Elmer/Ice News

Sensitivity of centennial mass loss projections of the Amundsen basin to the friction law

julien2019Reliable projections of ice sheets’ future contributions to sea-level rise require models that are able to accurately simulate grounding-line dynamics, starting from initial states consistent with observations. In this new article, we use Elmer/Ice to simulate the centennial evolution of the Amundsen Sea Embayment (the most active drainage basin of the antarctic ice sheet) in response to a prescribed perturbation in order to assess the sensitivity of mass loss projections to the chosen friction law, depending on the initialisation strategy. To this end, we take advantage of inverse methods implemented in Elmer/Ice to construct three different model states by inferring both the initial basal shear stress and viscosity fields with various relative weights. Then, starting from each of these model states, prognostic simulations are carried out using a Weertman, a Schoof and a Budd friction law, with different parameter values. These experiments demonstrate that although the sensitivity of projections to the chosen friction law tends to decrease when more weight is put on viscosity during initialisation, it remains significant for the most physically acceptable of the constructed model states. In addition, it turns out that independently of the considered model state, the Weertman law systematically predicts the lowest mass losses. Finally, because of its particular dependence on effective pressure, the Budd friction law induces significantly different grounding-line retreat patterns than the other laws and predicts significantly higher mass losses. These conclusions urge the scientific community to undertake major efforts in order to get a better understanding of processes at the root of basal motion, with the aim of developing reliable friction laws that could be used in models to produce accurate estimates of future contribution of ice sheets to sea level rise.

Read more: Brondex J., F. Gillet-Chaulet and O. Gagliardini, 2019. Sensitivity of centennial mass loss projections of the Amundsen basin to the friction law, The Cryosphere, 13, 177-195, https://doi.org/10.5194/tc-13-177-2019

The de-neutralized equilibrium

Gladstone etal 2018 equilibriaPoor 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.

Using Elmer/Ice on idealised grounding line evolution experiments, it is shown 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.

Read more: Gladstone, R. M., Xia, Y., and Moore, J.,2018. Neutral equilibrium and forcing feedbacks in marine ice sheet modelling. The Cryosphere, 12, 3605-3615. doi:10.5194/tc-12-3605-2018

Coupling Shallow Shelf with full-stress/Stokes

By the nature of the computational effort imposed by solving the Stokes equations in connection with the strong shear thinning viscosity of ice, the shallow ice approximation (SIA) and shallow shelf approximation (SSA) as well as a combination of both are the common choice for ice-sheet simulations exceeding the century scale. This comes with the caveat that they are of limited accuracy for certain parts of an ice sheet, which would rise motivation for the deployment of full-Stokes (FS) computations coupled to these approximations over such regions. In this new article the authors report on a novel way of iteratively coupling FS and SSA that has been implemented in Elmer/Ice and applied to conceptual marine ice sheets. Applied to MISMIP type of experiments, the FS–SSA coupling appears to be very accurate; the relative error in velocity compared to FS is below 0.5% for diagnostic runs and below 5% for prognostic runs. Results for grounding line dynamics obtained with the FS–SSA coupling are similar to those obtained from an FS model in an experiment with a periodical temperature forcing over 3000 years that induces grounding line advance and retreat. The rapid convergence of the FS–SSA coupling shows a large potential for reducing computation time, such that modelling a marine ice sheet for thousands of years should could become feasible. Despite inefficient matrix assembly in the current implementation, computation time is reduced by 32% in a 3-D ice shelf setup.

Read more:

van Dongen, E. C. H., Kirchner, N., van Gijzen, M. B., van de Wal, R. S. W., Zwinger, T., Cheng, G., Lötstedt, P., and von Sydow, L., 2018. Dynamically coupling full Stokes and shallow shelf approximation for marine ice sheet flow using Elmer/Ice (v8.3). Geosci. Model Dev., 11, 4563-4576. doi:10.5194/gmd-11-4563-2018

Elmer/Ice project © 2018 -- Conception : iGrafic