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Sensitivity of grounding line dynamics to the choice of the friction law

Written by Olivier Gagliardini on .

julien2017Basal slip accounts for a large part of the flow of ice streams draining ice from Antarctica and Greenland into the ocean. Therefore, an appropriate representation of basal slip in ice flow models is a prerequisite for accurate sea level rise projections. Various friction laws have been proposed to describe basal slip in models. Here, we compare the influence on grounding line (GL) dynamics of four friction laws: the traditional Weertman law and three effective pressure-dependent laws, namely the Schoof, Tsai and Budd laws. It turns out that, even when they are tuned to a common initial reference state, the Weertman, Budd and Schoof laws lead to thoroughly different steady-state positions, although the Schoof and Tsai laws lead to much the same result. In particular, under certain circumstances, it is pos- sible to obtain a steady GL located on a reverse slope area using the Weertman law. Furthermore, the predicted transient evolution of the GL as well as the projected contributions to sea level rise over a 100-year time horizon vary significantly depending on the friction law. We conclude on the importance of choosing an appropriate law for reliable sea level rise projections and emphasise the need for a coup- ling between ice flow models and physically based subglacial hydrological models.

Read more: Brondex, J., O. Gagliardini, F. Gillet-Chaulet and G. Durand, 2017. Sensitivity of grounding line dynamics to the choice of the friction law, Journal of Glaciology, 63(241), 854-866, doi: 10.1017/jog.2017.51.

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Comparing SIA and Stokes on a large Antarctic basin

Written by Thomas Zwinger on .

Full Stokes (FS) formulation and the most Seddik Shiraze ElmerIceWebsimplified shallow ice approximation (SIA) are implemented in the ice flow model Elmer/Ice, which enables the authors to compare them by applying the model to the East Antarctic Shirase drainage basin. Using the control inverse method to infer the distribution of basal friction with FS, they compare FS and SIA by simulating the flow of the drainage basin under present-day conditions and for three scenarios 100 years into the future defined by the SeaRISE (Sea-level Response to Ice Sheet Evolution) project. To no big surprise, FS reproduces the observed flow pattern of the drainage basin well, in particular the zone of fast flow near the grounding line, while SIA generally overpredicts the surface velocities. Looking at transient scenarios, the ice volume change (relative to the constant-climate control run) of the surface climate experiment is nearly the same for FS and SIA, while for the basal sliding experiment (halved basal friction), the ice volume change is  ∼  30 % larger for SIA than for FS. This confirms findings of earlier studies that, in order to model ice sheet areas containing ice streams and outlet glaciers with high resolution and precision, careful consideration must be given to the choice of a suitable force balance.

Read more: Seddik, H., R. Greve, T. Zwinger, and S. Sugiyama, 2017. Regional modeling of the Shirase drainage basin, East Antarctica: full Stokes vs. shallow ice dynamics, The Cryosphere, 11, 2213-2229, doi:https://doi.org/10.5194/tc-11-2213-2017.

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Reconstructing glacier thickness for test geometries on Svalbard

Written by Olivier Gagliardini on .

furst2017In this study, we present a two-step reconstruction approach for mapping glacier thickness that solves mass conservation over single or several connected drainage basins. The approach is applied to a variety of test geometries with abundant thickness measurements including marine- and land-terminating glaciers as well as an ice cap on Svalbard. In the first step, a geometrically controlled, non-local flux solution is converted into thickness values relying on the shallow ice approximation (SIA). In a second step, the thickness field is updated along fast-flowing glacier trunks on the basis of velocity observations. Both steps account for available thickness measurements. Each thickness field is presented together with an error-estimate map based on a formal propagation of input uncertainties. For Vestfonna ice cap, a previous ice volume estimate based on the same measurement record as used here has to be corrected upward by 22 %. We also find that a 13% area-fraction of the ice cap is in fact grounded below sea level. The former 5%-estimate from a direct measurement interpolation exceeds the aggregate error range of 6–23%.

Read more: Fürst, J. J., F. Gillet-Chaulet, T. J. Benham, J. A. Dowdeswell, M. Grabiec, F. Navarro, R. Pettersson, G. Moholdt, G., C. Nuth, B. Sass, K. Aas, X. Fettweis, C. Lang, T. Seehaus and M. Braun, 2017. Application of a two-step approach for mapping ice thickness to various glacier types on Svalbard, The Cryosphere, 11, 2003-2032, doi:10.5194/tc-11-2003-2017.

 

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