Elmer/Ice News

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New Beginner Elmer/Ice course this fall

Written by Olivier Gagliardini on .

We are organizing a 3-day beginner Elmer/Ice course from Monday October 31 to Wednesday November 2, 2016, at the University of Oslo (the week after the IGS Nordic Branch meeting which will be held at the Fram Centre in Tromsø, Norway, from Wednesday October 26 to Friday October 28).

This 3-days course is dedicated to students or researchers aiming to start working with Elmer/Ice. During the course, you will learn how to set up a simple ice flow problem for a flow line geometry as well as for a real mountain glacier. The third day will be dedicated to more advanced topics like the coupling of ice flow and temperature or inverse methods. For those interested, this last day might also be dedicated to start setting up your own problem with our help. 

The course is sponsored by the Department of Geosciences of the University of Oslo, the eScience tools for investigating climate change (eSTICC), CSC, LGGE and the Labex OSUG@2020.

The number of places is limited to 20, and will be given on the basis of the first registered, first served. To register, send an email to Olivier Gagliardini with you name, affiliation, position and few lines of motivations to attend the course. There will be no registration fees, but students will have to take care of their lodging and attend the course with their own laptop. More information will be given later on the Elmer/Ice website.

Elmer/Ice teachers: Adrien Gilbert, Fabien Gillet-Chaulet, Thomas Zwinger and Olivier Gagliardini
Local organising committee: Thomas V. Schuler

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Simultaneous inversion of basal friction and bed elevation

Written by Olivier Gagliardini on .

mosbeux gmd2016Ice flow models are now routinely used to forecast the ice sheets’ contribution to 21st century sea-level rise. For such short term simulations, the model response is greatly affected by the initial conditions. Data assimilation algorithms have been developed to invert for the friction of the ice on its bedrock using observed surface velocities. A drawback of these methods is that remaining uncertainties, especially in the bedrock elevation, lead to non-physical ice flux divergence anomalies resulting in undesirable transient effects. In this study, we compare two different assimilation algorithms based on adjoints and nudging to constrain both bedrock friction and elevation. Using synthetic twin experiments with realistic observation errors, we show that the two algorithms lead to similar performances in reconstructing both variables and allow the flux divergence anomalies to be significantly reduced.

More informations: Mosbeux, C., F. Gillet-Chaulet and O. Gagliardini, 2016. Comparison of adjoint and nudging methods to initialise ice sheet model basal conditions, Geosci. Model Dev., 9, 2549-2562, doi:10.5194/gmd-9-2549-2016.

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Can we apply 2.5-D model at a Dome?

Written by Olivier Gagliardini on .

2p5modelThree-dimensional ice flow modelling requires a large number of computing resources and observation data, such that 2-D simulations are often preferable. However, when there is significant lateral divergence, this must be accounted for (2.5-D models), and a flow tube is considered (volume between two horizontal flowlines). In the absence of velocity observations, this flow tube can be derived assuming that the flowlines follow the steepest slope of the surface, under a few flow assumptions. This method typically consists of scanning a digital elevation model (DEM) with a moving window and computing the curvature at the centre of this window. The ability of the 2.5-D models to account properly for a 3-D state of strain and stress has not clearly been established, nor their sensitivity to the size of the scanning window and to the geometry of the ice surface, for example in the cases of sharp ridges. Here, we study the applicability of a 2.5-D ice flow model around a dome, typical of the East Antarctic plateau conditions. A twin experiment is carried out, comparing 3-D and 2.5-D computed velocities, on three dome geometries, for several scanning windows and thermal conditions. The chosen scanning window used to evaluate the ice surface curvature should be comparable to the typical radius of this curvature. For isothermal ice, the error made by the 2.5-D model is in the range 0–10 % for weakly diverging flows, but is 2 or 3 times higher for highly diverging flows and could lead to a non-physical ice surface at the dome. For non-isothermal ice, assuming a linear temperature profile, the presence of a sharp ridge makes the 2.5-D velocity field unrealistic. In such cases, the basal ice is warmer and more easily laterally strained than the upper one, the walls of the flow tube are not vertical, and the assumptions of the 2.5-D model are no longer valid.

More information in  Passalacqua O., O. Gagliardini, F. Parrenin, J. Todd, F. Gillet-Chaulet and C. Ritz, 2016. Performance and applicability of a 2.5-D ice-flow model in the vicinity of a dome, Geosci. Model Dev., 9, 2301-2313, doi:10.5194/gmd-9-2301-2016.

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