Elmer/Ice News


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.


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.


Elmer/Ice at EGU 2016

Written by Olivier Gagliardini on .


Don't miss the talks and posters making use of Elmer/Ice:

** Monday
* CR1.4/AS4.3 Glaciers and ice caps under climate change (co-organized)
Poster: 17:30–19:00 / Hall X4
- Martina Schäfer, Marco Möller, Thomas Zwinger, and John Moore. The influence of topographic feedback on a coupled mass balance and ice-flow model for Vestfonna ice-cap, Svalbard

** Tuesday
* CR5.1 Modelling ice sheets and glaciers
Poster: 17:30–19:00 / Hall X4
- Julien Brondex, Olivier Gagliardini, Fabien Gillet-Chaulet, and Gael Durand. Influence of damage and basal friction on the grounding line dynamics

- Josefin Ahlkrona, Per Lötstedt, and Thomas Zwinger. Dynamically coupling the full Stokes equations and the SIA - the ISCAL method

* CR5.2 Subglacial Environments of Ice Sheets and Glaciers
Poster 17:30–19:00 / Hall X4

- Dorothée Vallot, Rickard Pettersson, Doug Benn, Adrian Luckman, Olivier Gagliardini, Fabien Gillet-Chaulet, Thomas Zwinger, Jack Kohler, Ward van Pelt, and Björn Claremar. Seasonal subglacial state under Kronebreen, Svalbard - inversion and sliding laws.

* CR5.3/OS2.10 Ice shelves and tidewater glaciers - dynamics, interactions, observations, modelling (co-organized)
Oral: 13:30–17:00 / Room L2
- 15h30-15h45: Martin O'Leary, Bernd Kulessa, Adam Booth, Paul Holland, Daniela Jansen, Ed King, Adrian Luckman, Dan McGrath, and Thomas Zwinger. Inferring the viscous and elastic properties of a suture zone in Larsen C

** Friday
* CR 13 Reconstructing paleo ice dynamics: Comparing and combining field-based evidence and numerical modeling
Oral: 10:30–12:00 / Room L2
- 10h30-10h45: Denis Cohen, Fabien Gillet-Chaulet, Thomas Zwinger, Horst Machguth, Wilfried Haeberli, and Urs H. Fischer Basal conditions and flow dynamics of the Rhine glacier, Alps, at the Last Glacial Maximum.

Poster: 13:30–15:00 / Hall X4
- Guillaume Jouvet, Denis Cohen, Julien Seguinot, and Fabien Gillet-Chaulet. Inconsistent Last Glacial Maximum ice thickness of the Rhine glacier between geomorphological reconstructions and two numerical models.




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