The ISMIP-HOM intercomparison exercise, launched in 2006, aims at comparing so-called higher-orde... more The ISMIP-HOM intercomparison exercise, launched in 2006, aims at comparing so-called higher-order ice sheet models to analytical solutions and at setting out a benchmark for such models. Higher-order models are models that incorporate further mechanical effects, principally longitudinal stress gradients, or the full Stokes system. These stresses become increasingly important in transition zones between ice sheets and ice shelves (ice streams), but also at the ice divide and in areas of complex basal topography. The proposed experiments are made accessible for a variety of model types, i.e. flowline models, vertically integrated planform models, as well as full three-dimensional models. The experiments are valid for both finite difference (FD) and finite element (FE) models. Furthermore, the grid type (regular or not) is unimportant. All thermomechanical effects are neglected and an isotherm ice mass is considered. Experiments include ideal geometry tests as well as a real case experiment on Haut Glacier d'Arolla. Most experiments are diagnostic, i.e. time evolution is not considered. This means that for a given geometry of the ice mass, a Glen-type flow law, and given appropriate boundary conditions, the stress and velocity field can be calculated. One experiment considers time-dependent response (the experiment is run until the free surface and velocity field reach a steady state) for a constant viscosity (linear flow law). For this experiment analytical solutions exist that are developed by Gudmundsson (2003). 28 numerical models of varying physical complexity participated in the exercise. The results show a very good convergence of the different models at different resolutions. At higher resolutions - and conform theory - a clear distinction can be made between higher-order models and those that solve the full system of equations (full Stokes models). The model results seem not to be influenced by the used numerical approaches, which is clearly demonstrated by the comparison of the different full Stokes models.
Recent satellite observations of the Antarctic and Greenland ice sheets show accelerated ice flow... more Recent satellite observations of the Antarctic and Greenland ice sheets show accelerated ice flow and associated ice sheet thinning along coastal outlet glaciers in contact with the ocean. Both processes are the result of grounding line retreat due to melting at the grounding line (the grounding line is the contact of the ice sheet with the ocean, where it starts to float and forms an ice shelf or ice tongue). Such rapid ice loss is not yet included in large-scale ice sheet models used for IPCC projections, as most of the complex processes are poorly understood. Here we report on the state-of-the art of grounding line migration in marine ice sheets and address different ways in which grounding line migration can be attributed and represented in ice sheet models. Using one-dimensional ice flow models of the ice sheet/ice shelf system we carried out a number of sensitivity experiments with different spatial resolutions and stress approximations. These are verified with semi-analytical steady state solutions. Results show that, in large-scale finite-difference models, grounding line migration is dependent on the numerical treatment (e.g. staggered/non-staggered grid) and the level of physics involved (e.g. shallow-ice/shallow-shelf approximation).
The stability of marine ice sheets is largely controlled by the dynamic behaviour of the groundin... more The stability of marine ice sheets is largely controlled by the dynamic behaviour of the grounding line, i.e., the contact of the bottom of the ice sheet resting on the bedrock with the ocean water. Marine ice sheet instability implies that an ice sheet on a downward sloping bedrock towards the interior will never find stable equilibria, hence leading to ice sheet collapse, unless an upward slope is reached (Schoof, 2007). The latter study shows that steady state solutions using a boundary layer theory for ice flux are in very close agreement with numerical resolutions that resolve the transition zone. However, the time dependent response of grounding line migration is not predicted by this theory. Precise knowledge of this response is essential for assessing the short term impact of accelerated ice discharge on sea level rise. Here we present the results of MISMIP-type (Marine Ice Sheet Model Intercomparison Project) experiments with different sets of numerical flowline models (fixed and moving grid) that solve the stress field in the transition zone according to different approximations to the Stokes equations. These models include shallow-ice (SIA0), shallow-shelf (L1L2) and higher-order (LMLa) approximations, and combinations of these types. All experiments are run at different spatial resolutions and for different sizes of the transition zone (high to low friction). The comparison of several stress approximants allows us to evaluate which stress components in the flow direction are important to the general behaviour of grounding line migration.
The ISMIP-HOM intercomparison exercise, launched in 2006, aims at comparing so-called higher-orde... more The ISMIP-HOM intercomparison exercise, launched in 2006, aims at comparing so-called higher-order ice sheet models to analytical solutions and at setting out a benchmark for such models. Higher-order models are models that incorporate further mechanical effects, principally longitudinal stress gradients, or the full Stokes system. These stresses become increasingly important in transition zones between ice sheets and ice shelves (ice streams), but also at the ice divide and in areas of complex basal topography. The proposed experiments are made accessible for a variety of model types, i.e. flowline models, vertically integrated planform models, as well as full three-dimensional models. The experiments are valid for both finite difference (FD) and finite element (FE) models. Furthermore, the grid type (regular or not) is unimportant. All thermomechanical effects are neglected and an isotherm ice mass is considered. Experiments include ideal geometry tests as well as a real case experiment on Haut Glacier d'Arolla. Most experiments are diagnostic, i.e. time evolution is not considered. This means that for a given geometry of the ice mass, a Glen-type flow law, and given appropriate boundary conditions, the stress and velocity field can be calculated. One experiment considers time-dependent response (the experiment is run until the free surface and velocity field reach a steady state) for a constant viscosity (linear flow law). For this experiment analytical solutions exist that are developed by Gudmundsson (2003). 28 numerical models of varying physical complexity participated in the exercise. The results show a very good convergence of the different models at different resolutions. At higher resolutions - and conform theory - a clear distinction can be made between higher-order models and those that solve the full system of equations (full Stokes models). The model results seem not to be influenced by the used numerical approaches, which is clearly demonstrated by the comparison of the different full Stokes models.
Recent satellite observations of the Antarctic and Greenland ice sheets show accelerated ice flow... more Recent satellite observations of the Antarctic and Greenland ice sheets show accelerated ice flow and associated ice sheet thinning along coastal outlet glaciers in contact with the ocean. Both processes are the result of grounding line retreat due to melting at the grounding line (the grounding line is the contact of the ice sheet with the ocean, where it starts to float and forms an ice shelf or ice tongue). Such rapid ice loss is not yet included in large-scale ice sheet models used for IPCC projections, as most of the complex processes are poorly understood. Here we report on the state-of-the art of grounding line migration in marine ice sheets and address different ways in which grounding line migration can be attributed and represented in ice sheet models. Using one-dimensional ice flow models of the ice sheet/ice shelf system we carried out a number of sensitivity experiments with different spatial resolutions and stress approximations. These are verified with semi-analytical steady state solutions. Results show that, in large-scale finite-difference models, grounding line migration is dependent on the numerical treatment (e.g. staggered/non-staggered grid) and the level of physics involved (e.g. shallow-ice/shallow-shelf approximation).
The stability of marine ice sheets is largely controlled by the dynamic behaviour of the groundin... more The stability of marine ice sheets is largely controlled by the dynamic behaviour of the grounding line, i.e., the contact of the bottom of the ice sheet resting on the bedrock with the ocean water. Marine ice sheet instability implies that an ice sheet on a downward sloping bedrock towards the interior will never find stable equilibria, hence leading to ice sheet collapse, unless an upward slope is reached (Schoof, 2007). The latter study shows that steady state solutions using a boundary layer theory for ice flux are in very close agreement with numerical resolutions that resolve the transition zone. However, the time dependent response of grounding line migration is not predicted by this theory. Precise knowledge of this response is essential for assessing the short term impact of accelerated ice discharge on sea level rise. Here we present the results of MISMIP-type (Marine Ice Sheet Model Intercomparison Project) experiments with different sets of numerical flowline models (fixed and moving grid) that solve the stress field in the transition zone according to different approximations to the Stokes equations. These models include shallow-ice (SIA0), shallow-shelf (L1L2) and higher-order (LMLa) approximations, and combinations of these types. All experiments are run at different spatial resolutions and for different sizes of the transition zone (high to low friction). The comparison of several stress approximants allows us to evaluate which stress components in the flow direction are important to the general behaviour of grounding line migration.
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Papers by Laura Perichon