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Author Topic: Glaciology Basics and Risks - Uncertainties  (Read 47089 times)

rituparna

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #100 on: March 27, 2015, 05:39:37 PM »
Thank you for the help.
.
.
Once tried to understand the Wikipedia things, but....couldn't get those. Anyways, will try again and let you know !!

rituparna

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #101 on: March 27, 2015, 05:42:10 PM »
Can you please clearly define : "What is Newtonian fluid?" "Their characteristics"
.
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This time please no links of definitions..... I want some simple sentence definition.

Thanks in advance.

rituparna

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #102 on: March 27, 2015, 05:43:36 PM »
also can you provide
                                 ----- "the List of Notations that are used for glacier modelling studies ?"

Thanks in advance

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #103 on: March 27, 2015, 05:55:41 PM »
Can you please clearly define : "What is Newtonian fluid?" "Their characteristics"
.
.
This time please no links of definitions..... I want some simple sentence definition.

Thanks in advance.

A Newtonian fluid is a fluid in which the viscous stresses arising from its flow, at every point, are linearly proportional to the local strain rate—the rate of change of its deformation over time.
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #104 on: March 27, 2015, 06:04:06 PM »
also can you provide
                                 ----- "the List of Notations that are used for glacier modelling studies ?"

Thanks in advance

Unfortunately, as I am a civil engineer and not a glaciologist I cannot provide a comprehensive list of notations.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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rituparna

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #105 on: March 27, 2015, 06:04:32 PM »
Why is it called Newtonian fluid ?

Laurent

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #106 on: March 27, 2015, 06:14:52 PM »
May be you could have looked for it by yourself ?
https://en.wikipedia.org/wiki/Newtonian_fluid
More precisely, a fluid is Newtonian only if the tensors that describe the viscous stress and the strain rate are related by a constant viscosity tensor that does not depend on the stress state and velocity of the flow.

Oh yes why Mister Newton ?
Newtonian fluids are named after Isaac Newton, who first derived the relation between the rate of shear strain rate and shear stress for such fluids in differential form.

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #107 on: March 27, 2015, 06:17:03 PM »
Why is it called Newtonian fluid ?

Newton's laws of mechanics are linear and Newtonian fluids respond linearly to local strain rates.  Non-Newtonian fluids response depends (non-linearly) on the shear rate and/or the shear history (like corn starch).
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rituparna

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #108 on: March 27, 2015, 06:45:19 PM »
Anyways, thanks for the help. Along with that, sorry for asking silly questions.

rituparna

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #109 on: March 27, 2015, 06:46:54 PM »
It will be helpful if anyone can provide me any list of glacial flow model notations.

Thanks

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #110 on: July 07, 2015, 02:17:23 AM »
The linked reference discusses the finding of a relatively new altimetric tool for measuring land-ice elevation changes in Antarctica:

Young, Duncan A.; Lindzey, Laura E.; Blankenship, Donald D.; Greenbaum, Jamin S.; Garcia De Gorordo, Alvaro; Kempf, Scott D.; Roberts, Jason L.; Warner, Roland C.; Van Ommen, Tas; Siegert, Martin J.; Le Meur, Emmanuel (February 2015), "Land-ice elevation changes from photon-counting swath altimetry: first applications over the Antarctic ice sheet", Journal of Glaciology, Volume 61, Number 225, pp. 17-28(12), DOI: http://dx.doi.org/10.3189/2015JoG14J048

http://www.ingentaconnect.com/content/igsoc/jog/2015/00000061/00000225/art00003

Abstract: "Satellite altimetric time series allow high-precision monitoring of ice-sheet mass balance. Understanding elevation changes in these regions is important because outlet glaciers along ice-sheet margins are critical in controlling flow of inland ice. Here we discuss a new airborne altimetry dataset collected as part of the ICECAP (International Collaborative Exploration of the Cryosphere by Airborne Profiling) project over East Antarctica. Using the ALAMO (Airborne Laser Altimeter with Mapping Optics) system of a scanning photon-counting lidar combined with a laser altimeter, we extend the 2003–09 surface elevation record of NASA's ICESat satellite, by determining cross-track slope and thus independently correcting for ICESat's cross-track pointing errors. In areas of high slope, cross-track errors result in measured elevation change that combines surface slope and the actual Δz/Δt signal. Slope corrections are particularly important in coastal ice streams, which often exhibit both rapidly changing elevations and high surface slopes. As a test case (assuming that surface slopes do not change significantly) we observe a lack of ice dynamic change at Cook Ice Shelf, while significant thinning occurred at Totten and Denman Glaciers during 2003–09."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #111 on: July 11, 2015, 12:49:43 PM »
The linked reference (& associated image) provides a more coherent interpretation of the past 25,000 years of Antarctic ice sheet history; which, will hopefully improve our understanding of ice-ocean-geothermal history:
http://planetearth.nerc.ac.uk/features/story.aspx?id=1774&cookieConsent=A
Extract: "The Scientific Committee for Antarctic Research (SCAR) commissioned an international team of 78 scientists from 14 countries to compile the most complete review of the history of the ice sheet to date. The team includes scientists from many backgrounds, including marine geologists, those studying changes recorded in lake muds, ice-sheet modellers, terrestrial glacial geologists and glaciologists.
The review looked at the ice sheet's extent and thickness at the Last Glacial Maximum – the peak of the last ice age, 25,000 years ago – and then every 5,000 years until the present. The challenge we faced was to take information from many sources and many different international teams, and turn it into a unified history of the Antarctic ice cap over the last few tens of thousands of years."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

wili

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #112 on: July 13, 2015, 04:54:26 PM »
Study finds surprisingly high geothermal heating beneath West Antarctic Ice Sheet

UC Santa Cruz team reports first direct measurement of heat flow from deep within the Earth to the bottom of the West Antarctic ice sheet

Lead author Andrew Fisher, professor of Earth and planetary sciences at UC Santa Cruz, emphasized that the geothermal heating reported in this study does not explain the alarming loss of ice from West Antarctica that has been documented by other researchers.

“The ice sheet developed and evolved with the geothermal heat flux coming up from below–it’s part of the system. But this could help explain why the ice sheet is so unstable. When you add the effects of global warming, things can start to change quickly,” he said.


http://news.ucsc.edu/2015/07/antarctic-heating.html

(Apologies if this was already posted. Thanks to COBob at robertscribbler's blog for this.)
"A force de chercher de bonnes raisons, on en trouve; on les dit; et après on y tient, non pas tant parce qu'elles sont bonnes que pour ne pas se démentir." Choderlos de Laclos "You struggle to come up with some valid reasons, then cling to them, not because they're good, but just to not back down."

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #113 on: September 27, 2015, 04:09:54 PM »
The linked article discusses another basic tool for assessment pre-cliff failure/hydrofracturing ice flow across the AIS.  The skillful use of such a tool could assist in the determination of model parameters that will help to determine when main phase cliff failure and hydrofracturing will begin for key Antarctic marine glaciers (like those in the ASE):

Felix S. L. Ng (2015), "Spatial complexity of ice flow across the Antarctic Ice Sheet", Nature Geoscience, doi:10.1038/ngeo2532


http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2532.html

Abstract: "Fast-flowing ice streams carry ice from the interior of the Antarctic Ice Sheet towards the coast. Understanding how ice-stream tributaries operate and how networks of them evolve is essential for developing reliable models of the ice sheet’s response to climate change.  A particular challenge is to unravel the spatial complexity of flow within and across tributary networks. Here I define a measure of planimetric flow convergence, which can be calculated from satellite measurements of the ice sheet’s surface velocity, to explore this complexity. The convergence map of Antarctica clarifies how tributaries draw ice from its interior. The map also reveals curvilinear zones of convergence along lateral shear margins of streaming, and abundant ripples associated with nonlinear ice rheology and changes in bed topography and friction. Convergence on ice-stream tributaries and their feeding zones is uneven and interspersed with divergence. For individual drainage basins, as well as the ice sheet as a whole, fast flow cannot converge or diverge as much as slow flow. I therefore deduce that flow in the ice-stream networks is subject to mechanical regulation that limits flow-orthonormal strain rates. These findings provide targets for ice-sheet simulations and motivate more research into the origin and dynamics of tributarization."
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #114 on: October 04, 2015, 02:55:52 AM »
The linked reference presents both analysis and field observations of a major calving event at Greenland’s Helheim Glacier.  The research identified the physical mechanisms operating during calving events, which result in "… glacial earthquakes, globally detectable seismic events whose proper interpretation will allow remote sensing of calving processes occurring at increasing numbers of outlet glaciers in Greenland and Antarctica."  This include remote monitoring of calving for key outlet glaciers like Jakobshavn, PIG and Thwaites.

T. Murray, M. Nettles, N. Selmes, L. M. Cathles, J. C. Burton, T. D. James, S. Edwards, I. Martin, T. O’Farrell, R. Aspey, I. Rutt and T. Baugé (17 July 2015, Published Online June 25 2015), " Reverse glacier motion during iceberg calving and the cause of glacial earthquakes", Science; Vol. 349, no. 6245, pp. 305-308, DOI: 10.1126/science.aab0460


http://www.sciencemag.org/content/349/6245/305.abstract?sid=76ef658e-614c-409d-848f-b8e53f184c97


Abstract: "Nearly half of Greenland’s mass loss occurs through iceberg calving, but the physical mechanisms operating during calving are poorly known and in situ observations are sparse. We show that calving at Greenland’s Helheim Glacier causes a minutes-long reversal of the glacier’s horizontal flow and a downward deflection of its terminus. The reverse motion results from the horizontal force caused by iceberg capsize and acceleration away from the glacier front. The downward motion results from a hydrodynamic pressure drop behind the capsizing berg, which also causes an upward force on the solid Earth. These forces are the source of glacial earthquakes, globally detectable seismic events whose proper interpretation will allow remote sensing of calving processes occurring at increasing numbers of outlet glaciers in Greenland and Antarctica."

See also:
http://www.latimes.com/science/sciencenow/la-sci-sn-glacier-earthquake-iceberg-greenland-ice-20150625-story.html
http://www.npr.org/sections/thetwo-way/2015/06/25/417457888/study-reveals-what-happens-during-a-glacial-earthquake
http://phys.org/news/2015-08-glacial-earthquakes-sea-level.html

Extract: "It is only recently that scientists learned of the existence of glacial earthquakes–measurable seismic rumblings produced as massive chunks fall off the fronts of advancing glaciers into the ocean. In Greenland, these quakes have grown sevenfold over the last two decades and they are advancing northward, suggesting that ice loss is increasing as climate warms. But exactly what drives the quakes has been poorly understood. Now, a new study elucidating the quakes' mechanics may give scientists a way to measure ice loss remotely, and thus refine predictions of future sea-level rise. The study appears this week in the early online edition of the leading journal Science.
It shows that as the glacier front falls off into the water, or calves, there is a kickback. The rest of the glacier moves rapidly downward and backward–something like a skateboard that slips out from under a rider's feet and goes backward as the rider falls forward. This is what produces the quake, say the researchers. The force of that kickback can be so great, it can reverse the glacier's flow for a few minutes, from the equivalent of about 95 feet per day forward to about 130 feet per day backward. Earlier studies have shown that glaciers often speed up after calving, but did not show the more immediate backward motion that apparently produces the quakes.

"This gives us a far better explanation for the source of the earthquakes than we had before," said Meredith Nettles, a seismologist at Columbia University's Lamont-Doherty Earth Observatory and a coauthor of the study. "It will move us a long way towards being able to use remotely detectable seismic signals to estimate mass loss from a major class of events in both Greenland and Antarctica.""
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #115 on: March 12, 2016, 03:44:05 AM »
The linked article discusses field work about the influence of tides on Antarctic ice shelves in order to develop better ice shelf/ice sheet models.  I note that the Nansen Ice Shelf is about to break free:

http://phys.org/news/2016-03-nasa-tracking-tides-ice-shelves.html

Extract: "he NASA scientists worked with personnel from the Korea Polar Research Institute to install instruments on the Nansen Ice Shelf, a roughly 30-mile-long ice shelf sticking out from the coast of Antarctica's Victoria Land. The ice shelf is near the new South Korean Jang Bogo Station, where Walker and Dow stayed during their field campaign.
"Nansen is a smaller ice shelf but we're hoping that it's representative of many of the smaller ice shelves that ring Antarctica," Walker said. "We also hope that the techniques that we're testing out in this campaign can be used in the future on larger ice shelves."
"Ice shelves are very important for holding back ice flow behind them because what they're essentially doing is acting as a plug; as soon as you remove them, there's nothing there preventing the ice mass from moving quickly down," Dow said. "It's a particular worry at the moment that the ice shelves around Antarctica are going to break up, and we're going to see an unprecedented speed-up in the ice coming from the center of the ice sheet."he NASA scientists worked with personnel from the Korea Polar Research Institute to install instruments on the Nansen Ice Shelf, a roughly 30-mile-long ice shelf sticking out from the coast of Antarctica's Victoria Land. The ice shelf is near the new South Korean Jang Bogo Station, where Walker and Dow stayed during their field campaign.
"Nansen is a smaller ice shelf but we're hoping that it's representative of many of the smaller ice shelves that ring Antarctica," Walker said. "We also hope that the techniques that we're testing out in this campaign can be used in the future on larger ice shelves."
"Ice shelves are very important for holding back ice flow behind them because what they're essentially doing is acting as a plug; as soon as you remove them, there's nothing there preventing the ice mass from moving quickly down," Dow said. "It's a particular worry at the moment that the ice shelves around Antarctica are going to break up, and we're going to see an unprecedented speed-up in the ice coming from the center of the ice sheet.""


“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 #116 on: April 09, 2016, 05:04:45 PM »
Here are some more basics w.r.t. glacial dynamics:

Krabbendam, M.: Basal sliding of temperate basal ice on a rough, hard bed: pressure melting, creep mechanisms and implications for ice streaming, The Cryosphere Discuss., doi:10.5194/tc-2016-52, in review, 2016.


http://www.the-cryosphere-discuss.net/tc-2016-52/

Abstract. Basal ice motion is crucial to ice dynamics of ice sheets. The Weertman sliding model for basal sliding over bedrock obstacles proposes that sliding velocity is controlled by pressure melting and/or ductile flow, whichever is the fastest; it further assumes that stoss-side melting is limited by heat flow through the obstacle and ductile flow is controlled by Power Law Creep. These last two assumptions, it is argued here, are invalid if a substantial basal layer of temperate (T ~ Tmelt) ice is present. In that case, frictional melting results in excess basal meltwater and efficient water flow, leading to near-thermal equilibrium. Stoss-side melting is controlled by melt water production, heat advection by flowing meltwater to the next obstacle, and heat conduction through ice/rock over half the obstacle height. No heat flow through the obstacle is required. High temperature ice creep experiments have shown a sharp weakening of a factor 5–10 close to Tmelt, implying breakdown of Power Law Creep and probably caused by a deformation-mechanism switch to grain boundary pressure melting. Ice streaming over a rough, hard bed, as likely in the Northeast Greenland Ice Stream, may be explained by enhanced basal motion in a thick temperate ice layer.

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solartim27

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #117 on: April 21, 2016, 05:00:19 AM »
Nansen broke apart on Apr 8th.  Very considerate to do so before we lost the view to winter.
http://earthobservatory.nasa.gov/IOTD/view.php?id=87859

http://earthobservatory.nasa.gov/IOTD/view.php?id=87657
« Last Edit: April 21, 2016, 05:09:49 AM by solartim27 »
FNORD

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #118 on: June 29, 2016, 05:56:41 PM »
The linked reference on low-porosity crusts at the WAIS Divide site in West Antarctica could help future interpretations of paleoclimate data from this area:

Fegyveresi, J. M., Alley, R. B., Muto, A., Orsi, A. J., and Spencer, M. K.: Surface formation, preservation, and history of low-porosity crusts at the WAIS Divide site, West Antarctica, The Cryosphere Discuss., doi:10.5194/tc-2016-155, in review, 2016.

http://www.the-cryosphere-discuss.net/tc-2016-155/

Abstract. Observations at the WAIS Divide site show that near-surface snow is strongly altered by weather-related processes such as strong winds and temperature fluctuations, producing features that are recognizable in the deep ice core. Prominent "glazed" surface crusts develop frequently at the site during summer seasons. Surface, snow pit, and ice core observations made in this study during summer field seasons from 2008–09 to 2012–13, supplemented by Automated Weather Station (AWS) data with insolation sensors, revealed that such crusts formed during relatively low-wind, low-humidity, clear-sky periods with intense daytime sunshine. After formation, such glazed surfaces typically developed cracks in a polygonal pattern with few-meter spacing, likely from thermal contraction at night. Cracking was commonest when several clear days occurred in succession, and was generally followed by surface hoar growth; vapor escaping through the cracks during sunny days may have contributed to the high humidity that favored nighttime formation of surface hoar. Temperature and radiation observations showed that daytime solar heating often warmed the near-surface snow above the air temperature, contributing to mass transfer favoring crust formation and then surface hoar formation. Subsequent investigation of the WDC06A deep ice core revealed that crusts are preserved through the bubbly ice, and some occur in snow accumulated during winters, although not as commonly as in summertime deposits. Although no one has been on site to observe crust formation during winter, it may be favored by greater wintertime wind-packing from stronger peak winds, high temperatures and steep temperature gradients from rapid midwinter warmings reaching as high as −15 °C, and perhaps longer intervals of surface stability. Time-variations in crust occurrence in the core may provide paleoclimatic information, although additional studies are required. Discontinuity and cracking of crusts likely explain why crusts do not produce significant anomalies in other paleoclimatic records.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #119 on: July 06, 2016, 06:30:45 PM »
The linked (open access) reference provides guidance for improved modelling of marine ice sheets:

Gladstone, R. M., Warner, R. C., Galton-Fenzi, B. K., Gagliardini, O., Zwinger, T., and Greve, R.: Marine ice sheet model performance depends on basal sliding physics and sub-shelf melting, The Cryosphere Discuss., doi:10.5194/tc-2016-149, in review, 2016

http://www.the-cryosphere-discuss.net/tc-2016-149/

Abstract. Computer models are necessary for understanding and predicting marine ice sheet behaviour. However, there is uncertainty over implementation of physical processes at the ice base, both for grounded and floating glacial ice. Here we implement several sliding relations in a marine ice sheet flowline model accounting for all stress components, and demonstrate that model resolution requirements are strongly dependent on both the choice of basal sliding relation and the spatial distribution of ice shelf basal melting.

Sliding relations that reduce the magnitude of the step change in basal drag from grounded ice to floating ice (where basal drag is set to zero) show reduced dependence on resolution compared to a commonly used relation, in which basal drag is purely a power law function of basal ice velocity. Sliding relations in which basal drag goes smoothly to zero as the grounding line is approached from inland (due to a physically motivated incorporation of effective pressure at the bed) provide further reduction to resolution dependence.

A similar issue is found with the imposition of basal melt under the floating part of the ice shelf: melt parameterisations that reduce the abruptness of change in basal melting from grounded ice (where basal melt is set to zero) to floating ice provide improved convergence with resolution compared to parameterisations in which high melt occurs adjacent to the grounding line. Thus physical processes, such as sub-glacial outflow (which could cause high melt near the grounding line), would impact on capability to simulate marine ice sheets.

For any given marine ice sheet the basal physics, both grounded and floating, governs the feasibility of simulating the system. The combination of a physical dependency of basal drag on effective pressure and low ice shelf basal melt rates near the grounding line mean that some marine ice sheet systems can be reliably simulated at a coarser resolution than currently thought necessary.
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #120 on: September 12, 2016, 07:23:13 PM »
The linked reference addresses the balance of driving stress vs basal drag on the Siple Coast Ice Streams; however, if future global warming leads to hydrofracturing of the RIS, these indicated balance of stresses will be lost:

Wohland, J., Albrecht, T., and Levermann, A.: Balance between driving stress and basal drag results in linearity between driving stress and basal yield stress in Antarctica's Siple Coast Ice Streams, The Cryosphere Discuss., doi:10.5194/tc-2016-191, in review, 2016.

http://www.the-cryosphere-discuss.net/tc-2016-191/

Abstract. Ice streams are distinct, fast-flowing regimes within ice sheets that exhibit fundamentally different characteristics as compared to the slow-moving inner parts of the ice sheets. While along-flow surface profiles of ice sheets are typically convex, some ice streams show linearly sloping or even concave surface profiles. We use observational data of the Siple Coast in Antarctica to inversely calculate membrane stresses, driving stresses and basal yield stresses based on the Shallow Shelf Approximation. Herein we assume that these marine-based ice streams are isothermal and in neglecting vertical shear we assume that their flow is dominated by sliding. We find that in the Siple Coast ice streams the membrane stresses are negligible and the driving stress balances the basal drag. It follows directly that in the Coulomb limit (i.e. basal drag independent of velocity) the driving stress is linear in the basal yield stress. In addition, we find that the ice topography and the basal conditions developed such that the driving stress is linear in the basal yield stress regardless of the choice of the pseudo plastic exponent in the basal drag parameterization.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #121 on: October 06, 2016, 12:09:36 PM »
The linked research is being used to improve the ice sheet model used by ACME (Accelerated Climate Modeling for Energy):


Zhu H, Petra N, Stadler G, Isaac T, Hughes TJR, Ghattas O. "Inversion of Geothermal Heat Flux in a Thermo-Mechanically Coupled Nonlinear Stokes Ice Sheet Model.". 2016.

http://www.the-cryosphere.net/10/1477/2016/


Abstract: “We address the inverse problem of inferring the basal geothermal heat flux from surface velocity observations using a steady-state thermomechanically coupled nonlinear Stokes ice flow model. This is a challenging inverse problem since the map from basal heat flux to surface velocity observables is indirect: the heat flux is a boundary condition for the thermal advection–diffusion equation, which couples to the nonlinear Stokes ice flow equations; together they determine the surface ice flow velocity. This multiphysics inverse problem is formulated as a nonlinear least-squares optimization problem with a cost functional that includes the data misfit between surface velocity observations and model predictions. A Tikhonov regularization term is added to render the problem well posed. We derive adjoint-based gradient and Hessian expressions for the resulting partial differential equation (PDE)-constrained optimization problem and propose an inexact Newton method for its solution. As a consequence of the Petrov–Galerkin discretization of the energy equation, we show that discretization and differentiation do not commute; that is, the order in which we discretize the cost functional and differentiate it affects the correctness of the gradient. Using two- and three-dimensional model problems, we study the prospects for and limitations of the inference of the geothermal heat flux field from surface velocity observations. The results show that the reconstruction improves as the noise level in the observations decreases and that short-wavelength variations in the geothermal heat flux are difficult to recover. We analyze the ill-posedness of the inverse problem as a function of the number of observations by examining the spectrum of the Hessian of the cost functional. Motivated by the popularity of operator-split or staggered solvers for forward multiphysics problems – i.e., those that drop two-way coupling terms to yield a one-way coupled forward Jacobian – we study the effect on the inversion of a one-way coupling of the adjoint energy and Stokes equations. We show that taking such a one-way coupled approach for the adjoint equations can lead to an incorrect gradient and premature termination of optimization iterations. This is due to loss of a descent direction stemming from inconsistency of the gradient with the contours of the cost functional. Nevertheless, one may still obtain a reasonable approximate inverse solution particularly if important features of the reconstructed solution emerge early in optimization iterations, before the premature termination.”
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

A-Team

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #122 on: October 08, 2016, 03:12:45 PM »
linked research is being used to improve the ice sheet model
Not according to the abstract. The authors explored an ambitious approach but found the mathematical obstacles insuperable. The details of why things didn't work out will prove very interesting to others in the field but ultimately nothing has been learned about geothermal heat flux under Greenland that will improve understanding of the past, present or future of this ice sheet.

Meanwhile, there has been quite a bit of experimental progress reported lately on geothermal heat flux and the equally important melt state of the bottom ice. Ultimately the lack of data on matters such as temperature, hydration state and topography of basal till are essential boundary conditions. PDEs propagate from there, they can't go anywhere without them.

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #123 on: October 08, 2016, 06:03:39 PM »
linked research is being used to improve the ice sheet model

Not according to the abstract. The authors explored an ambitious approach but found the mathematical obstacles insuperable. The details of why things didn't work out will prove very interesting to others in the field but ultimately nothing has been learned about geothermal heat flux under Greenland that will improve understanding of the past, present or future of this ice sheet.

Meanwhile, there has been quite a bit of experimental progress reported lately on geothermal heat flux and the equally important melt state of the bottom ice. Ultimately the lack of data on matters such as temperature, hydration state and topography of basal till are essential boundary conditions. PDEs propagate from there, they can't go anywhere without them.


Not that I have any desire to over-sell the value of "Inversion of Geothermal Heat Flux in a Thermo-Mechanically Coupled Nonlinear Stokes Ice Sheet Model"; however, the abstract does state: "Nevertheless, one may still obtain a reasonable approximate inverse solution particularly if important features of the reconstructed solution emerge early in optimization iterations, before the premature termination.”  Therefore, the research did provide some guidance in certain "optimized" circumstances.  Thus even if these finds are of little practical value for either the GIS or the EAIS; they may well be of some value for the WAIS (which is currently more critical, w.r.t. collapse risk); as the following references indicate that the geothermal heat flux beneath portions of the WAIS (particularly near the West Antarctic Rift System (WARS) and the Marie Byrd Land Dome (MBLD).  Also, for what is worth, ACME continues to provide funding for this research; thus what is impracticable today, may (with more findings) be practicable in the not too distant future:

C. Ramirez, A. Nyblade, S.E. Hansen, D.A. Wiens, S. Anandakrishnan, R.C. Aster, A.D. Huerta, P. Shore and T. Wilson (March, 2016) "Crustal and upper-mantle structure beneath ice-covered regions in Antarctica from S-wave receiver functions and implications for heat flow", Geophys. J. Int., 204 (3): 1636-1648. doi: 10.1093/gji/ggv542

https://gji.oxfordjournals.org/content/204/3/1636.refs?related-urls=yes&legid=gji;204/3/1636

Abstract: "S-wave receiver functions (SRFs) are used to investigate crustal and upper-mantle structure beneath several ice-covered areas of Antarctica. Moho S-to-P (Sp) arrivals are observed at ∼6–8 s in SRF stacks for stations in the Gamburtsev Mountains (GAM) and Vostok Highlands (VHIG), ∼5–6 s for stations in the Transantarctic Mountains (TAM) and the Wilkes Basin (WILK), and ∼3–4 s for stations in the West Antarctic Rift System (WARS) and the Marie Byrd Land Dome (MBLD). A grid search is used to model the Moho Sp conversion time with Rayleigh wave phase velocities from 18 to 30 s period to estimate crustal thickness and mean crustal shear wave velocity. The Moho depths obtained are between 43 and 58 km for GAM, 36 and 47 km for VHIG, 39 and 46 km for WILK, 39 and 45 km for TAM, 19 and 29 km for WARS and 20 and 35 km for MBLD. SRF stacks for GAM, VHIG, WILK and TAM show little evidence of Sp arrivals coming from upper-mantle depths. SRF stacks for WARS and MBLD show Sp energy arriving from upper-mantle depths but arrival amplitudes do not rise above bootstrapped uncertainty bounds. The age and thickness of the crust is used as a heat flow proxy through comparison with other similar terrains where heat flow has been measured. Crustal structure in GAM, VHIG and WILK is similar to Precambrian terrains in other continents where heat flow ranges from ∼41 to 58 mW m−2, suggesting that heat flow across those areas of East Antarctica is not elevated. For the WARS, we use the Cretaceous Newfoundland–Iberia rifted margins and the Mesozoic-Tertiary North Sea rift as tectonic analogues. The low-to-moderate heat flow reported for the Newfoundland–Iberia margins (40–65 mW m−2) and North Sea rift (60–85 mW m−2) suggest that heat flow across the WARS also may not be elevated. However, the possibility of high heat flow associated with localized Cenozoic extension or Cenozoic-recent magmatic activity in some parts of the WARS cannot be ruled out."

See also:
Andrew T. Fisher et. al. (July 2015), "High geothermal heat flux measured below the West Antarctic Ice Sheet", Science Advances 1(6):e1500093-e1500093, DOI: 10.1126/sciadv.1500093

http://advances.sciencemag.org/content/1/6/e1500093
http://advances.sciencemag.org/content/advances/1/6/e1500093.full.pdf

Abstract: "The geothermal heat flux is a critical thermal boundary condition that influences the melting, flow, and mass balance of ice sheets, but measurements of this parameter are difficult to make in ice-covered regions. We report the first direct measurement of geothermal heat flux into the base of the West Antarctic Ice Sheet (WAIS), below Subglacial Lake Whillans, determined from the thermal gradient and the thermal conductivity of sediment under the lake. The heat flux at this site is 285 ± 80 mW/m(2), significantly higher than the continental and regional averages estimated for this site using regional geophysical and glaciological models. Independent temperature measurements in the ice indicate an upward heat flux through the WAIS of 105 ± 13 mW/m(2). The difference between these heat flux values could contribute to basal melting and/or be advected from Subglacial Lake Whillans by flowing water. The high geothermal heat flux may help to explain why ice streams and subglacial lakes are so abundant and dynamic in this region."


See also:

Theresa M. Damiani, Tom A. Jordan, Fausto Ferraccioli, Duncan A. Young, and Donald D. Blankenship, (2014), "Variable crustal thickness beneath Thwaites Glacier revealed from airborne gravimetry, possible implications for geothermal heat flux in West Antarctica", Earth and Planetary Science Letters Volume 407, 1, Pages 109–122, DOI: 10.1016/j.epsl.2014.09.023

http://www.sciencedirect.com/science/article/pii/S0012821X14005780

“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 #124 on: January 30, 2017, 06:58:25 PM »
The linked reference presents a sensitivity study of SLR to the loss of Antarctic ice shelves.

Pattyn, F.: Sea-level response to melting of Antarctic ice shelves on multi-centennial time scales with the fast Elementary Thermomechanical Ice Sheet model (f.ETISh v1.0), The Cryosphere Discuss., doi:10.5194/tc-2017-8, in review, 2017.

http://www.the-cryosphere-discuss.net/tc-2017-8/
&
http://www.the-cryosphere-discuss.net/tc-2017-8/tc-2017-8.pdf

Abstract: "The magnitude of the Antarctic ice sheet's contribution to global sea-level rise is dominated by the potential of its marine sectors to become unstable and collapse as a response to ocean (and atmospheric) forcing. This paper presents Antarctic sea-level response to sudden atmospheric and oceanic forcings on multi-centennial time scales with the newly developed fast Elementary Thermomechanical Ice Sheet (f.ETISh) model. The f.ETISh model is a vertically integrated hybrid ice sheet/ice shelf model with an approximate implementation of ice sheet thermomechanics, making the model two-dimensional. Its marine boundary is represented by two different flux conditions, coherent with power-law basal sliding and Coulomb basal friction. The model has been compared to a series of existing benchmarks.

Modelled Antarctic ice sheet response to forcing is dominated by sub-ice shelf melt and the sensitivity is highly dependent on basal conditions at the grounding line. Coulomb friction in the grounding-line transition zone leads to significantly higher mass loss in both West and East Antarctica on centennial time scales, leading to 2 m sea level rise after 500 years for a moderate melt scenario of 20 m a−1 under freely-floating ice shelves, up to 6 m for a 50 m a−1 scenario. The higher sensitivity is attributed to higher driving stresses upstream from the grounding line.

Removing the ice shelves altogether results in a disintegration of the West Antarctic ice sheet and (partially) marine basins in East Antarctica. After 500 years, this leads to a 4.5 m and a 12.2 m sea level rise for the power-law basal sliding and Coulomb friction conditions at the grounding line, respectively. The latter value agrees with simulations by DeConto and Pollard (2016) over a similar period (but with different forcing and including processes of hydro-fracturing and cliff failure)."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson