scholarly journals Marine ice-sheet experiments with the Community Ice Sheet Model

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.

scholarly journals Marine ice sheet experiments with the Community Ice Sheet Model

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.

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

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

<p>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 ∼2 km (arguably 4 km) when the connectivity is low or moderate, and ∼1 km (arguably 2 km) when the connectivity is strong.</p>

Coupling subglacial hydrology to basal friction in an Antarctic ice sheet model

2020 ◽  
Author(s):  
Elise Kazmierczak ◽  
Lars Zipf ◽  
Frank Pattyn
Keyword(s):  
Time Scales ◽  
Ice Sheet ◽  
Effective Pressure ◽  
Antarctic Ice Sheet ◽  
Friction Law ◽  
Friction Laws ◽  
Basal Friction ◽  
Direct Observations ◽  
Modelling Studies ◽  
The Antarctic

<p>Due to the lack of direct observations, subglacial hydrology is still marginally considered in Antarctic ice sheet modelling studies, albeit that several approaches exist (e.g., LeBrocq, Bueler and Van Pelt). Subglacial hydrology impacts basal friction through a reduction in effective pressure and through changing properties of subglacial sediments, both factors influencing the lubrication at the bottom of the ice sheet. Several approaches exist to represent subglacial hydrology in ice sheet models (Bueler and Brown, 2009, Goeller et al., 2013) and are generally coupled to either a Coulomb or a Weertman friction law. However, the type of subglacial process determines to a large extent the sensitivity of Antarctic mass change (Sun et al, submitted).</p><p> </p><p>In this study we investigate the sensitivity of subglacial dynamics on the behaviour of the Antarctic ice sheet on centennial time scales. For this purpose we employ a subglacial hydrology model for subglacial water routing (Lebrocq et al., 2009) coupled to a thermomechanical ice-sheet model (f.ETISh; Pattyn, 2017). We consider different parametrizations and representations of effective pressure and till water content at the base.  We also consider the combination of different friction laws and hydrological models (sheet flow, till deformation) depending on estimates of the subglacial conditions of the Antarctic ice sheet. Results show that the way of coupling subglacial hydrology influences the sensitivity of the ice-sheet system on centennial time scales. However, the type and power of the friction law (Coulomb versus Weertman)  has the most dominant impact on ice sheet sensitivity.</p>

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

2017 ◽  
Vol 63 (241) ◽  
pp. 854-866 ◽  
Author(s):  
JULIEN BRONDEX ◽  
OLIVIER GAGLIARDINI ◽  
FABIEN GILLET-CHAULET ◽  
GAËL DURAND
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Basal Slip ◽  
Ice Streams ◽  
Ice Flow ◽  
Friction Law ◽  
Flow Models ◽  
Friction Laws ◽  
Grounding Line ◽  
Physically Based

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.

scholarly journals Parameterization of basal hydrology near grounding lines in a one-dimensional ice sheet model

2014 ◽  
Vol 8 (1) ◽  
pp. 363-419
Author(s):  
G. R. Leguy ◽  
X. S. Asay-Davis ◽  
W. H. Lipscomb
Keyword(s):  
Transition Zone ◽  
Ice Sheet ◽  
Smooth Transition ◽  
Ice Shelf ◽  
Ice Shelves ◽  
One Dimensional ◽  
Basal Friction ◽  
Hydrological System ◽  
Intercomparison Project ◽  
Basal Shear

Abstract. Ice sheets and ice shelves are linked by the transition zone, the region where the grounded ice lifts off the bedrock and begins to float. Adequate resolution of the transition zone is necessary for numerically accurate ice sheet–ice shelf simulations. The required resolution depends on how the basal physics is parameterized. We propose a new, simple parameterization of the basal hydrology in a one-dimensional vertically integrated model. This parameterization accounts for connectivity between the basal hydrological system and the ocean in the transition zone. Our model produces a smooth transition between finite basal friction in the ice sheet and zero basal friction in the ice shelf. Through a set of experiments based on the Marine Ice Sheet Model Intercomparison Project (MISMIP), we show that a smoother basal shear stress, in addition to adding physical realism, significantly improves the numerical accuracy of our fixed-grid model, allowing for reliable grounding-line dynamics at resolutions ~1 km.

scholarly journals Results of the third Marine Ice Sheet Model Intercomparison Project (MISMIP+)

2020 ◽  
Vol 14 (7) ◽  
pp. 2283-2301 ◽  
Author(s):  
Stephen L. Cornford ◽  
Helene Seroussi ◽  
Xylar S. Asay-Davis ◽  
G. Hilmar Gudmundsson ◽  
Rob Arthern ◽  
...  
Keyword(s):  
Ice Sheet ◽  
Model Intercomparison ◽  
Ice Streams ◽  
Important Distinction ◽  
Ice Flow ◽  
Flow Models ◽  
The Third ◽  
Grounding Line ◽  
Intercomparison Project ◽  
The Difference

Abstract. We present the result of the third Marine Ice Sheet Model Intercomparison Project, MISMIP+. MISMIP+ is intended to be a benchmark for ice-flow models which include fast sliding marine ice streams and floating ice shelves and in particular a treatment of viscous stress that is sufficient to model buttressing, where upstream ice flow is restrained by a downstream ice shelf. A set of idealized experiments first tests that models are able to maintain a steady state with the grounding line located on a retrograde slope due to buttressing and then explore scenarios where a reduction in that buttressing causes ice stream acceleration, thinning, and grounding line retreat. The majority of participating models passed the first test and then produced similar responses to the loss of buttressing. We find that the most important distinction between models in this particular type of simulation is in the treatment of sliding at the bed, with other distinctions – notably the difference between the simpler and more complete treatments of englacial stress but also the differences between numerical methods – taking a secondary role.

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 Projecting Antarctic ice discharge using response functions from SeaRISE ice-sheet models

2014 ◽  
Vol 5 (2) ◽  
pp. 271-293 ◽  
Author(s):  
A. Levermann ◽  
R. Winkelmann ◽  
S. Nowicki ◽  
J. L. Fastook ◽  
K. Frieler ◽  
...  
Keyword(s):  
Sea Level ◽  
Ice Sheet ◽  
Response Functions ◽  
Ice Shelf ◽  
Uncertainty Range ◽  
Basal Ice ◽  
Grounding Line ◽  
Rcp 8.5 ◽  
Intercomparison Project ◽  
Global Sea Level

Abstract. The largest uncertainty in projections of future sea-level change results from the potentially changing dynamical ice discharge from Antarctica. Basal ice-shelf melting induced by a warming ocean has been identified as a major cause for additional ice flow across the grounding line. Here we attempt to estimate the uncertainty range of future ice discharge from Antarctica by combining uncertainty in the climatic forcing, the oceanic response and the ice-sheet model response. The uncertainty in the global mean temperature increase is obtained from historically constrained emulations with the MAGICC-6.0 (Model for the Assessment of Greenhouse gas Induced Climate Change) model. The oceanic forcing is derived from scaling of the subsurface with the atmospheric warming from 19 comprehensive climate models of the Coupled Model Intercomparison Project (CMIP-5) and two ocean models from the EU-project Ice2Sea. The dynamic ice-sheet response is derived from linear response functions for basal ice-shelf melting for four different Antarctic drainage regions using experiments from the Sea-level Response to Ice Sheet Evolution (SeaRISE) intercomparison project with five different Antarctic ice-sheet models. The resulting uncertainty range for the historic Antarctic contribution to global sea-level rise from 1992 to 2011 agrees with the observed contribution for this period if we use the three ice-sheet models with an explicit representation of ice-shelf dynamics and account for the time-delayed warming of the oceanic subsurface compared to the surface air temperature. The median of the additional ice loss for the 21st century is computed to 0.07 m (66% range: 0.02–0.14 m; 90% range: 0.0–0.23 m) of global sea-level equivalent for the low-emission RCP-2.6 (Representative Concentration Pathway) scenario and 0.09 m (66% range: 0.04–0.21 m; 90% range: 0.01–0.37 m) for the strongest RCP-8.5. Assuming no time delay between the atmospheric warming and the oceanic subsurface, these values increase to 0.09 m (66% range: 0.04–0.17 m; 90% range: 0.02–0.25 m) for RCP-2.6 and 0.15 m (66% range: 0.07–0.28 m; 90% range: 0.04–0.43 m) for RCP-8.5. All probability distributions are highly skewed towards high values. The applied ice-sheet models are coarse resolution with limitations in the representation of grounding-line motion. Within the constraints of the applied methods, the uncertainty induced from different ice-sheet models is smaller than that induced by the external forcing to the ice sheets.

scholarly journals ESD Ideas: Propagation of high-frequency forcing to ice age dynamics

2019 ◽  
Vol 10 (2) ◽  
pp. 257-260 ◽  
Author(s):  
Mikhail Y. Verbitsky ◽  
Michel Crucifix ◽  
Dmitry M. Volobuev
Keyword(s):  
Conservation Laws ◽  
High Frequency ◽  
Ice Sheet ◽  
Ice Age ◽  
Ice Flow ◽  
Age Dynamics ◽  
Nonlinear Character ◽  
Continuous Background ◽  
Ice Sheet Dynamics ◽  
Flow Conservation

Abstract. Palaeoclimate records display a continuous background of variability connecting centennial to 100 kyr periods. Hence, the dynamics at the centennial, millennial, and astronomical timescales should not be treated separately. Here, we show that the nonlinear character of ice sheet dynamics, which was derived naturally from the ice-flow conservation laws, provides the scaling constraints to explain the structure of the observed spectrum of variability.

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