Sensitivity of grounding line dynamics to the choice of the friction law

ABSTRACTBasal slip accounts for a large part of the flow of ice streams draining ice from Antarctica and Greenland into the ocean. Therefore, an appropriate representation of basal slip in ice flow models is a prerequisite for accurate sea level rise projections. Various friction laws have been proposed to describe basal slip in models. Here, we compare the influence on grounding line (GL) dynamics of four friction laws: the traditional Weertman law and three effective pressure-dependent laws, namely the Schoof, Tsai and Budd laws. It turns out that, even when they are tuned to a common initial reference state, the Weertman, Budd and Schoof laws lead to thoroughly different steady-state positions, although the Schoof and Tsai laws lead to much the same result. In particular, under certain circumstances, it is possible to obtain a steady GL located on a reverse slope area using the Weertman law. Furthermore, the predicted transient evolution of the GL as well as the projected contributions to sea level rise over a 100-year time horizon vary significantly depending on the friction law. We conclude on the importance of choosing an appropriate law for reliable sea level rise projections and emphasise the need for a coupling between ice flow models and physically based subglacial hydrological models.

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Marine ice sheet experiments with the Community Ice Sheet Model

The Cryosphere ◽

10.5194/tc-15-3229-2021 ◽

2021 ◽

Vol 15 (7) ◽

pp. 3229-3253

Author(s):

Gunter R. Leguy ◽

William H. Lipscomb ◽

Xylar S. Asay-Davis

Keyword(s):

Ice Sheet ◽

Ice Flow ◽

Friction Law ◽

Friction Laws ◽

Basal Friction ◽

Grounding Line ◽

Hydrological System ◽

Flow Approximation ◽

Intercomparison Project ◽

Ice Sheet Dynamics

Abstract. Ice sheet models differ in their numerical treatment of dynamical processes. Simulations of marine-based ice are sensitive to the choice of Stokes flow approximation and basal friction law and to the treatment of stresses and melt rates near the grounding line. We study the effects of these numerical choices on marine ice sheet dynamics in the Community Ice Sheet Model (CISM). In the framework of the Marine Ice Sheet Model Intercomparison Project 3d (MISMIP3d), we show that a depth-integrated, higher-order solver gives results similar to a 3D (Blatter–Pattyn) solver. We confirm that using a grounding line parameterization to approximate stresses in the grounding zone leads to accurate representation of ice sheet flow with a resolution of ∼2 km, as opposed to ∼0.5 km without the parameterization. In the MISMIP+ experimental framework, we compare different treatments of sub-shelf melting near the grounding line. In contrast to recent studies arguing that melting should not be applied in partly grounded cells, it is usually beneficial in CISM simulations to apply some melting in these cells. This suggests that the optimal treatment of melting near the grounding line can depend on ice sheet geometry, forcing, or model numerics. In both experimental frameworks, ice flow is sensitive to the choice of basal friction law. To study this sensitivity, we evaluate friction laws that vary the connectivity between the basal hydrological system and the ocean near the grounding line. CISM yields accurate results in steady-state and perturbation experiments at a resolution of ∼2 km (arguably 4 km) when the connectivity is low or moderate and ∼1 km (arguably 2 km) when the connectivity is strong.

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Marine ice-sheet experiments with the Community Ice Sheet Model using the MISMIP+ experimental framework.

10.5194/egusphere-egu21-6608 ◽

2021 ◽

Author(s):

Gunter Leguy ◽

William Lipscomb ◽

Xylar Asay-Davis

Keyword(s):

Ice Sheet ◽

Ice Flow ◽

Friction Law ◽

Friction Laws ◽

Basal Friction ◽

Grounding Line ◽

Hydrological System ◽

Flow Approximation ◽

Intercomparison Project ◽

Ice Sheet Dynamics

Ice sheet models differ in their numerical treatment of dynamical processes. Simulations of marine-based ice are sensitive to the choice of Stokes flow approximation and basal friction law, and to the treatment of stresses and melt rates near the grounding line. We present the effects of these numerical choices on marine ice-sheet dynamics in the Community Ice Sheet Model (CISM). In the experimental framework of the Marine Ice Sheet Model Intercomparison Project (MISMIP+), we compare different treatments of sub-shelf melting near the grounding line. In contrast to recent studies arguing that melting should not be applied in partly grounded cells, it is usually beneficial in CISM simulations to apply some melting in these cells. This suggests that the optimal treatment of melting near the grounding line can depend on ice-sheet geometry, forcing, or model numerics. In the MISMIP+ framework, the ice flow is also sensitive to the choice of basal friction law. To study this sensitivity, we evaluate friction laws that vary the connectivity between the basal hydrological system and the ocean near the grounding line. CISM yields accurate results in steady-state and perturbation experiments at a resolution of &#8764;2 km (arguably 4 km) when the connectivity is low or moderate, and &#8764;1 km (arguably 2 km) when the connectivity is strong.

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Retreat of Thwaites Glacier, West Antarctica, over the next 100 years using various ice flow models, ice shelf melt scenarios and basal friction laws

The Cryosphere ◽

10.5194/tc-12-3861-2018 ◽

2018 ◽

Vol 12 (12) ◽

pp. 3861-3876 ◽

Cited By ~ 12

Author(s):

Hongju Yu ◽

Eric Rignot ◽

Helene Seroussi ◽

Mathieu Morlighem

Keyword(s):

Sea Level ◽

Initial Conditions ◽

Numerical Models ◽

West Antarctica ◽

Ice Shelf ◽

Flow Models ◽

Friction Laws ◽

Basal Friction ◽

Grounding Line ◽

Western Side

Abstract. Thwaites Glacier (TG), West Antarctica, has experienced rapid, potentially irreversible grounding line retreat and mass loss in response to enhanced ice shelf melting. Results from recent numerical models suggest a large spread in the evolution of the glacier in the coming decades to a century. It is therefore important to investigate how different approximations of the ice stress balance, parameterizations of basal friction and ice shelf melt parameterizations may affect projections. Here, we simulate the evolution of TG using ice sheet models of varying levels of complexity, different basal friction laws and ice shelf melt to quantify their effect on the projections. We find that the grounding line retreat and its sensitivity to ice shelf melt are enhanced when a full-Stokes model is used, a Budd friction is used and ice shelf melt is applied on partially floating elements. Initial conditions also impact the model results. Yet, all simulations suggest a rapid, sustained retreat of the glacier along the same preferred pathway. The fastest retreat rate occurs on the eastern side of the glacier, and the slowest retreat occurs across a subglacial ridge on the western side. All the simulations indicate that TG will undergo an accelerated retreat once the glacier retreats past the western subglacial ridge. Combining all the simulations, we find that the uncertainty of the projections is small in the first 30 years, with a cumulative contribution to sea level rise of 5 mm, similar to the current rate. After 30 years, the contribution to sea level depends on the model configurations, with differences up to 300 % over the next 100 years, ranging from 14 to 42 mm.

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Contrasting the modelled sensitivity of the Amundsen Sea Embayment ice streams

Journal of Glaciology ◽

10.1017/jog.2016.40 ◽

2016 ◽

Vol 62 (233) ◽

pp. 552-562 ◽

Cited By ~ 31

Author(s):

ISABEL J. NIAS ◽

STEPHEN L. CORNFORD ◽

ANTONY J. PAYNE

Keyword(s):

Sea Level Rise ◽

Sea Level ◽

Adaptive Mesh Refinement ◽

Adaptive Mesh ◽

Mass Conservation ◽

Physical Parameters ◽

Ice Streams ◽

Amundsen Sea ◽

West Antarctic ◽

Grounding Line

AbstractPresent-day mass loss from the West Antarctic ice sheet is centred on the Amundsen Sea Embayment (ASE), primarily through ice streams, including Pine Island, Thwaites and Smith glaciers. To understand the differences in response of these ice streams, we ran a perturbed parameter ensemble, using a vertically-integrated ice flow model with adaptive mesh refinement. We generated 71 sets of three physical parameters (basal traction coefficient, ice viscosity stiffening factor and sub-shelf melt rate), which we used to simulate the ASE for 50 years. We also explored the effects of different bed geometries and basal sliding laws. The mean rate of sea-level rise across the ensemble of simulations is comparable with current observed rates for the ASE. We found evidence that grounding line dynamics are sensitive to features in the bed geometry: simulations using BedMap2 geometry resulted in a higher rate of sea-level rise than simulations using a rougher geometry, created using mass conservation. Modelled grounding-line retreat of all the three ice streams was sensitive to viscosity and basal traction, while the melt rate was more important in Pine Island and Smith glaciers, which flow through more confined ice shelves than Thwaites, which has a relatively unconfined shelf.

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Deriving a Physically-Based Calving Rate Law for Marine Ice-Cliff Instability

10.5194/egusphere-egu2020-15790 ◽

2020 ◽

Author(s):

Anna Crawford ◽

Joe Todd ◽

Doug Benn ◽

Jan Åström ◽

Thomas Zwinger

Keyword(s):

Viscous Flow ◽

Sea Level Rise ◽

Sea Level ◽

Large Scale ◽

Ice Sheet ◽

Rate Law ◽

Grounding Line ◽

Physically Based ◽

Calving Rate ◽

Global Sea Level

Rapid grounding line retreat at marine-terminating glaciers could expose ice cliffs with heights greater than those on observational record. However, the finite strength of ice places a limit on the height of subareal cliffs. It is proposed that marine ice-cliff instability (MICI) will begin once a stable height threshold is exceeded. If a glacier is situated over a retrograde slope, as is the case for Thwaites Glacier and much of the West Antarctic Ice Sheet, MICI can be expected to accelerate as retreat progresses and increasingly tall and unstable ice cliffs are formed. This is consequential for global sea level rise, yet large uncertainties remain in the prediction of MICI retreat rates. We investigate MICI by pairing the full Stokes continuum model Elmer/Ice and the Helsinki Discrete Element Model (HiDEM). Viscous flow, simulated in Elmer/Ice, is found to be a necessary pre-condition for MICI collapse. Forward advance and bulging lead to ice-front instability and pervasive crevassing in HiDEM. This culminates in full-thickness calving events. We do not observe calving at ice faces prior to viscous deformation. HiDEM simulations that implement viscous flow (HiDEM-ve) also show forward advance and waterline bulging, similar to the Elmer/Ice simulations. However, the importance of granular shear is highlighted by pronounced shear bands and patterns of surface lowering in HiDEM-ve output. These results emphasize the importance and complexity of viscous and brittle process interaction during MICI.A simulation matrix of grounded termini shows that calving frequency and magnitude increase with the thickness of the calving front. The time required for viscous flow to recreate unstable conditions is influenced by thickness as well as ice temperature and basal friction. Simulations of buoyant termini are seen to calve through basal-crevassing and block-rotation, as opposed to incising surface-crevasses. Lastly, we observe that buttressing m&#233;lange can suppress retreat rate if a sufficient resistive force is delivered to the calving front. A physically-based law for MICI retreat rate is derived from our simulation matrix; this calving rate law can be incorporated into large-scale ice sheet models to constrain projections of Antarctic retreat and associated global sea level rise. Our results will also be used to investigate the future retreat of Thwaites Glacier, which is vulnerable to MICI due to a retreating grounding line, fragile floating ice shelf, and precarious positioning above an overdeepening basin.

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Sensitivity of Greenland ice sheet projections to spatial resolution in higher-order simulations: the AWI contribution to ISMIP6-Greenland using ISSM

10.5194/tc-2019-329 ◽

2020 ◽

Cited By ~ 1

Author(s):

Martin Rückamp ◽

Heiko Goelzer ◽

Angelika Humbert

Keyword(s):

Sea Level Rise ◽

Sea Level ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Higher Order ◽

Initial State ◽

Ice Flow ◽

Outlet Glacier ◽

Flow Models ◽

High Emission

Abstract. Projections of the contribution of the Greenland ice sheet to future sea-level rise include uncertainties primarily due to the imposed climate forcing and the initial state of the ice sheet model. Several state-of-the-art ice flow models are currently being employed on various grid resolutions to estimate future mass changes in the framework of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). Here we investigate the sensitivity to grid resolution on centennial sea-level contributions from the Greenland ice sheet and study the mechanism at play. To this end, we employ the finite-element higher-order ice flow model ISSM and conduct experiments with four different horizontal resolutions, namely 4, 2, 1 and 0.75 km. We run the simulation based on the ISMIP6 core GCM MIROC5 under the high emission scenario RCP8.5 and consider both atmospheric and oceanic forcing in full and separate scenarios. Under the full scenarios, finer simulations unveil up to ~5 % more sea-level rise compared to the coarser resolution. The sensitivity depends on the magnitude of outlet glacier retreat, which is implemented as a series of retreat masks following the ISMIP6 protocol. Without imposed retreat under atmosphere-only forcing, the resolution dependency exhibits an opposite behaviour with about ~ 5 % more sea-level contribution in the coarser resolution. The sea-level contribution indicates a converging behaviour

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Marine ice-sheet experiments with the Community Ice Sheet Model

10.5194/tc-2020-304 ◽

2020 ◽

Author(s):

Gunter R. Leguy ◽

William H. Lipscomb ◽

Xylar S. Asay-Davis

Keyword(s):

Ice Sheet ◽

Ice Flow ◽

Friction Law ◽

Friction Laws ◽

Basal Friction ◽

Grounding Line ◽

Hydrological System ◽

Flow Approximation ◽

Intercomparison Project ◽

Ice Sheet Dynamics

Abstract. Ice sheet models differ in their numerical treatment of dynamical processes. Simulations of marine-based ice are sensitive to the choice of Stokes flow approximation and basal friction law, and to the treatment of stresses and melt rates near the grounding line. We study the effects of these numerical choices on marine ice-sheet dynamics in the Community Ice SheetModel (CISM). In the framework of the Marine Ice Sheet Model Intercomparison Project 3d (MISMIP3d), we show that a depth-integrated, higher-order solver gives results similar to a 3D (Blatter-Pattyn) solver. We confirm that using a grounding-line parameterization to approximate stresses in the grounding zone leads to accurate representation of ice sheet flow with a resolution of ∼2 km, as opposed to ∼0.5 km without the parameterization. In the MISMIP+ experimental framework, we compare different treatments of sub-shelf melting near the grounding line. In contrast to recent studies arguing that melting should not be applied in partly grounded cells, it is usually beneficial in CISM simulations to apply some melting in these cells. This suggests that the optimal treatment of melting near the grounding line can depend on ice-sheet geometry, forcing, or model numerics. In both experimental frameworks, ice flow is sensitive to the choice of basal friction law. To study this sensitivity, we evaluate friction laws that vary the connectivity between the basal hydrological system and the ocean near the grounding line. CISM yields accurate results in steady-state and perturbation experiments at a resolution of ∼2 km (arguably 4 km) when the connectivity is low or moderate, and ∼1 km (arguably 2 km) when the connectivity is strong.

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Recent acceleration of Denman Glacier (1972–2017), East Antarctica, driven by grounding line retreat and changes in ice tongue configuration

The Cryosphere ◽

10.5194/tc-15-663-2021 ◽

2021 ◽

Vol 15 (2) ◽

pp. 663-676

Author(s):

Bertie W. J. Miles ◽

Jim R. Jordan ◽

Chris R. Stokes ◽

Stewart S. R. Jamieson ◽

G. Hilmar Gudmundsson ◽

...

Keyword(s):

Numerical Modelling ◽

Sea Level Rise ◽

Sea Level ◽

Flow Speed ◽

Recent Change ◽

East Antarctica ◽

Ice Flow ◽

Geological Evidence ◽

Flow Acceleration ◽

Grounding Line

Abstract. After Totten, Denman Glacier is the largest contributor to sea level rise in East Antarctica. Denman's catchment contains an ice volume equivalent to 1.5 m of global sea level and sits in the Aurora Subglacial Basin (ASB). Geological evidence of this basin's sensitivity to past warm periods, combined with recent observations showing that Denman's ice speed is accelerating and its grounding line is retreating along a retrograde slope, has raised the prospect that its contributions to sea level rise could accelerate. In this study, we produce the first long-term (∼50 years) record of past glacier behaviour (ice flow speed, ice tongue structure and calving) and combine these observations with numerical modelling to explore the likely drivers of its recent change. We find a spatially widespread acceleration of the Denman system since the 1970s across both its grounded (17±4 % acceleration; 1972–2017) and floating portions (36±5 % acceleration; 1972–2017). Our numerical modelling experiments show that a combination of grounding line retreat, ice tongue thinning and the unpinning of Denman's ice tongue from a pinning point following its last major calving event are required to simulate an acceleration comparable with observations. Given its bed topography and the geological evidence that Denman Glacier has retreated substantially in the past, its recent grounding line retreat and ice flow acceleration suggest that it could be poised to make a significant contribution to sea level in the near future.

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Glaciological setting of the Queen Mary and Knox coasts, East Antarctica, over the past 60 years, and implied dynamic stability of the Shackleton system

10.5194/tc-2021-265 ◽

2021 ◽

Author(s):

Sarah Susan Thompson ◽

Bernd Kulessa ◽

Stephen Cornford ◽

Adrian Luckman ◽

Jacqueline Halpin

Keyword(s):

Sea Level Rise ◽

Surface Structure ◽

Sea Level ◽

Temporal Frequency ◽

Flow Speed ◽

East Antarctica ◽

Small Contribution ◽

Ice Flow ◽

Geological Conditions ◽

Grounding Line

Abstract. The discovery of the deepest subglacial trough beneath the Denman Glacier, combined with high rates of basal melt at the grounding line, have caused significant concern over its vulnerability to retreat. Recent attention has therefore been focusing on understanding the governing dynamic controls, although knowledge of the wider regional context and timescales over which the future responses may occur remains poor. Here we consider the whole Shackleton system, comprising of the Shackleton ice shelf, Denman Glacier and adjacent Scott, Northcliffe, Roscoe and Apfel glaciers, about which almost nothing is known. We widen the context of previously observed dynamic changes in the Denman Glacier into the wider region of the Queen Mary and Knox coasts; with a multi-decadal timeframe and an improved biannual temporal frequency of observations in the last seven years (2014–21). We integrate new satellite observations of ice structure, changes in ice front position and ice-flow velocities to investigate changes in the system. We furthermore use the BISICLES ice sheet model to assess the sensitivity and simulate the response times of the Queen Mary and Knox coasts to hypothetical disintegration of its floating ice areas, in response to coupled ocean and atmospheric forcing. Over the 60-year period of observation, the Queen Mary and Knox coasts do not appear to have changed significantly and higher frequency observations have not revealed any significant annual or sub-annual variations in ice flow. A previously observed increase in the ice flow speed of the Denman Glacier has not continued beyond 2008, and we cannot identify any related change in the surface structure of the system since then. We do, however, observe more significant change in the Scott Glacier, with an acceleration in ice flow associated with calving and progressing from the ice front along the floating tongue since early 2020. No changes in surface structure or ice flow speed are observed closer to the grounded ice. Our upper limit numerical simulations for a 400-year period are consistent with noticeable grounding line retreat in the Denman Glacier in the next two centuries if all floating ice were lost, before stabilising again in the third century from now. This equates to around 6 cm of sea level rise, a small contribution when compared to other areas of East Antarctica expected to change over the same time frame. It is clear that current knowledge is insufficient to explain the observed spatial and temporal changes in the dynamic behaviour of the grounded and floating sections in the Shackleton system. Given the potential vulnerability of the system to accelerating retreat better data recording the glaciological, oceanographic, and geological conditions in the Queen Mary and Knox coasts are required to improve the certainty of numerical model predictions. With access to these remote coastal regions a major challenge, coordinated internationally collaborative efforts are required to quantify how much the Queen Mary and Knox coastal region is likely contribute to sea level rise in the coming centuries.

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

Journal of Glaciology ◽

10.3189/002214310794457344 ◽

2010 ◽

Vol 56 (199) ◽

pp. 781-794 ◽

Cited By ~ 157

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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