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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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ITMIX: How accurate are estimates of glacier ice thickness?

Written by Olivier Gagliardini on .

itmixElmer/Ice used to build 3 synthetic cases for the ITMIX intercomparisoon exercice, but also as a participating models. 

Abstract: Knowledge of the ice thickness distribution of glaciers and ice caps is an important prerequisite for many glaciological and hydrological investigations. A wealth of approaches has recently been presented for inferring ice thickness from characteristics of the surface. With the Ice Thickness Models Intercomparison eXperiment (ITMIX) we performed the first coordinated assessment quantifying individual model performance. A set of 17 different models showed that individual ice thickness estimates can differ considerably – locally by a spread comparable to the observed thickness. Averaging the results of multiple models, however, significantly improved the results: on average over the 21 considered test cases, comparison against direct ice thickness measurements revealed deviations on the order of 10 ± 24 % of the mean ice thickness (1σ estimate). Models relying on multiple data sets – such as surface ice velocity fields, surface mass balance, or rates of ice thickness change – showed high sensitivity to input data quality. Together with the requirement of being able to handle large regions in an automated fashion, the capacity of better accounting for uncertainties in the input data will be a key for an improved next generation of ice thickness estimation approaches.

Read more:  Farinotti, D. and others, 2017. How accurate are estimates of glacier ice thickness? Results from ITMIX, the Ice Thickness Models Intercomparison eXperiment, The Cryosphere, 11, 949-970, doi:10.5194/tc-11-949-2017.

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