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Discussion papers | Copyright
https://doi.org/10.5194/gmd-2017-322
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Development and technical paper 08 Feb 2018

Development and technical paper | 08 Feb 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Geoscientific Model Development (GMD).

A rapidly converging spin-up method for the present-day Greenland ice sheet using the GRISLI ice-sheet model

Sébastien Le clec'h1, Aurélien Quiquet1, Sylvie Charbit1, Christophe Dumas1, Masa Kageyama1, and Catherine Ritz2 Sébastien Le clec'h et al.
  • 1Laboratoire des Sciences du Climat et de l’Environnement, LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
  • 2Univ. Grenoble Alpes, CNRS, IGE, 38000 Grenoble, France

Abstract. Providing reliable projections of the ice-sheet contribution to future sea-level rise has become one of the main challenges of the ice-sheet modelling community. To increase confidence in future projections, a good knowledge of the present-day state of the ice flow dynamics, which is critically dependent on basal conditions, is strongly needed. The main difficulty is tied to the scarcity of observations at the ice-bed interface at the scale of the whole ice sheet, resulting in poorly constrained parameterisations in ice-sheet models. To circumvent this drawback, inverse modelling approaches can be developed and validated against available data to infer reliable initial conditions of the ice sheet. Here, we present a spin-up method for the Greenland ice sheet using the thermo-mechanical hybrid GRISLI ice-sheet model. Our approach is based on the adjustment of the basal drag coefficient that relates the sliding velocities at the ice-bed interface to basal shear stress in unfrozen bed areas. This method relies on an iterative process in which the basal drag is periodically adjusted in such as way that the simulated ice thickness matches the observed one. The process depends on three parameters controlling the duration and the number of iterations. The best spin-up parameters are chosen according to two criteria to minimize errors in sea-level projections: the final difference between the simulated and the observed Greenland ice volume as well as the final ice volume trend which must both be as low as possible. To increase confidence in the inferred parameters, we also make sure that the final ice thickness root mean square error from the observations is not greater than a few tens of meters. Our best results are obtained after only 420 years of simulation, highlighting a rapid convergence and demonstrating that our method can be used for computationally expensive ice sheet models.

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Sébastien Le clec'h et al.
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Short summary
Providing reliable projections of the ice-sheet contribution to future sea-level rise is a challenge for the ice-sheet model community. To increase confidence, ice-sheet model must simulate the observed ice sheet present-day state. Using a low computational iterative inverse spin-up method, based on the adjustment of the basal drag coefficient, we rapidly minimize the errors between the simulated and the observed Greenland ice volume, ice thickness and minimize the simulated ice volume trend.
Providing reliable projections of the ice-sheet contribution to future sea-level rise is a...
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