scholarly journals Modeling dynamic controls on ice streams: a Bayesian statistical approach

2008 ◽  
Vol 54 (187) ◽  
pp. 705-714 ◽  
Author(s):  
L.M. Berliner ◽  
K. Jezek ◽  
N. Cressie ◽  
Y. Kim ◽  
C.Q. Lam ◽  
...  
Keyword(s):  
Statistical Approach ◽  
Stochastic Equation ◽  
Bayes Theorem ◽  
Surface Velocity ◽  
Ice Streams ◽  
Ice Stream ◽  
Modeling Assumption ◽  
Using Data ◽  
Basal Shear ◽  
Analyze Data

AbstractOur main goal is to exemplify the study of ice-stream dynamics via Bayesian statistical analysis incorporating physical, though imperfectly known, models using data that are both incomplete and noisy. The physical–statistical models we propose account for these uncertainties in a coherent, hierarchical manner. The initial modeling assumption estimates basal shear stress as equal to driving stress, but subsequently includes a random corrector process to account for model error. The resulting stochastic equation is incorporated into a simple model for surface velocities. Use of Bayes’ theorem allows us to make inferences on all unknowns given basal elevation, surface elevation and surface velocity. The result is a posterior distribution of possible values that can be summarized in a number of ways. For example, the posterior mean of the stress field indicates average behavior at any location in the field, and the posterior standard deviations describe associated uncertainties. We analyze data from the ‘Northeast Greenland Ice Stream’ and illustrate how scientific conclusions may be drawn from our Bayesian analysis.

Surface velocity and mass balance of Ice Streams D and E, West Antarctica

1996 ◽  
Vol 42 (142) ◽  
pp. 461-475 ◽  
Author(s):  
Robert Bindschadler ◽  
Patricia Vornberger ◽  
Donald Blankenship ◽  
Ted Scambos ◽  
Robert Jacobel
Keyword(s):  
Mass Balance ◽  
Surface Velocity ◽  
Random Errors ◽  
West Antarctica ◽  
Velocity Measurements ◽  
Ice Streams ◽  
Ice Stream ◽  
Data Density ◽  
Basal Shear ◽  
Collection Methods

AbstractOver 75 000 surface-velocity measurements are extracted from sequential satellite imagery of Ice Streams D and E to reveal a complex pattern of flow not apparent from previous measurements. Horizontal and vertical strain rates, calculated from surface velocity, indicate that the bed experiences larger basal shear where the surface of these ice streams is rougher. Ten airborne-radar profiles and one surface-based radar profile of ice thickness make possible the calculation of mass balance for longitudinal sections of each ice stream. Improved data-collection methods increase data density, substantially reducing random errors in velocity. However, systematic errors continue to limit the ability of the flux-differencing technique used here to resolve local variations in mass balance. Nevertheless, significant local variations in mass balance are revealed, while, overall, Ice Streams D and E are in approximate equilibrium. An earlier estimate of the net mass balance for Ice Stream D is improved.

Surface velocity and mass balance of Ice Streams D and E, West Antarctica

1996 ◽  
Vol 42 (142) ◽  
pp. 461-475 ◽  
Author(s):  
Robert Bindschadler ◽  
Patricia Vornberger ◽  
Donald Blankenship ◽  
Ted Scambos ◽  
Robert Jacobel
Keyword(s):  
Mass Balance ◽  
Surface Velocity ◽  
Random Errors ◽  
West Antarctica ◽  
Velocity Measurements ◽  
Ice Streams ◽  
Ice Stream ◽  
Data Density ◽  
Basal Shear ◽  
Collection Methods

AbstractOver 75 000 surface-velocity measurements are extracted from sequential satellite imagery of Ice Streams D and E to reveal a complex pattern of flow not apparent from previous measurements. Horizontal and vertical strain rates, calculated from surface velocity, indicate that the bed experiences larger basal shear where the surface of these ice streams is rougher. Ten airborne-radar profiles and one surface-based radar profile of ice thickness make possible the calculation of mass balance for longitudinal sections of each ice stream. Improved data-collection methods increase data density, substantially reducing random errors in velocity. However, systematic errors continue to limit the ability of the flux-differencing technique used here to resolve local variations in mass balance. Nevertheless, significant local variations in mass balance are revealed, while, overall, Ice Streams D and E are in approximate equilibrium. An earlier estimate of the net mass balance for Ice Stream D is improved.

scholarly journals Interpretation of radar-detected internal layer folding in West Antarctic ice streams

1993 ◽  
Vol 39 (133) ◽  
pp. 528-537 ◽  
Author(s):  
W. Jacobel Robert ◽  
M. Gades Anthony ◽  
L. Gottschling David ◽  
M. Hodge Steven ◽  
L. Wright David
Keyword(s):  
Flow Direction ◽  
Low Frequency ◽  
Internal Layers ◽  
West Antarctica ◽  
Ice Streams ◽  
West Antarctic ◽  
Ice Stream ◽  
High Resolution Images ◽  
Basal Shear ◽  
Inland Ice

AbstractLow-frequency surface-based radar-profiling experiments on Ice Streams Β and C, West Antarctica, have yielded high-resolution images which depict folding of the internal layers that can aid in the interpretation of ice-stream dynamics. Unlike folding seen in most earlier radar studies of ice sheets, the present structures have no relationship to bedrock topography and show tilting of their axial fold planes in the flow direction. Rather than being standing waves created by topography or local variations in basal shear stress, the data show that these folds originate upstream of the region of streaming flow and are advected into the ice streams. The mechanism for producing folds is hypothesized to be changes in the basal boundary conditions as the ice makes the transition from inland ice to ice-stream flow. Migration of this transition zone headward can then cause folds in the internal layering to be propagated down the ice streams.

scholarly journals Force, Mass, and Energy Budgets of the Crary Ice Rise Complex, Antarctica

1987 ◽  
Vol 33 (114) ◽  
pp. 218-230 ◽  
Author(s):  
D.R. Macayeal ◽  
R.A. Bindschadler ◽  
S. Shabtaie ◽  
S. Stephenson ◽  
C.R. Bentley
Keyword(s):  
Potential Energy ◽  
Snow Accumulation ◽  
Back Pressure ◽  
Surface Strain ◽  
Surface Velocity ◽  
Pressure Force ◽  
Ice Shelf ◽  
Ice Streams ◽  
Ice Stream ◽  
Work Done

AbstractResistive force exerted by the Crary Ice Rise on its ice-shelf/ice-stream environment and back-pressure force transmitted across the grounding lines of Ice Streams A and B are calculated from airborne radio echo-sounding data and measurements of surface strain-rates. Resistance generated by the ice rise ranges in magnitude between 45 and 51% of the back-pressure force on the ice streams (depending on the flow law). The mechanical-energy budget of the ice rise is computed by considering work done against frictional forces at the perimeter of the ice rise and gravitational potential energy fluxes associated with changing mass distribution in the ice/ocean system. Energy dissipated by flow surrounding the ice rise is balanced by potential energy released within Ice Streams A and B, and accounts for between 15 and 49% of the work done by the ice streams against ice-shelf back pressure at their grounding lines. Mass balance of the ice rise, and the discharge of Ice Streams A and B, are calculated from surface-velocity and snow-accumulation measurements. The ice rise and its immediate environment gain mass by advection and snowfall at a rate equivalent to an area-averaged thickening rate of 0.44 ± 0.06 m/year. This mass gain may be balanced by regional basal melting (which we do not measure), or could contribute to ice-rise expansion through regional thickening and ice-shelf grounding. Approximately 1/4 to 1/2 of the excess volume discharged by Ice Streams A and B above snow accumulation in their catchment areas is deposited in the vicinity of the ice rise (or melted from the bottom of the ice shelf). This suggests that the ice rise may have formed as a consequence of recent ice-stream acceleration, and that its continued growth may eventually reverse this trend of ice-stream discharge.

scholarly journals Ribbed bedforms on palaeo-ice stream beds resemble regular patterns of basal shear stress (‘traction ribs’) inferred from modern ice streams

2016 ◽  
Vol 62 (234) ◽  
pp. 696-713 ◽  
Author(s):  
CHRIS R. STOKES ◽  
MARTIN MARGOLD ◽  
TIMOTHY T. CREYTS
Keyword(s):  
Shear Stress ◽  
Ice Sheets ◽  
Western Canada ◽  
Coupled Flow ◽  
Ice Streams ◽  
Ice Stream ◽  
Stream Beds ◽  
Basal Shear ◽  
Subglacial Meltwater ◽  
Regular Patterns

Rapidly-flowing ice streams are an important mechanism through which ice sheets lose mass, and much work has been focussed on elucidating the processes that increase or decrease their velocity. Recent work using standard inverse methods has inferred previously-unrecognised regular patterns of high basal shear stress (‘sticky spots’ >200 kPa) beneath a number of ice streams in Antarctica and Greenland, termed ‘traction ribs’. They appear at a scale intermediate between smaller ribbed moraines and much larger mega-ribs observed on palaeo-ice sheet beds, but it is unclear whether they have a topographic expression at the bed. Here, we report observations of rib-like bedforms from DEMs along palaeo-ice stream beds in western Canada that resemble both the pattern and dimensions of traction ribs. Their identification suggests that traction ribs may have a topographic expression that lies between, and partly overlaps with, ribbed moraines and much larger mega-ribs. These intermediate-sized bedforms support the notion of a ribbed bedform continuum. Their formation remains conjectural, but our observations from palaeo-ice streams, coupled with those from modern ice masses, suggest they are related to wave-like instabilities occurring in the coupled flow of ice and till and modulated by subglacial meltwater drainage. Their form and pattern may also involve glaciotectonism of subglacial sediments.

scholarly journals ‘Sticky spots’ and subglacial lakes under ice streams of the Siple Coast, Antarctica

2011 ◽  
Vol 52 (58) ◽  
pp. 18-22 ◽  
Author(s):  
Olga V. Sergienko ◽  
Christina L. Hulbe
Keyword(s):  
Numerical Models ◽  
Water Source ◽  
Surface Velocity ◽  
Ice Streams ◽  
Ice Flow ◽  
Stream Surface ◽  
Topographic Features ◽  
West Antarctic ◽  
Ice Stream ◽  
Subglacial Lakes

AbstarctLocations of subglacial lakes discovered under fast-moving West Antarctic ice streams tend to be associated with topographic features of the subglacial bed or with areas that have strong variations in basal conditions. Inversion of ice-stream surface velocity indicates that basal conditions under ice streams can be highly variable and that there can be widespread regions where basal traction is high. To seek an explanation for why lakes appear to be sited near areas with high basal traction, we use numerical models to simulate ice-stream dynamics, thermodynamics and subglacial water flow. We demonstrate that the ice flow over high basal traction areas produces favourable conditions for the ponding of meltwater. Energy dissipation associated with ice sliding over a region with high basal traction constitutes a water source supplying a lake, and ice-thickness perturbations induced by ice flow over variable traction create local minima in hydraulic potential. Variations in thermodynamic processes caused by such ice flow could be responsible for limiting the horizontal extent of the subglacial lakes.

scholarly journals Surface undulations of Antarctic ice streams tightly controlled by bedrock topography

2012 ◽  
Vol 6 (5) ◽  
pp. 4485-4516 ◽  
Author(s):  
J. De Rydt ◽  
G. H. Gudmundsson ◽  
H. F. J. Corr ◽  
P. Christoffersen
Keyword(s):  
Spatial Scales ◽  
Radar Data ◽  
Surface Velocity ◽  
Ice Thickness ◽  
Ice Streams ◽  
Slip Ratio ◽  
Flow Models ◽  
Bedrock Topography ◽  
Ice Stream ◽  
The Mean

Abstract. Full Stokes models predict that fast-flowing ice streams transmit information about their bedrock topography most efficiently to the surface for basal undulations with length scales between 1 and 20 times the mean ice thickness. This typical behaviour is independent on the precise values of the flow law and sliding law exponents, and should be universally observable. However, no experimental evidence for this important theoretical prediction has been obtained so far, hence ignoring an important test for the physical validity of current-day ice flow models. In our work we use recently acquired airborne radar data for the Rutford Ice Stream and Evans Ice Stream, and we show that the surface response of fast-flowing ice is highly sensitive to bedrock irregularities with wavelengths of several ice thicknesses. The sensitivity depends on the slip ratio, i.e. the ratio between mean basal sliding velocity and mean deformational velocity. We find that higher values of the slip ratio generally lead to a more efficient transfer, whereas the transfer is significantly dampened for ice that attains most of its surface velocity by creep. Our findings underline the importance of bedrock topography for ice stream dynamics on spatial scales up to 20 times the mean ice thickness. Our results also suggest that local variations in the flow regime and surface topography at this spatial scale cannot be explained by variations in basal slipperiness.

Integrating satellite observations with modelling: basal shear stress of the Filcher-Ronne ice streams, Antarctica

2006 ◽  
Vol 364 (1844) ◽  
pp. 1795-1814 ◽  
Author(s):  
Ian Joughin ◽  
Jonathan L Bamber ◽  
Ted Scambos ◽  
Slawek Tulaczyk ◽  
Mark Fahnestock ◽  
...  
Keyword(s):  
Shear Stress ◽  
Ice Sheet ◽  
Inverse Methods ◽  
Satellite Observations ◽  
Major Influence ◽  
Ice Streams ◽  
Ice Stream ◽  
Subglacial Topography ◽  
Basal Shear ◽  
Thickness Measurements

Using inverse methods constrained by recent satellite observations, we have produced a comprehensive estimate of the basal shear stress beneath the Filchner-Ronne ice streams. The inversions indicate that a weak bed (approx. 4–20 kPa) underlies much of these ice streams. Compared to the Ross ice streams, the distribution of weak subglacial till is more heterogeneous, with ‘sticky spots’ providing much of the resistance to flow. A weak bed beneath Recovery ice stream extends several hundred kilometres inland with flow. Along this ice stream, discrepancies between thickness measurements and flux estimates suggest the existence of a deep (−1400 m) trough not resolved by existing maps of subglacial topography. We hypothesize that the presence of this and other deep troughs is a major influence on this sector of the ice sheet that is not fully incorporated in current models of ice-sheet evolution.

scholarly journals Surface undulations of Antarctic ice streams tightly controlled by bedrock topography

2013 ◽  
Vol 7 (2) ◽  
pp. 407-417 ◽  
Author(s):  
J. De Rydt ◽  
G. H. Gudmundsson ◽  
H. F. J. Corr ◽  
P. Christoffersen
Keyword(s):  
Flow Line ◽  
Spatial Scales ◽  
Radar Data ◽  
Surface Velocity ◽  
Ice Thickness ◽  
Ice Streams ◽  
Slip Ratio ◽  
Bedrock Topography ◽  
Ice Stream ◽  
The Mean

Abstract. Full Stokes flow-line models predict that fast-flowing ice streams transmit information about their bedrock topography most efficiently to the surface for basal undulations with length scales between 1 and 20 times the mean ice thickness. This typical behaviour is independent of the precise values of the flow law and sliding law exponents, and should be universally observable. However, no experimental evidence for this important theoretical prediction has been obtained so far, hence ignoring an important test for the physical validity of current-day ice flow models. In our work we use recently acquired airborne radar data for the Rutford Ice Stream and Evans Ice Stream, and we show that the surface response of fast-flowing ice is highly sensitive to bedrock irregularities with wavelengths of several ice thicknesses. The sensitivity depends on the slip ratio, i.e. the ratio between mean basal sliding velocity and mean deformational velocity. We find that higher values of the slip ratio generally lead to a more efficient transfer, whereas the transfer is significantly dampened for ice that attains most of its surface velocity by creep. Our findings underline the importance of bedrock topography for ice stream dynamics on spatial scales up to 20 times the mean ice thickness. Our results also suggest that local variations in the flow regime and surface topography at this spatial scale cannot be explained by variations in basal slipperiness.

scholarly journals Force, Mass, and Energy Budgets of the Crary Ice Rise Complex, Antarctica

1987 ◽  
Vol 33 (114) ◽  
pp. 218-230 ◽  
Author(s):  
D.R. Macayeal ◽  
R.A. Bindschadler ◽  
S. Shabtaie ◽  
S. Stephenson ◽  
C.R. Bentley
Keyword(s):  
Potential Energy ◽  
Snow Accumulation ◽  
Back Pressure ◽  
Surface Strain ◽  
Surface Velocity ◽  
Pressure Force ◽  
Ice Shelf ◽  
Ice Streams ◽  
Ice Stream ◽  
Work Done

AbstractResistive force exerted by the Crary Ice Rise on its ice-shelf/ice-stream environment and back-pressure force transmitted across the grounding lines of Ice Streams A and B are calculated from airborne radio echo-sounding data and measurements of surface strain-rates. Resistance generated by the ice rise ranges in magnitude between 45 and 51% of the back-pressure force on the ice streams (depending on the flow law). The mechanical-energy budget of the ice rise is computed by considering work done against frictional forces at the perimeter of the ice rise and gravitational potential energy fluxes associated with changing mass distribution in the ice/ocean system. Energy dissipated by flow surrounding the ice rise is balanced by potential energy released within Ice Streams A and B, and accounts for between 15 and 49% of the work done by the ice streams against ice-shelf back pressure at their grounding lines. Mass balance of the ice rise, and the discharge of Ice Streams A and B, are calculated from surface-velocity and snow-accumulation measurements. The ice rise and its immediate environment gain mass by advection and snowfall at a rate equivalent to an area-averaged thickening rate of 0.44 ± 0.06 m/year. This mass gain may be balanced by regional basal melting (which we do not measure), or could contribute to ice-rise expansion through regional thickening and ice-shelf grounding. Approximately 1/4 to 1/2 of the excess volume discharged by Ice Streams A and B above snow accumulation in their catchment areas is deposited in the vicinity of the ice rise (or melted from the bottom of the ice shelf). This suggests that the ice rise may have formed as a consequence of recent ice-stream acceleration, and that its continued growth may eventually reverse this trend of ice-stream discharge.

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