Elmer/Ice News

A subgrid scheme for grounding line migration

gmd 13 2245 2020 avatar web

The flow of large ice sheets and glaciers can be simulated by solving the full Stokes equations using a finite element method. The simulation is particularly sensitive to the discretization of the grounding line, which separates ice resting on bedrock and ice floating on water, and is moving with time. The boundary conditions at the ice base are enforced by Nitsche's method and a subgrid treatment of the grounding line element. Simulations with the method in two dimensions for an advancing and a retreating grounding line illustrate the performance of the method. The computed grounding line position is compared to previously published data with a fine mesh, showing that similar accuracy is obtained using subgrid modeling with more than 20-times-coarser meshes. This subgrid scheme is implemented in the two-dimensional version of the open-source code Elmer/ICE.

Read more: Cheng, G., P. Lötstedt and L. von Sydow, 2020. A full Stokes subgrid scheme in two dimensions for simulation of grounding line migration in ice sheets using Elmer/ICE (v8.3), Geosci. Model Dev., 13, 2245–2258, doi:10.5194/gmd-13-2245-2020

A Micro-Mechanical Model for the Transformation of Dry Polar Firn Into Ice Using the Level-Set Method

Fourteau 2020Interpretation of greenhouse gas records in polar ice cores requires a good understanding of the mechanisms controlling gas trapping in polar ice, and therefore of the processes of densification and pore closure in firn (compacted snow). Current firn densification models are based on a macroscopic description of the firn and rely on empirical laws and/or idealized geometries to obtain the equations governing the densification and pore closure.

Here, we propose a physically-based methodology explicitly representing the porous structure and its evolution over time. In order to handle the complex geometry and topological changes that occur during firn densification, we rely on a Level-Set representation of the interface between the ice and the pores. Two mechanisms are considered for the displacement of the interface: (i) mass surface diffusion driven by local pore curvature and (ii) ice dislocation creep. For the latter, ice is modeled as a viscous material and the flow velocities are solutions of the Stokes equations.

First applications show that the model is able to densify firn and split pores. Using the model in cold and arid conditions of the Antarctic plateau, we show that gas trapping models do not have to consider the reduced compressibility of closed pores compared to open pores in the deepest part of firns. Our results also suggest that the mechanism of curvature-driven surface diffusion does not result in pore splitting, and that ice creep has to be taken into account for pores to close. Future applications of this type of model could help quantify the evolution and closure of firn porous networks for various accumulation and temperature conditions. 

Read More: Fourteau, K., F. Gillet-Chaulet, P. Martinerie and X. Faïn, 2020. A Micro-Mechanical Model for the Transformation of Dry Polar Firn Into Ice Using the Level-Set Method, Frontiers in Earth Science, 8, doi:10.3389/feart.2020.00101

A viscoelastic solid earth deformation model to Elmer/Ice

keyfig GMD croppedIn a recent development funded by the University of Tasmania, Hobart, Australia, the linear Elasticity solver in Elmer has been modified to account for Maxwell type of viscoelastic materials. Furthermore, the most simplest implementation of a a viscoelastic solid earth model, neglecting sphericity and self-gravitation has been set up in its weak form. In contrary to for GIA adapted commercial codes, this means that Elmer does not need special treatment of boundaries between layers of different density, which makes the setup of a layered solid earth model much easier and - since Elmer is open source - independent from any license fees that would be imposed by commercial codes. The paper mentioned below itself does not include results from Elmer/Ice, but we would like to utilize this news-feed to point out the possibility to apply the new module, Elmer/Earth, in combination with Elmer/Ice, as the transfer of ice-loads and deformations are easily achieved by the common code-basis Elmer. Imposed by the earlier mentioned simplifications, computations should be confined to regional studies. In future we might think of including missing components to deploy this model on continental scale ice-sheets.

Read more:

Zwinger, T., Nield, G. A., Ruokolainen, J., and King, M. A., 2020. A new open-source viscoelastic solid earth deformation module implemented in Elmer (v8.4),  Geosci. Model Dev., 14, 1155–1164, doi:10.5194/gmd-13-1155-2020

 

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