Elmer/Ice News

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Where shelf-ice turns passive

Written by Olivier Gagliardini on .

 

NCC JohannesThe decadal record of ice-shelf recession and break-up on the Antarctic Peninsula reveals that these vast slabs of floating ice restrain the sea-level relevant outflow from the upstream, well-grounded tributary glaciers. One of the key questions in projecting this outflow for the whole of Antarctica is how far ice-front recession may progress before important dynamic consequences are anticipated. This question is addressed in a recently published article in Nature Climate Change which relies on a continental-scale data assimilation with Elmer/Ice. Details of this assimilation were already presented in an accompanying article published earlier in The Cryosphere. The aim of the data assimilation was to quantify the buttressing effect on ice shelves relying on the inferred stress regime. If maximum buttressing showed values below a threshold, which itself was inferred from generic calving experiments, ice was considered dynamically passive. As long as the calving only removes this passive shelf-ice (PSI) portion, no important dynamic consequences are expected. The results show that 13 % of the total ice-shelf surface of Antarctica hold PSI with contrasting results across the continent. The presented analysis draws again the attention to the Amundsen and Bellingshausen sea sectors, where the PSI area fraction is notably lower, with regional averages of 7 and 5 %, respectively.

News written by Johannes J. Fürst.

Fürst, J. J., G. Durand, F. Gillet-Chaulet, L. Tavard, M. Rankl, M. Braun and O. Gagliardini, 2016. The safety band of Antarctic ice shelves, Nature Climate Change, doi:10.1038/NCLIMATE2912.

 

 

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Impact of mesh resolution for MISMIP and MISMIP3d experiments

Written by Olivier Gagliardini on .

frictionThe dynamical contribution of marine ice sheets to sea level rise is largely controlled by grounding line (GL) dynamics. Two marine ice sheet model intercomparison exercises, namely MISMIP and MISMIP3d, have been proposed to the community to test and compare the ability of models to capture the GL dynamics. Both exercises are known to present a discontinuity of the friction at the GL, which is believed to increase the model sensitivity to mesh resolution. Here, using Elmer/Ice, the only Stokes model which completed both intercomparisons, the sensitivity to the mesh resolution is studied from an extended MISMIP experiment in which the friction continuously decreases over a transition distance and equals zero at the GL. Using this MISMIP-like setup, it is shown that the sensitivity to the mesh resolution is not improved for a vanishing friction at the GL. For the original MISMIP experiment, i.e. for a discontinuous friction at the GL, we further show that the results are moreover very sensitive to the way the friction is interpolated in the close vicinity of the GL. In the light of these new insights, and thanks to increased computing resources, new results for the MISMIP3d experiments obtained for higher resolutions than previously published are made available for future comparisons as the Supplement.

Gagliardini O., J. Brondex, F. Gillet-Chaulet, L. Tavard, V. Peyaud and G. Durand, 2016. Brief communication: Impact of mesh resolution for MISMIP and MISMIP3d experiments using Elmer/Ice, The Cryosphere, 10, 307-312, doi:10.5194/tc-10-307-2016.

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Best of both worlds: SIA and full-Stokes

Written by Thomas Zwinger on .

In a recent article in the Journal of Computational Physics, Ahlkrona et al. present a method (ISCAL - Ice  Sheet  Coupled Approximation Levels) implemented in Elmer/Ice that uses an error estimate to identify areas where the shallow ice approximation (SIA) can and cannot be deployed. According to this estimate, those regions, where the shallowness assumption prevails can be switched to a - compared to the full-Stokes solution - extremely cheap SIA (simply by replacing the matrix entries in the system matrix of the linearized and discretized full-Stokes problem. In their article the authors demonstrate a reduction of CPU time by ISCAL on an in size reduced real world case of up to 75% compared to a full-Stokes simulation run of the same problem.

Ahlkrona, J., P. Lötstedt, N. Kirchner, and T. Zwinger, 2016. Dynamically coupling the non-linear Stokes equations with the shallow ice approximation in glaciology: Description and first applications of the ISCAL method. J. Comp. Phys., 308, 1-19, doi:10.1016/j.jcp.2015.12.025.

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