scholarly journals Results of the Marine Ice Sheet Model Intercomparison Project, MISMIP

2012 ◽  
Vol 6 (1) ◽  
pp. 267-308 ◽  
Author(s):  
F. Pattyn ◽  
C. Schoof ◽  
L. Perichon ◽  
R. C. A. Hindmarsh ◽  
E. Bueler ◽  
...  
Keyword(s):  
Steady State ◽  
Adaptive Mesh Refinement ◽  
Ice Sheet ◽  
Adaptive Mesh ◽  
Stable Steady State ◽  
Fixed Grid ◽  
Approximate Analytical Solutions ◽  
Grounding Line ◽  
Intercomparison Project ◽  
Line Positions

Abstract. Predictions of marine ice-sheet behaviour require models that are able to robustly simulate grounding line migration. We present results of an intercomparison exercise for marine ice-sheet models. Verification is effected by comparison with approximate analytical solutions for flux across the grounding line using simplified geometrical configurations (no lateral variations, no effects of lateral buttressing). Unique steady-state grounding line positions exist for ice sheets on a downward sloping bed, while hysteresis occurs across an overdeepened bed, and stable steady state grounding line positions only occur on the downward-sloping sections. Models based on the shallow ice approximation, which does not resolve extensional stresses, do not reproduce the approximate analytical results unless appropriate parameterizations for ice flux are imposed at the grounding line. For extensional-stress resolving "shelfy stream" models, differences between model results were mainly due to the choice of spatial discretization. Moving grid methods were found to be the most accurate at capturing grounding line evolution, since they track the grounding line explicitly. Adaptive mesh refinement can further improve accuracy, including in fixed-grid models that generally perform poorly at coarse resolution. Fixed grid models with nested grid representations of the grounding line are able to generate accurate steady-state positions, but can be inaccurate over transients. Only one full Stokes model was included in the intercomparison, and consequently the accuracy of shelfy stream models as approximations of full Stokes models remains to be determined in detail, especially during transients.

scholarly journals Results of the Marine Ice Sheet Model Intercomparison Project, MISMIP

2012 ◽  
Vol 6 (3) ◽  
pp. 573-588 ◽  
Author(s):  
F. Pattyn ◽  
C. Schoof ◽  
L. Perichon ◽  
R. C. A. Hindmarsh ◽  
E. Bueler ◽  
...  
Keyword(s):  
Steady State ◽  
Adaptive Mesh Refinement ◽  
Ice Sheet ◽  
Adaptive Mesh ◽  
Stable Steady State ◽  
Fixed Grid ◽  
Approximate Analytical Solutions ◽  
Grounding Line ◽  
Intercomparison Project ◽  
Line Positions

Abstract. Predictions of marine ice-sheet behaviour require models that are able to robustly simulate grounding line migration. We present results of an intercomparison exercise for marine ice-sheet models. Verification is effected by comparison with approximate analytical solutions for flux across the grounding line using simplified geometrical configurations (no lateral variations, no effects of lateral buttressing). Unique steady state grounding line positions exist for ice sheets on a downward sloping bed, while hysteresis occurs across an overdeepened bed, and stable steady state grounding line positions only occur on the downward-sloping sections. Models based on the shallow ice approximation, which does not resolve extensional stresses, do not reproduce the approximate analytical results unless appropriate parameterizations for ice flux are imposed at the grounding line. For extensional-stress resolving "shelfy stream" models, differences between model results were mainly due to the choice of spatial discretization. Moving grid methods were found to be the most accurate at capturing grounding line evolution, since they track the grounding line explicitly. Adaptive mesh refinement can further improve accuracy, including fixed grid models that generally perform poorly at coarse resolution. Fixed grid models, with nested grid representations of the grounding line, are able to generate accurate steady state positions, but can be inaccurate over transients. Only one full-Stokes model was included in the intercomparison, and consequently the accuracy of shelfy stream models as approximations of full-Stokes models remains to be determined in detail, especially during transients.

scholarly journals Neutral equilibrium and forcing feedbacks in marine ice sheet modelling

2018 ◽  
Vol 12 (11) ◽  
pp. 3605-3615 ◽  
Author(s):  
Rupert M. Gladstone ◽  
Yuwei Xia ◽  
John Moore
Keyword(s):  
Steady State ◽  
False Positive ◽  
Ice Sheet ◽  
Stable Steady State ◽  
Model Configuration ◽  
Grounding Line ◽  
Neutral Equilibrium ◽  
Poor Convergence ◽  
Line Positions ◽  
Additional Constraints

Abstract. Poor convergence with resolution of ice sheet models when simulating grounding line migration has been known about for over a decade. However, some of the associated numerical artefacts remain absent from the published literature. In the current study we apply a Stokes-flow finite-element marine ice sheet model to idealised grounding line evolution experiments. We show that with insufficiently fine model resolution, a region containing multiple steady-state grounding line positions exists, with one steady state per node of the model mesh. This has important implications for the design of perturbation experiments used to test convergence of grounding line behaviour with resolution. Specifically, the design of perturbation experiments can be under-constrained, potentially leading to a “false positive” result. In this context a false positive is an experiment that appears to achieve convergence when in fact the model configuration is not close to its converged state. We demonstrate a false positive: an apparently successful perturbation experiment (i.e. reversibility is shown) for a model configuration that is not close to a converged solution. If perturbation experiments are to be used in the future, experiment design should be modified to provide additional constraints to the initialisation and spin-up requirements. This region of multiple locally stable steady-state grounding line positions has previously been mistakenly described as neutral equilibrium. This distinction has important implications for understanding the impacts of discretising a forcing feedback involving grounding line position and basal friction. This forcing feedback cannot, in general, exist in a region of neutral equilibrium and could be the main cause of poor convergence in grounding line modelling.

scholarly journals Neutral equilibrium and forcing feedbacks in marine ice sheet modelling

2018 ◽  
Author(s):  
Rupert Gladstone ◽  
Yuwei Xia ◽  
John Moore
Keyword(s):  
Steady State ◽  
False Positive ◽  
Ice Sheet ◽  
Stable Steady State ◽  
Model Configuration ◽  
Grounding Line ◽  
Neutral Equilibrium ◽  
Poor Convergence ◽  
Line Positions ◽  
Additional Constraints

Abstract. Poor convergence with resolution of ice sheet models when simulating grounding line migration has been known about for over a decade. However, some of the associated numerical artifacts remain absent from the published literature. In the current study we apply a Stokes-flow finite element marine ice sheet model to idealised grounding line evolution experiments. We show that with insufficiently fine model resolution, a region containing multiple steady state grounding line positions exists, with one steady state per node of the model mesh. This has important implications for the design of perturbation experiments used to test convergence of grounding line behaviour with resolution. Specifically, the design of perturbation experiments can be under-constrained, potentially leading to a "false positive" result. In this context a false positive is an experiment that appears to achieve convergence when in fact the model configuration is not close to its converged state. We demonstrate a false positive: an apparently successful perturbation experiment (i.e. reversibility is shown) for a model configuration that is not close to a converged solution. If perturbation experiments are to be used in the future, experiment design should be modified to provide additional constraints to the initialisation/spin up requirements. This region of multiple locally stable steady state grounding line positions has previously been mistakenly described as neutral equilibrium. This distinction has important implications for understanding the impacts of discretizing a forcing feedback involving grounding line position and basal friction. This forcing feedback can not, in general, exist in a region of neutral equilibrium, and could be the main cause of poor convergence in grounding line modelling.

scholarly journals Suppression of marine ice sheet instability

2018 ◽  
Vol 857 ◽  
pp. 648-680 ◽  
Author(s):  
Samuel S. Pegler
Keyword(s):  
Steady State ◽  
Rapid Assessment ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Stable Steady State ◽  
Ice Shelf ◽  
West Antarctic Ice Sheet ◽  
West Antarctic ◽  
Grounding Line ◽  
The Stability

A long-standing open question in glaciology concerns the propensity for ice sheets that lie predominantly submerged in the ocean (marine ice sheets) to destabilise under buoyancy. This paper addresses the processes by which a buoyancy-driven mechanism for the retreat and ultimate collapse of such ice sheets – the marine ice sheet instability – is suppressed by lateral stresses acting on its floating component (the ice shelf). The key results are to demonstrate the transition between a mode of stable (easily reversible) retreat along a stable steady-state branch created by ice-shelf buttressing to tipped (almost irreversible) retreat across a critical parametric threshold. The conditions for triggering tipped retreat can be controlled by the calving position and other properties of the ice-shelf profile and can be largely independent of basal stress, in contrast to principles established from studies of unbuttressed grounding-line dynamics. The stability and recovery conditions introduced by lateral stresses are analysed by developing a method of constructing grounding-line stability (bifurcation) diagrams, which provide a rapid assessment of the steady-state positions, their natures and the conditions for secondary grounding, giving clear visualisations of global stabilisation conditions. A further result is to reveal the possibility of a third structural component of a marine ice sheet that lies intermediate to the fully grounded and floating components. The region forms an extended grounding area in which the ice sheet lies very close to flotation, and there is no clearly distinguished grounding line. The formation of this region generates an upsurge in buttressing that provides the most feasible mechanism for reversal of a tipped grounding line. The results of this paper provide conceptual insight into the phenomena controlling the stability of the West Antarctic Ice Sheet, the collapse of which has the potential to dominate future contributions to global sea-level rise.

scholarly journals Implementation and performance of adaptive mesh refinement in the Ice Sheet System Model (ISSM v4.14)

2019 ◽  
Vol 12 (1) ◽  
pp. 215-232 ◽  
Author(s):  
Thiago Dias dos Santos ◽  
Mathieu Morlighem ◽  
Hélène Seroussi ◽  
Philippe Remy Bernard Devloo ◽  
Jefferson Cardia Simões
Keyword(s):  
Adaptive Mesh Refinement ◽  
Mesh Refinement ◽  
Computational Cost ◽  
Ice Sheet ◽  
Adaptive Mesh ◽  
System Model ◽  
Computational Time ◽  
Error Estimator ◽  
Fine Mesh ◽  
Grounding Line

Abstract. Accurate projections of the evolution of ice sheets in a changing climate require a fine mesh/grid resolution in ice sheet models to correctly capture fundamental physical processes, such as the evolution of the grounding line, the region where grounded ice starts to float. The evolution of the grounding line indeed plays a major role in ice sheet dynamics, as it is a fundamental control on marine ice sheet stability. Numerical modeling of a grounding line requires significant computational resources since the accuracy of its position depends on grid or mesh resolution. A technique that improves accuracy with reduced computational cost is the adaptive mesh refinement (AMR) approach. We present here the implementation of the AMR technique in the finite element Ice Sheet System Model (ISSM) to simulate grounding line dynamics under two different benchmarks: MISMIP3d and MISMIP+. We test different refinement criteria: (a) distance around the grounding line, (b) a posteriori error estimator, the Zienkiewicz–Zhu (ZZ) error estimator, and (c) different combinations of (a) and (b). In both benchmarks, the ZZ error estimator presents high values around the grounding line. In the MISMIP+ setup, this estimator also presents high values in the grounded part of the ice sheet, following the complex shape of the bedrock geometry. The ZZ estimator helps guide the refinement procedure such that AMR performance is improved. Our results show that computational time with AMR depends on the required accuracy, but in all cases, it is significantly shorter than for uniformly refined meshes. We conclude that AMR without an associated error estimator should be avoided, especially for real glaciers that have a complex bed geometry.

scholarly journals Hydrostatic grounding line parameterization in ice sheet models

2014 ◽  
Vol 8 (6) ◽  
pp. 2075-2087 ◽  
Author(s):  
H. Seroussi ◽  
M. Morlighem ◽  
E. Larour ◽  
E. Rignot ◽  
A. Khazendar
Keyword(s):  
Steady State ◽  
High Spatial Resolution ◽  
Numerical Models ◽  
Ice Sheet ◽  
System Model ◽  
Line Position ◽  
Grounding Line ◽  
Mesh Resolution ◽  
Averaged Model ◽  
Line Positions

Abstract. Modeling of grounding line migration is essential to accurately simulate the behavior of marine ice sheets and investigate their stability. Here, we assess the sensitivity of numerical models to the parameterization of the grounding line position. We run the MISMIP3D benchmark experiments using the Ice Sheet System Model (ISSM) and a two-dimensional shelfy-stream approximation (SSA) model with different mesh resolutions and different sub-element parameterizations of grounding line position. Results show that different grounding line parameterizations lead to different steady state grounding line positions as well as different retreat/advance rates. Our simulations explain why some vertically depth-averaged model simulations deviate significantly from the vast majority of simulations based on SSA in the MISMIP3D benchmark. The results reveal that differences between simulations performed with and without sub-element parameterization are as large as those performed with different approximations of the stress balance equations in this configuration. They also demonstrate that the reversibility test is passed at relatively coarse resolution while much finer resolutions are needed to accurately capture the steady-state grounding line position. We conclude that fixed grid SSA models that do not employ such a parameterization should be avoided, as they do not provide accurate estimates of grounding line dynamics, even at high spatial resolution. For models that include sub-element grounding line parameterization, in the MISMIP3D configuration, a mesh resolution finer than 2 km should be employed.

scholarly journals Implementation and Performance of Adaptive Mesh Refinement in the Ice Sheet System Model (ISSM v4.14)

2018 ◽  
Author(s):  
Thiago Dias dos Santos ◽  
Mathieu Morlighem ◽  
Hélène Seroussi ◽  
Philippe Remy Bernard Devloo ◽  
Jefferson Cardia Simões
Keyword(s):  
Adaptive Mesh Refinement ◽  
Mesh Refinement ◽  
Computational Cost ◽  
Ice Sheet ◽  
Adaptive Mesh ◽  
System Model ◽  
Computational Time ◽  
Error Estimator ◽  
Grounding Line ◽  
Refined Meshes

Abstract. Accurate projections of the evolution of ice sheets in a changing climate require a fine mesh/grid resolution to correctly capture fundamental physical processes, such as the evolution of the grounding line, the region where grounded ice starts to float. The evolution of the grounding line indeed plays a major role in ice sheet dynamics, as it is a fundamental control on marine ice sheet stability. Numerical modeling of grounding line requires significant computational resources since the accuracy of its position depends on grid or mesh resolution. A technique that improves accuracy with reduced computational cost is the adaptive mesh refinement approach, AMR. We present here the implementation of the AMR technique in the finite element Ice Sheet System Model (ISSM) to simulate grounding line dynamics under two different benchmarks, MISMIP3d and MISMIP+. We test different refinement criteria: (a) distance around grounding line, (b) a posteriori error estimator, the Zienkiewicz-Zhu (ZZ) error estimator, and (c) different combinations of (a) and (b). We find that for MISMIP3d setup, refining 5 km around the grounding line, both on grounded and floating ice, is sufficient to produce AMR results similar to the ones obtained with uniformly refined meshes. However, for the MISMIP+ setup, we note that there is a minimum distance of 30 km around the grounding line required to produce accurate results. We find this AMR mesh-dependency is linked to the complex bedrock topography of MISMIP+. In both benchmarks, the ZZ error estimator presents high values around the grounding line. Particularly for MISMIP+ setup, the estimator also presents high values in the grounded part of the ice sheet, following the complex shape of the bedrock geometry. This estimator helps guide the refinement procedure such that AMR performance is improved. Our results show that computational time with AMR depends on the required accuracy, but in all cases, it is significantly shorter than for uniformly refined meshes. We conclude that AMR without an associated error estimator should be avoided, especially for real glaciers that have a complex bed geometry.

scholarly journals Resolution requirements for grounding-line modelling: sensitivity to basal drag and ice-shelf buttressing

2012 ◽  
Vol 53 (60) ◽  
pp. 97-105 ◽  
Author(s):  
Rupert M. Gladstone ◽  
Antony J. Payne ◽  
Stephen L. Cornford
Keyword(s):  
Ice Sheet ◽  
Horizontal Resolution ◽  
Channel Width ◽  
Ice Shelf ◽  
Fixed Grid ◽  
Grounding Line ◽  
Poor Convergence ◽  
Set Up ◽  
Line Positions ◽  
Resolution Requirements

AbstractSimulations of grounding-line migration in ice-sheet models using a fixed grid have been shown to exhibit poor convergence at achievable resolutions. We present a series of ‘shelfy-stream’ flowline model experiments using an idealized set-up. We assess the performance of a range of grounding-line parameterizations (GLPs) over a large input space by varying bedrock gradient, rate factor, basal drag coefficient and net accumulation. The relative performance of GLPs is similar to Gladstone and others (2010a) except at low basal drag, in which case the grounding-line errors are very small for all GLPs. We find that grounding-line errors are far more sensitive to basal drag than to the other inputs or to choice of GLP. We then quantify grounding-line errors as a function of resolution while varying basal drag and channel width (using a parameterization to represent buttressing). Reducing either basal drag or channel width reduces the errors associated with the grounding line. Our results suggest that a structured fixed-grid shelfy-stream ice-sheet model would need to run at a horizontal resolution of ~1–2km to accurately simulate grounding-line positions of marine ice-sheet outlet glaciers such as Pine Island Glacier, Antarctica.

scholarly journals Grounding line transient response in marine ice sheet models

2012 ◽  
Vol 6 (5) ◽  
pp. 3903-3935 ◽  
Author(s):  
A. S. Drouet ◽  
D. Docquier ◽  
G. Durand ◽  
R. Hindmarsh ◽  
F. Pattyn ◽  
...  
Keyword(s):  
Steady State ◽  
Sea Level ◽  
Ice Sheet ◽  
Numerical Approach ◽  
Surface Velocity ◽  
Ice Shelf ◽  
Transient Behaviour ◽  
Surface Elevation Change ◽  
Grounding Line ◽  
Intercomparison Project

Abstract. Marine ice sheet stability is mostly controlled by the dynamics of the grounding line, i.e., the junction between the grounded ice sheet and the floating ice shelf. Grounding line migration has been investigated in the framework of MISMIP (Marine Ice Sheet Model Intercomparison Project), which mainly aimed at investigating steady state solutions. Here we focus on transient behaviour, executing short-term simulations (200 yr) of a steady ice sheet perturbed by the release of the buttressing restraint exerted by the ice shelf on the grounded ice upstream. The transient grounding line behaviour of four different flowline ice sheet models has been compared. The models differ in the physics implemented (full-Stokes and Shallow Shelf Approximation), the numerical approach, as well as the grounding line treatment. Their overall response to the loss of buttressing is found to be consistent in terms of grounding line position, rate of surface elevation change and surface velocity. However, large discrepancies (>100%) are observed in terms of ice sheet contribution to sea level. Despite the recent important improvements of marine ice sheet models in their ability to compute steady-state configurations, our results question models' capacity to compute reliable sea-level rise projections.

scholarly journals A comparison of two Stokes ice sheet models applied to the Marine Ice Sheet Model Intercomparison Project for plan view models (MISMIP3d)

2016 ◽  
Author(s):  
Tong Zhang ◽  
Stephen Price ◽  
Lili Ju ◽  
Wei Leng ◽  
Julien Brondex ◽  
...  
Keyword(s):  
Boundary Conditions ◽  
Ice Sheet ◽  
Model Intercomparison ◽  
Plan View ◽  
Grounding Line ◽  
Intercomparison Project ◽  
Simulation Results ◽  
Line Positions ◽  
Measure Of Uncertainty ◽  
Diagnostic Experiment

Abstract. We present a comparison of the numerics and simulation results for two "full" Stokes ice sheet models, FELIX-S (Leng et al., 2012) and Elmer/Ice (Gagliardini et al., 2013). The models are applied to the Marine Ice Sheet Model Intercomparison Project for planview models (MISMIP3D). For the diagnostic experiment (P75D) the two models give very similar results (<2 % difference with respect to along-flow velocities) when using identical geometries and computational meshes, which we interpret as an indication of inherent consistencies and similarities between the two models. For the Stnd, P75S, and P75R prognostic experiments, we find that FELIX-S (Elmer/Ice) grounding lines are relatively more retreated (advanced), results that are consistent with minor differences observed in the diagnostic experiment results and largely due to slightly different choices in the implementation of basal boundary conditions used by the two models. Based on current understanding, neither set of implementations is more or less favorable, in which case we propose that the span of different grounding line positions from these two models provides a measure of uncertainty when treating the results from full-Stokes models as a metric for accuracy in model intercomparison experiments. More importantly, we show that as grid resolution increases the grounding line positions for FELIX-S and Elmer/Ice appear to converge. We conclude that future model intercomparisons using full-Stokes models as a metric should include more than one model, to provide both additional confidence in the results from full-Stokes models and a measure of their uncertainty.

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