Elmer/Ice News

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

SVIFT1.0: The Ice-Free Topography of Svalbard

shmipSvalbard is an archipelago in the Arctic, north of Norway, which is comparable in size to the New York metropolitan area. Roughly half of it is covered by glacier ice. Yet to this day, the ice volume stored in the many glaciers on Svalbard is not well known. Many attempts have been made to infer a total volume estimate, but results differ substantially. This surprises because of the long research activity in this area. A large record of more than 1 million thickness measurements exists, making Svalbard an ideal study area for the application of a state-of-the-art mapping approach for glacier ice thickness. The mapping approach computes an ice volume that will raise global sea level by more than half an inch if instantaneously melted. If spread over the metropolitan area, New York would be buried beneath a 100-m ice cover. The asset of this approach is that it provides not only a thickness map for each glacier on the archipelago but also an error map that defines the likely local thickness range. Finally, we provide the first well-informed estimate of the ice front thickness of all marine-terminating glaciers that loose icebergs to the ocean. The archipelago-wide mean ice front cliff is 135 m. The first version of the Svalbard ice-free topography (SVIFT1.0) is publicly available here

Read More:  Fürst, J. J., Navarro, F., Gillet‐Chaulet, F., Huss, M., Moholdt, G., Fettweis, X., et al. 2018. The ice‐free topography of Svalbard. Geophysical Research Letters, 45. doi:10.1029/2018GL079734

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