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Author Topic: Glaciology Basics and Risks - Uncertainties  (Read 67434 times)

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #150 on: September 08, 2018, 07:50:53 PM »
The linked article discusses the scientific value of a new high resolution elevation model of Antarctica.  Such a tool could be very valuable in verifying/calibrating cliff failure and hydrofracturing models of ice sheets:

Title: "New map of Antarctica shows the icy continent in 'stunning detail'"

https://www.usatoday.com/story/tech/science/2018/09/07/antarctica-new-map-shows-icy-continent-stunning-detail/1224078002/

Extract: "Scientists from Ohio State University and the University of Minnesota have created what they say is the best, most complete and accurate map ever made of the frozen continent at the bottom of the world …

“Now we’ll be able to see changes in melting and deposition of ice better than ever before,” Morin said. “That will help us understand the impact of climate change and sea level rise. We’ll be able to see it right before our eyes.”"
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #151 on: October 31, 2018, 04:51:30 PM »
The linked reference discusses basic mechanics of ice cliff failures, and the attached image illustrates the relationship of acceleration of calving rate vs freeboard and relative water depth of an ice cliff for a marine glacier and/or a marine-terminating glacier:

Tanja Schlemm and Anders Levermann (2018), "A simple stress-based cliff-calving law", The Cryosphere Discuss., https://doi.org/10.5194/tc-2018-205

https://www.the-cryosphere-discuss.net/tc-2018-205/tc-2018-205.pdf

Abstract. Over large coastal regions in Greenland and Antarctica the ice sheet calves directly into the ocean. In contrast to ice-shelf calving, an increase in cliff calving directly contributes to sea-level rise and a monotonously increasing calving rate with ice thickness can constitute a self-amplifying ice loss mechanism that may significantly alter sea-level projections both of Greenland and Antarctica. Here we seek to derive a minimalistic stress-based parameterization for cliff calving. To this end we compute the stress field for a glacier with a simplified two-dimensional geometry from the two-dimensional Stokes equation. First we assume a constant yield stress to derive the failure region at the glacier front from the stress field within the ice sheet. Secondly, we assume a constant response time of ice failure due to exceedance of the yield stress. With this strongly constraining but very simple set of assumption we propose a cliff-calving law where the calving rate follows a power-law dependence on the freeboard of the ice with exponents between 2 and 3 depending on the relative water depth at the calving front. The critical freeboard below which the ice front is stable decreases with increasing relative water depth of the calving front. For a dry water front it is, for example, 75m. The purpose of this study is not to provide a comprehensive calving law, but to derive a particularly simple equation with a transparent and minimalistic set of assumptions.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #152 on: November 08, 2018, 12:23:23 AM »
nother tool for estimating iceberg calving from the face of a marine (or marine terminating) glacier:

Trevers, M., Payne, A. J., Cornford, S. L., and Moon, T.: Buoyant forces promote tidewater glacier iceberg calving through large basal stress concentrations, The Cryosphere Discuss., https://doi.org/10.5194/tc-2018-212, in review, 2018.

https://www.the-cryosphere-discuss.net/tc-2018-212/
&
https://www.the-cryosphere-discuss.net/tc-2018-212/tc-2018-212.pdf

Abstract. Iceberg calving parameterisations currently implemented in ice sheet models do not reproduce the full observed range of calving behaviours. For example, though buoyant forces at the ice front are known to trigger full-depth calving events on major Greenland outlet glaciers, a multi-stage iceberg calving event at Jakobshavn Isbræ is unexplained by existing models. To explain this and similar events, we propose a notch-triggered rotation mechanism whereby a relatively small subaerial calving event triggers a larger full-depth calving event due to the abrupt increase in buoyant load and the associated stresses generated at the ice-bed interface. We investigate the notch-triggered rotation mechanism by applying a geometric perturbation to the subaerial section of the calving front in a diagnostic flowline model of an idealised glacier snout, using the full-Stokes, finite element method code Elmer/Ice. Different sliding laws and water pressure boundary conditions are applied at the ice-bed interface. Water pressure has a big influence on the likelihood of calving, and stress concentrations large enough to open crevasses were generated in basal ice. Significantly, the location of stress concentrations produced calving events of approximately the size observed, providing support for future application of the notch-triggered rotation mechanism in ice-sheet models.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson