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Classification of different calving modes

Written by Thomas Zwinger on .

BennJGlac2017 picture

Fracture of ice is an inherently discontinuous process and consequently difficult to be included in continuum ice-flow models. There are existing models that deal with the physics of fracture, with the drawback of the demand for high spatial (in the size of the cracks) and temporal (spatial scales divided by speed of sound) resolution. In this paper the authors develop a new strategy for formulating calving laws,using a model of the latter kind, the Helsinki Discrete Element Model (HiDEM) to explicitly model fracture and calving. Simultaneously, the Elmer/Ice is applied to the same geometries to identify critical stress states that can be linked to computed calving events in HiDEM. The final goal is to develop improved calving laws for continuum models, as in last consequence only continuum models are capable to cover ice dynamics exceeding a few hours. Usually, calving induced by buoyancy and undercutting is under-represented in existing continuum calving laws. This study helps to identify stress states from a full Stokes model (Elmer/Ice) that can help to indicate characteristic stress/strain patterns that would be input for improved calving laws in future.

Read more: Benn, D.I., J. Åström, T. Zwinger, J. Todd, F.M. Nick, S. Cook, N.R.J. Hulton, and A. Luckman, 2017. Melt-under-cutting and buoyancy-driven calving from tidewater glaciers: new insights from discrete element and continuum model simulations, Journal of Glaciology, 1-12, doi:10.1017/jog.2017.41.

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Investigation of a rapidly rising jökulhlaup

Written by Thomas Zwinger on .

Mesh of Elmer/Ice of the Skáfta cauldronThe authors investigate a rapidly rising glacial outburst flood (jökulhlaup) at the western Skaftá cauldron, Vatnajökull, Iceland, in September 2006. Outflow from the subglacial lake, flood discharge at the glacier terminus and the transient subglacial volume of floodwater during the jökulhlaup are derived from measured discharge and water temperature measurements in the pro-glacial river and the lowering of the ice over subglacial lake. Elmer/Ice is used to simulate an axi-symmetric approximation of the ice above the subglacial lake, which acts similar to an ice-shelf reacting on the elevation change of the water level. Consideration on the available thermal and potential energy along the 40 km initial subglacial water-path indicate that the jökulhlaup propagates by lifting and deformation of the overlying ice, induced by water pressure in excess of the ice overburden pressure and that melting of ice due to the heat of the floodwater from the subglacial lake and frictional heat generated by the dissipation of potential energy in the flow played a smaller role. Therefore this event and other rapidly rising jökulhlaups cannot be explained by the jökulhlaup theory of Nye (1976).

Read more: Einarsson, B., T. Jóhannesson, T. Thorsteinsson, E. Gaidos, and T. Zwinger, 2017. Subglacial flood path development during a rapidly rising jökulhlaup from the western Skaftá cauldron, Vatnajökull, Iceland, Journal of Glaciology, 1-13, doi:10.1017/jog.2017.33.

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Determining surface mass balance from DEM's

Written by Thomas Zwinger on .

Accum all annotThis article deals with the reconstruction of surface mass balance patterns based on digital elevation models (DEM's). Using two consecutive DEMs, Elmer/Ice has been used as a diagnostic model to evaluate the emergence velocity for a geometry defined as the arithmetic mean between these two DEMs. Using in addition a linearly between the DEMs interpolated local elevation change, one is able to reconstruct an average surface mass balance (SMB) for the period of interest. As the object of study, Midtre Lovénbreen (a small glacier in Svalbard, close to the Nye Ålesund station), has an accompanying excellent record of stake data measurements, the authors were able to compare the results from their new approach with in-situ observations. The advantages of the presented approach clearly are given by the facts that (provided the bedrock is known) surface DEMs can be obtained using modern remote sensing techniques and lead to a result for the inverted SMB covering the whole glacier, whereas in-situ measurements usually are confined to a limited area of the glacier.

Read more:  Välisuo, I., T. Zwinger and J. Kohler, 2017. Inverse solution of surface mass balance of Midtre Lovénbreen, Svalbard, Journal of Glaciology, 1-10, doi:10.1017/jog.2017.26.

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