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AbruptSLR

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Glaciology Basics and Risks - Uncertainties
« on: May 16, 2013, 08:44:40 PM »
I am opening this new thread both to provide some discussion of glaciology basic concepts/terminology for basic readers, and also to create a forum to illustrate situations where traditional thinking on glaciology may result in imposing unnecessary risks/uncertainties on the global society with regard to abrupt sea level rise, ASLR.

First I provide the following definitions:

Mass balance is the change in the mass of a glacier or ice body, or part thereof, over a stated span of time.  The term mass budget is a synonym. The span of time is often a year or a season. A seasonal mass balance is nearly always either a winter balance or a summer balance. The (cumulative) mass balance, b, is the sum of accumulation, c, and ablation, a (the ablation
is defined here as negative). The symbol, b (for point balances) and B (for glacierwide balances)
has traditionally been used in studies of surface mass balance of valley glaciers.  Mass balance is often treated as a rate, b or B dot.

Accumulation
1. All processes that add to the mass of the glacier.
2. The mass gained by the operation of any of the processes of sense, expressed as a positive
number.
Components:
- Snow fall (usually the most important).
- Deposition of hoar (a layer of ice crystals, usually cup-shaped and facetted, formed by
vapor transfer (sublimation followed by deposition) within dry snow beneath the snow
surface), freezing rain, solid precipitation in forms other than snow (re-sublimation
composes 5-10% of the accumulation on Ross Ice Shelf, Antarctica).
- Gain of windborne blowing snow and drifting snow
- Avalanching
- Basal freeze-on (usually beneath floating ice)
- Internal accumulation.
Note: Unless it freezes, rainfall does not constitute accumulation, and nor does the addition of
debris by avalanching, ashfall or similar processes.

Ablation
1. All processes that reduce the mass of the glacier.
2. The mass lost by the operation of any of the processes of sense, expressed as a negative
number.
Components:
- Melting (usually the most important on land-based glaciers. Melt water that re-freezes
onto another part of the glacier is not referred to as ablation).
- Calving (or, when the glacier nourishes an ice shelf, ice discharge across the grounding
line): Calving is iceberg discharge into seas or lakes; important, for example, in
Greenland and Antarctica, where approximately 50% and 90%, respectively, of all
ablation occurs via calving.
- Loss of windborne blowing snow and drifting snow
- Avalanching
- Sublimation (important, for example, in dry climates, and on blue-ice zones in Antarctica; is a function of vapor pressure)
Note the difference between a) precipitation (includes solid precipitation and rain) and surface accumulation (does not include rain).  Note, that in contrast to what is natural in dynamic glaciology and glacial geomorphology, for mass-balance purposes the glacier consists only of frozen water. Sediment carried by the glacier is deemed to be outside the glacier. Meltwater in transit or in storage, for example in supraglacial lakes or subglacial cavities, is also regarded as being outside the glacier.
b) Meltwater and Meltwater runoff (A portion of melt may refreeze; the latter refers to the
meltwater that does not refreeze)
c) Meltwater runoff and Runoff (the latter includes rain or any other source of water other than
meltwater.
d) Accumulation and Net accumulation (the latter is a balance, i.e. accumulation plus ablation.
It is identical to the mass balance in case the balance is positive. It equals zero in case the
balance is negative).

Next, I briefly discuss the attached image showing some basic glaciological concepts for a marine terminating glacier contribution ice mass to SLR.  This figure shows how mass balance "dot b" integrated over area "A" gives the input of ice mass "Q" into the upstream end of the glacial flow (also "Q" but for U integrated over the glacier's cross section), thus causing a gravity force that is partially resisted by basal (and side) friction (and an allowance for energy dissipation from internal work of deforming and internal melting of the glacial ice as it flows down hill).  The value of Qcalving is intended in this image to represent the discharge (ice volume per unit time) of ice mass contributing to SLR associated with glacial flow velocity "U".  However, for simplicity this figure does not illustrate ice mass contribution to SLR from: (a) ice surface melting and run-off; (b) basal meltwater discharge, and (c) grounding line retreat due to advective melting of the grounded ice.
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #1 on: May 16, 2013, 10:03:56 PM »
I provide the following definitions of traditional glaciology terms related to glacial movement, not because I use them very much, but because they reflect the fact that most glaciologists were trained to think about mountain glaciers; and also because these term illustrate the complex/dynamic nature of glacial ice movement including: (a) both brittle and viscous behavior; (b) surface, internal, and basal ice melting; (c) ice - geo. interaction; (d) ice - atmos. interaction; and (e) ice - ocean/lake interaction:

Traditional Terms Related to Glacial Movement:
Ablation
wastage of the glacier through sublimation, ice melting and iceberg calving.
Ablation zone
Area of a glacier in which the annual loss of ice through ablation exceeds the annual gain from precipitation.
Arête
an acute ridge of rock where two cirques abut.
Bergshrund
crevasse formed near the head of a glacier, where the mass of ice has rotated, sheared and torn itself apart in the manner of a geological fault.
Cirque, corrie or cwm
bowl shaped depression excavated by the source of a glacier.
Creep
adjustment to stress at a molecular level.
Flow
movement (of ice) in a constant direction.
Fracture
brittle failure (breaking of ice) under the stress raised when movement is too rapid to be accommodated by creep. It happens for example, as the central part of a glacier moves faster than the edges.
Horn
spire of rock, also known as a pyramidal peak, formed by the headward erosion of three or more cirques around a single mountain. It is an extreme case of an arête.
Plucking/Quarrying
where the adhesion of the ice to the rock is stronger than the cohesion of the rock, part of the rock leaves with the flowing ice.
Tarn
a post-glacial lake in a cirque.
Tunnel valley
The tunnel that is formed by hydraulic erosion of ice and rock below an ice sheet margin. The tunnel valley is what remains of it in the underlying rock when the ice sheet has melted.
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #2 on: May 17, 2013, 03:47:38 PM »
My prior two posts in this thread focus on the "Traditional" glaciological thinking that chararterized IPCC AR4; which focused on Mountain glacier contributions to SLR and assumed that any ice mass loss from both the GIS and the AIS would be offset by an increase of snowfall in Antarctica. 

This post focuses on the "Neo-Traditional" glaciological thinking that will be characterized by IPCC AR5 (based on the SOD); which acknowledges a limited amount of SLR from both the GIS and the AIS based on the "Neo-Traditional" glacier models presented here, with the first attached image showing a comparison of an Antarctic type marine glacier (such as the PIG, which rests on the seafloor) vs a Greenland type marine terminating glacier (such as the Petermann Glacier, which terminates on the seafloor but rests primarily on the land).  This first image (a re-post) also shows the importance of interaction with warm ocean water, subglacial hydrology, and the ice shelf (or ice melange).

The second image (a re-post) illustrates the importance of the advective "saline pump" action on Antarctic marine glaciers, where the warm CDW approaches the grounding line, GL, through a trough and melts some of the ice there both causing the GL to retreat, and producing relative light (low salinity) meltwater that floats up along the underside of the ice shelf (causing more ice melt) further driving the advective inflow of more warm CDW.  This process thins both the ice shelf and the downstream edge of the marine glacier causing the ice flow velocity to accelerate, resulting in more iceberg calving from the ice shelf (leading to reduced buttressing from the ice shelf).

The third image (also a re-post) from Willis and Church, 2012, summarizes many additional  features of the "neo-traditional" conceptual models for the Antarctic type marine glacier (think PIG) including: (a) the fingerprint effect of the local sea elevation dropping due to a reducing of gravitational attraction associated with ice mass loss; (b) a change in the sea ice formation and associate saltwater rejection due to the increasing icemelt water at the surface, which reduces the formation of Antarctic Bottom Water, AABW, which is warming bottom water temperatures around the world; (c) Katabatic winds that can blow fresh fallen snow into the ocean; and (d) changes in circumpolar winds and associated currents that is affecting upwelling of warm circumpolar deepwater, CDW, which is currently accelerating the advective process shown in the first, second and third attached images.  While looking at this third image, I would like to note that in cases such as the PIG with a relatively narrow glacial valley the advective process can melt glacial ice faster than the glacier can thin sufficiently to float, in which case a subglacial cavity can extend beneath the glacial ice (which rests on the seafloor by arching across the subglacial cavity from one side of the glacial valley to the other).  I would also like to know while the Thwaites Glacier gateway trough currently also allows for such arches across a subglacial cavity; in the future when the east-side (PIG side) shear/buttress action is degraded, it will no longer be possible for the glacial ice to arch across a subglacial cavity, which in equilibrium cases will require the ice shelf to be sufficiently thin to float above the submerged mount on the east-side of the Thwaites Glacier gateway (see the PIG/Thwaites 2012 - 2060 thread).

The fourth attached image presents a conceptual model of how the inherent gravitational instability of Antarctic type marine glacier can cause the ice discharge, q, to increase when the GL retreats down the negative slope of the seafloor. In the two panels of this image, a. In a steady state, the groundling-line discharge, q (red curve), which is dependent on the thickness of the grounding line, must match the balance flux, ax (blue line), which in this 2D example is the product of an accumulation rate, a, and the upstream catchment length, x.  Steady state is achieved where q=ax, which occurs at three points in this example (indicated by green, yellow and red vertical lines), which also correspond to the ice-sheet steady-state profiles shown in b.
« Last Edit: May 17, 2013, 03:54:05 PM by AbruptSLR »
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #3 on: May 18, 2013, 01:50:11 AM »
As the neo-traditional (AR5) methodology is based on the same principles as the traditional (AR4) methodology, therefore this, and my next series of, posts will address some basic glacier movement/motion/flux principles.

The first image provides a visual presentation for a land-based glacier of many of the terms that I presented in my second post in this thread.

The second image shows how ice flow can scour and pluck out material from the glacier bed.

The third image shows the ice particle flow trajectories for a valley glacier, together with conceptual particle velocities and zones of extending and compressive flow relative to the equilibrium line (SMB).

The fourth image shows the balance rate and total balance rate along the length of a valley glacier.
« Last Edit: May 18, 2013, 02:08:32 AM by AbruptSLR »
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #4 on: May 18, 2013, 02:16:24 AM »
The first image presents a summary of some of the key glacial motion response characteristics and parameters (including shear, viscosity, temperature, slope, geometry, etc) governing typical land-based ice flow.

The second image illustrates the mechanical relationships of the response shown in the first image.

The third image illustrates the relationship of shear stress and strain with ice flow velocity distribution with the depth of the land-based glacier.

The fourth image illustrates how shear stress typically varies with glacier depth and slope.
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #5 on: May 18, 2013, 02:27:15 AM »
Regarding constituative properties:

The first image shows ice velocity as a function of viscosity.

The second image shows ice viscosity as a function of shear stress.

The third images shows the temperature dependence of ice flow rate.

The fourth image shows typical creep behavior of ice.
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #6 on: May 18, 2013, 02:36:36 AM »
The first image gives a freebody of flux(discharge) relationships for a valley glacier.

The second image give the relationship of glacial weight on magma in the mantle which creates a need for the glacial isostatic adjustment as glaciers accrete and recede.

The third image gives an idea of this influence of glacial ice mass loss on nearby active faults.
« Last Edit: May 18, 2013, 02:41:45 AM by AbruptSLR »
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #7 on: May 18, 2013, 06:51:12 PM »
Discussion (and downloads) related to the neo-traditional (AR5) ice sheet model:  Ice Sheet System Model (ISSM) and the associated Design Analysis Kit for Optimization and Terascale Applications (DAKOTA), can be found at:
http://issm.jpl.nasa.gov/

ISSM is the result of a collaboration between the Jet Propulsion Laboratory and University of California at Irvine. Its purpose is to tackle the challenge of modeling the evolution of the polar ice caps in Greenland and Antarctica.  These tools are used to focus current IceBridge investigation, and their mathematical sophistication is too detailed to present here but they include: Viscous flow, elastic response, brittle behavior, basal friction, momentum, boundary conditions, thermo-mechanical properties of ice, grounding line migration, sensitivity analyses, error investigations, etc.

While I agree that such neo-traditional models have done a good job of modeling the extant behavior of Antarctic ice sheet behavior; efforts such as those presented in the "RCM" thread by Gladestone et al 2013 indicate that still more sophisticated models capturing local/regional circulation models (which are still in developmental stages) are needed to reasonably project the probabilities of future ice mass loss scenarios.

Therefore on the topic of "risks and uncertainties", examples of factors that such ice sheet models must do a better job of before we can rely upon them to accurately estimate the risks of say the Thwaites Glacier collapsing this century include:
- A better understanding of ocean-atmosphere-ice-land interaction (as is currently being developed by the Bisicles and CISM software, see the RCM thread for discussion).  This is particularly important to capture: (a)  the accelerated warming of the CDW during the El Nino hiatus period; (b) indications of future increased upwelling around the Antarctic coasts with increased global warming (including the influence of the rapid accumulation of CH4 over the Antarctic continent, see the "Antarctic Methane" thread); (c) the strong influence of glacial meltwater on the formation of Antarctic sea ice and ocean water circulation patterns; and (d) the influence of the projected early break-up of Antarctic sea ice beginning circa 2050-2060.
- Consideration that for the past 13 years the El Nino hiatus period has limited the onshore winds being driven into the ASE from the Amundsen Sea Low (see the discussing in the "Weather and Meteorology" thread) that occurs during strong El Nino periods, which should drive considerable more warm CDW water into the troughs leading to both the PIG and Thwaites Glacier (as did occur during last period of strong El Nino events in the 1990's).
- Better modeling of the influence of subglacial hydrological systems (particularly the influence of activation of the newly identified subglacial lake within the extensive Thwaites Glacier, TG, subglacial hydrological system).
- The influence of future surface meltwater (note that the onshore wind from an Amundsen Sea Low system is warm can could promote extensive surface melting in the TG basin within the next ten years) in the ASE area carrying heat from the surface directly down to the basal zone through ice fissures, thus reducing basal friction and increasing the influence of the subglacial hydrological system on ice flow.
- The influence of the recently identified high geothermal heat input into the base of Byrd Subglacial Basin.
- Possible increase of snowfall at the upstream end of the TG prior to 2050 that could maintain steep ice surface gradients that could help to trigger a "Jacobshaven Effect" type of rapid ice mass loss in the TG due to local gravitational instabilities of the calving surface, if future buttressing action of the TG ice shelf is reduced (see the PIG/Thwaites 2012 to 2060" thread discussion).
- Possible future isostatic rebound triggered earthquakes (see the images in my last post), as most ice is lost from the ASE glaciers.
- A probably loss of boundary restraint on the east (PIG side) of the Thwaites Gateway (possibly due to advective processes through the Thwaites trough, see the PIG/Thwaites 2012 to 2060 thread), resulting in a rapid acceleration of the ice flux from the TG.
- Reductions in ice viscosity, and an increase in internal ice melting, as the ice streams accelerate due to various mechanisms (including enthapy modeling).
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #8 on: June 22, 2013, 10:13:52 PM »
The accompanying three images come from:

Mass Balance of West Antarctic Ice Sheet from ICESat Measurements
By H. Jay Zwally, Jun Li, John Robbins, Jack L. Saba, Donghui Yi
WAIS Workshop, Colorado, September 22, 2011

The first image shows that ICESat measures changes in surface elevation, and the figure illustrates the considerations required to determine the quantity of interest: ice Mass Balance.

The second image shows ICESat measurement for the West Antarctic for the period from 2003 to 2008.

The third image shows the specific considerations that must be considered when interpreting the ICESat measurement, including: firn compaction, bed elevation changes, ice elevation changes, ablation, and accumulation.
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icebgone

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #9 on: June 23, 2013, 12:21:29 AM »
Excellent post ASLR.  PIG and TG have potential to destabilize quickly if conditions warrant.  In-depth measurements and analysis point to a need for installation of long-term self-sustaining field equipment in the critical zones.  I understand better now why you fear their early collapse before the end of this century.  The more so if we begin experiencing strong El Nino events before the end of this decade.  I fear any strong El Nino because it can create havoc above and beyond what has already been happening.  My glaciology knowledge is a bit old but still useful.  I wonder if it would be possible to reduce development time and money by borrowing existing measurements and technologies from pavement strain experiments used by Engineers to build highways and bridges in various environments? 
There is nothing like watching and hearing glaciers flow.  I recommend everyone to do this at least once in their life.  Each glacier also has a unique geochemistry taking place between the glacier, basal objects carried by the glacier and the underlying rock on which it flows.  It is difficult for me to believe that within a few lifetimes glaciers, the insatiable ice dragons of the earth, could be destroyed by the fires of man is sad indeed.     

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #10 on: June 23, 2013, 04:11:57 AM »
icebgone,

Thank you for your thoughful words.  I trust that the Antarctic field researchers make the best use possible of their limited research funds, and that they continue make excellent progress.  My biggest concern is not with the researchers themselves but with the administrators who are reticient to acknowlege the true risks of rapid sea level rise contributions from Antarctica; and that by the time sufficient data is accumulated to overcome this reticience that thermal inertia will result in unacceptable levels of SLR.  I sincerely hope that I am wrong; but I hope to shine what limited daylight that I can on the relevant new findings (most of which seem to be supporting my concerns) as I find them, in the hope that some decision makers will that this matter more seriously in the future.

Best,
ASLR
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #11 on: June 24, 2013, 02:09:43 PM »
My biggest concern remains the stability of Ross? Comparable periods of earth history with similar CO2/temps show us that we can expect to lose ross at some point and see East and West Antarctic become separated by a channel from Ross to Weddell?

I remember some research (BAS?) in the early noughties that show a 'ruck' in the ice around Ross as if the momentum of a fast flowing ice mass was interupted by the formation of the 'buttress' that ross now is. I have to worry that this structure shows us what will happen to the feed glaciers for the embayment once that 'buttress' has been removed?

I am awaiting the next major calve from ross and have been eyeing the largr fissure from Roosevelt Island to the central section of the shelf for over 4 years now. when i first became interested in this feature I contact Bob Grumbine to see what he knew and he informed me that they had just installed sensors along the length of the feature to monitor changes. Currently ,were this to fail, it would be a berg 4 times larger than the last major loss from the shelf.

With the Arrival of warm bottom waters into the area ( worked around from PIG) I also worry that the 'grounding line' will now rapidly retreat putting more of a strain onto this feature and lead to it's early demise?
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #12 on: June 24, 2013, 05:10:16 PM »
Gray-Wolf,

I believe that your concerns are all too real as I discuss in the "FRIS - RIS 2012 to 2060" thread.  Certainly, I believe that calving from the Ross Ice Shelf, RIS, will probably accelerate as the face of the ice shelf thins over the coming decades; and I believe that around 2050 to 2060 warm circumpolar deep water, CDW, will enter the Ross embayment, which would change RSI from a cold ice shelf to a warm ice shelf; which I believe will lead to the collapse of the RIS before the end of the century; which in turn will accelerate ice mass loss (VAF) to contribute to SLR.

That said, I still believe that the situations around the Amundsen Sea Embayment, ASE, and the Filchner Ronne Ice Shelf, FRIS, are even more critical than that for the RIS; but in any case it is likely that all of these areas may contribute to the collapse of the WAIS in rapid sequence (in one order or another) to each other.

Best,
ASLR
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #13 on: July 11, 2013, 01:47:06 AM »
For anyone who wants to know some of the latest thinking about Antarctic firn densification, the following reference provided input into the Ice2sea program:

An improved semi-empirical model for the densification of Antarctic firn
by: S. R. M. Ligtenberg, M. M. Helsen, and M. R. van den Broeke; The Cryosphere, 5, 809–819, 2011; www.the-cryosphere.net/5/809/2011/; doi:10.5194/tc-5-809-2011

"Abstract. A firn densification model is presented that simulates steady-state Antarctic firn density profiles, as well as the temporal evolution of firn density and surface height. The model uses an improved firn densification expression that is tuned to fit depth-density observations. Liquid water processes (meltwater percolation, retention and refreezing) are also included. Two applications are presented. First, the steady-state model version is used to simulate the strong spatial variability in firn layer thickness across the Antarctic ice sheet. Second, the time-dependent model is run for 3 Antarctic locations with different climate conditions. Surface height changes are caused by a combination of accumulation, melting and firn densification processes. On all 3 locations, an upward trend of the surface during autumn, winter and spring is present, while during summer there is a more rapid lowering of the surface. Accumulation and (if present) melt introduce large inter-annual variability in surface height trends, possibly hiding ice dynamical thickening and thinning."
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sidd

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Re: better GIA models for Antarctica
« Reply #14 on: July 18, 2013, 08:03:21 PM »
http://www.the-cryosphere-discuss.net/7/3497/2013/tcd-7-3497-2013.html

" A range of different GRACE gravity models were evaluated, as well as a new ICESat surface height trend map computed using an overlapping footprint approach. When the GIA models created from the combination approach were compared to in-situ GPS ground station displacements, the vertical rates estimated showed consistently better agreement than existing GIA models. In addition, the new empirically derived GIA rates suggest the presence of strong uplift in the Amundsen Sea and Philippi/Denman sectors, as well as subsidence in large parts of East Antarctica."

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #15 on: July 18, 2013, 11:33:27 PM »
Sidd,

This is a great find and the conclusions for the ASE by Ligtenberg et al 2013 seem to match those present by Groh et al, using preliminary GPS data in:

An investigation of Glacial Isostatic Adjustment over the Amundsen Sea sector, West Antarctica
by: A. Groh; H. Ewert, M. Scheinert, M. Fritsche, A. Rülke, A. Richter, R. Rosenau, R. Dietrich
http://dx.doi.org/10.1016/j.gloplacha.2012.08.001

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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #16 on: July 20, 2013, 03:10:08 PM »
The following information about calving is taken from:

http://en.wikipedia.org/wiki/Ice_calving

"Ice calving, also known as glacier calving or iceberg calving, is the breaking off of chunks of ice at the edge of a glacier. It is a form of ice ablation or ice disruption. It is the sudden release and breaking away of a mass of ice from a glacier, iceberg, ice front, ice shelf, or crevasse. The ice that breaks away can be classified as an iceberg, but may also be a growler, bergy bit, or a crevasse wall breakaway.
Calving of glaciers is often accompanied by a loud cracking or booming sound before blocks of ice up to 60 metres (200 ft) high break loose and crash into the water. The entry of the ice into the water causes large, and often hazardous waves. The waves formed in locations like Johns Hopkins Glacier can be so large that boats cannot approach closer than 3 kilometres (1.9 mi). These events have become major tourist attractions in locations such as Alaska.
Many glaciers terminate at oceans or freshwater lakes which results naturally with the calving of large numbers of icebergs. Calving of Greenland's glaciers produce 12,000 to 15,000 icebergs each year alone.
Calving of ice shelves is usually preceded by a rift. These events are not often observed.
Etymologically, calving is cognatic with calving as in birthing a calf.

Causes:

It is useful to classify causes of calving into first, second, and third order processes. First order processes are responsible for the overall rate of calving at the glacier scale. The first order cause of calving is longitudinal stretching, which controls the formation of crevasses. When crevasses penetrate the full thickness of the ice, calving will occur. Longitudinal stretching is controlled by friction at the base and edges of the glacier, glacier geometry and water pressure at the bed. These factors, therefore, exert the primary control on calving rate.
Second and third order calving processes can be considered to be superimposed on the first order process above, and control the occurrence of individual calving events, rather than the overall rate. Melting at the waterline is an important second order calving process as it undercuts the subaerial ice, leading to collapse. Other second order processes include tidal and seismic events, buoyant forces and melt water wedging.
When calving occurs due to waterline melting, only the subaerial part of the glacier will calve, leaving a submerged 'foot'. Thus, a third order process is defined, whereby upward buoyant forces cause this ice foot to break off and emerge at the surface. This process is extremely dangerous, as it has been known to occur, without warning, up to 300m from the glacier terminus.

Calving Law:

Though many factors that contribute to calving have been identified, a reliable predictive mathematical formula is still under development. Data is currently being assembled from ice shelves in Antarctica and Greenland to help establish a 'calving law'. Variables used in models include properties of the ice such as thickness, density, temperature, c-axis fabric, impurity loading, though 'ice front normal spreading stress', is likely the most important variable, however it is usually not measured.
There are currently several concepts upon which to base a predictive law. One theory states that the calving rate is primarily a function of the ratio of tensile stress to vertical compressive stress, i.e., the calving rate is a function of the ratio of the largest to smallest principle stress.  Another theory, based on preliminary research, shows that the calving rate increases as a power of the spreading rate near the calving front.

Major Calving Events:

Filchner-Ronne Ice Shelf
In October, 1988, the A-38 iceberg broke away from the Filchner-Ronne Ice Shelf. It was about 150 km x 50 km, a mass of ice bigger than the area of Delaware. A second calving occurred in May 2000 and created an iceberg 167 km x 32 km.

Amery Ice Shelf
A major calving event occurred in 1962 to 1963. Currently, there is a section at the front of the shelf referred to as the 'loose tooth'. This section, about 30 km by 30 km is moving at about 12 meters per day and is expected to eventually calve away."

This post may help readers to better understand the recent iceberg calving event for the Pine Island Ice Shelf, PIIS; and to better understand the risks of potential future calving events, particularly for PIIS and the Filchner Ice Shelf. Understanding ice calving is fundamental to better understanding the risks of abrupt SLR.
“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 #17 on: August 20, 2013, 08:13:18 PM »
For those interested in the historical glacial data that the National Snow and Ice Data Center compiles on Antarctica please check-out their new beta interface to this data at the following weblink:

http://nsidc.org/agdc/acap/
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #18 on: August 21, 2013, 06:47:53 PM »
The following reference concludes that "… basal sliding is widespread beneath the Antarctic Ice Sheet"; which implies that only a very small fraction of the AIS needs to warm-up in order to significantly accelerate ice mass loss from the AIS (particularly the WAIS); which could occur either due to increased basal friction as the ice velocity increases and/or due to an increase in basal meltwater volume:

http://onlinelibrary.wiley.com/doi/10.1002/jgrf.20125/abstract

Inversion of basal friction in Antarctica using exact and incomplete adjoints of a higher-order model; M. Morlighem, H. Seroussi, E. Larour, & E. Rignot; 2013; Journal of Geophysical Research: Earth Surface; DOI: 10.1002/jgrf.20125

Abstract:
"Basal friction beneath ice sheets remains poorly characterized and yet is a fundamental control on ice mechanics. Here, we use a complete map of surface velocity of the Antarctic Ice Sheet to infer the basal friction over the entire continent by combining these observations with a three-dimensional, thermo-mechanical, higher-order ice sheet numerical model from the Ice Sheet System Model (ISSM) open source software. We demonstrate that inverse methods can be readily applied at the continental scale with appropriate selections of the cost function and of the scheme of regularization, at a spatial resolution as high as 3 km along the coastline. We compare the convergence of two descent algorithms with the exact and incomplete adjoints to show that the incomplete adjoint is an excellent approximation. The results reveal that the driving stress is almost entirely balanced by the basal shear stress over 80% of the ice sheet. The basal friction coefficient, which relates basal friction to basal velocity, is however significantly heterogeneous: it is low on fast moving ice and high near topographic divides. Areas with low values extend far out into the interior, along glacier and ice stream tributaries, almost to the flanks of topographic divides, suggesting that basal sliding is widespread beneath the Antarctic Ice Sheet."
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #19 on: August 22, 2013, 01:18:36 AM »
The linked reference on the high sensitivity of tidewater outlet glacier dynamics to shape, while theoretical, is of high importance for glaciers such as the Thwaites Glacier, that meets all of the papers criteria for high instability.  The link provides a free pdf:


http://www.the-cryosphere.net/7/1007/2013/tc-7-1007-2013.html


Enderlin, E. M., Howat, I. M., and Vieli, A.: High sensitivity of tidewater outlet glacier dynamics to shape, The Cryosphere, 7, 1007-1015, doi:10.5194/tc-7-1007-2013, 2013

"Abstract. Variability in tidewater outlet glacier behavior under similar external forcing has been attributed to differences in outlet shape (i.e., bed elevation and width), but this dependence has not been investigated in detail. Here we use a numerical ice flow model to show that the dynamics of tidewater outlet glaciers under external forcing are highly sensitive to width and bed topography. Our sensitivity tests indicate that for glaciers with similar discharge, the trunks of wider glaciers and those grounded over deeper basal depressions tend to be closer to flotation, so that less dynamically induced thinning results in rapid, unstable retreat following a perturbation. The lag time between the onset of the perturbation and unstable retreat varies with outlet shape, which may help explain intra-regional variability in tidewater outlet glacier behavior. Further, because the perturbation response is dependent on the thickness relative to flotation, varying the bed topography within the range of observational uncertainty can result in either stable or unstable retreat due to the same perturbation. Thus, extreme care must be taken when interpreting the future behavior of actual glacier systems using numerical ice flow models that are not accompanied by comprehensive sensitivity analyses."
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #20 on: August 23, 2013, 08:15:09 PM »
The following linked reference provides insight about glacial earthquakes of the Whillans Ice Stream:

http://onlinelibrary.wiley.com/doi/10.1002/grl.50130/abstract


Winberry J. P., S. Anandakrishnan, D. A. Wiens, and R. B. Alley (2013), Nucleation and seismic tremor associated with the glacial earthquakes of Whillans Ice Stream, Antarctica, Geophys. Res. Lett., 40, 312–315, doi:10.1002/grl.50130.


Abstract:

"The ability to monitor transient motion along faults is critical to improving our ability to understand many natural phenomena such as landslides and earthquakes. Here, we usedata from a GPS and seismometer network that were deployed to monitor the regularly repeating glacial earthquakes of Whillans Ice Stream, West Antarctica to show that a unique pattern of precursory slip precedes complete rupture along the bed of the ice stream. Additionally, we show that rupture can be independently tracked by increased levels of microseismic activity, including harmonic tremor, that are coincident with the onset of slip at any location, thus providing a remote means of monitoring stress and rupture propagation during the glacial earthquakes."
« Last Edit: August 24, 2013, 05:11:55 PM by AbruptSLR »
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sidd

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #21 on: August 23, 2013, 11:00:26 PM »
I attach fig 5 from Morlighem (2013). The blue areas indicate basal sliding, extending fingers deep into the ice. To me this implies that ice melt at the edge will be replaced with ice from the thinning interior. And we have seen surface melt quite deep into Antarctica also.

sidd

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #22 on: August 23, 2013, 11:05:23 PM »
Enderlin (2013) notes :

"... glaciers with the widest outlets reach flotation above the basal depression, triggering
a much larger retreat and discharge increase."

"Further, we suggest that similar sensitivity analyses should be completed using two- or three-dimensional models in order to assess the influence of glacier shape on grounding line stability for glaciers and ice streams with strong lateral convergence along their trunks."

"... glaciers with wider steady-state grounding lines and those with deeper basal depressions will tend to be closer to flotation in the depression than narrower or shallow glaciers, and thus less dynamic thinning will be required to bring the ice within the depression to flotation."

Each of these sentences reminded me strongly of Thwaites.

sidd


AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #23 on: August 24, 2013, 05:34:18 PM »
Sidd,


The Morlighem et al (2013) figure that you posted, and the Enderlin et al (2013) quotes that you cite, bring into sharp focus the seriousness of the risk for ASLR that the world faces circa 2050 by which time the ocean water advection and the acceleration of basal sliding has had a chance to destablize large portions of the WAIS.  I find it particularly disturbing that some of the dark blue basal sliding fingers are starting to converge from different sides of the WAIS (eg the PIG fingers with those from the Weddell Sea basin, and the Ferrigno finger with the PIG fingers).

Best,
ASLR
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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sidd

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #24 on: August 24, 2013, 07:32:28 PM »
Agreed on convergence of the sliding streams. For some detail compare surface velocities in the attached figure 1 from  Rignot(2008, doi:10.1038/ngeo102).

In addition to the ones pointed out already, I note the Siple coast area streams reaching towards  Thwaites under Mercer, Whillans, Kamb, Bindschadler, MacAyeal and Echelmeyer. The fingers behind Byrd and David facing the Siple coast shock me as they extend behind the Transantarctic ridge. Cook, Ninnis, Moscow U and Totten are disturbing. I don't even want to look at Amery.


AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #25 on: August 25, 2013, 02:57:03 AM »
Sidd,

When I think about the 118.1k years ago spike in sea level that O'Leary et al 2013 provide evidence for (see the discussion in the "Timing" thread and the "Paleo-evidence" thread); I can only conclude that if O'Leary et al 2013 are correct then the portions of the EAIS that you highlight in you post must be primed for collapse shortly after the collapse of the WAIS otherwise it would be impossible to reach the 9m eustatic SLR value cited by O'Leary et al 2013 in the timeframe that they discuss.  I am not certain whether an equable climate would be needed to quickly trigger the marine-terminating portions of the EAIS that you cite; but it is disturbing to think about this possibility (such as a 1m surge of sea level by 2055 due to a partial collapse of the WAIS forcing warm Pacific ocean water into the Arctic Ocean, thus rapidly extending the ice free season for the Arctic Ocean, leading to a rapid build-up of specific humidity in the Arctic atmosphere, leading to a rapid transition to an equable climate, leading to a rapid collapse of the marine-terminating glaciers/ice streams shortly after 2100.

I do not mean to sound alarmist, but I do not have any confidence that because the current models do not show a meaninful risk of such a scenario, that society is actual safe from such an occurrence.

Best,
ASLR
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #26 on: August 25, 2013, 04:23:08 PM »
Agreed on convergence of the sliding streams. For some detail compare surface velocities in the attached figure 1 from  Rignot(2008, doi:10.1038/ngeo102).

Rignot's group has produced a new map with much improved coverage, see here:

http://nsidc.org/data/nsidc-0484.html
http://www.sciencemag.org/content/333/6048/1427

sidd

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #27 on: August 25, 2013, 09:30:02 PM »
As nukefix points out, Rignot(2011) has a better picture, which I have attached

AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #28 on: September 01, 2013, 12:15:55 AM »
The following linked reference provides insight into the glaciofluvial processes in the Garwood Valley, Antarctica:

http://gsabulletin.gsapubs.org/content/125/9-10/1484.abstract


Garwood Valley, Antarctica: A new record of Last Glacial Maximum to Holocene glaciofluvial processes in the McMurdo Dry Valleys;
by: Joseph S. Levy, Andrew G. Fountain, Jim E. O’Connor, Kathy A. Welch and W. Berry Lyons; June 7, 2013, doi: 10.1130/B30783.1 v. 125 no. 9-10 p. 1484-1502


"Abstract
We document the age and extent of late Quaternary glaciofluvial processes in Garwood Valley, McMurdo Dry Valleys, Antarctica, using mapping, stratigraphy, geochronology, and geochemical analysis of sedimentary and ice deposits. Geomorphic and stratigraphic evidence indicates damming of the valley at its Ross Sea outlet by the expanded Ross Sea ice sheet during the Last Glacial Maximum. Damming resulted in development of a proglacial lake in Garwood Valley that persisted from late Pleistocene to mid-Holocene time, and in the formation of a multilevel delta complex that overlies intact, supraglacial till and buried glacier ice detached from the Ross Sea ice sheet. Radiocarbon dating of delta deposits and inferred relationships between paleolake level and Ross Sea ice sheet grounding line positions indicate that the Ross Sea ice sheet advanced north of Garwood Valley at ca. 21.5 ka and retreated south of the valley between 7.3 and 5.5 ka. Buried ice remaining in Garwood Valley has a similar geochemical fingerprint to grounded Ross Sea ice sheet material elsewhere in the southern Dry Valleys. The sedimentary sequence in Garwood Valley preserves evidence of glaciofluvial interactions and climate-driven hydrological activity from the end of the Pleistocene through the mid-Holocene, making it an unusually complete record of climate activity and paleoenvironmental conditions from the terrestrial Antarctic."
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #29 on: September 03, 2013, 05:16:58 PM »
The following linked reference (with a free pdf) discusses the effects of entrained debris on the basal sliding stability of glaciers:

http://www3.geosc.psu.edu/~cjm38/papers_talks/ZoetJGR2013.pdf


Zoet, L. K., B. Carpenter, M. Scuderi, R. B. Alley, S. Anandakrishnan, C. Marone, and M. Jackson (2013), The effects of entrained debris on the basal sliding stability of a glacier, J. Geophys. Earth Surf., 118, doi:10.1002/jgrf.20052.

Abstract:
"New laboratory experiments exploring likely subglacial conditions reveal controls on the transition between stable sliding and stick-slip motion of debris-laden ice over rock, with implications for glacier behavior. Friction between a rock substrate and clasts in ice generates heat, which melts nearby ice to produce lubricating water. An increase in sliding speed or an increase in entrained debris raises heat generation and thus meltwater production. Unstable sliding is favored by low initial lubrication followed by rapid meltwater production in response to a velocity increase. Low initial lubrication can result from cold or drained conditions, whereas rapid increase in meltwater generation results from strong frictional heating caused by high sliding velocity or high debris loads.  Strengthening of the interface (healing) during “stick” intervals between slip events occurs primarily through meltwater refreezing. When healing and unstable sliding are taken together, the experiments reported here suggest that stick-slip behavior is common from motion of debris-laden glacier ice over bedrock."
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #30 on: September 03, 2013, 05:24:44 PM »
The linked reference discusses accelerated subglacial erosion in response to stick-slip motion:


http://geology.gsapubs.org/content/41/2/159.short



Accelerated subglacial erosion in response to stick-slip motion; by: L.K. Zoet, R.B. Alley, S. Anandakrishnan and K. Christianson; 2012; Geology; v. 41 no. 2 p. 159-162; doi: 10.1130/G33624.1


"Abstract
Subglacial stick-slip motion speeds erosion by hydrofracturing and in other ways, as determined from analysis of the growing body of field data. Microearthquake monitoring commonly detects subglacial earthquakes, likely mostly from stick-slip motion of debris-laden ice over bedrock. Source parameters show that many quakes cause enough motion to greatly lower water pressure in cavities on the lee sides of bedrock steps. We calculate that the resulting expansion of higher-pressure water in nearby cracks promotes hydrofracturing, with even relatively small cracks growing unstably under thick glaciers and all cracks growing faster than for aseismic behavior. This mechanism also helps generate the step-like topography favoring block plucking. This stick-slip glacier-erosion hypothesis suggests that the erosion rate will increase with ice thickness as well as basal shear stress, ice-flow velocity, and water supply."
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #31 on: September 05, 2013, 12:52:01 AM »
The following link presents research that West Antarctica began to glaciate about 32 million years ago, when more of this area was at a higher elevation than at the present time:

http://phys.org/news/2013-09-west-antarctica-ice-sheet-million.html
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #32 on: September 07, 2013, 09:27:59 PM »
The following abstracts are taken from the proceedings of the following IGSOC sponsored symposia, and they all relate to fundamental glaciology and the mechanics of associated ice shelves:


International Symposium on Changes in Glaciers and Ice Sheets: observations, modelling and environmental interactions; 28 July–2 August; Beijing, China; Contact: Secretary General, International Glaciological Society


http://www.igsoc.org/symposia/2013/beijing/proceedings/procsfiles/procabstracts_62.htm


A particle-based simulation model for glacier dynamics
J.C. MOORE, J.A. ÅSTRÖM, T. RIIKILÄ, T. TALLINEN, T. ZWINGER, D. BENN, J. TIMONEN
Corresponding author: J.C. MOORE
Corresponding author e-mail:
"A particle-based computer simulation model has been developed with the objective of investigating the dynamics of glaciers. To describe ice dynamics in a realistic fashion, a model must include elastic deformation, granular flow and fracture of large ice bodies. In spite of several simplifications, which include restriction to two dimenions only and simplified rheology for ice and water, the present model is able to reproduce iceberg and small debris size distributions observed in calving. The observed size distributions are well approximated by universal scaling laws. On a moderate slope a large ice-block (here 50 m high and 200 m long) is stable as long there is enough friction with the substrate. This is a quiescent state. At a critical extent of frictional contact there is an onset of global sliding and the model glacier begin to surge. During the surge, the glacier is fragmented into small pieces. In this case the fragment size distribution has the shape that is typically observed for grinding processes."


A new calving law based on continuous damage and fracture mechanics
J. KRUG, J. WEISS, G. DURAND, O. GAGLIARDINI
Corresponding author: J. Krug
Corresponding author e-mail: jean.krug@ujf-grenoble.fr
"A number of studies have shown that mechanical ice loss through calving is responsible for most of the ice discharge from glaciers and ice sheets. However calving processes are complex and still poorly understood. Representation of calving in ice-sheet models is still limited and the estimation of future ice loss from the Greenland and Antarctic ice sheets is therefore inaccurate. This is the reason why the last IPPC report asked for a better understanding and representation of calving processes. Several approaches are necessary to represent at the same time the slow deformation of the ice, the initiation of crevasses and their rapid propagation preceding the calving event. First, the effect of damage and small-scale fracturing on this slow viscous deformation can be represented by continuous damage mechanics (CDM). CDM describes the evolution of damage in the ice from a state in which the ice has no defect to the appearance of a macro-crack. This evolution depends on the stress field and is advected with the flow of ice. Second, the fast propagation of pre-existing crevasses into the media can be satisfyingly described using linear elastic fracture mechanics (LEFM) for which ice is considered as an elastic medium. This approach allows us to deal with the stress concentration at the tip of the crack and so differs from the traditionally used Nye’s criterion. Together, they may propose a complete and versatile calving law that covers first-order processes (related to longitudinal stretching and surface velocity gradients) as well as second-order processes occurring at the glacier front (subaqueous melting, force imbalance, etc.). These two approaches are combined and implemented into the Elmer/Ice full-Stokes ice-flow model. An initial grounded terminated glacier is perturbed by an increase in frontal subaqueous melting. This process, called undercutting, results in a block of ice overhanging the sea, followed by the calving of the aerial part. Strength of the model to various physical parameters is tested, such as the ratio between water depth and ice thickness, the inlet velocity and the temperature of the ice. The shape of the bedrock is also investigated."



Dynamics of meltwater plumes under ice shelves: frictional geostrophy and melt-channelling instability
Felix NG, Adrian JENKINS
Corresponding author: Felix Ng
Corresponding author e-mail: f.ng@sheffield.ac.uk
"The flow of buoyant meltwater plumes under several ice shelves has been reproduced in numerical simulations that represent such plumes as a well-mixed layer, and researchers have begun using these simulations to explore how ocean warming could cause irreversible melting and retreat of ice shelves. Here we present a new mathematical theory for the coupled ice-shelf–meltwater-plume system to illuminate two aspects of this problem. The first aspect concerns the two-dimensional flow field of the plume and how it determines the spatial distribution of the rate of sub-ice-shelf basal melt. By starting with the equations used in the simulations, we derive a simplified model of plume physics that explains how sub-shelf friction and Coriolis force conspire to govern the plume water flux and deflection angle. This leads us to explain why the plumes always transition from friction-dominated flow near the grounding line towards geostrophic flow farther out under the shelf. This theory identifies plume buoyancy, the Coriolis parameter and shelf-base slope as key factors of the transition and elucidates how they impact the sub-shelf melt rate. The second aspect of the problem concerns the origin of basal melt channels that have been observed under many ice shelves, including the floating tongue of Petermann Glacier in Greenland and the ice shelf fed by Pine Island Glacier in Antarctica. By conducting a linear stability analysis of our model, we find that with typical parameters the coupled shelf–plume system is unstable to perturbations so that incipient channels with spacing of the order of kilometres form across the ice-shelf base. This analysis establishes how model parameters control the incipient-channel spacing and orientation and pinpoints the mechanisms behind the instability. These results inform future studies that seek to understand how fully developed basal channels affect how an ice shelf evolves."
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #33 on: September 07, 2013, 10:57:04 PM »
The following abstracts are taken from the proceedings of the following IGSOC sponsored symposia, and they all relate remote sensing of information needed for fundamental glaciology in Antarctica:

International Symposium on Changes in Glaciers and Ice Sheets: observations, modelling and environmental interactions; 28 July–2 August; Beijing, China; Contact: Secretary General, International Glaciological Society


http://www.igsoc.org/symposia/2013/beijing/proceedings/procsfiles/procabstracts_62.htm

Mass balance of Antarctic ice sheet 1992–2008 from ERS and ICESat: gains exceed losses
H. Jay ZWALLY, Jun LI, John ROBBINS, Jack L. SABA, Donghui YI, Anita BRENNER
Corresponding author: H. Jay ZWALLY
Corresponding author e-mail: zwally@icesat2.gsfc.nasa.gov
"During 2003–2008, the mass gain of the Antarctic ice sheet from snow accumulation exceeded the loss from ice discharge by 73±23 Gt a–1 (3.7% of input), as derived from ICESat laser altimetry. The 131 Gt a–1 gain in East Antarctica (EA) and the 70 Gt a–1 gain in four drainage systems (DS) of West Antarctic (WA2) exceeded combined losses of 98 Gt a–1 from three coastal DS of West Antarctic (WA1) and 28 Gt a–1 from the Antarctic Peninsula (AP). Re-analysis of ERS radar-altimeter data, including a new post-glacial-rebound correction, indicates an even larger overall gain of 120 ± 51 Gt a–1 during 1992–2001. In WA2 and EA, persistent dynamic thickening (deficiency of ice flow relative to long-term accumulation) contributed more than 200 Gt a–1 to the net positive balance in both periods. Consistent with observed outlet-glacier accelerations, loss increases of 38 Gt a–1 in WA1 and 21 Gt a–1 in AP from increased dynamic thinning dominated a gain increase of 9 Gt a–1 from positive accumulation anomalies in WA1 and AP. These decadal-scale changes are small relative to the long-term dynamic thickening in EA and WA2, which may buffer additional dynamic thinning for several decades."

Modeling dynamic thickening in East Antarctica as observed from ICESat
Weili WANG, H. Jay ZWALLY, Jun LI
Corresponding author: Weili WANG
Corresponding author e-mail: weili.wang@nasa.gov
"Mass changes of the Antarctic ice sheet derived from ICESat laser altimetry show that during 2003–08 mass gains from snow accumulation exceeded losses from ice discharge by 73 Gt a–1 (0.20 mm a–1 sea level depletion). Results from ERS radar altimetry give a similar net gain of 120 Gt a–1 for 1992–2001. In East Antarctica and four West Antarctic drainage systems, most of the net mass gain is caused by persistent dynamic thickening (excess of long-term accumulation relative to ice flow) at a rate of 207 Gt a–1, and not by contemporaneous increases in snowfall. To investigate the dynamic thickening rate, we apply a 3-D ice-sheet model to the Antarctic ice sheet for the sensitivity experiments with climate change. The model results indicate that the East Antarctic ice sheet has been growing due to increased snowfall after the last ice age. The modeled thickening rate near Vostok is 2.5 cm a–1 for the present time, which is consistent with the observations from ICESat and ERS data. Overall, the model and observations indicate a long-term mass gain for East Antarctica and the interior of West Antarctic, which has been offsetting dynamic losses that have increased in the Antarctic Peninsula and West Antarctica during the last two decades."



Improved Antarctic surface mass-balance remote sensing using ASCAT
Alexander D. FRASER, Simon WOTHERSPOON, Hiroyuki ENOMOTO, Neal W. YOUNG
Corresponding author: Alexander D. Fraser
Corresponding author e-mail: adfraser@utas.edu.au
"Large-scale distribution of Antarctic surface mass balance (SMB) is currently poorly understood. High-quality in situ measurements of SMB are sparse, particularly in the interior of the continent. Remote sensing can be used to guide interpolation between in situ measurements. Previously, passive microwave polarization ratio, which is sensitive to the density of horizons of different dielectric properties in the upper snowpack (a proxy for SMB), has been used to guide interpolation of SMB points in Antarctica. We present evidence that maps of alternative parameters may be more suitable maps upon which to base interpolated fields. These maps come from the EUMETSAT Advanced Scatterometer (ASCAT) C-band scatterometer, which was launched in 2007. In particular, we use the ‘A’ (isotropic component of backscatter, sensitive to grain size within the C-band penetration depth of ~20 m) and ‘B’ (linear component of backscatter dependence on incidence angle, sensitive to grain-size profile). Importantly, these maps are sensitive to recently mapped extensive areas of surface wind glaze, which are areas of near-zero net accumulation and thus are less prone to overestimation of SMB compared with earlier large-scale SMB maps. A further focus of this work is a comparison of several statistical interpolation methods, including a careful consideration of the statistical treatment of negative SMB values. A primary output of this work is a new SMB map of the Antarctic continent based on these improved fields."

Synoptic-timescale observations of Antarctic snowfall/wind redistribution events from scatterometer data
Alexander D. FRASER, Melissa A. NIGRO, John CASSANO, Neal W. YOUNG, Benoit LEGRESY, Hiroyuki ENOMOTO
Corresponding author: Alexander D. Fraser
Corresponding author e-mail: adfraser@utas.edu.au
"The orbit and swath configuration of the EUMETSAT Advanced Scatterometer (ASCAT) instrument gives C-band backscatter measurements from a wide range of azimuth and incidence angles over most of the Antarctic continent. A 5 day orbital subcycle combined with this excellent observation angle diversity means that complete maps of accumulation-sensitive parameters can be produced on a 5 day basis. Analysis of time series of these parameters reveals several abrupt changes in localized regions, particularly in the ‘A’ parameter (isotropic component of backscatter, which is sensitive to snow grain size) and the ‘M2’ parameter (the magnitude of the second-order Fourier term describing the near-bi-sinusoidal azimuthal response, which is an indicator of the presence/magnitude of sastrugi/other surface microrelief). Using 15 km grid spacing Antarctic Mesoscale Prediction System (AMPS) numerical weather prediction model data, we show these abrupt changes in the ‘A’ and ‘M2’ parameters are associated with snowfall events arising from incursions of air from lower latitudes. Both the ‘A’ and ‘M2’ parameters show a complex response to precipitation events, with both the sign and magnitude of the response depending on wind reworking/redistribution. This observation of changes in near-surface snowpack conditions complements recent results from other authors using GRACE-derived gravity and CloudSat-derived snowfall observations to detect similar snowfall events in East Antarctica."



How accurately can radar altimetry contribute to estimate the Antarctic ice sheet volume and mass balance?
B. LEGRÉSY, M. HORWATH, S.R.M. LIGTENBERG, M.R. VAN DEN BROEKE, F. BLAREL
Corresponding author: B. Legresy
Corresponding author e-mail: benoit.legresy@legos.obs-mip.fr
"Knowing the interannual variations of the Antarctic ice sheet net snow accumulation, or surface mass balance (SMB), is essential for analyzing and interpreting present-day observations. For example, accumulation events like the one in East Antarctica in 2009 challenge our ability to interpret observed decadal-scale trends in terms of long-term changes versus natural fluctuations. We developed a higher accuracy time series of radar altimetry with ERS2 and Envisat data from 1995 to 2010. We will present the surface topography variations, the internal error levels for both altimeters and the radar echo and ground miss-repeat corrections made. We show that a different echo correction has to be applied to ERS2 and Envisat as the firn changed in between the two periods of observation. Therefore the possibility to correct the radar altimetry data for echo shape changes from the echo shape is limited, limiting the attainable accuracy of volume change estimates. We illustrate the great potential and limitations of radar altimetry by internal assessment and by comparing with other changes estimates as temporal gravity variations and atmospheric modeling of firn densification. We evaluate the limits of techniques depending on the temporal and spatial scales of interest. SMB variations cause changes in the firn density structure, which need to be accounted for when converting volume trends from satellite altimetry into mass trends. Recent assessments of SMB and firn volume variations mainly rely on atmospheric modeling and firn densification modeling. The modeling results need observational validation, which has been limited until now. Geodetic observations by satellite altimetry and satellite gravimetry reflect interannual firn volume and mass changes, among other signals like changes in ice-flow dynamics. Therefore, these observations provide a means of validating modeling results over the observational period. We present comprehensive comparisons between seasonal and interannual volume variations from radar altimetry and firn densification modeling, and between interannual mass variations from SMB."


Estimated ICESat inter-campaign bias and its impact on the determination of ice-sheet mass balance
Donghui YI, H. Jay ZWALLY, John W. ROBBINS, Jun LI, Jack L. SABA, Jinlun ZHANG
Corresponding author: Donghui Yi
Corresponding author e-mail: donghui.yi@nasa.gov
"ICESat operated for 18 campaign periods from March 2003 to October 2009. Most of the operational periods were between 34 and 38 days long. Because of laser failure and orbit transition from 8 day to 91 day orbit, there were four periods lasting 57, 16, 23 and 12 days. Owing to laser characteristic changes (three different lasers, laser energy decreasing with time, the changes in laser pulse shape and beam pattern, etc.), there are range biases (D) between ICESat campaign periods. The long-term trend of the inter-campaign biases (dD/dt) directly affects the derived ice-sheet mass-balance results. In this study, we used the ICESat measured mean sea level over the sea-ice-covered Arctic Ocean to estimate ICESat inter-campaign biases and evaluate the impact of the inter-campaign biases on ice-sheet mass balance. The mean sea level was calculated by averaging the elevation of the leads (open water and thin ice) within the Arctic Ocean sea-ice pack, with waveform saturation correction, inverse barometer correction, dry and wet troposphere corrections, and tidal corrections applied. The ocean dynamic topography effect was also evaluated. We adjusted the derived D by a trend of 0.31 ± 0.07 cm a–1 to account for the current rate of sea-level rise. The resulting mean inter-campaign bias trend (dDsl/dt) from September 2003 to November 2008 (the four full year period of ICESat’s 91 day orbit operation) is –1.60 ± 0.77 cm a–1. Converting this to a volume change rate (dV/dt), we get about 28 km3 a–1 for Greenland and about 198 km3 a–1 for Antarctica. Comparing ICESat elevation profiles over Lake Vostok, Antarctica, with ERS elevation profile over the same region, the bias-corrected ICESat profiles show more consistency than the profiles with no bias correction. The ICESat data used in this study are release version 633 with transmitted pulse Gaussian/Centroid peak location correction (G-C) applied."
« Last Edit: September 07, 2013, 11:07:47 PM by AbruptSLR »
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #34 on: September 08, 2013, 03:16:41 AM »
A free access pdf of the referenced paper is available at the following link.  This paper presents theoretical work that is applicable to the Thwaites Glacier, TG, and indicates that mechanisms such as: ocean-ice interaction, crevasses, basal meltwater,  ice shelf collapse, and/or surface melting is required to destabilize the TG, and that simple geometry is most likely not sufficient:


http://www.the-cryosphere.net/7/647/2013/tc-7-647-2013.pdf


Ice-shelf buttressing and the stability of marine ice sheets
by: G. H. Gudmundsson; The Cryosphere, 7, 647–655, 2013; www.the-cryosphere.net/7/647/2013/; doi:10.5194/tc-7-647-2013


Abstract. Ice-shelf buttressing and the stability of marine type ice sheets are investigated numerically. Buttressing effects are analysed for a situation where a stable grounding line is located on a bed sloping upwards in the direction of flow. Such grounding-line positions are known to be unconditionally unstable in the absence of transverse flow variations.  It is shown that ice-shelf buttressing can restore stability under these conditions. Ice flux at the grounding line is, in general, not a monotonically increasing function of ice thickness.  This, possibly at first somewhat counterintuitive result, is found to be fully consistent with recent theoretical work.  Grounding lines on retrograde slopes are conditionally stable, and the stability regime is a non-trivial function of bed and ice-shelf geometry. The stability of grounding lines cannot be assessed from considerations of local bed slope only."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #35 on: September 08, 2013, 03:30:10 AM »
A free access pdf of the referenced paper is available at the following link.  This paper presents both theoretical and field evidence that the surface response of fast-flowing ice is highly sensitive to bedrock irregularities with wavelengths of several ice thicknesses:


http://www.the-cryosphere.net/7/407/2013/tc-7-407-2013.pdf



Surface undulations of Antarctic ice streams tightly controlled by bedrock topography;
by: J. De Rydt, G. H. Gudmundsson, H. F. J. Corr, and P. Christoffersen; The Cryosphere, 7, 407–417, 2013; www.the-cryosphere.net/7/407/2013/; doi:10.5194/tc-7-407-2013


"Abstract. Full Stokes flow-line models predict that fast flowing ice streams transmit information about their bedrock topography most efficiently to the surface for basal undulations with length scales between 1 and 20 times the mean ice thickness. This typical behaviour is independent of the precise values of the flow law and sliding law exponents, and should be universally observable. However, no experimental evidence for this important theoretical prediction has been obtained so far, hence ignoring an important test for the physical validity of current-day ice flow models. In our work we use recently acquired airborne radar data for the Rutford Ice Stream and Evans Ice Stream, and we show that the surface response of fast-flowing ice is highly sensitive to bedrock irregularities with wavelengths of several ice thicknesses. The sensitivity depends on the slip ratio, i.e. the ratio between mean basal sliding velocity and mean deformational velocity.  We find that higher values of the slip ratio generally lead to a more efficient transfer, whereas the transfer is significantly dampened for ice that attains most of its surface velocity by creep. Our findings underline the importance of bedrock topography for ice stream dynamics on spatial scales up to 20 times the mean ice thickness. Our results also suggest that local variations in the flow regime and surface topography at this spatial scale cannot be explained by variations in basal slipperiness."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #36 on: September 08, 2013, 04:25:07 AM »
The following link leads to a free pdf related to an Antarctic ice sheet model that has matched ice mass balance trends observed by the GRACE satellite:

http://link.springer.com/content/pdf/10.1007%2Fs00382-012-1464-3.pdf


Verification of model simulated mass balance, flow fields and tabular calving events of the Antarctic ice sheet against remotely sensed observations;
by: Diandong Ren, Lance M. Leslie, & Mervyn J. Lynch; Climate Dynamics; June 2013, Volume 40, Issue 11-12, pp 2617-2636

"Abstract
The Antarctic ice sheet (AIS) has the greatest potential for global sea level rise. This study simulates AIS ice creeping, sliding, tabular calving, and estimates the total mass balances, using a recently developed, advanced ice dynamics model, known as SEGMENT-Ice. SEGMENT-Ice is written in a spherical Earth coordinate system. Because the AIS contains the South Pole, a projection transfer is performed to displace the pole outside of the simulation domain. The AIS also has complex ice-water-granular material-bedrock configurations, requiring sophisticated lateral and basal boundary conditions. Because of the prevalence of ice shelves, a ‘girder yield’ type calving scheme is activated. The simulations of present surface ice flow velocities compare favorably with InSAR measurements, for various ice-water-bedrock configurations. The estimated ice mass loss rate during 2003–2009 agrees with GRACE measurements and provides more spatial details not represented by the latter. The model estimated calving frequencies of the peripheral ice shelves from 1996 (roughly when the 5-km digital elevation and thickness data for the shelves were collected) to 2009 compare well with archived scatterometer images. SEGMENT-Ice’s unique, non-local systematic calving scheme is found to be relevant for tabular calving. However, the exact timing of calving and of iceberg sizes cannot be simulated accurately at present. A projection of the future mass change of the AIS is made, with SEGMENT-Ice forced by atmospheric conditions from three different coupled general circulation models. The entire AIS is estimated to be losing mass steadily at a rate of ~120 km3/a at present and this rate possibly may double by year 2100."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #37 on: September 08, 2013, 02:24:27 PM »
The following reference presents methodology for improving ice-sheet modeling:

Indirect inversions;
by: Olga Sergienko; Geophysical Research Abstracts; Vol. 15, EGU2013-5301, 2013; EGU General Assembly 2013

Abstract:
"Since Doug MacAyeal’s pioneering studies of the ice-stream basal traction optimizations by control methods, inversions for unknown parameters (e.g., basal traction, accumulation patterns, etc) have become a hallmark of the present-day ice-sheet modeling. The common feature of such inversion exercises is a direct relationship between optimized parameters and observations used in the optimization procedure. For instance, in the standard optimization for basal traction by the control method, ice-stream surface velocities constitute the control data.  The optimized basal traction parameters explicitly appear in the momentum equations for the ice-stream velocities (compared to the control data). The inversion for basal traction is carried out by minimization of the cost (or objective, misfit) function that includes the momentum equations facilitated by the Lagrange multipliers. Here, we build upon this idea, and demonstrate how to optimize for parameters indirectly related to observed data using a suite of nested constraints (like Russian dolls) with additional sets of Lagrange multipliers in the cost function.  This method opens the opportunity to use data from a variety of sources and types (e.g., velocities, radar layers, surface elevation changes, etc.) in the same optimization process."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #38 on: September 08, 2013, 02:40:18 PM »
The following link provides a free pdf of the linked reference which discusses the topic of icequakes as related to global warming induced ice stream velocities:

http://www.igsoc.org/annals/54/64/a64A033.pdf


Deformation in Rutford Ice Stream, West Antarctica: measuring shear-wave anisotropy from icequakes
by: S.R. HARLAND, J.-M. KENDALL, G.W. STUART, G.E. LLOYD, A.F. BAIRD,
A.M. SMITH, H.D. PRITCHARD, and A.M. BRISBOURNE; Annals of Glaciology 54(64) 2013 doi:10.3189/2013AoG64A033

"ABSTRACT. Ice streams provide major drainage pathways for the Antarctic ice sheet. The stress distribution and style of flow in such ice streams produce elastic and rheological anisotropy, which informs ice-flow modelling as to how ice masses respond to external changes such as global warming.  Here we analyse elastic anisotropy in Rutford Ice Stream, West Antarctica, using observations of shearwave splitting from three-component icequake seismograms to characterize ice deformation via crystalpreferred orientation. Over 110 high-quality measurements are made on 41 events recorded at five stations deployed temporarily near the ice-stream grounding line. To the best of our knowledge, this is the first well-documented observation of shear-wave splitting from Antarctic icequakes. The magnitude of the splitting ranges from 2 to 80ms and suggests a maximum of 6% shear-wave splitting. The fast shear-wave polarization direction is roughly perpendicular to ice-flow direction. We consider three mechanisms for ice anisotropy: a cluster model (vertical transversely isotropic (VTI) model); a girdle model (horizontal transversely isotropic (HTI) model); and crack-induced anisotropy (HTI model).  Based on the data, we can rule out a VTI mechanism as the sole cause of anisotropy – an HTI component is needed, which may be due to ice crystal a-axis alignment in the direction of flow or the alignment of cracks or ice films in the plane perpendicular to the flow direction. The results suggest a combination of mechanisms may be at play, which represent vertical variations in the symmetry of ice crystal anisotropy in an ice stream, as predicted by ice fabric models."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #39 on: September 08, 2013, 07:19:39 PM »
The following abstracts come from the linked sources and are relevant to the glaciology:

www.igsoc.org/symposia/2013/kansas/proceedings/procsfiles/procabstracts_63.htm
Contact: Secretary General, International Glaciological Society


67A025
Integrating RES and remotely sensed ice surface data to enhance survey design, mapping and characterization of the ice-sheet bed
Neil ROSS, Martin J. SIEGERT, Stewart S.R. JAMIESON, Hugh CORR, David RIPPIN, Fausto FERRACCIOLI, Rob G. BINGHAM, Tom A. JORDAN, Anne LE BROCQ, Kathryn ROSE
Corresponding author: Neil Ross
Corresponding author e-mail: neil.ross@ncl.ac.uk
Although commonly used to identify the locations and spatial extent of subglacial lakes and to map past and present ice-flow regimes, ice surface imagery is typically an oft-overlooked resource when major aerogeophysical surveys are designed, acquired and interpreted. Satellite-derived ice velocity data are now often used in the design of major airborne geophysical campaigns and the gridding of the resultant data (e.g. using mass conservation approaches). We propose and advocate that the careful targeted analysis and application of ice-sheet surface imagery can also provide considerable useful information at both pre- and post-survey stages. Better integration of remote-sensing imagery with radio-echo sounding (RES) data can increase the efficiency of, and enhance the scientific output from, both local and large-scale geophysical surveys of sub-ice conditions. Combining MODIS Mosaic of Antarctica and/or RADARSAT imagery with ground- and airborne RES data, we show that remote-sensing products that characterize the ice-sheet surface contain important, spatially continuous information on bed topography, sub-ice geology and basal conditions. We will outline the opportunities, benefits and limitations of the integration of ice-sheet surface imagery within the design and interpretation of major aerogeophysical campaigns, describing methods through which the information extracted from ice surface imagery can be enhanced and quantitatively analysed. We illustrate the importance of these data with examples from Antarctica and Greenland, discussing the present and past glaciological implications of our findings. Improved methodologies for the analysis of ice-sheet surface data products may unlock the potential for these high-resolution spatially contiguous datasets to enhance gridding of subglacial topography from sparse RES measurements and as input data for numerical ice-sheet models.

67A053
Sounding of subglacial nunatak ridges in West Antarctica using a UHF radar
Cameron LEWIS, Howard CONWAY, John STONE, Perry SPECTOR, John PADEN, Prasad GOGINENI
Corresponding author: Cameron Lewis
Corresponding author e-mail: cameronlewis@ku.edu
We developed an ultra-high-frequency (UHF) radar that operates over the frequency range of 600–900 MHz for surface-based measurements with a virtual antenna array of 16 elements. We used this radar to collect data on ice-covered nunataks in the Pirrit Hills of West Antarctica during the 2012/13 Antarctic summer season. These data were collected to generate fine-resolution 3-D bedrock topography maps for identifying the position and depth of the subglacial ridges of the Harter and John nunataks in preparation for future drilling campaigns. Data were collected over a grid for two areas: one approximately 0.6 km &mult; 0.3 km and the other approximately 1.2 km &mult; 0.5 km. We completed preliminary processing of the collected data, which included coherent integration, pulse compression and geo-synchronization. The processed data were used to generate radar echograms that revealed bedrock features at depths between 100 and 400 m below the surface. We also observed the presence of significant range hyperbolae indicating that a long aperture can be synthesized to obtain fine along-track resolution. Virtual phase center (tomographic) techniques will be applied to obtain fine resolution in the cross-track direction for producing 3-D maps of the bedrock topography. The combination of the radar system and data-processing techniques demonstrates the ability to image the ice–bedrock interface with fine resolution. This radar can also be used for fine-resolution imaging of the ice–water interface of ice shelves. We have successfully sounded 500–600 m thick ice shelves with an airborne version of the radar operating with a single receiver. Time-separated in situ measurements can provide single point analysis of ice-shelf basal melt rates; however, continuous wide-area coverage is needed to accurately evaluate the integrity of an ice shelf. Application of the virtual antenna array to the airborne version of the radar using the aforementioned data-processing techniques, with time-separated data collection campaigns, helps fill this data gap.
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #40 on: September 08, 2013, 10:39:31 PM »
Ren et al. (2012)

Clim Dyn (2013) 40:2617–2636 DOI 10.1007/s00382-012-1464-3 "Verification of model simulated mass balance, flow fields and tabular calving events of the Antarctic ice sheet against remotely sensed observations"
Ren et al.

I have issues:

1) "For example, the central parts of the Eastern Antarctic Ice Sheet typically cannot find a path way to ocean, which also is the case for the central section of the West Antarctic Ice Sheet (WAIS)."

I dispute that.

2)They admit:
"... ice shelf melting is enhanced by intrusions of warm circumpolar deep water onto the continental shelf and down into deep troughs carved into the sea floor during past ice ages (P. Molnar and J. Chen, personal communications, 2009). With 5-km resolution data (*3 km in longitudinal direction at this latitude), the ice model cannot fully resolve the troughs."

But we know CDW intrusion is a huge player.

3)They estimate a doubling of the 120GT/yr AIS mass loss by 2100. I think they are far,far, too low

sidd

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #41 on: September 09, 2013, 01:23:55 AM »
Sidd,

Thanks for your comments (all of which I agree with).  I feel remiss, in that I have not taken the time to comment sufficiently to point out the many limitations (and to highlight the positives) of the various references that I am providing.  All readers should think critically of any published model projections of ice mass loss though 2100, as no computer program now (or probably for 20 to 30 years from now) can adequately capture all of the various lightly synergistic and nonlinear mechanisms that are likely to lead to AIS negative ice mass balance by 2100.  Therefore, when we read such projections, we should not think that we are reading a prediction (like we get from well estabished science), but rather we should think: "What behavior is this model capturing and what is it leaving out."

Certainly, any model claiming to make a +/- 25% estimate of AIS ice mass loss by 2100 would need to accurately model not only the full influence of warm CDW (probably for RCP 8.5 95%CL) but also the full influence of:

(a) surface and basal crevasses; (b) subglacial hydrological systems; (c) suface meltwater influences on both ice shelves and ice sheets; (d) basal friction and geothermal basal effects; (e) changes in ocean current circulation patterns; (f) storm and infragravity wave effects; (g) SAM, ENSO, PDO, etc oscillational effects; (h) geometric and local instability effects similar to the "Jakobshavn Effect"; plus all of the other effects that I have referred to in the various threads in the Antarctic folder.

A big problem is that the public is use to science having solved so many problems to a degree of accuracy that the public can feel comfortable relying on the scientific "answers" that the public is not use to being given a partial answer by scientists.  It is also a problem that if the scientist does cite all the limitations of a model projection (e.g. AR4 SLR projections clearly stated they they did not include dynamic ice mass loss contributions); then either the public ignores the limits cited by the scientist, or they discount out of hand all of the hardwork that the scientist did to produce the partial answer.
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #42 on: September 10, 2013, 02:09:56 AM »
The linked reference has a free pdf, and I agree with Drouet et al that:
"Despite the recent important improvements of marine ice-sheet models in their ability to compute steady state configurations, our results question the capacity of these models to compute short-term reliable sea-level rise projections."

http://www.the-cryosphere.net/7/395/2013/tc-7-395-2013.html

Drouet, A. S., Docquier, D., Durand, G., Hindmarsh, R., Pattyn, F., Gagliardini, O., and Zwinger, T.: Grounding line transient response in marine ice sheet models, The Cryosphere, 7, 395-406, doi:10.5194/tc-7-395-2013, 2013.


"Abstract. Marine ice-sheet stability is mostly controlled by the dynamics of the grounding line, i.e. the junction between the grounded ice sheet and the floating ice shelf. Grounding line migration has been investigated within the framework of MISMIP (Marine Ice Sheet Model Intercomparison Project), which mainly aimed at investigating steady state solutions. Here we focus on transient behaviour, executing short-term simulations (200 yr) of a steady ice sheet perturbed by the release of the buttressing restraint exerted by the ice shelf on the grounded ice upstream. The transient grounding line behaviour of four different flowline ice-sheet models has been compared. The models differ in the physics implemented (full Stokes and shallow shelf approximation), the numerical approach, as well as the grounding line treatment. Their overall response to the loss of buttressing is found to be broadly consistent in terms of grounding line position, rate of surface elevation change and surface velocity. However, still small differences appear for these latter variables, and they can lead to large discrepancies (> 100%) observed in terms of ice sheet contribution to sea level when cumulated over time. Despite the recent important improvements of marine ice-sheet models in their ability to compute steady state configurations, our results question the capacity of these models to compute short-term reliable sea-level rise projections."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #43 on: September 13, 2013, 10:58:01 PM »
As Antarctic glacial speeds accelerate their basal ice will tend to become more debris-rich; which implies that the following research on anisotropy of magnetic susceptibility (AMS) will become increasingly useful in monitoring strains around the margins of accelerating Antarctic Ice Streams such as the Thwaites Glacier:

http://onlinelibrary.wiley.com/doi/10.1002/jgrf.20144/abstract

Magnetic fabrics in the basal ice of a surge-type glacier;
by: Edward J. Fleming, Harold Lovell, Carl T.E. Stevenson, Michael S. Petronis, Douglas I. Benn, Michael J. Hambrey, Ian J. Fairchild; 2013; Journal of Geophysical Research: Earth Surface; DOI: 10.1002/jgrf.20144


"Abstract
Anisotropy of magnetic susceptibility (AMS) has been shown to provide specific useful information regarding the kinematics of deformation within subglacially deformed sediments. Here we present results from debris-rich basal glacier ice to examine deformation associated with glacier motion. Basal ice samples were collected from Tunabreen, a polythermal surge-type glacier in Svalbard. The magnetic fabrics recorded show strong correlation with structures within the ice, such as sheath folds and macroscopic stretching lineations. Thermomagnetic, low-temperature susceptibility, varying field susceptibility and isothermal remanent magnetism (IRM) acquisition experiments reveal that the debris-rich basal ice samples have a susceptibility and anisotropy dominated by paramagnetic phases within the detrital sediment. Sediment grains entrained within the basal ice are inferred to have rotated into a preferential alignment during deformation associated with flow of the glacier. An up-glacier directed plunge of magnetic lineations and subtle deviation from bulk glacier flow at the margins highlight the importance of non-coaxial strain during surge propagation. The results suggest that AMS can be used as an ice petrofabric indicator for investigations of glacier deformation and interactions with the bed."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #44 on: September 14, 2013, 02:28:51 AM »
The linked reference (with a pdf) indicates the importance of accounting for the influence of tidal action on Antarctic glacial ice velocities when interpreting satellite measurements:


http://www.the-cryosphere.net/7/1375/2013/tc-7-1375-2013.html


Marsh, O. J., Rack, W., Floricioiu, D., Golledge, N. R., and Lawson, W.: Tidally induced velocity variations of the Beardmore Glacier, Antarctica, and their representation in satellite measurements of ice velocity, The Cryosphere, 7, 1375-1384, doi:10.5194/tc-7-1375-2013, 2013

Abstract. Ocean tides close to the grounding line of outlet glaciers around Antarctica have been shown to directly influence ice velocity, both linearly and non-linearly. These fluctuations can be significant and have the potential to affect satellite measurements of ice discharge, which assume displacement between satellite passes to be consistent and representative of annual means. Satellite observations of horizontal velocity variation in the grounding zone are also contaminated by vertical tidal effects, the importance of which is highlighted here in speckle tracking measurements. Eight TerraSAR-X scenes from the grounding zone of the Beardmore Glacier are analysed in conjunction with GPS measurements to determine short-term and decadal trends in ice velocity. Diurnal tides produce horizontal velocity fluctuations of >50% on the ice shelf, recorded in the GPS data 4 km downstream of the grounding line. This variability decreases rapidly to <5% only 15 km upstream of the grounding line. Daily fluctuations are smoothed to <1% in the 11-day repeat pass TerraSAR-X imagery, but fortnightly variations over this period are still visible and show that satellite-velocity measurements can be affected by tides over longer periods. The measured tidal displacement observed in radar look direction over floating ice also allows the grounding line to be identified, using differential speckle tracking where phase information cannot be easily unwrapped."
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #45 on: October 24, 2013, 01:47:57 AM »
This article indicates that due to the highly non-linear response of marine terminating glaciers w.r.t. ice mass loss, that researchers should do a more thorough job on sensitivity analysis where presenting related research results:

http://www.the-cryosphere.net/7/1579/2013/tc-7-1579-2013.html

Enderlin, E. M., Howat, I. M., and Vieli, A.: The sensitivity of flowline models of tidewater glaciers to parameter uncertainty, The Cryosphere, 7, 1579-1590, doi:10.5194/tc-7-1579-2013, 2013

Abstract. Depth-integrated (1-D) flowline models have been widely used to simulate fast-flowing tidewater glaciers and predict change because the continuous grounding line tracking, high horizontal resolution, and physically based calving criterion that are essential to realistic modeling of tidewater glaciers can easily be incorporated into the models while maintaining high computational efficiency. As with all models, the values for parameters describing ice rheology and basal friction must be assumed and/or tuned based on observations. For prognostic studies, these parameters are typically tuned so that the glacier matches observed thickness and speeds at an initial state, to which a perturbation is applied. While it is well know that ice flow models are sensitive to these parameters, the sensitivity of tidewater glacier models has not been systematically investigated. Here we investigate the sensitivity of such flowline models of outlet glacier dynamics to uncertainty in three key parameters that influence a glacier's resistive stress components. We find that, within typical observational uncertainty, similar initial (i.e., steady-state) glacier configurations can be produced with substantially different combinations of parameter values, leading to differing transient responses after a perturbation is applied. In cases where the glacier is initially grounded near flotation across a basal over-deepening, as typically observed for rapidly changing glaciers, these differences can be dramatic owing to the threshold of stability imposed by the flotation criterion. The simulated transient response is particularly sensitive to the parameterization of ice rheology: differences in ice temperature of ~ 2 °C can determine whether the glaciers thin to flotation and retreat unstably or remain grounded on a marine shoal. Due to the highly non-linear dependence of tidewater glaciers on model parameters, we recommend that their predictions are accompanied by sensitivity tests that take parameter uncertainty into account.
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AbruptSLR

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #46 on: November 22, 2013, 04:38:02 PM »
The following links leads to an website, explaining that the IceBridge Antarctic Mission is underway in an expanded format (operating from McMurdo instead of Chile); so that the airborne science mission can partially fill the void between the defunct ICESat satellite and the planned ICESat-2, which is slated to launch in 2016.


http://www.nbcnews.com/science/nasas-icebridge-mission-back-action-over-antarctica-2D11624553
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #47 on: November 26, 2013, 04:35:46 PM »
The following link and summary cites NSF funded research about how cracks along the edges of the ice shelves in the Amundsen Sea Sector may provide a positive feedback factor for ice mass loss from this area (as I have discussed previously):

https://www.research.gov/research-portal/appmanager/base/desktop;jsessionid=1JyLSXcYvTKLnT6mLffJJjRtvsvnBK0tTyjB2hSY8tYtQP2pnJtL!-1826466010!941996275?_nfpb=true&_windowLabel=awardSummary_1&_urlType=action&wlpawardSummary_1_id=%2FresearchGov%2FAwardHighlight%2FPublicAffairs%2F23322_WestAntarcticiceshelvesslip-slidingaway.html&wlpawardSummary_1_action=selectAwardDetail

Research Summary:

"Nearly 40 years of satellite imagery suggests that west Antarctic ice shelves floating in the Amundsen Sea are steadily losing their grip on adjacent bay walls. This pattern of retreat could potentially amplify an already accelerating loss of ice to the sea, according to glaciologists at The University of Texas at Austin.
The record created in this study will give scientists a better understanding of the recent evolution of west Antarctica's ice shelves. Knowing why these changes have occurred is critical for predicting future changes. Previously, most computer models have neglected this specific pattern of ice-shelf retreat, partly because it involves fracture, but also because no comprehensive record of this pattern existed.
The Amundsen Sea Embayment is one of the few places in Antarctica with good long-term satellite coverage of its coastline. This comprehensive record shows clearly that the ice shelves changed substantially between the beginning of the Landsat satellite record in 1972 and late 2011. These changes were especially rapid during the past decade, and the affected ice shelves include the floating extensions of the rapidly thinning Thwaites and Pine Island glaciers.
Normally, the ice shelves grip onto rocky bay walls or slower ice masses at their very edge. However, as that grip continues to loosen, these already-thinning ice shelves will be even less able to hold back ice upstream. The fractured edges are retreating inland, resembling a cracked mirror in satellite imagery until the detached icebergs finally drift out to the open sea. This pattern is believed to be a symptom, rather than a trigger, of the recent glacier acceleration in this region. However, it could also generate additional acceleration."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

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Re: Glaciology Basics and Risks - Uncertainties
« Reply #48 on: December 12, 2013, 12:58:48 AM »
The following link leads to an interesting article about the recently completed 2013 Antarctic IceBridge campaign:

http://phys.org/news/2013-12-icebridge-successful-antarctic-campaign.html
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Glaciology Basics and Risks - Uncertainties
« Reply #49 on: January 08, 2014, 12:01:01 AM »
The following link leads to an interesting article about the formation of blue-ice areas in Eastern Antarctica:

http://www.igsoc.org/journal/60/219/t13J116.html
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson