Interpretation 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
In 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.
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
Marine-based sectors of the Antarctic Ice Sheet are increasingly contributing to sea level rise. The basal conditions exert an important control on the ice dynamics and can be propitious to instabilities in the grounding line position. Because the force balance is non-inertial, most ice flow models are now equipped with time-independent inverse methods to constrain the basal conditions from observed surface velocities. However, transient simulations starting from this initial state usually suffer from inconsistencies and are not able to reproduce observed trends. Here, using a synthetic flow line experiment, we assess the performance of an ensemble Kalman filter for the assimilation of transient observations of surface elevation and velocities in a marine ice sheet model. The model solves the shallow shelf equation for the force balance and the continuity equation for ice thickness evolution. The position of the grounding line is determined by the floatation criterion. The filter analysis estimates both the state of the model, represented by the surface elevation, and the basal conditions, with the simultaneous inversion of the basal friction and topography. The idealised experiment reproduces a marine ice sheet that is in the early stage of an unstable retreat. Using observation frequencies and uncertainties consistent with current observing systems, we find that the filter allows the accurate recovery of both the basal friction and topography after few assimilation cycles with relatively small ensemble sizes. In addition it is found that assimilating the surface observations has a positive impact on constraining the evolution of the grounding line during the assimilation window. Using the initialised state to perform century-scale forecast simulations, we show that grounding line retreat rates are in agreement with the reference; however remaining uncertainties in the basal conditions may lead to significant delays in the initiation of the unstable retreat. These results are encouraging for the application to real glacial systems.
Read more: Gillet-Chaulet, F., 2020. Assimilation of surface observations in a transient marine ice sheet model using an ensemble Kalman filter, The Cryosphere 14, 811–832, doi:10.5194/tc-14-811-2020