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Author Topic: What's new in Greenland?  (Read 87654 times)

Laurent

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What's new in Greenland?
« on: December 15, 2014, 08:45:03 PM »
« Last Edit: November 26, 2015, 04:21:54 PM by Neven »

Lennart van der Linde

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Sigmetnow

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Re: what's new in Greenland ?
« Reply #2 on: December 16, 2014, 02:02:08 PM »
Hidden Movements of Greenland Ice Sheet, Runoff Revealed
New NASA video has a nice overview of Greenland topography, satellite and Ice Bridge data collection, and glacier melt.
http://www.nasa.gov/content/goddard/hidden-movements-of-greenland-ice-sheet-runoff-revealed/
People who say it cannot be done should not interrupt those who are doing it.

Laurent

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Re: what's new in Greenland ?
« Reply #3 on: December 18, 2014, 06:00:14 PM »

Laurent

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Re: what's new in Greenland ?
« Reply #4 on: December 19, 2014, 11:24:52 AM »

A-Team

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Re: what's new in Greenland ?
« Reply #5 on: December 19, 2014, 04:52:50 PM »
Whoa, this is really sensational to have access to 0.5 m WorldView panchromatic -- three cheers for DigitalGlobe and NGI for not hoarding Greenland imagery! After processing stereo pairs 45 seconds apart in satellite travel time into DEMs, it is 2 m resolution; the spectacular sample image of the west central Greenland coast is hill-shaded from that.

I located the region covered by the image in google earth -- quite the difference. The caption mentions Sermeq Silardleq (Silarleq) and Kangigdleq (Kangilleq). I added the available CReSIS radar tracks and a bit of the surface velocity map.

 the researchers are steadily processing it all with new Ohio State software called SETSM (for Surface Extraction from TIN-based Search Minimization). Ohio State research associate Myoung-Jong Noh created the software, which builds 1-gigabyte "tiles" representing regions 7 kilometers on a side and assembles them into mosaics depicting land, sea and ice elevation.

It seems at the meeting that Howat also showed Jakobshavn imagery having cracks hundreds of kilometers inland. The location and rate of widening indicates the direction and magnitude of ice accleration. 

Ohio State has finished only a quarter of the Greenland Ice Sheet which has taken a year of computing time using MJ Noh software called SETSM (Surface Extraction from TIN-based Search Minimization) where TIN stands for Triangulated Irregular Network. The tiles are 7 x 7 km but already 1-gigabyte in size.  Greenland is ~2.3 million sq km, so that amounts to 47,000 tiles. 

Commercial imagery vendor DigitalGlobe licensed it for U.S. federal use via the NSA's National Geospatial Intelligence Agency, which sends it to the Polar Geospatial Center at the University of Minnesota. At any given time, the Ohio Supercomputer Center is processing a 30-terabyte.

It seems that U Minn hosts a publicly accessible website off http://www.pgc.umn.edu/. I don't know if that is up and running yet or whether the interface requires whole tile downloads at full resolution or is more like EarthExplorer. 7 km doesn't get you very far up Jakobshavn for a gigabyte.

The full size resolution of the sample imagery is online at 1440 x 3093 pixels, mysteriously as a color jpg (it is 8-bit grayscale). The second image below shows a small piece of it, the full image is at http://cdn.phys.org/newman/gfx/news/hires/2014/thegreenland.jpg

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Re: what's new in Greenland ?
« Reply #6 on: December 19, 2014, 07:33:23 PM »
I found the SETSM site and got registered; no password, products are free. I am not sure if the Greenland forums are a proceeding or just a blog -- maybe if we have a dedicated forum to SETSM it would be worth a head's up:

I agree to inform Dr. Ian Howat if this dataset is to be presented in any publication, presentation or proceeding and to acknowledge the Byrd Polar Research Center (BPRC) Glacier Dynamics Research Group, Ohio State University by name in presentations and publications arising from use of these data.

http://www.pgc.umn.edu/elevation/stereo

The portal is well designed and provides enough levels of preview that you don't have to download a large pig in a poke only to find you wanted the next one over. They have quite a good gallery. http://www.pgc.umn.edu/elevation/stereo/gallery These are also available as kmz for layering onto Google Earth, hooray.

A-Team

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Re: what's new in Greenland ?
« Reply #7 on: December 20, 2014, 02:42:19 PM »
The new PNAS paper Lennart mentions above is available there as full free text, which will give a clearer account than newspaper feeds. The authors have a long track record of research in what might be called fusion geomatics, putting together information from a mix of disparate, temporally spotty, terrestrially sparse remote sensing data.

Here they are after Greenland's contribution to sea level rise (taken as ice sheet mass loss), a sore point with researchers because best estimates were blown off by an IPCC 2007 panel as too uncertain (though today seen as spot-on).

There was enough radar altimetry to develop a time series for ice thickness of Greenland and partition that into contributions from surface mass balance (snowfall accumulation minus melt etc) and ice dynamics (gravity-driven flow, ie calving). However satellite coverage issues (ICESat ended Oct 2009; IceBridge tracks are sparse, ICESat 2 not launching until 2017) meant no annual updates for the 5 most recent years -- SETSM might help but all-Greenland processing is still a year off.

Greenland is best broken up into lots of individual iceshed basins; the 3-4 largest contributors cannot really serve as proxies because each represents a one-off situation, eg NEGIS icestreaming or Jakobshavn overdeepening. The authors make the important point that since physical inter-basin couplings are weak across ice divides,  annual changes in tandem must be attributable to climate.

The main internal products of the study are first the behavioral classification of individual glaciers (Fig.2, a portion shown below) which prove surprisingly heterogenous, and second the annual time series for six consecutive years 2004-2009.

The latter result is fairly difficult to convey effectively via scientific illustration. It involves 18 images representing SMB + Dynamic = Total for the six years. The authors provide these at low resolution and as a movie (blended slide show) whose frames are high resolution. The color palette is not completely satisfactory; a log scale might have better represented big coastal vs small interior changes. I tried a couple of things below but didn't improve conveyance greatly.

http://www.pnas.org/content/early/2014/12/12/1411680112.full.pdf+html?with-ds=yes

sidd

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Re: what's new in Greenland ?
« Reply #8 on: December 21, 2014, 07:45:34 AM »
Greenland section from nsidc report card
http://www.arctic.noaa.gov/reportcard/greenland_ice_sheet.html

two points the struck me
a)albedo continues to drop
b)GRACE mass loss is only 6 Gton for June2013-June2014, as compared to nearly 500 the previous year. A reprieve, but for how long ?

sidd

Wipneus

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Re: what's new in Greenland ?
« Reply #9 on: December 21, 2014, 07:47:45 AM »
I did not see this mentioned elsewhere.

The Danish Polar Portal has issued a 2014 season report, available in Danish, English and Greenlandic.

The summary:
Year 2014 comes in above average for the amount of melting from the Greenland Ice Sheet in the period since 2002. On the other hand, Arctic sea ice was strengthened in 2014.

The most important results of climate
monitoring in the Arctic in 2014 are:

• The Greenland Ice Sheet contributed
approximately 1.2 mm to sea-level rise;
• Below average reflection of sunlight is
associated with increased melting from the
Greenland Ice Sheet in 2014;
• The surface mass balance of the Greenland
Ice Sheet was lower than normal, but not
record low;
• Arctic sea ice strengthened in 2014;
• A new temperature record was established
in west Greenland in June 2014;
• There were no exceptional changes in the
movements of glacier fronts in Greenland.


About Zachariae:

Zachariae Glacier is one of the 20 largest
glaciers that changed the most during the 2014
melt season. Already in April several km2 had
broken off the terminus, but were nonetheless
partially rebuilt by the glacier flow from the ice
sheet. However, between July and August an
additional roughly 20 km2 of the glacier’s
terminus was broken off to form floating
icebergs. This especially affected the central
and northern parts of the glacier, which lost 4
km between June and September.

Even though there were no major changes in
the terminus of 79N Glacier, there were both
more and larger meltwater lakes on the surface
of the glacier in the middle of August in
comparison with the same period in 2013.

Wipneus

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Re: what's new in Greenland ?
« Reply #10 on: December 21, 2014, 10:04:41 AM »
In The Cryosphere Discussion Brief Communication: 2014 velocity and flux for five major Greenland outlet glaciers using mGRAFT and Landsat-8 by A. Messerli, N. B. Karlsson, and A. Grinsted.

Abstract. This study presents average velocity fields, mass flux estimates and central flowline profiles for five major Greenland outlet glaciers; Jakobshavn Isbræ, Nioghalvfjerdsbræ, Kangerdlugssuaq, Helheim and Petermann glaciers, spanning the period (August) 2013–(September) 2014. The results are produced by the feature tracking toolbox, ImGRAFT using Landsat-8, panchromatic data. The resulting velocity fields agree with the findings of existing studies. Furthermore, our results show an unprecedented speed of over 50 m day−1 at Jakobshavn Isbræ as it continues to retreat. All the processed data will be freely available for download at http://imgraft.glaciology.net.


ImGRAFT is (unfortunately for me) written in matlab, otherwise I would love to apply it at Zachariae.

A-Team

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Re: what's new in Greenland ?
« Reply #11 on: December 21, 2014, 05:21:46 PM »
ImGRAFT is (unfortunately for me) written in matlab, otherwise I would love to apply it at Zachariae.
You might email the authors and ask them to please run your favorite pair of Zachariae Landsat-8's (matched viewing geometry, minimal clouds, id numbers like LC80090112014040LGN00 supplied). Explain that you plan to write a blog article on Zachariae citing their software. Myself, I hope they start to offer this as an online service (like the 301 G'Mac filters), running matlab in the background.

Or try ImageJ2 (FIJI). It comes pre-loaded with automatic feature-tracking plugins and can process Landsat8 in 12-bit mode (32 bit mode supported for SETSM above, 64-bit supported for a few things).

It is becoming possible to run a pair of Landsat-8's with a full year of separation (good for slower moving upstream and peripheral areas. T Scambos AGU 2014 abstract indicates this has been done already far upstream for Jakobshavn with yet other software. More ambitiously, run all of them, compare and contrast algorithm output.

Meanwhile, back at 0.6 m DigiGlobe and really high resolution velocity maps, I've made some inroads downloading and processing the 32-bit SETSM DEMs from Noh and Howat which in some cases use WorldView 1 and 2 of different dates.

Here even the sub-tiles (20 km x 20 km) would benefit from a wall-sized monitor (in a room with a tall ceiling) being 10,100 pixels on a side. So far I have just tried shadowing (poor man's hill shading), contouring, norm gradient and jakobean.

The  ImGRAFT paper makes some interesting points. Note flux gates have been "constructed" elsewhere at both 1700 and 2000 m contours around the whole of Greenland.

Our data indicate yet a further speedup of Jakobshavn Isbræ in July 2014 peaking at a record 52 meters per day. This was manually verified using a simple triangulation of selected features near the terminus ... The current flux gate at the grounding line for Jakobshavn Isbræ sees ~30 cubic km per year go by... Joughin 2014 suggest that a tenfold increase in this estimate in the future is plausible.

A noticeable observation at Petermann is the distinct separation between the main trunk and the northern marginal slower flow which has been described in Münchow 2014. The large tributary that flows into the main glacier forms a slower flowing part of the glacier tongue. Petermann and Nioghalvfjerdsbræ display highest speeds not at the terminus but at approximately 45 and 70 km from the calving front respectively

Below Jakobshavn and Nioghalvfjerdsbræ are shown bumped up slightly from native submitted resolution. Below that, ImGRAFT and IceTools together in GoogleEarth.

Below that, a gradient of a SETSM tile with middle Jakobshavn lower left; and lastly an animation shadowing 32-bit Noh and Howat DEM, more rounds of shadowing bring out relief in low-relief areas (at the same time overdoing it in moderate relief areas). The direction of light chosen, from the NE, is roughly a flow line so orthogonal to surface wave features. With incredible resolution comes incredible file sizes ... hopefully we little desktops can stay in the game.

sidd

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Re: what's new in Greenland ?
« Reply #12 on: December 21, 2014, 09:33:26 PM »
Re;Matlab

Try Octave. I have translated some fairly complicated Matlab code to run on Octave, with small difficulty.

sidd

sidd

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Re: what's new in Greenland ?
« Reply #13 on: December 21, 2014, 09:39:33 PM »
That report fron Denmark polar portal does not agree with the NSIDC report i linked to, which shows only 6Gton for 2013-2014 mass loss for GRIS from GRACE data. Nebbe because the former is based on   reflectivity based studies, the latter on gravitational signal ?

Maybe Prof. Box will chime in. His name is on the NSIDC report, and he works at GEUS i think these days.

sidd

A-Team

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Re: what's new in Greenland ?
« Reply #14 on: December 22, 2014, 01:54:43 PM »
With incredible resolution comes incredible file sizes ... hopefully we little desktops can stay in the game.
Improving ground resolution is very important to more reliable modelling of the Greenland Ice Sheet -- but file sizes go as the square (from 5km --> 1 km is 25x the pixels). A time series of Landsat-8s is already a full plate; the new SETSM DEM is 10 GB per tile x 2300 tiles.

At what point will we lose the ability to replicate results and just have to take them on faith?

If someone should steal my current computer so I could rationalize getting a new one, would newer capabilities make any difference? For $3k, there has been a huge improvement in the screens: the current 27" iMac has 5120 x 2880 = 14.7 million pixels (218 ppi pixel density), a 7x improvement but it can also supports an external display of 3840 by 2160 resolution, another 8.3 million pixels. There is a big productivity improvement from getting the operational interface off the main screen.

The graphics card offers 3.5 teraflops -- large resolution raster image manipulation has benefited from the video gamer market. Yosemite OS can take advantage of high-end displays. The quad-core 4.0GHz Intel Core i7 processor can be overclocked to 4.4GHz, not much of an improvement over the 3.1 GHz I have now. The RAM limit is 32 GB (vs 16 MB now) though some sites are saying 64. I don't really store much locally but the 1.0 TB drive has faster retrieving. The 802.11ac supports data rates of 1.3 Gbps which is a mismatch to my mediocre connection and data plan.

There is a happy coincidence in that Greenland in polar stereographic projection has height:width ratio of 1.84 which, viewed sideways, is an excellent match to the 1.77 of the monitor. The icesheet itself covers 1,755,637 km^2 so a sq km could be represented by 3 x 3 pixels. How though would a neat image be shared -- at 700 pxl blog width?

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Re: what's new in Greenland ?
« Reply #15 on: December 22, 2014, 02:46:07 PM »
Might try matLab in Octave

Interesting suggestion, sidd. Octave just thinks of images as matrices, stacked one per color channel. This makes sense as a lot of raster image processing amounts to very basic matrix manipulations. As mentioned before, the BMP file format interconverts image display on the monitor with its numerical matrix equivalent. Octave now has a GUI to get out of command line; I haven' tried it.
https://www.gnu.org/software/octave/doc/interpreter/index.html#SEC_Contents

report from Denmark polar portal does not agree with NSIDC which shows only 6 Gton for 2013-2014 mass loss from GRACE data.

Indeed, something is not right here. The polar portal offers an explanation in Box 2, page 3: power was shut off to the GRACE satellite in July to spare the battery which couldn't have come at worse time for Greenland mass balance. I'm inclined to go with the graph in Fig.3 below. The pdf also has a good summary of weather aspects.

Satellite observations since 2002 show that the Greenland Ice Sheet is not in balance and that the loss of ice from calving of icebergs and surface melting exceeds the overall mass input from snowfall. The Greenland Ice Sheet has lost about 250 Gt/year of mass over the past decade. One Gt is 1 billion tonnes and is equivalent to 1 cubic kilometer of water. A loss of mass of 100 Gt of ice corresponds to a sea level rise of 0.28 mm.

The annual melting season is normally at its peak in July or at the beginning of August, and 2014 was a year with greater melting than normal—although less than the highest level so far, 2012. According to our reflectivity based estimate, the ice sheet lost mass equivalent to approximately 1.7 mm sea-­‐level rise during the period of greatest sunlight from May to September 2014.

This is about 50% more than the average for the years 2002 to 2013 and only about 5% less than the loss of mass in the record year 2012. This loss of mass puts the year 2014 in third place in relation to melting since 2002. Second place is year 2010. When the average addition of mass equivalent to 0.4 mm during the winter period from October to March is considered, it is estimated that in 2014 the Greenland Ice Sheet contributed about 1.2 mm to sea-­‐level rise over the entire period.

Changes in the overall mass of the ice sheet are determined by two different methods. One method builds on measurements from the GRACE satellite of changes in the gravitational pull of the ice sheet, which decreases when there is less ice. However, it takes up to two to three months to analyze these data and GRACE data are unavailable mid-­‐2014 due to satellite power problems.

Therefore, researchers from GEUS have developed a supplementary method that is faster but not quite as precise as measurements of the gravity. This method is based on measurements of the albedo effect, that is, the reflection of sunlight from the ice sheet. This can be used because a statistical relationship has been found between the albedo effect and the gravity of the ice sheet. In this way a rapid, but provisional assessment can be made of the loss of mass from the ice sheet, while the more precise data are being analyzed.

http://polarportal.dk/fileadmin/user_upload/PolarPortal/season_report_2014/PolarPortal2014-EN.pdf

Wipneus

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Re: what's new in Greenland ?
« Reply #16 on: December 22, 2014, 07:40:45 PM »
Before shutting down, this is what I did today:

- borrowed a matlab installation
- installed the ImGRAFT software and figured out how to run a demo example
- got two Landsat B&W images with the same path and row (July 17 and August 29)
- played with the parameters to improve the picture

Latest result is attached. Scale is in meters/day.

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Re: what's new in Greenland ?
« Reply #17 on: December 22, 2014, 10:49:47 PM »
Bravo! Do we know of some reason for enhanced motion in the area outlined below? (Please excuse the blur but it seems to give an overall picture.)

Laurent

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Re: what's new in Greenland ?
« Reply #18 on: December 23, 2014, 12:40:13 PM »

A-Team

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Re: what's new in Greenland ?
« Reply #19 on: December 23, 2014, 03:22:50 PM »
The AGU has not concerned itself greatly with providing online resources covering the meeting. I cannot understand how uploading slides can be optional for presenters since their research, salary, retirement, medical, meeting expenses etc etc are done entirely on the taxpayers' dime -- in terms of disadvantaging the competition (does this even make sense in a climate crisis), the audience has lots and lots of camcorders.

Even the poster session posters were not posted!!! (4 out of 30 below). Those posters had to have been finalized at the time of submission to the session. I'd say automatically upload those at the close of the meeting and let authors over-write with any corrections and updates. Topics like cryosphere -- wouldn't a tar.zip of the whole package be vastly more efficient than pecking through one pdf at a time?

I looked for a list of Cryosphere posters and panel talks which were either video-archived online or where the authors had uploaded a pdf or ppt.  This simple task proves to be a total nuisance, clicking through the entire archive one session at a time looked for 'ePoster' links. A lot of the main Greenland players appear not to have submitted anything -- or rather, name search failed to link up their ePosters (often submitted under unknown grad student name).

I did find two relevant to Laurent's newsy link above but the AGU did not have the pdf links formatted correctly (as far as Chrome or Firefox were concerned)! I eventually did get them open, very worthwhile. It is not possible to walk up their file system from the initial ePoster. In fact I haven't seen a pattern yet in their ePoster directory system.

C21B-0329 Firn and percolation conditions in the vicinity of recently formed high elevation supra-glacial lakes on the Greenland Ice Sheet assessed by airborne radar
ePoster - https://agu.confex.com/data/handout/agu/fm14/Paper_24929_handout_1496_0.pdf

The western region of the Greenland Ice Sheet around and above the equilibrium line is characterized by relatively high accumulation rates with short-lasting melt events of variable intensity during the summer months. During melt season, supra-glacial lakes are formed at least temporarily in depressions found in the topography of the ice.

These ponds can form and drain rapidly, affecting the dynamics of the ice below. Recent warming trends have gradually increased the amount of meltwater found every summer over the ice sheet, with melt regimes migrating to higher altitudes. Consequentially, supra-glacial lakes are being found at higher elevations, yet it is unclear what mechanisms control their formation over firn.

We used data from different radar systems acquired by Operation Icebridge around and over lakes formed above the equilibrium line of the Greenland Ice Sheet to study internal features of identified frozen/drained supra-glacial lakes, and to investigate near-surface snow and firn conditions in the vicinity of the ponds by radar-mapping internal snowpack structure. Airborne radar and additional field observations revealed extensive and impermeable ice layers 20-70 cm thick formed at elevations between 1500 m and 2200 m.

Buried by winter accumulation, these ice layers prevent further meltwater to percolate deeper during melt season, limiting firn capacity to absorb meltwater and causing near-surface snowpack saturation, thus facilitating the transport of meltwater to newly-formed basins above the equilibrium line. Ice penetrating capabilities from the different radar systems allow the survey of different firn layers and internal features created by refrozen meltwater. IceBridge data is acquired in early spring, when no liquid water content is found over this region ensuring adequate radar response.

C21B-0316Massive Perched Ice Layers in the Shallow Firn of Greenland's Lower Accumulation Area Inhibit Percolation and Enhance Runoff
ePoster - https://agu.confex.com/data/handout/agu/fm14/Paper_10527_handout_506_0.pdf

Greenland's recent trend of record-breaking melt seasons (2012, 2010, 2007, 2002, et al.) have substantially increased the amount of melt water generated in the ice sheet's lower accumulation area. Due to this enhanced refreezing in the firn, regions with low accumulation rates have formed multi-annual ice layers 5-10+ meters thick in the thermally active shallow firn that overlies porous firn at depth.

The loss of pore space in the firn prevents the majority of melt water from percolating to depth and results in surface runoff where water previously would have refrozen. Here we present evidence from in situ ground-penetrating radar, firn cores and airborne radar from NASA's Operation IceBridge, collected both before and after Greenland's 2012 melt season, to illustrate the mechanism by which southwest Greenland's runoff zone in 2012 extended 20 kilometers inland from the long-term saturation line.

Additional evidence from satellite imagery, firn temperature profiles and modeling support the notion that these layers blocked percolation and contributed to Greenland's record runoff in 2012. Should Greenland's trend of anomalously warm summers persist, these massive lenses are likely to grow thicker and extend further inland, resulting in enhanced runoff and rapid upslope migration of the equilibrium line. These results illustrate the vital importance of understanding subsurface firn changes in order to accurately predict Greenland's future runoff in a changing climate.
« Last Edit: December 23, 2014, 04:29:20 PM by A-Team »

Wipneus

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Re: what's new in Greenland ?
« Reply #20 on: December 24, 2014, 07:44:53 PM »
Reporting on my progress with the ImGRAFT software.
This time I tried the Sentinel images, these are lower in resolution (as currently available) 40m and more of the 'SAR quality'. More high frequency noise than in optical images.
So this was more a challenge but I think I get reasonable result now.
What was done:
- filtered the input images in matlab using hi- and lo-pass filters (with the help of some suggestions in ImGRAFT documentation);
- Used quite a large 'template size', the pieces in the source image for which a match is sought in the target. It is about 140 pixels wide, judge by the left marging in the attached picture;
- Did away with the signal/noise filtering used in the examples;
- Probed a piece of solid rock to measure the alignment of the two images, used this to correct the flow field. As the pixel offset was not a whole number this is not an easy job (for me) in the Gimp;

The result has some encouraging things to say:
- the speed near the calving front is about 6m/day, just what we expected;
- part of the  Nioghalvfjerdsfjorden looks quite good;
Other localized aberrations in the flow field are visible, hopefully we can say more about these when we follow the glacier longer and in better resolution.
 

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Re: what's new in Greenland ?
« Reply #21 on: December 25, 2014, 02:01:46 PM »
Nice! It would take n well-distributed rocks to really rectify image geometry (ImageJ2 distortion plugin) -- even when orbital parameters are 'known' they have too much uncertainty for geodesy (Howat 2014).

The ImGRAFT output is an evenly gridded vector field yet it can only find corresponding features in the two images here and there. Can it also output just the primary match arrows it uses to make the grid? Can it output RGB where as HSV the V is velocity magnitude and H is 360º direction? With 3 successive image dates, can it display acceleration dv/dt?

I rescaled and rotated imagery from Latour 2014 which uses Zachariae velocities from Rignot 2012 which uses InSar from still earlier to more or less fit the ImGRAFT scene above. Nothing there corresponds to the upstream oddities which ImGRAFT is reporting from both Landsat and Sentinel.

nukefix

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Re: what's new in Greenland ?
« Reply #22 on: December 25, 2014, 04:54:34 PM »
If the S-1 images are from the same track they are already in the same sensor geometry and should match very well after just applying a shift (no rotation needed). If we had a DEM it should be possible to geocode images from different tracks to map coordinates without having to use GCPs...

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Re: what's new in Greenland ?
« Reply #23 on: December 26, 2014, 11:07:37 AM »
Nice! It would take n well-distributed rocks to really rectify image geometry (ImageJ2 distortion plugin) -- even when orbital parameters are 'known' they have too much uncertainty for geodesy (Howat 2014).

From the Sentinel-1 User Handbook:

In particular for interferometry, SENTINEL-1 requires stringent orbit control. Satellite positioning along the orbit must be accurate, with pointing and timing/synchronisation between interferometric pairs. Orbit positioning control for SENTINEL-1 is defined using an orbital Earth fixed "tube", 50 m (RMS) wide in radius, around a nominal operational path. The satellite is kept inside this "tube" for most of its operational lifetime.

50m/693km seems to me impressive, does Howat consider such precision?

The ImGRAFT software contains a module "camera.m" that can rectify for differences in cameras (focal length, distortions) and positions. They don't use it on the Landsat images though, neither do I.

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Re: what's new in Greenland ?
« Reply #24 on: December 26, 2014, 11:41:35 AM »
The ImGRAFT output is an evenly gridded vector field yet it can only find corresponding features in the two images here and there. Can it also output just the primary match arrows it uses to make the grid? Can it output RGB where as HSV the V is velocity magnitude and H is 360º direction? With 3 successive image dates, can it display acceleration dv/dt?


Core of the ImGRAFT software is "templatematch.m" a routine that give 2 images, an array of x,y coordinates in the first image will calculate the displacements to matching point in the second image. Everything else is a demo: the evenly grid, the output image with colors and arrows. The user can  change those as you like it, as long as it is programmable.
Moreover there is nothing in the software that is aware that the displacements are a (u,v) vector field with some (dictated by physics) continuity in space and time (when more than 2 images are available). Those features seem desirable but non-trivial to add.

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Re: what's new in Greenland ?
« Reply #25 on: December 26, 2014, 05:26:51 PM »
Everything else is add-on: the evenly grid, the output image with colors and arrows. The user can change those as long as it is programmable. nothing in the software is aware displacements are a (u,v) vector field with dictated by physics)

This modularity is a good thing ... users can build various paths through various options for interpolation and display in something like a PENTAHO drag'n'drop environment. Whether you end up with something the corresponds to reality as well as interferometric SAR (which inherently provides continuous velocity contours) isn't frequently validated on the ground (eg fiberglass pole array to 80 m depth Swiss Camp flow line).

On bringing in ice physics (see 14 talks of Morlighem at AGU 2014), for me the question is what we can measure with surface displacements versus what we want to measure, namely how the velocity and acceleration fields in 3D are responding to climate change, without just assuming surface motion is a satisfactory proxy for them.
 
The ImGRAFT software contains a module "camera.m" that can rectify for differences in cameras (focal length, distortions) and positions. They don't use it on the Landsat images though, neither do I.

The Landsat-8 has a group of image geometric attributes, things like ROLL_ANGLE = -0.001 which I've never seen vary to any interesting extent.

What got my attention above was Poh and Howat 2014, mss in prep http://www.pgc.umn.edu/system/files/SETSM_Product_Sheet_v1.1.pdf, which seeks to "extract a stereo-photogrammetric DEM from pairs of images without any user-defined or a-priori information and using only the [satellite] sensor Rational Polynomial Coefficients (RPC) for geometric constraints."

Surface Extraction with TIN-based Search-space Minimization (SETSM)
Fully automatic stereo-photogrammetric Digital Elevation Model (DEM) extraction from pushbroom satellite imagery

Since the geolocation accuracy of RPCs without ground control for WorldView-1 and 2 is 5m CE90 (DigitalGlobe, 2013), there is an offset between corresponding points projected by the vertical line locus. Where large enough, this offset can result in matching failure. Relative RPC updating provides an adaptive method for mitigating this error.

For any RPC-constrained DEM extraction algorithm there will be two common and dominant sources of error: blunders and RPC errors. Blunders are caused by incorrect matching of features between images, resulting in surface outliers. The iterative restriction of the search area in SETSM, as well as the blunder detection algorithm in step 4 above, substantially reduce blunders.

RPC errors are typically the result of errors in satellite positioning and look geometry, increasing with sensor look angle. These errors have two main effects: First, they result in misalignment of the stereo pair in the matching routine, resulting in poor match returns, which is mitigated in SETSM through iterative refinement of the search space.

The second effect is a bias in DEM elevations. Cheng and Chappel (2008), found an average bias of 5 m in several test DEMS extracted from WorldView-1 imagery. We are still testing for this bias using both Worldview 1 and 2 image pairs ... We anticipate further accuracy gains by determining which look geometries result in the best RCP determinations. The most effective way to reduce geometrically induced error, however, is by using ground control points (GCP).

The interometric velocity  below is adapted from http://posters.unh.edu/gallery/view/3177/  "Investigating the cause of the 2012 Acceleration of Jakobshavn Isbrae, Greenland Using High Resolution Observations of the Glacier Terminus" by Ryan Cassotto
« Last Edit: December 26, 2014, 06:02:55 PM by A-Team »

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Re: what's new in Greenland ?
« Reply #26 on: December 27, 2014, 01:04:45 PM »
I collected all the Greenland and Arctic talks at AGU 2014 that had an attached pdf poster. Less than 1 in 10 offered anything beyond an abstract; whole poster sessions went by without a single poster posted! Worse, the .php formatting prevented advance google search from seeing the abstracts or finding the ePoster attachments; some talks had an attached graphic not listed anywhere.

This is about the lamest excuse for a scientific meeting I have ever come across, unless it be AGU 2013.

https://agu.confex.com/data/handout/agu/fm14/Paper_28053_handout_1869_0.pdf # Greenland # Saturated Crevasses along Shear Margins of Jakobshavn # A Ring # "temporal increase in lateral drag seems to indicate  long-­‐term stress loading of the shear margins as the ice  stream response to down stream mass perturbaAons at  the terminus. Differences in transects indicate that  regions where water-­‐filled crevasses are found the  magnitude of lateral drag is less and the rate of drag  increase is smaller than regions without water. Given  this, preliminary results suggest that water-­‐filled  crevasses are weakening the shear margins and likely  resulAng in enhanced stress loading in other parts of  the shear margins that are devoid of water."

https://agu.confex.com/data/handout/agu/fm14/Paper_10527_handout_506_0.pdf # Greenland # Perched Ice Layers In Shallow Firn Inhibit Percolation And Enhance Runoff # MJ MacFerrin # "mapping: some regions have formed multi-annual ice layers several meters thick in the top 20 meters of firn. When such layers grow thick enough to be impermeable, they prevent water from percolating to depth and cause surface runof. thick refrozen ice layers in the top 20 meters of firn in Greenland’s lower accumulation zone. •   When thick enough, these perched layers become impervious to further melt and quickly migrate the runoff line inland, even when pore space still exists below that could otherwise absorb water. If warming continues, these layers are expected to grow inland. Radar enables us to map and monitor these massive perched ice layers

https://agu.confex.com/data/handout/agu/fm14/Paper_24929_handout_1496_0.pdf # Greenland # Firn and percolation in high elevation supra-glacial lakes # Peña Howat # "assessment of percolation in  accumulation zone of western Greenland after the intense melt episodes of 2010 and 2012. Concentrations of ice has increased dramatically over the firn of the GIS, limiting the buffering capacity of the firn and facilitating meltwater transport and retention as ponds. The area covered by percolation features has expanded more than twice in 10 years, and melt intensity above 2000 m in 2012 was found to be an order of magnitude greater than at the end of the 20th Century... abrupt densification due to percolation processes a larger factor in altimetry–derived studies."

https://agu.confex.com/data/handout/agu/fm14/Paper_8948_handout_2231_0.pdf # Greenland # Disko Bay icebergs # J Scheick # "algorithm to delineate and extract quantitative information (location, geometry) about icebergs in optical satellite imagery. Future work will improve the cloud mask apply the algorithm to a long time series of Landsat images (1994-present), map changes in iceberg distributions with time (do export pathways remain constant or change depending on iceberg size?)"

https://agu.confex.com/data/handout/agu/fm14/Paper_11555_handout_734_0.pdf # Greenland # Rapid drainage of supraglacial lakes # S Adhikari # "Some Greenland Ice Sheet supraglacial lakes drain rapidly within the timescale of a few hours. The vertical discharge of water during these events may find a pre-­existing film of water potentially within a system of linked cavities. Here, we present a model for subglacial flooding in these circumstances."

https://agu.confex.com/data/handout/agu/fm14/Paper_17159_handout_1223_0.pdf # Greenland # Snow/firn density distribution on Devon Ice Cap, Canadian Arctic from airborne radar reflectometry # A Rutishauser # "Scattering and reflectivity distributions demonstrate the potential of using RES data to characterize the near-surface snow/firn properties. We hypothesize that the scattering values indicate a pseudo dry snow zone above ~1800 m asl, and the transition between the SI and glacier ice zone at ~1300 m asl. From the reflectivity values, we estimate the winter snow pack thicknesses, showing highest snow accumulation (~70-120 cm) in the southeast sector of DIC. The derived RSR dataset might help understanding the complex nature of the internal layering pattern. Especially the distribution of refrozen ice lenses/layers might explain complex internal layers resulting from a distorted snow/firn deposition pattern."

https://agu.confex.com/data/handout/agu/fm14/Paper_17824_handout_1269_0.pdf # Greenland # High frequency seismic waves recorded by the greenland ice sheet monitoring network (glisn) during the drainage of a supraglacial lake # EJ Orantes # "Supraglacial lake drainage is a major source of subglacial water under the Greenland Ice Sheet, impacting the ice dynamics at different temporal and spatial scales. Previous studies have shown that fast drainage of a supraglacial lake can produce high frequency seismic waves that are detected by local seismometers; however, little work has been done on the regional detection of such waves. Here we present the results of a study focusing on seismic data and their potential linkage to the drainage of a supraglacial lake (Lake Ponting) in the Paakitsoq region of the West Greenland ice sheet. The corrected seismograms show similar waveforms for arrivals on a single line supporting the idea that each line represents a traveling wave. The velocities derived from the trendlines are too low for the waves to be traveling through either the rock or the solid ice. Our current hypothesis is that they are traveling in a low-velocity channel of till underneath the ice. This would be consistent with the low attenuation required for the propagation of high frequency energy over regional distances."

https://agu.confex.com/data/handout/agu/fm14/Paper_19489_handout_1454_0.pdf # Greenland # Modeling of subaqueous melting in Petermann fjord, northwestern Greenland using an ocean general circulation model # C Cai # "Petermann Glacier drains approximate 6.1% (73,927k m2 of total 1,209,280 km2) of Greenland Ice Sheet. Basal melting of the floating tongue of Petermann Glacier is by far the largest process of mass ablation. Melting of the floating tongue is controlled by the buoyancy of the melt water plume, the pressure dependence of the melting point of sea ice, and the mixing of warm subsurface water with fresh buoyant subglacial discharge. In prior simulations of this melting process, the role of subglacial discharge has been neglected because in the Antarctic surface runoff is negligible; this is however not true in Greenland. In this work, we simulate the melting process of the ice shelf by MITgcm) at high resolution including outflow. We use varying oceanic thermal forcing and new bathymetry from Operation IceBridge.

The shape of the ice shelf cavity influences the ice shelf melt rate, especially in summer. ith the OIB-derived bathymetry, the melt rate is 34% higher in summer compared to winter. Between the 1990s and the 2010s, runoff increased by 20% and ice shelf melting should have increased by 16%. Between the 1990s and the 2010s, ocean temperature warmed by 0.5-0.9 C, the melt rate should have increased by 7%~20% . Taken together, these numbers may explain the recent break up of Petermann ice tongue, which is indicative of ice thinning. We will pursue this work in 3-D to include the assymetry in the bathymetry of Petermann Fjord.
"
https://agu.confex.com/data/handout/agu/fm14/Paper_24220_handout_713_0.pdf # Greenland # Predicting the stability of ice sheets with crevasses: a numerical experiment # Y Ma # "Iceberg calving accounts for ~50% of the mass lost from the Greenland Ice Sheet. Increased calving rates can lead to rapid sea level rise. Calving is not well parameterized in numerical ice sheet models. Water plays a negative role in the growth of crevasses and thus calving events, by both creating a more negative largest principle stress field and slowing down both the thinning process and flow speed of the glacier. The depth of a crevasse is inversly related to water depth but calving rate can be directly related to flow speed. The same buttressing effect applies to ice shelves, melange, etc. When water is absent, surface crevasses may reach the bed."

https://agu.confex.com/data/handout/agu/fm14/Paper_24717_handout_1434_0.pdf # Greenland # Improving estimates of cloud radiative forcing over Greenland # W Wang # "Larger uncertainty in SW, especially in Shelf and Coastal areas.  Positive (warm the surface). Negative (cool the surface): Shelf Average total forcing over both Inland: ~28 W/m2 ≈ 0.89 m snowmelt (liquid water) from May to Aug Low Clouds: liquid-only clouds High Clouds: mixed and ice clouds. Surface Ratidation Fluxes > Cloud Radiative Forcing because CRF bias is partially reduced when clear-sky fluxes are removed from the all-sky fluxes. No dataset is outstanding in all fluxes: CERES does a fairly good job in CRF. In Inland areas (warming area), low clouds warm the surface better than high clouds."

https://agu.confex.com/data/handout/agu/fm14/Paper_26038_handout_952_0.pdf # Greenland # Thermo-­mechanically coupled modeling of high elevation regions of the Greenland Ice Sheet # A Sommers # "Thermo-mechanical simulations were conducted on 54 flowlines in western and southeastern Greenland (shown on map) from the main divide past the 2,000 m elevation PARCA stake location. The mean absolute velocity difference between modeled and observed surface velocity at each PARCA stake is 21 m/a. Geothermal heat flux To examine the sensitivity of model results to the magnitude of prescribed geothermal heat flux, simulations were conducted with the geothermal heat flux increased or decreased. Surface velocity decreases in cases with lower geothermal heat flux (colder ice, lower viscosity). Quite surprisingly, surface velocity also decreases as the geothermal heat flux is increased for stake locations with temperate bed. The driving mechanism for this behavior may be that when the viscosity very near the bed is decreased, higher advection rates of cold ice from upstream result, producing colder temperatures and thus smaller velocity gradients in the slightly higher locations in the ice column. Bedrock elevation To examine the influence of uncertainty in bedrock elevation, the prescribed data (Bamber et al., 2013) were perturbed by adding Gaussian white noise (with standard deviation of 125 m) and smoothing using a moving average function to generate ‘similar but different’ bed profiles. Importance of thermo-mechanical coupling A substantial underestimation of surface velocities results from isothermal calculations (assuming ice temperature -5°C) with no enhancement to the flow law parameter for Wisconsin ice. The isothermal model tends to overpredict surface velocity when the enhancement factor is included."

https://agu.confex.com/agu/fm14/meetingapp.cgi#Paper/21416 # Greenland # Isochronal Ice Sheet Model: a new approach to tracer transport by explicitly tracing accumulation layers # A Born # "The long, high-resolution and largely undisturbed depositional record of polar ice sheets is one of the greatest resources in paleoclimate research. The vertical profile of isotopic and other geochemical tracers provides a full history of depositional and dynamical variations. Numerical simulations of this archive could afford great advances both in the interpretation of these tracers as well as to help improve ice sheet models themselves, as show successful implementations in oceanography and atmospheric dynamics. However, due to the slow advection velocities, tracer modeling in ice sheets is particularly prone to numerical diffusion, thwarting efforts that employ straightforward solutions. Previous attemps to circumvent this issue follow conceptually and computationally extensive approaches that augment traditional Eulerian models of ice flow with a semi-Lagrangian tracer scheme. Here, we propose a new vertical discretization for ice sheet models that eliminates numerical diffusion entirely. Vertical motion through the model mesh is avoided by mimicking the real-world ice flow as a thinning of underlying layers (see figure). A new layer is added to the surface at equidistant time intervals (isochronally). Therefore, each layer is uniquely identified with an age. Horizontal motion follows the shallow ice approximation using an implicit numerical scheme. Vertical diffusion of heat which is physically desirable is also solved implicitly. A simulation of a two-dimensional section through the Greenland ice sheet will be discussed."

A-Team

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Re: what's new in Greenland ?
« Reply #27 on: December 29, 2014, 03:10:44 PM »
ImGRAFT so far has gotten two anonymous reviews, one a bit cantankerous and the other constructive, which could lead to (needed) substantive improvements in the paper. http://www.the-cryosphere-discuss.net/8/6235/2014/tcd-8-6235-2014-discussion.html

... if the center of the displacement vector is used versus the vector tail (starting center point of the source sub-scene), here glaciers flow a kilometer between image pairs – enough so that the strain field traversed by the tracked features becomes important.

On errors, a single value of ±2 m/d is used, for the 16-day repeat pairs using the same path-row, a systematic error based on geo-location issues. However, adjacent (non- identical) path-rows were used, with time-separations varying quite a bit. The greater error with non-identical viewing geometry is mentioned, but without elaboration.

Error bars need to be shown in Figure 2 - and this will show that 2m/d error will blur quite a bit of the seasonal signal you appear to be mapping. Also show the seasonal variation relative to the merged mean Landsat 8 velocity or to the InSAR mean. But this will point out uncorrected errors in the Landsat 8 mapping. 

Figure 2: should really re-design Fig2 for Niog and Petermann – there’s no detail visible, and a lot of white space.
On Figure 1, are the centerlines correct? picking centerlines with some of the data having systematic errors in flow direction is risky. The centerline for Niog seems off?  it appears as though nunataks are sitting within the ice stream.

Results are like those already reported with the exception of the slightly greater flow speed for Jacobshavn (±2m/d makes this suspect). How about differencing the new map with the InSAR data -- it would reveal errors, but might reveal evolution.

Meanwhile, never mind this vector cross-correlation software, here is NASA printing out DEM slope vectors and drawing drainage divides by hand, then re-digitizing: http://icesat4.gsfc.nasa.gov/cryo_data/ant_grn_drainage_systems.php

Drainage divides were digitized using ArcGIS v 9.3. For the majority of the divides, the digitization was done from paper maps of the slope vectors via an Altek Datatab Pro Line puck digitizer. However, in regions where the slope vectors did not yield useful information, imagery was used as a guide. Vector maps showing the downslope maximum-gradient direction were generated from this DEM.

Drainage system divides were drawn on these maps, primary divides along major ridges, and secondary divides starting at points of interest, drawn upslope until meeting a primary divide or another secondary divide. The drainage systems and sub-systems include all basins and sub-basins in each.

The drainage system outlines used here are defined in WGS84 coordinates. This was ignored in this work, and the coordinates were treated as if they were Topex coordinates. The maximum difference between Topex and WGS84 latitude occurs at a latitude of 45° and is approximately 1.37 cm.

Laurent

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Re: what's new in Greenland ?
« Reply #28 on: December 30, 2014, 12:10:30 PM »

A-Team

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Re: what's new in Greenland ?
« Reply #29 on: December 30, 2014, 04:47:05 PM »
Models of Greenland Ice Melting Could Be Way Low


We've now covered these same two campus press releases a dozen times beginning on 15 Dec (top). Headlining Florida is odd as the climate model re-runs have not yet been performed and neither paper mentioned this topic. Ditto sled dog video.

NBC does add value with 3rd party interviews -- commenter T Scambos has been at this too for decades: "What Csatho's group has done," he said, "is truly admirable — marshaling a huge amount of data to reveal not just where, but when and how much, ice is being lost from every glacier system in Greenland."

Nukefix writes, Hmmm box-plots still for SMB, I think more research is needed to produce plots with mass-balance each year.

I chased down where this all stood back in 1998-2001. As it happens, B Csatho, the lead author of the PNAS paper above co-authored three earlier versions of Greenland surface mass balance:

Thickening of the western part of the Greenland Ice Sheet.
R Thomas, B. Csatho, S. Gogineni, K. Jezek, and K. Kuivinen.
Journal of Glaciology 44: 653-658 1998 http://www.igsoc.org:8080/journal/44/148/igs_journal_vol44_issue148_pg653-658.pdf

Mass Balance of the Greenland Ice Sheet at High Elevations
R. Thomas, T. Akins, B. Csatho, M. Fahnestock,  P. Gogineni et al
Science 21 July 2000: DOI: 10.1126/science.289.5478.426 (registration req'd)

Greenland Ice Sheet: High-Elevation Balance and Peripheral Thinning
W. Krabill et al
Science 21 July 2000 http://www.uio.no/studier/emner/matnat/geofag/GEO4420/h06/undervisningsmateriale/papers/Krabill.pdf

Mass balance of higher-elevation parts of the Greenland ice sheet
R. Thomas, B. Csatho, C. Davis, C. Kim, W. Krabill, S. Manizade, J. McConnell  and J. Sonntag
JGR 106 707-33,716 2001 http://onlinelibrary.wiley.com/doi/10.1029/2001JD900033/pdf

Espen

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Re: what's new in Greenland ?
« Reply #30 on: December 30, 2014, 05:07:35 PM »
As I think I have mentioned some times before, I believe more interest, for reasons, is now centering around the situation at Zachariae (Shfaqat A. Khan from DTU Space) , where there is big changes ahead this map below (from the new studies) shows how big the catchment area is compared to other more prominent places:
« Last Edit: December 30, 2014, 05:15:01 PM by Espen »
Have a ice day!

Laurent

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Re: what's new in Greenland ?
« Reply #31 on: December 30, 2014, 05:30:01 PM »
Hello Espen,

What did you say is the flow of ice of zachariae compare to Jakobshavn ?

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Re: what's new in Greenland ?
« Reply #32 on: December 31, 2014, 10:11:58 AM »
Hello Laurent,

I have mentioned the great changes seen at Zachariae since the early 2000s, and I expect far more to come in the next few years, probably a retreat of 25 - 50 km from the current position:

https://forum.arctic-sea-ice.net/index.php/topic,400.msg14600.html#msg14600

https://forum.arctic-sea-ice.net/index.php/topic,400.msg11377.html#msg11377
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Re: what's new in Greenland ?
« Reply #33 on: December 31, 2014, 11:00:18 AM »
It's a good idea not to mix up the overall Zachariae basin with NEGIS, the Northeast Greenland Ice Stream that, today, provides almost all the ice passing through the flux gate near Zachariae's calving front.

NEGIS is the only feature of its type on Greenland. It is narrow, fast-moving and completely lacking in contributing tributaries due to its sharp shear walls, often attributed to an elevated geothermal gradient in the bedrock below the summit ridge. Three important field studies on upper NEGIS were published this summer -- those results have been reviewed multiple times on other forums.

The other 85% of the iceshed draining to Nioghalvfjerdsfjorden or Zachariae may be waking up as well but at this point is still in slow motion and not contributing much quantitatively to ice discharge.

Even though 79N seems like too far north to play an early role in Greenland's contribution to sea level rise, two other factors come into play: warming of the Fram Strait seas (which may have more dramatic climatic consequences than more southerly Baffin Bay) and markedly less net snow accumulation NE of the summit ridge (which means more pronounced ice sheet thinning and so faster discharge). So I'm on the same page with Espen in terms of this area's importance.

However to predict the near-term future with any degree of reliability, we need to have a better factual understanding of basal and englacial conditions under this sector of Greenland. Until that arrives, it's worth looking at various models (and their conflicts with received wisdom).

Figures below adapted from a 2012 Aschwanden paper (http://www.iac.ethz.ch/doc/publications/andy3.pdf) treating latent heat in temperate ice ('enthalpy') are used in the blockbuster AGU 2014 ePoster of Sommers above that provides the first 3D model of englacial temperature isotherms above the PARCA flux gate perimeter. Sommers also had access to the LAYERMAP paper, J MacGregor et al. (in revision, not yet online), Radiostratigraphy and age structure of the Greenland Ice Sheet, J. Geophys. Res. Earth Surf.

This will be a game-changer to have a 3D coordinate system of isotherms, isochrons and slope/elevation/thickness ... 2015 will be the year of foliations, principal curvatures and moving Darboux frames.

Espen

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Re: what's new in Greenland ?
« Reply #34 on: December 31, 2014, 05:07:55 PM »
A-Team,

I completely agree with you conclusion, however, my gut feeling tells me something big is on ( not very scientific though) :

"However to predict the near-term future with any degree of reliability, we need to have a better factual understanding of basal and englacial conditions under this sector of Greenland. Until that arrives, it's worth looking at various models (and their conflicts with received wisdom)." 

Happy New Year from Berlin! ;)
Have a ice day!

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Re: what's new in Greenland ?
« Reply #35 on: December 31, 2014, 06:26:04 PM »
however, my gut feeling tells me something big is on -- though it might just be those Jägermeisters
Below is a crumpled handout that supposedly a janitor picked up after a secretive closed-door AGU14 session that predicts something big going on at Zachariae ... can you make out what it is saying???

Espen

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Re: what's new in Greenland ?
« Reply #36 on: January 01, 2015, 01:11:08 AM »
Yeah right, that's how I read it,

Espen reporting from Berlin, just came from a +1 million party, now waiting for the hang-over to arrive ???
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Re: what's new in Greenland ?
« Reply #37 on: January 01, 2015, 07:00:19 PM »
Greenland in 3D has arrived! At least for the west central Greenland. Soon we will be able to fly through the interior enjoying the temperature display, ice velocities, colored markups of the slush, and times of deposition (thanks to a mashup of Sommers modelling and MacGregor's stratigraphy) at least for the area above the 2000 m Parca stake gate.

So far only 5 of the Parca transects have been released. They, like Cresis radargrams, come packed in a fixed sized box which forces them all to have different scales both horizontally and vertically. I resized three of them to a constant physical scale suitable for our around-the-island tour of the interior. Only 5 of the 161 slices have been released to date.

The first image finds the radar flight lines that best approximate the Parca flow lines. The second is from Sommers AGU 2014 ePoster, and the third is a fix for that. I do not yet have access to the kml files for the ice flow lines terminating at the Parca ice stakes (still offline and unpublished, 18 years and counting); their length and elevations are needed to script rescaling.

While 'just' a model, it will provide us with a concrete picture of englacial conditions. I myself would not have chosen te 2005 geothermal heat model since a suite of more recent (see above) only means more operator-unattended runs. Ditto for only using the first of the Aschwanden diagrams and skimping on basal sliding regime.

They do bring in a very significant result from MacGregor's forthcoming radar stratigraphy paper, the scalar constant in Glen's Law (enhancement factor) of 3 being said appropriate to softer pre-11 kyr ice (not to be confused with the 3 taken by convention in the exponent).

They did not simply accept the new bedrock DEM as if it were written in stone but instead looked at model sensitivity to bedrock re-sampling (adding Gaussian white noise of appropriated radius to generate equally probable bed profiles).

The 2000 m perimeter flux gate is a very good idea because it isolates the mostly orderly behavior of the main body of the ice sheet from the case-by-case situations at the ocean margins. As mentioned earlier, B Csatho hosts csv files tantamount to the Paca stake kml polygon, whose interior area then follows from an online GoogE auxillary tool and whose ice volume results from masking the thickness DEM and toting up with the ImageJ2 volume plugin.
« Last Edit: January 01, 2015, 09:05:23 PM by A-Team »

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Re: what's new in Greenland ?
« Reply #38 on: January 02, 2015, 02:57:23 PM »
During the 21 years of pursuit of bedrock topography, surprisingly very few flight lines followed either the Parca perimeter polygon or flowlines ending to the Parca stakes (which were placed in the ice in 1995 or so and their movement measured a year later).

However 5 segments beginning with Cresis 20130409_01_052 do follow Parca #foo7 up to the summit divide. Recall #foo7 was one of 4 examples provided in the Sommers ePoster above.

I rescaled the radar stratigraphy imagery to a depth of 2500 m which suffices to catch bedrock the entire way, then rescaled to match the temperature prediction for #foo7 (which did not go to bedrock). The image below shows, rather provisionally, the temperature isotherms overlying the stratigraphic isochrons. Ice velocity isotachs could be done as well.

All of these will need interpolating between the 161 flow lines to actually build the 3D englacial model showing these intersecting surfaces (whose principal curvatures tell almost the whole story). The only intermediate data consists of measured surface elevation and velocity, ice thickness, and the sometimes sparse radar tracks. However, after carving out upheavals, NEGIS and the SE margin, the ice above the 2000 m contour is fairly well behaved.

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Re: what's new in Greenland ?
« Reply #39 on: January 03, 2015, 02:50:33 PM »
I located a 5th Parca flowline output graphic on the Boulder website of the ePosters. The Parca stake wasn't labelled but stated to be on west Greenland. I measured the elevation at the stake from their graphic (which is reversed relative to the others); by sorting B Csatho's stake elevation database, the mystery stake turned out to be wpo7, way to the north.

It would be a good idea to re-name the stakes more systematically and stick with upper or lower case; 'wp' -- used for 31 stakes in 2.5 drainages -- was a poor choice because kml file readers mistake it for way point which it probably meant originally.

I rescaled horizontally and vertically it to fit the west side Parca graphics above. The flow line is the longest of the set, over 400 km to the summit ridge. Distortion of the letters gives some sense of the rescalings.

 
Temperature and velocity profiles inferred by thermal flowline modeling for high elevation regions of the Greenland Ice Sheet
AN Sommers, H Rajaram,  WT Colgan
http://hydrosciences.colorado.edu/symposium/abstract_details.php?abstract_id=7

...Most recent changes in the surface mass balance and ice dynamics of the Greenland ice sheet have been restricted to elevations below 2,000m. Substantial computational efficiency can be gained by limiting numerical modeling efforts to this lower elevation periphery, where changes in ice sheet form and flow are most pronounced, rather than modeling the entire ice sheet from the main divide to the margin. Accurately modeling the lower elevations with this approach is dependent on prescribing accurate velocity and temperature profiles at the upstream boundary... Without corresponding velocity and temperature profiles, however, these data alone are insufficient to serve as upstream boundary conditions for lower elevation thermo-mechanical modeling.

Using a two-dimensional, enthalpy-based thermal flowline model, we generate velocity and temperature profiles across the ice sheet depth at the PARCA stake locations. While prescribing ice surface and bedrock elevation, observed surface velocities at the stake locations and the ice discharge calculated from surface mass balance serve as modeling targets. We employ an iterative procedure between mechanical and thermal calculations; ice velocities found by solving the momentum equation (via the Shallow Ice Approximation, which is valid for these high-elevation domains) inform the energy equation to solve for temperature and liquid water content, which then inform the velocity calculations, and so on until convergence.

Preliminary results suggest that observed surface velocities in some regions of Greenland can only be reproduced with a temperate bed at high elevations...

A-Team

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Re: what's new in Greenland ?
« Reply #40 on: January 05, 2015, 01:58:18 PM »
The graphic below shows that the Parca stake boundary at ~2000 m served as a Cresis ice penetrating radar flight line in multiple years. This means a continuous radar 'curtain' could be constructed from the surface down to bedrock around the entire perimeter.

Unfortunately flights up the flow lines through the flux gate stations were never a specific goal and are rarely available as noted above, that stratigraphy can be filled in by interpolation from nearby (oblique) tracks.

Jakobshavn is an especially favorable case because of gridded 2008 flight coverage including the Parka flux gates and continuing ~200 km up towards the summit. Some of these could be 'held back' to test the accuracy of interpolation schemes. However this is a plain vanilla layer cake region of the ice sheet so would not validate the procedure for more complex sites with upheaval features.
« Last Edit: January 05, 2015, 02:15:53 PM by A-Team »

A-Team

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Re: what's new in Greenland ?
« Reply #41 on: January 06, 2015, 02:04:53 PM »
I located a set of Cresis ice penetrating radar scenes that furnish a curtain around the whole island at the 2000 m Parca stake perimeter. Actually, as shown below, that perimeter was only nominally at 2000 m -- it varied from 1637 to 2878 averaging 2164 m (sd dev 358), I suppose to avoid crevasses and exposed rock.

These are not so easy to combine into a single image as they come from different years and are of vastly different scales and resolution -- not to mention gigantic image dimensions if all tiled up together. Perhaps LAYERMAP will provide a convenient way of this in terms of lines of dated stratigraphy.

At the time the Parca stakes were placed, there was not comprehensive information available on ice thickness or depth to bedrock. Today, we might choose the perimeter along a contour of equal driving stress, which amounts to thickness and density profile of the ice adjusted for buttressing slope.

Everything being the same, it will take quite a few iso's from geology, geometry, meteorology, glaciology, and related fields to describe the various substructures of englacial Greenland. These include isobar, isobathytherm, isocaloric, isocheimal, isochor, isochron, isoclinal, isocontour, isocurvature, isodensity, isodiabatic, isodiffusivity, isodrosotherm, isoelastic, isoelectric, isoenergetic, isoenthalpic, isogeotherm, isohel, isohyet, isointensity, isokinetic, isomagnetic, isometric, isopach, isophot, isopleth, isopotential, isopycnal, isospectral, isostasy, isostress, isosurface, isotach, isothere, isotherm, isovelocity, isoviscous, isovolumetric.

Here are the specific scenes needed from Cresis for the Parca curtain: it is easy to build out the urls from https://data.cresis.ku.edu/data/rds

20100513_04_004_2echo_picks.jpg   19970521_01_024_2echo_picks.jpg   19950524_01_003_2echo_picks.jpg
20100513_04_005_2echo_picks.jpg   19970521_01_025_2echo_picks.jpg   20060609_01_008_2echo_picks.jpg
20100513_04_006_2echo_picks.jpg   19970523_01_006_2echo_picks.jpg   20060609_01_007_2echo_picks.jpg
20100513_04_007_2echo_picks.jpg   19970523_01_007_2echo_picks.jpg   20060609_01_006_2echo_picks.jpg
20100513_04_008_2echo_picks.jpg   19970523_01_008_2echo_picks.jpg   20060609_01_005_2echo_picks.jpg
20100513_04_001_2echo_picks.jpg   19970523_01_009_2echo_picks.jpg   20060602_01_003_2echo_picks.jpg
19970511_01_012_2echo_picks.jpg   19970523_01_010_2echo_picks.jpg   20060602_01_002_2echo_picks.jpg
19970511_01_011_2echo_picks.jpg   19970523_01_011_2echo_picks.jpg   20060602_01_001_2echo_picks.jpg
19970511_01_010_2echo_picks.jpg   19970523_01_012_2echo_picks.jpg   20060529_01_003_2echo_picks.jpg
19970511_01_009_2echo_picks.jpg   19970523_01_013_2echo_picks.jpg   20060529_01_002_2echo_picks.jpg
19970511_01_008_2echo_picks.jpg   19970523_01_014_2echo_picks.jpg   20060528_01_006_2echo_picks.jpg
19970511_01_007_2echo_picks.jpg   19970523_01_015_2echo_picks.jpg   20060528_01_007_2echo_picks.jpg
19970511_01_006_2echo_picks.jpg   19950524_01_009_2echo_picks.jpg   20060528_01_008_2echo_picks.jpg
19970511_01_005_2echo_picks.jpg   19950524_01_008_2echo_picks.jpg   20080715_04_014_2echo_picks.jpg
19970521_01_023_2echo_picks.jpg   19950524_01_007_2echo_picks.jpg   20080715_04_013_2echo_picks.jpg
19970521_01_022_2echo_picks.jpg   19950524_01_006_2echo_picks.jpg   20080715_04_012_2echo_picks.jpg
19970521_01_021_2echo_picks.jpg   19950524_01_005_2echo_picks.jpg   20080715_04_011_2echo_picks.jpg
19970521_01_020_2echo_picks.jpg   19950524_01_004_2echo_picks.jpg

Lennart van der Linde

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Re: what's new in Greenland ?
« Reply #42 on: January 06, 2015, 11:36:37 PM »
Applegate et al 2014:
http://link.springer.com/article/10.1007/s00382-014-2451-7

Abstract
"Damages from sea level rise, as well as strategies to manage the associated risk, hinge critically on the time scale and eventual magnitude of sea level rise. Satellite observations and paleo-data suggest that the Greenland Ice Sheet (GIS) loses mass in response to increased temperatures, and may thus contribute substantially to sea level rise as anthropogenic climate change progresses. The time scale of GIS mass loss and sea level rise are deeply uncertain, and are often assumed to be constant. However, previous ice sheet modeling studies have shown that the time scale of GIS response likely decreases strongly with increasing temperature anomaly. Here, we map the relationship between temperature anomaly and the time scale of GIS response, by perturbing a calibrated, three-dimensional model of GIS behavior. Additional simulations with a profile, higher-order, ice sheet model yield time scales that are broadly consistent with those obtained using the three-dimensional model, and shed light on the feedbacks in the ice sheet system that cause the time scale shortening. Semi-empirical modeling studies that assume a constant time scale of sea level adjustment, and are calibrated to small preanthropogenic temperature and sea level changes, may underestimate future sea level rise. Our analysis suggests that the benefits of reducing greenhouse gas emissions, in terms of avoided sea level rise from the GIS, may be greatest if emissions reductions begin before large temperature increases have been realized. Reducing anthropogenic climate change may also allow more time for design and deployment of risk management strategies by slowing sea level contributions from the GIS."

Supplementary material:
http://link.springer.com/content/esm/art:10.1007/s00382-014-2451-7/file/MediaObjects/382_2014_2451_MOESM1_ESM.pdf

GIS could be gone in three centuries, it seems, if we keep forcing it hard enough.
« Last Edit: January 06, 2015, 11:49:03 PM by Lennart van der Linde »

A-Team

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Re: what's new in Greenland ?
« Reply #43 on: January 07, 2015, 02:23:03 PM »
The authors have chosen to bury their publication behind the largest paywall I have ever seen, declining the open access option. Instead of doing science, grant money is going to this. Very few universities can subscribe to this journal -- and a personal subscription is unheard of. Has anyone in the history of the universe ever paid $39.95 for an itty-bitty pdf?

The article seems directed to policymakers but these people won't have access at all, much less read about scenarios 300 years in the future, much less take action based on what somebody says is revealed by many runs of complex model code.

For blog suitability (everyone has google abstract alerts), readers here need to not only have access but some ability to replicate the results locally (doable on a desktop) or at least look under the hood at the long list of underlying assumptions and simplifications. And I don't mean commented code with that.

The question to ask about these sweeping models of Greenland is: what exactly do they descend to, in terms of predictions or assumption, about particular glaciers in coming years? I don't doubt that Greenland will melt out on the course the planet is on but where are the testable predictions here? That means next 5-10 years, not 300.

Greenland cannot be taken as one big ice sheet -- it can only be understood as the sum of individual glacial parts. And these have very heterogeneous behavior indeed, for example very little seasonal synchronicity. Englacial melt lakes and re-frozen blobs in the firn -- good luck modelling the consequences of that even this coming year.

We've been struggling mightily, even with great access to Landsat8 and Sentinel data, to understand what happened to Jakobshavn, Zachariae, Petermann etc in summer 2014. How could these guys go out to 2314? Could I actually learn how much Jakobshavn will accelerate even in 2015 from taking apart their models?  I sincerely doubt it.

What happens three years from now at Jakobshavn when the calving front retreats past the Big Bend resistance and then the entrance of the first sub-stream? Do they incorporate the speed-up in their 300 year model? I sincerely doubt it.

Let's look at an open source article published the same day concerning the Store Glacier. It's very well-written and provides clear explanations, diagrams and discussion of the future consequences of the one-off (aren't they all) geometry of the Store sill, stoss-side stress, buttressing, throttling and ice convergence.

J Todd and P Christoffersen
http://www.the-cryosphere.net/8/2353/2014/tc-8-2353-2014.pdf
Are seasonal calving dynamics forced by buttressing from ice mélange or undercutting by melting?

I've always wondered about sub-stream convergence at Jakobshavn (that is, wondered why it was never discussed). The fast branch is only contributing a third of the calving front width. How are the two main side streams brought up to a common speed where they come in, does not this generate a truly massive amount of frictional heat, how is the extra volume accommodated given steep bedrock variance underneath and incompressibility of ice without giving rise to an overt surface feature?

The authors wonder too about this absence of prior discussion and respond with a Stokes model term to amend it in section 3.3:
This convergence term represents an important 3-D effect, ensures that mass balance is maintained throughout the model domain, and allows for realistic evolution of mass and momentum near the terminus. We note that this prescribed flux convergence differs from implementation of flow convergence in earlier work with flow line models, where the additional mass is added as an input to the surface mass balance. Although the latter will result in correct flux, it neglects the direct effect of the additional flux on the velocity field and may consequently underestimate velocity change while overestimating elevation change.

« Last Edit: January 07, 2015, 02:37:19 PM by A-Team »

sidd

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Re: what's new in Greenland ?
« Reply #44 on: January 07, 2015, 09:34:36 PM »
The mechanism is interesting, i speculated a few years ago that steepening slopes would increase driving stress, the authors seem to agree.

Applegate(2014):
" ... small to moderate increases in surface temperatures result in a progressive and accelerating surface lowering near the ice sheet margins (Born and Nisancioglu 2012), which steepens ice surface slopes near the margin of the ice sheet. This steepening increases the driving stress, which speeds ice flow into the ablation zone and generates a wave of thinning that propagates toward the central parts of the ice sheet (Huybrechts and de Wolde 1999; Parizek and Alley 2004)."

I enclose a copy of Fig 2a, showing the decrease in e-folding time with global average temperature increase.

For those who want a copy call or email the corresponding author: patrick.applegate@psu.edu
at Penn State. Or call Alley, he is on the paper, and quite responsive.


Lennart van der Linde

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Re: what's new in Greenland ?
« Reply #45 on: January 08, 2015, 05:41:46 PM »
Applegate et al 2014 assumes a polar amplification factor of 1.5 for Greenland, with a range up to about 2. See their fig.2 attached below.

But could this factor not be significantly higher? Say up to 3 or even 4? See for example fig.3a in Masson-Delmotte et al 2006 (also attached below):
http://pubs.giss.nasa.gov/docs/2006/2006_MassonDelmotte_etal_1.pdf

Or would that imply lower global mean warming? Does anyone have more (recent) info about potential polar amplification factors?

Lennart van der Linde

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Re: what's new in Greenland ?
« Reply #46 on: January 08, 2015, 08:29:48 PM »
Just wondering if for example Brigham-Grette et al 2013 could be reason to assume the possibility of an amplification factor of maybe 2.5 to 3:
http://www.researchgate.net/profile/Volker_Wennrich/publication/236872407_Science-2013-Brigham-Grette-science.1233137/links/00b7d519c6bd6f3c93000000

Maybe not, but if so, then even 3-4C global warming could possibly cause very rapid deglaciation of GIS, it seems.

From a risk perspective we should maybe need to take a risk into account of almost full deglaciation within 5-7 centuries in such a case? Average discharge could then be 1-1.4m per century, with peak discharge maybe up to 1.5-2 meters in a century?

I speculate that the first 1-2m SLR-contribution from GIS could be discharged at faster speed than the next 5-6m, because the forcing would be strongest in the beginning and the most vulnerable, marine based ice would go first, together with the most vulnerable non-marine based ice.

So peak discharge over a century, say from 2100-2200, could be some more than the average over full deglaciation, I suppose.

Or would that not be a plausible line of reasoning?

sidd

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Re: what's new in Greenland ?
« Reply #47 on: January 08, 2015, 08:34:00 PM »
Polar Amplification from IPCC AR5 Fig 12.41 attached. 2 looks better than 1.5 for Greenland

sidd

sidd

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Re: what's new in Greenland ?
« Reply #48 on: January 08, 2015, 08:47:48 PM »
In the previous post, I should have written, 2 looks better for the zonal mean and 1.5 for Greenland. Personally, I think both those numbers are optimistic. The projected amplification is already huge for the peripheral ice shelf northwest of GRIS

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AbruptSLR

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Re: what's new in Greenland ?
« Reply #49 on: January 09, 2015, 01:48:45 AM »
The link leads to an article about GISS measured global temperatures that includes the attached 2008-2012 temperature change map that shows measured Arctic Amplification around Greenland.

http://www.giss.nasa.gov/research/news/20130115/
« Last Edit: January 09, 2015, 04:12:18 PM by AbruptSLR »
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