Sensitivity of basal conditions in an inverse model: Vestfonna ice cap, Nordaustlandet/Svalbard

Schäfer, Martina; Zwinger, Thomas; Christoffersen, Poul; Gillet-Chaulet, Fabien; Laakso, Kati; Pettersson, Rickard; Pohjola, Veijo; Strozzi, Tazio; Moore, John C.

doi:10.5194/tc-6-771-2012

Cited by 39 publications

(57 citation statements)

References 49 publications

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“…As for glaciological applications, velocities and strain rates can vary by several orders of magnitude inside the domain, and we have observed that including the nodal weights in the definition of the Fréchet derivatives leads to good convergence properties when using an unstructured mesh where large elements correspond to low-velocity areas, and vice versa. Possible alternatives are, for the control inverse method (Morlighem et al, 2010), to use a cost function that measures the logarithm of the misfit or, for the Robin inverse method (Arthern and Gudmundsson, 2010), to use a spatially varying step size rather than a fixed step in the gradient descent algorithm, as proposed in Schäfer et al (2012).…”

Section: Minimisationmentioning

confidence: 99%

“…Elmer/Ice was further used as a reference for the later MISMIP3d experiments (Pattyn et al, 2013). Recently, data assimilation was implemented within Elmer/Ice (Jay-Allemand et al, 2011;Schäfer et al, 2012;Gillet-Chaulet et al, 2012) to infer poorly known parameters such as basal drag. Today, Elmer/Ice is the only three-dimensional full-Stokes model that solves the grounding line dynamics (Favier et al, 2012), and it will be the only full-Stokes model able to run forecast simulations for the whole Greenland ice sheet for the coming AR5 IPPC report, in the framework of both SeaRISE and ice2sea Shannon et al, 2013;Edwards et al, 2013) programmes.…”

Section: Introductionmentioning

confidence: 99%

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Capabilities and performance of Elmer/Ice, a new-generation ice sheet model

et al. 2013

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Abstract. The Fourth IPCC Assessment Report concluded that ice sheet flow models, in their current state, were unable to provide accurate forecast for the increase of polar ice sheet discharge and the associated contribution to sea level rise. Since then, the glaciological community has undertaken a huge effort to develop and improve a new generation of ice flow models, and as a result a significant number of new ice sheet models have emerged. Among them is the parallel finite-element model Elmer/Ice, based on the opensource multi-physics code Elmer. It was one of the first fullStokes models used to make projections for the evolution of the whole Greenland ice sheet for the coming two centuries. Originally developed to solve local ice flow problems of high mechanical and physical complexity, Elmer/Ice has today reached the maturity to solve larger-scale problems, earning the status of an ice sheet model. Here, we summarise almost 10 yr of development performed by different groups. Elmer/Ice solves the full-Stokes equations, for isotropic but also anisotropic ice rheology, resolves the grounding line dynamics as a contact problem, and contains various basal friction laws. Derived fields, like the age of the ice, the strain rate or stress, can also be computed. Elmer/Ice includes two recently proposed inverse methods to infer badly known parameters. Elmer is a highly parallelised code thanks to recent developments and the implementation of a block preconditioned solver for the Stokes system. In this paper, all these components are presented in detail, as well as the numerical performance of the Stokes solver and developments planned for the future.

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Section: Minimisationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Capabilities and performance of Elmer/Ice, a new-generation ice sheet model

et al. 2013

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“…The research area and observational data have been presented in detail by Schäfer et al (2012). Key features and additional data are described in Sect.…”

Section: Introductionmentioning

confidence: 99%

Assessment of heat sources on the control of fast flow of Vestfonna ice cap, Svalbard

et al. 2014

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Abstract. Understanding the response of fast flowing ice streams or outlet glaciers to changing climate is crucial in order to make reliable projections of sea level change over the coming decades. Motion of fast outlet glaciers occurs largely through basal motion governed by physical processes at the glacier bed, which are not yet fully understood. Various subglacial mechanisms have been suggested for fast flow but common to most of the suggested processes is the requirement of presence of liquid water, and thus temperate conditions.We use a combination of modelling, field, and remote observations in order to study links between different heat sources, the thermal regime and basal sliding in fast flowing areas on Vestfonna ice cap. A special emphasis lies on Franklinbreen, a fast flowing outlet glacier which has been observed to accelerate recently. We use the ice flow model Elmer/Ice including a Weertman type sliding law and a Robin inverse method to infer basal friction parameters from observed surface velocities. Firn heating, i.e. latent heat release through percolation of melt water, is included in our model; its parameterisation is calibrated with the temperature record of a deep borehole. We found that strain heating is negligible, whereas friction heating is identified as one possible trigger for the onset of fast flow. Firn heating is a significant heat source in the central thick and slow flowing area of the ice cap and the essential driver behind the ongoing fast flow in all outlets.Our findings suggest a possible scenario of the onset and maintenance of fast flow on the Vestfonna ice cap based on thermal processes and emphasise the role of latent heat released through refreezing of percolating melt water for fast flow. However, these processes cannot yet be captured in a temporally evolving sliding law. In order to simulate correctly fast flowing outlet glaciers, ice flow models not only need to account fully for all heat sources, but also need to incorporate a sliding law that is not solely based on the basal temperature, but also on hydrology and/or sediment physics.

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“…These methods have been implemented and applied with success in ice flow models of different complexity in order to infer the basal drag, one of the most uncertain model parameters (e.g. Morlighem et al, 2010;Jay-Allemand et al, 2011;Schäfer et al, 2012;GilletChaulet et al, 2012).…”

Section: Introductionmentioning

confidence: 99%

Comparison of adjoint and nudging methods to initialise ice sheet model basal conditions

2016

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Abstract. Ice 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.

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Sensitivity of basal conditions in an inverse model: Vestfonna ice cap, Nordaustlandet/Svalbard

Cited by 39 publications

References 49 publications

Capabilities and performance of Elmer/Ice, a new-generation ice sheet model

Capabilities and performance of Elmer/Ice, a new-generation ice sheet model

Assessment of heat sources on the control of fast flow of Vestfonna ice cap, Svalbard

Comparison of adjoint and nudging methods to initialise ice sheet model basal conditions

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