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Evidence for uncharted pinning points in Antarctica

Written by Olivier Gagliardini on .

furst pinningIn ice flow modelling, the use of control methods to assimilate the dynamic and geometric state of an ice body has become common practice. These methods have primarily focussed on inverting for one of the two least known properties in glaciology, namely the basal friction coefficient or the ice viscosity parameter. Here, we present an approach to infer both properties simultaneously for the whole of the Antarctic ice sheet. After the assimilation, the root-mean-square deviation between modelled and observed surface velocities attains 8.7 m a−1 for the entire domain, with a slightly higher value of 14.0 m a−1 for the ice shelves. An exception in terms of the velocity mismatch is the Thwaites Glacier Ice Shelf, where the RMS value is almost 70 m a−1. The reason is that the underlying Bedmap2 geometry ignores the presence of an ice rise, which exerts major control on the dynamics of the eastern part of the ice shelf. On these grounds, we suggest an approach to account for pinning points not included in Bedmap2 by locally allowing an optimisation of basal friction during the inversion. In this way, the velocity mismatch on the ice shelf of Thwaites Glacier is more than halved. A characteristic velocity mismatch pattern emerges for unaccounted pinning points close to the marine shelf front. This pattern is exploited to manually identify seven uncharted features around Antarctica that exert significant resistance to the shelf flow. Potential pinning points are detected on Fimbul, West, Shackleton, Nickerson and Venable ice shelves. As pinning points can provide substantial resistance to shelf flow, with considerable consequences if they became ungrounded in the future, the model community is in need of detailed bathymetry there. Our data assimilation points to some of these dynamically important features not present in Bedmap2 and implicitly quantifies their relevance.

 

Fürst J. J., G. Durand, F. Gillet-Chaulet, N. Merino, L. Tavard, J. Mouginot, N. Gourmelen and O. Gagliardini, 2015. Assimilation of Antarctic velocity observations provides evidence for uncharted pinning points, The Cryosphere, 9, 1427-1443, doi:10.5194/tc-9-1427-2015.

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Study of interaction between katabatic storms and blue ice areas

Written by Thomas Zwinger on .

Blue ice areas (BIAs) cover about 1% of the glaciated area of the Antarctic ice sheet. At higher altitude they exist because of reoccurring meteorological conditions that clear the ice surface from its snow or firn cover in a consistent and frequent manner.

Katabatic front above the blue ice area

BIAs, hence, can be very constant features and - by their flow properties - potentially also act as a source for easily accessible ice-core climate records in less interior regions of the ice-sheet. By using Elmer as a CFD code the authors investigate the interaction of a katabatic storm front with the ice surface at the high-elevation BIA at Scharffenberbotnen, Dronning Maud Land, Antarctica. The simulations were performed using the variatonal multi-scale (VMS) method in Elmer in a CFD-like simulation of the air-flow above the terrain. The main finding was that the present day geometry of the surrounding nunataks at the valley redirect the fast turbulent air flow exactly towards the region of the inner BIA. A simulation run on a modified terrain resembling the higher ice thickness (and smoother surface) at the Late Glacial Maximum revealed that this effect of redirection of the wind-impact occurred only after the lowering of the ice sheet.

 

Zwinger, T., T. Malm,  M. Schäfer, R. Stenberg, and J.C. Moore, 2015. Numerical simulations and observations of the role of katabatic winds in the creation and maintenance of Scharffenbergbotnen blue ice area, Antarctica, The Cryosphere, 9, 1415-1426, doi:10.5194/tc-9-1415-2015 [link to paper]

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Evolution of ice-shelf channels in Antarctic ice shelves

Written by Olivier Gagliardini on .

Pic4elmer

Ice shelves buttress the continental ice flux and mediate ice–ocean interactions. They are often traversed by channels in which basal melting is enhanced, impacting ice-shelf stability. Here, channel evolution is investigated using Elmer/Ice and modeling results are qualitatively compared with geophysical data collected on the Roi Baudouin Ice Shelf (RBIS), Antarctica. The modeling confirms basal melting as a feasible mechanism for channel creation, although channels may also advect without melting for many tens of kilometers. Channels can be out of hydrostatic equilibrium depending on their width and the upstream melt history. Inverting surface elevation for ice thickness using hydrostatic equilibrium in those areas is erroneous, and corresponding observational evidence is presented at RBIS by comparing the hydrostatically inverted ice thickness with radar measurements. The model shows that channelized melting imprints the flow field characteristically, which can result in enhanced horizontal shearing across channels. This is exemplified for a channel at RBIS using observed surface velocities and opens up the possibility to classify channelized melting from space, an important step towards incorporating these effects in ice–ocean models.

Drews R., 2015. Evolution of ice-shelf channels in Antarctic ice shelves, The Cryosphere, 9, 1169-1181, doi:10.5194/tc-9-1169-2015.

 

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