Elmer/Ice News

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

Elmer/Ice participated the SHMIP exercice

shmipOver recent decades, Greenland ice sheet surface melt has shown an increase both in intensity and spatial extent. Part of this water probably reaches the bed and can enhance glacier speed, advecting a larger volume of ice into the ablation area. In the context of a warming climate, this mechanism could contribute to the future rate of thinning and retreat of land-terminating glaciers of Greenland. These changes in ice flow conditions will in turn influence surface crevassing and thus the ability of water to reach the bed at higher elevations. Here, using a coupled basal hydrology and prognostic ice flow model, the evolution of a Greenland-type glacier subject to increasing surface melt is studied over a few decades. For different scenarios of surface melt increase over the next decades, the evolution of crevassed areas and the ability of water to reach the bed is inferred. Our results indicate that the currently observed crevasse distribution is likely to extend further upstream which will allow water to reach the bed at higher elevations. This will lead to an increase in ice flux into the ablation area which, in turn, accelerates the mass loss of land-terminating glaciers.

Read More:  De Fleurian, B., M. Werder, S. Beyer, D. Brinkerhoff, I. Delaney, C. Dow, C., J. Dows, O. Gagliardini, M.J. Hoffman, R. LeB Hooke, J. Seguinot, A.N. Sommers, 2018. SHMIP The subglacial hydrology model intercomparison Project. Journal of Glaciology, 1-20. doi:10.1017/jog.2018.78

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