scholarly journals Reassessing the flow law of glacier ice using satellite observations

2021 ◽  
Author(s):  
Joanna Millstein ◽  
Brent Minchew ◽  
Samuel Pegler
Keyword(s):  
Viscous Flow ◽  
Satellite Observations ◽  
Ice Flow ◽  
Ice Shelves ◽  
Accurate Representation ◽  
Flow Models ◽  
Glacier Dynamics ◽  
Glacier Ice ◽  
Flow Law ◽  
Range Of Values

Accurate representation of the viscous flow of ice is fundamental to understanding glacier dynamics and projecting sea-level rise. Ice viscosity is often described by a simple but largely untested and uncalibrated constitutive relation, Glen’s Flow Law, wherein the rate of deformation is proportional to stress raised to the power n. The value n = 3 is commonly prescribed in ice-flow models, though observations and experiments support a range of values across stresses and temperatures found on Earth. Here, we leverage recent remotely-sensed observations of Antarctic ice shelves to show that Glen’s Flow Law approximates the viscous flow of ice with n = 4.1 ± 0.4 in fast-flowing areas. The viscosity and flow rate of ice are therefore more sensitive to changes in stress than most ice-flow models allow.

scholarly journals Calving cycle of the Brunt Ice Shelf, Antarctica, driven by changes in ice shelf geometry

2019 ◽  
Vol 13 (10) ◽  
pp. 2771-2787 ◽  
Author(s):  
Jan De Rydt ◽  
Gudmundur Hilmar Gudmundsson ◽  
Thomas Nagler ◽  
Jan Wuite
Keyword(s):  
Large Scale ◽  
Ice Shelf ◽  
Ice Flow ◽  
Ice Shelves ◽  
Flow Models ◽  
Speed Up ◽  
The Antarctic ◽  
Detrimental Impact ◽  
Natural Laboratory

Abstract. Despite the potentially detrimental impact of large-scale calving events on the geometry and ice flow of the Antarctic Ice Sheet, little is known about the processes that drive rift formation prior to calving, or what controls the timing of these events. The Brunt Ice Shelf in East Antarctica presents a rare natural laboratory to study these processes, following the recent formation of two rifts, each now exceeding 50 km in length. Here we use 2 decades of in situ and remote sensing observations, together with numerical modelling, to reveal how slow changes in ice shelf geometry over time caused build-up of mechanical tension far upstream of the ice front, and culminated in rift formation and a significant speed-up of the ice shelf. These internal feedbacks, whereby ice shelves generate the very conditions that lead to their own (partial) disintegration, are currently missing from ice flow models, which severely limits their ability to accurately predict future sea level rise.

Relating polarimetric radar measurements of ice fabric to ice-flow enhancement of Rutford Ice Stream

2020 ◽  
Author(s):  
Thomas Jordan ◽  
Alex Brisbourne ◽  
Carlos Martin ◽  
Rebecca Schlegel ◽  
Dustin Schroeder ◽  
...  
Keyword(s):  
Polarimetric Radar ◽  
Ice Streams ◽  
Ice Flow ◽  
Near Surface ◽  
Flow Models ◽  
Relative Contribution ◽  
Ice Stream ◽  
Glacier Ice ◽  
Flow Enhancement ◽  
The Impact

<p>Lateral shear margins provide resistance to ice flow within ice streams and play an important role in the overall dynamics of ice sheets. The strength and location of shear margins are known to be influenced by both subglacial factors (e.g. bed roughness, meltwater availability) and ice rheology (ice temperature, ice fabric, and damage). Assessing the relative contribution of these factors upon ice-stream flow is complex but can be aided by geophysical measurements (e.g. radar-sounding and seismic imaging) of the ice-stream subsurface. There are, however, ongoing challenges in obtaining geophysical information in an appropriate form to be incorporated into ice-flow models. This is true of ice fabric, and its direction-dependent effect upon ice viscosity is typically neglected in models of ice streams.</p><p>Here we develop a framework to relate ice fabric measurements from polarimetric radar sounding to ice-flow enhancement within ice streams. First, we extend a `polarimetric coherence’ radar method to automate the extraction of ice fabric using quad-polarized data.  Second, using a previously developed anisotropic flow-law formulation, we relate the radar fabric measurements to direction-dependent enhancement factors of glacier ice. We demonstrate the approach using a radar ground survey, collected by the British Antarctic Survey, which traverses between the centre and shear margin of Rutford Ice Stream. The data indicate that a vertical girdle fabric is present in the near-surface of the ice stream (approximately the top 300 m) which azimuthally rotates and strengthens toward the shear margin. We then assess the effect that the girdle fabric has upon shear and compression and the impact upon ice-flow models of Rutford Ice Stream.</p><p> </p><p> </p><p> </p>

scholarly journals Calving cycle of the Brunt Ice Shelf, Antarctica, driven by changes in ice-shelf geometry

2019 ◽  
Author(s):  
Jan De Rydt ◽  
G. Hilmar Gudmundsson ◽  
Thomas Nagler ◽  
Jan Wuite
Keyword(s):  
Large Scale ◽  
Ice Shelf ◽  
Ice Flow ◽  
Ice Shelves ◽  
Flow Models ◽  
Speed Up ◽  
The Antarctic ◽  
Detrimental Impact ◽  
Natural Laboratory

Abstract. Despite the potentially detrimental impact of large-scale calving events on the geometry and ice flow of the Antarctic Ice Sheet, little is known about the processes that drive rift formation prior to calving, or what controls the timing of these events. The Brunt Ice Shelf in East Antarctica presents a rare natural laboratory to study these processes, following the recent formation of two rifts, each now exceeding 50 km in length. Here we use a unique 50-years' time series of in-situ and remote sensing observations, together with numerical modelling, to reveal how slow changes in ice shelf geometry over time caused build-up of mechanical tension far upstream of the ice front, and culminated in rift formation and a significant speed-up of the ice shelf. These internal feedbacks, whereby ice shelves generate the very conditions that lead to their own (partial) disintegration are currently missing from ice flow models, which severely limits their ability to accurately predict future sea level rise.

scholarly journals Assimilation of Antarctic velocity observations provides evidence for uncharted pinning points

2015 ◽  
Vol 9 (2) ◽  
pp. 1461-1502
Author(s):  
J. J. Fürst ◽  
G. Durand ◽  
F. Gillet-Chaulet ◽  
N. Merino ◽  
L. Tavard ◽  
...  
Keyword(s):  
Ice Shelf ◽  
Ice Flow ◽  
Ice Shelves ◽  
Basal Friction ◽  
A Value ◽  
Model Community ◽  
Glacier Ice ◽  
The Antarctic ◽  
Shelf Flow ◽  
Velocity Mismatch

Abstract. In ice flow modelling, the use of control methods to assimilate the dynamic and geometric state of an ice body has become common practice. These methods have primarily focussed on inverting for one of the two least known properties in glaciology, namely the basal friction coefficient or the ice viscosity parameter. Here, we present an approach to infer both properties simultaneously for the whole of the Antarctic ice sheet. During the assimilation, the root-mean-square deviation between modelled and observed surface velocities is reduced to 12.3 m a−1, with a value of 16.4 m a−1 for the ice shelves. An exception in terms of the velocity mismatch is the Thwaites Glacier ice shelf, where the RMS value attains almost 80 m a−1. The reason is that the underlying BEDMAP2 geometry ignores the presence of an ice rise, that exerts major control on the dynamics of the eastern part of the ice shelf. On these grounds, we suggest an approach to account for pinning points not included in BEDMAP2 by locally allowing an optimisation of basal friction during the inversion. In this way, the velocity mismatch on the Thwaites ice shelf is more than halved. A characteristic velocity mismatch pattern emerges for unaccounted pinning points close to the marine shelf front. This pattern is exploited to manually identify 7 uncharted features around Antarctica that exert significant resistance to the shelf flow. Potential pinning points are detected on Fimbul, West, Shakelton, Nickerson and Venable ice shelves. As pinning points can provide substantial resistance to shelf flow, with considerable consequences if they became ungrounded in the future, the model community is in need of detailed bathymetry there. Our data assimilation points to some of these dynamically important features, not present in BEDMAP2, and implicitly quantifies their relevance.

scholarly journals The control of an uncharted pinning point on the flow of an Antarctic ice shelf

2016 ◽  
Vol 62 (231) ◽  
pp. 37-45 ◽  
Author(s):  
SOPHIE BERGER ◽  
LIONEL FAVIER ◽  
REINHARD DREWS ◽  
JEAN-JACQUES DERWAEL ◽  
FRANK PATTYN
Keyword(s):  
Global Navigation Satellite Systems ◽  
Ice Shelf ◽  
Alos Palsar ◽  
Secondary Importance ◽  
Ice Flow ◽  
Ice Shelves ◽  
Satellite Systems ◽  
Flow Models ◽  
Ice Velocity ◽  
Global Navigation Satellite

ABSTRACTAntarctic ice shelves are buttressed by numerous pinning points attaching to the otherwise freely-floating ice from below. Some of these kilometric-scale grounded features are unresolved in Antarctic-wide datasets of ice thickness and bathymetry, hampering ice flow models to fully capture dynamics at the grounding line and upstream. We investigate the role of an 8.7 km2 pinning point at the front of the Roi Baudouin Ice Shelf, East Antarctica. Using ERS interferometry and ALOS-PALSAR speckle tracking, we derive, on a 125 m grid spacing, surface velocities deviating by −5.2 ± 4.5 m a−1 from 37 on-site global navigation satellite systems-derived velocities. We find no evidence for ice flow changes on decadal time scales and we show that ice on the pinning point virtually stagnates, deviating the ice stream and causing enhanced horizontal shearing upstream. Using the BISICLES ice-flow model, we invert for basal friction and ice rigidity with three input scenarios of ice velocity and geometry. We show that inversion results are the most sensitive to the presence/absence of the pinning point in the bathymetry; surface velocities at the pinning point are of secondary importance. Undersampling of pinning points results in erroneous ice-shelf properties in models initialised by control methods. This may impact prognostic modelling for ice-sheet evolution in the case of unpinning.

scholarly journals Ice flow at low deviatoric stress

2003 ◽  
Vol 49 (166) ◽  
pp. 359-369 ◽  
Author(s):  
Erin C. Pettit ◽  
Edwin D. Waddington
Keyword(s):  
Stress Exponent ◽  
Constitutive Relations ◽  
Surface Profile ◽  
Deviatoric Stress ◽  
Constitutive Relationship ◽  
Internal Layers ◽  
Flow Mechanism ◽  
Ice Flow ◽  
Flow Models ◽  
Flow Law

AbstractThe effective viscosity of ice depends upon many factors, including temperature, deviatoric stress, crystal orientation and impurities. A flow law that includes these factors and is simple to implement is a requirement for numerically efficient ice-flow models. The dominant microscale flow mechanism changes as temperature, deviatoric stress or grain-size changes. For both anisotropic and isotropic constitutive relations, this shift in dominant flow mechanism is expressed as a change in the stress exponent. We study the effects of this shift in stress exponent on ice flow using a two-term flow law for isotropic ice. Our stress–strain-rate relationship does not explicitly describe the microscale processes of ice deformation; however, it encompasses a range of deformation behaviors with a simple law. In terrestrial ice, a flow-mechanism shift may occur in low-deviatoric-stress regions near ice divides, resulting in a near-linear constitutive relationship for ice flow. Compared to a non-linear (Glen) divide, a divide dominated by a near-linear flow mechanism has vertical-velocity profiles that are similar at divide and flank sites, internal layers that do not develop a Raymond bump, and a steady-state surface profile that is more rounded near the divide.

scholarly journals Tidal bending of ice shelves as a mechanism for large-scale temporal variations in ice flow

2017 ◽  
Author(s):  
Sebastian H. R. Rosier ◽  
G. Hilmar Gudmundsson
Keyword(s):  
Large Scale ◽  
Temporal Variations ◽  
Mean Flow ◽  
Ice Shelf ◽  
Ice Flow ◽  
Ice Shelves ◽  
Glacier Ice ◽  
Bending Stresses ◽  
Viscoelastic Modelling ◽  
Shelf Flow

Abstract. GPS measurements reveal strong modulation of horizontal ice-shelf and ice-stream flow at a variety of tidal frequencies, most notably a fortnightly (Msf) frequency not present in the vertical tides themselves. Current theories largely fail to explain the strength and prevalence of this signal over floating ice shelves. We propose that tidal bending stresses, through the nonlinear rheology of glacier ice, can have a sufficiently large impact on the effective viscosity of ice along its floating margins to give rise to significant and widespread temporal variations in the horizontal velocity of ice shelves. Using full-Stokes viscoelastic modelling, we show that inclusion of tidal bending within the model accounts for much of the observed tidal modulation of ice-shelf flow. Furthermore, our model shows that, in the absence of vertical tidal forcing, the mean flow of the ice shelf is reduced by almost 30 % for the geometry that we consider.

scholarly journals Assimilation of Antarctic velocity observations provides evidence for uncharted pinning points

2015 ◽  
Vol 9 (4) ◽  
pp. 1427-1443 ◽  
Author(s):  
J. J. Fürst ◽  
G. Durand ◽  
F. Gillet-Chaulet ◽  
N. Merino ◽  
L. Tavard ◽  
...  
Keyword(s):  
Ice Shelf ◽  
Ice Flow ◽  
Ice Shelves ◽  
Basal Friction ◽  
Flow Potential ◽  
Model Community ◽  
Glacier Ice ◽  
The Antarctic ◽  
Shelf Flow ◽  
Velocity Mismatch

Abstract. In ice flow modelling, the use of control methods to assimilate the dynamic and geometric state of an ice body has become common practice. These methods have primarily focussed on inverting for one of the two least known properties in glaciology, namely the basal friction coefficient or the ice viscosity parameter. Here, we present an approach to infer both properties simultaneously for the whole of the Antarctic ice sheet. After the assimilation, the root-mean-square deviation between modelled and observed surface velocities attains 8.7 m a−1 for the entire domain, with a slightly higher value of 14.0 m a−1 for the ice shelves. An exception in terms of the velocity mismatch is the Thwaites Glacier Ice Shelf, where the RMS value is almost 70 m a−1. The reason is that the underlying Bedmap2 geometry ignores the presence of an ice rise, which exerts major control on the dynamics of the eastern part of the ice shelf. On these grounds, we suggest an approach to account for pinning points not included in Bedmap2 by locally allowing an optimisation of basal friction during the inversion. In this way, the velocity mismatch on the ice shelf of Thwaites Glacier is more than halved. A characteristic velocity mismatch pattern emerges for unaccounted pinning points close to the marine shelf front. This pattern is exploited to manually identify seven uncharted features around Antarctica that exert significant resistance to the shelf flow. Potential pinning points are detected on Fimbul, West, Shackleton, Nickerson and Venable ice shelves. As pinning points can provide substantial resistance to shelf flow, with considerable consequences if they became ungrounded in the future, the model community is in need of detailed bathymetry there. Our data assimilation points to some of these dynamically important features not present in Bedmap2 and implicitly quantifies their relevance.

Inferring ice rheology in Antarctic ice shelves from remotely sensed observations

2021 ◽  
Author(s):  
Joanna Millstein ◽  
Brent Minchew
Keyword(s):  
Stress Exponent ◽  
Ice Sheets ◽  
Remotely Sensed ◽  
Creep Mechanism ◽  
Surface Velocity ◽  
Dislocation Creep ◽  
Ice Flow ◽  
Ice Shelves ◽  
Creep Mechanisms ◽  
Flow Law

<p>Glaciers and ice sheets flow as a consequence of ice rheology. At the temperatures and pressures found on Earth, several creep mechanisms allow glacier ice to flow as a non-Newtonian (shear-thinning) viscous fluid. The semi-empirical constitutive relation known as Glen’s Flow Law is often used to describe ice flow and to provide a simple expression for an effective viscosity that decreases with increasing stress and deformation rate. Glen’s Flow Law is a power-law relation between effective strain rate and deviatoric stress, with two parameters defining the rheology of ice: a rate factor, A, and stress exponent, n. The rate factor depends on features such as temperature and grain size, while the stress exponent is primarily representative of the creep mechanism. Neither A nor n are well constrained in natural ice, and the stress exponent is typically assumed to be n = 3 everywhere. Here, we take advantage of recent improvements in remotely sensed observations of surface velocity and ice shelf thickness to infer the values of A and n in Antarctic ice shelves. We focus on areas of ice shelves that flow in a purely extensional regime, where extensional stresses are proportional to observed ice thickness, drag at the base of the ice is negligible, and extensional strain-rates are calculated from the gradients of observed surface velocity fields. In this manner, we use independent observational data to derive spatially dependent constraints on the rate factor A and stress exponent n in Glen's Flow Law. The robust spatial variability provides insights into the creep mechanisms of ice, thereby capturing rheological properties from satellite observations. Our analysis indicates that n ≈ 4 in most fast-flowing areas in an extensional regime, contrary to the prototypical value of n = 3. This finding implies higher non-linearity in ice flow than typically prescribed, influencing calculations of mass flux and the response of ice sheets to perturbations. Additionally, This result suggests that dislocation creep is the dominant creep mechanism in extensional regimes of Antarctic ice shelves, indicative of tertiary creep. This analysis unites theoretical work and synoptic-scale observations of ice flow, providing insights into the rheology and stress-states of ice shelves in Antarctica.</p>

scholarly journals A physically based calving model applied to marine outlet glaciers and implications for the glacier dynamics

2010 ◽  
Vol 56 (199) ◽  
pp. 781-794 ◽  
Author(s):  
F.M. Nick ◽  
C.J. Van Der Veen ◽  
A. Vieli ◽  
D.I. Benn
Keyword(s):  
Dynamical Behaviour ◽  
Ice Flow ◽  
Outlet Glacier ◽  
Ice Shelves ◽  
Flow Models ◽  
Grounding Line ◽  
Physically Based ◽  
Local Water ◽  
Bed Slope ◽  
Geometry Characteristic

AbstractWe present results from numerical ice-flow models that include calving criteria based on penetration of surface and basal crevasses, which in turn is a function of longitudinal strain rates near the glacier front. The position of the calving front is defined as the point where either (1) surface crevasses reach the waterline (model CDw), or (2) surface and basal crevasses penetrate the full thickness of the glacier (model CD). For comparison with previous studies, results are also presented for a height-above-buoyancy calving model. Qualitatively, both models CDw and CD produce similar behaviour. Unlike previous models for calving, the new calving criteria are applicable to both grounded termini and floating ice shelves and tongues. The numerical ice-flow model is applied to an idealized geometry characteristic of marine outlet glaciers. Results indicate that grounding-line dynamics are less sensitive to basal topography than previously suggested. Stable grounding-line positions can be obtained even on a reverse bed slope with or without floating termini. The proposed calving criteria also allow calving losses to be linked to surface melt and therefore climate. In contrast to previous studies in which calving rate or position of the terminus is linked to local water depth, the new calving criterion is able to produce seasonal cycles of retreat and advance as observed for Greenland marine outlet glaciers. The contrasting dynamical behaviour and stability found for different calving models suggests that a realistic parameterization for the process of calving is crucial for any predictions of marine outlet glacier change.

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