yr−1 to the D, respectively. However, the D’s uncertainty due to the remarkable ice thickness data gap, which is represented by the thickness difference between the BEDMAP2 and the BedMachine reaches 101.7 Gt yr−1, which indicates its dominant cause of the future D’s uncertainty. In addition, the interannual variability of D caused by the annual changes in the ice velocity and ice thickness are considerable compared with the target uncertainty of 15 Gt yr−1, which cannot be ignored in annual MB estimations.

scholarly journals Large-Scale Numerical Modelling of the Antarctic Ice Sheet

1982 ◽  
Vol 3 ◽  
pp. 42-49 ◽  
Author(s):  
W.F. Budd ◽  
I.N. Smith
Keyword(s):  
Sea Level ◽  
Large Scale ◽  
Accumulation Rate ◽  
Ice Sheet ◽  
Ice Thickness ◽  
Ice Flow ◽  
Antarctic Ice Sheet ◽  
Flow Parameters ◽  
The Antarctic ◽  
Thinning Rate

A large-scale dynamic numerical model of the Antarctic ice sheet has been developed to study its present state of ice flow and mass balance as well as its response to long-term changes of climate or sea-level.The flow of ice over a two-dimensional grid is determined from the ice thickness, the basal shear stress, the bedrock depth, and ice flow parameters derived from velocities of existing ice sheets. The change in ice thickness with time is governed by the continuity equation involving the ice flux divergence and the ice accumulation or ablation. At the ice sheet seaward boundary, a floating criterion and floating ice thinning rate apply. Bedrock depression with a time-delayed response dependent on the history of the ice load is also included.A 61 × 61 point grid with 100 km spacing has been used to represent the ice-sheet surface, bedrock, and accumulation rate. The model has been used to simul a te the growth of the present ice sheet and i ts reaction to changes of sea-level, bedrock depression, accumulation rate, ice flow parameters, and the iceshelf thinning rate.Preliminary results suggest that the present ice sheet is not in equilibrium but rather is still adjusting to changes of these parameters.

Impact of ice sheet reconstructions on the deglacial climate in iLOVECLIM

2021 ◽  
Author(s):  
Nathaelle Bouttes ◽  
Didier Roche ◽  
Fanny Lhardy ◽  
Aurelien Quiquet ◽  
Didier Paillard ◽  
...  
Keyword(s):  
Boundary Conditions ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Last Deglaciation ◽  
Antarctic Ice Sheet ◽  
Intermediate Complexity ◽  
Climate Evolution ◽  
The Last Deglaciation ◽  
The Antarctic ◽  
The Impact

<p>The last deglaciation is a time of large climate transition from a cold Last Glacial Maximum at 21,000 years BP with extensive ice sheets, to the warmer Holocene 9,000 years BP onwards with reduced ice sheets. Despite more and more proxy data documenting this transition, the evolution of climate is not fully understood and difficult to simulate. The PMIP4 protocol (Ivanovic et al., 2016) has indicated which boundary conditions to use in model simulations during this transition. The common boundary conditions should enable consistent multi model and model-data comparisons. While the greenhouse gas concentration evolution and orbital forcing are well known and easy to prescribe, the evolution of ice sheets is less well constrained and several choices can be made by modelling groups. First, two ice sheet reconstructions are available: ICE-6G (Peltier et al., 2015) and GLAC-1D (Tarasov et al., 2014). On top of topographic changes, it is left to modelling groups to decide whether to account for the associated bathymetry and land-sea mask changes, which is technically more demanding. These choices could potentially lead to differences in the climate evolution, making model comparisons more complicated.</p><p>We use the iLOVECLIM model of intermediate complexity (Goosse et al., 2010) to evaluate the impact of different ice sheet reconstructions and the effect of bathymetry changes on the global climate evolution during the Last deglaciation. We test the two ice sheet reconstructions (ICE-6G and GLAC-1D), and have implemented changes of bathymetry and land-sea mask. In addition, we also evaluate the impact of accounting for the Antarctic ice sheet evolution compared to the Northern ice sheets only.</p><p>We show that despite showing the same long-term changes, the two reconstructions lead to different evolutions. The bathymetry plays a role, although only few changes take place before ~14ka. Finally, the impact of the Antarctic ice sheet is important during the deglaciation and should not be neglected.</p><p>References</p><p>Goosse, H., et al., Description of the Earth system model of intermediate complexity LOVECLIM version 1.2, Geosci. Model Dev., 3, 603–633, https://doi.org/10.5194/gmd-3-603-2010, 2010</p><p>Ivanovic, R. F., et al., Transient climate simulations of the deglaciation 21–9 thousand years before present (version 1) – PMIP4 Core experiment design and boundary conditions, Geosci. Model Dev., 9, 2563–2587, https://doi.org/10.5194/gmd-9-2563-2016, 2016</p><p>Peltier, W. R., Argus, D. F., and Drummond, R., Space geodesy constrains ice age terminal deglaciation: The global ICE-6G_C (VM5a) model, J. Geophys. Res.-Sol. Ea., 120, 450–487, doi:10.1002/2014JB011176, 2015</p><p>Tarasov,L.,  et al., The global GLAC-1c deglaciation chronology, melwater pulse 1-a, and a question of missing ice, IGS Symposium on Contribution of Glaciers and Ice Sheets to Sea-Level Change, 2014</p>

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

1998 ◽  
Vol 27 ◽  
pp. 1-6 ◽  
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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