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A-Team

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #200 on: June 17, 2015, 02:09:22 PM »
Below is a 10 m resolution Sentinel radar image from 27 May 15 shown in false color and reduced resolution. At the original size, the image was very speckled; I could not find any way of usefully sharpening ground features (never the design intent). So it sees through clouds which is helpful but is not suitable for up-to-the minute rift tip monitoring.

Nukefix has a better way of processing using Sentinel toolbox software, step-by-step at link below. I've found the software easy to install, stable in operation, and having advanced commands for processing this type of imagery. The most useful thing for us will be interferometric velocity measurements but that entails a much higher level of processing sophistication.

http://forum.arctic-sea-ice.net/index.php/topic,154.msg54200.html#msg54200

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #201 on: June 18, 2015, 04:08:19 PM »
 Looking to see if the melt this year is more/less extensive than in the past, there were cloud-free Landsat-8 scenes  for June 15th for 2015 and 2014 and for June 10th of 2013. The path,rows do not match very well, so it did not prove feasible to align perfectly to the walls of the fjord (not to mention the 1.2 km yearly advance of the ice shelf).

Overall, surface melt water (ponds and drainage channels) is very sensitive to date. The peak date can occur mid to late June because the water can later work its way down through crevasses. Here the 2013 hardly shows any melt whereas 2014 and 2015 are quite similar.

The same specific ponds re-occur to a large extent. These are distributed systematically with respect to large scale longitudinal flow-line features, more on that below.

LC80 43 001 2013 163 LGN00
LC80 41 001 2014 168 LGN00
LC80 36 248 2015 168 LGN00

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #202 on: June 18, 2015, 05:17:13 PM »
The Petermann ice shelf has a number of fascinating surface features that provide a window to the forces acting on it. In part these are driven by rigid but somewhat deformable ice trying to flow through a narrowing, northerly curving funnel-shaped fjord (as described earlier by Espen).

The images below show the northwest corner by Faith Glacier. The first is just band 8 of the 15 Jun 15 Landsat-8; the second overlays a crude regional feature classification; the third provides some measure of objective support for that via a Canny edge filter.

Here the wall curvature and compressive forces from tributary glaciers across the fjord could give rise to elastic compression resulting in small amplitude longitudinal waves in the ice (to be contrasted with brittle fracture of tributary ice along the opposite fjord wall from Porsild to Belgrave).

These frozen waves hug the walls for a long ways but nearer the calving front fan out towards the center line. The visibility of these features is greatly enhanced by melt water collecting in the wave troughs. In fact ridging is very subtle and not seen in the late afternoon illumination of the 11 August 14 ultra high resolution WorldView image.

The third zone consists of a broad flowline feature defined by oblique shorter wavelength herringbone that extends back beyond the hinge line many km upstream. Note the wave pattern is almost 45º to the flow line but arcs around to perpendicular more towards the center (where it could even be split into a fourth zone of classification).

There are quite a few candidates for the underlying physical process: differential velocity across the shelf (fastest in the center), compensation for channels under the shelf hollowed out by circulating sea water, or a 'frozen rapid' caused by flow over a bedrock sill or bulge far upstream that is not indicative of forces acting locally far downstream.

« Last Edit: June 19, 2015, 01:36:46 AM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #203 on: June 19, 2015, 05:59:33 PM »
The animation below compares two Landsat-8 images of different years but the same path,row and approximate dates, namely  2015 153 43 001 and 2013 163 43 001 The images are rotated to put the fjord approximately horizontal and aligned along the central drainage (thus fjord walls move as the ice sheet advances).

The very fact that the left part of the images (the northwestern half of the fjord in Landsat view) are almost perfectly alignable after two full years means there is no velocity gradient across the ice shelf -- it is moving as a block other than for rifts and along the compression ridges along the rocks.

This contradicts a recent determination of velocities by a very experienced radar interferometry group (2nd image). This is troubling to say the least because of the number of glaciers where we are trusting the latter type of analysis. http://web.stanford.edu/group/radar/group.html

An earlier synoptic scale measurement of Petermann does not provide sufficient resolution in the channel. I contoured this with the G'mic online tool. (21st-Century Evolution of Greenland Outlet Glacier Velocities, T. Moon et al DOI: 10.1126/science.1219985)

The right part of the animations shows a very significant velocity drop across Sigurd Berg tributary line, along with some block rotation and deformation, in somewhat better agreement though not in details.

Of course, there is nothing quite like some GPS stations drilled into fixed positions in the ice by AP Ahlstrøm et al which cover up to July 2012. Note the ice sheet has to be thickening somewhat for the velocity to decrease between the lower two stations. http://www.earth-syst-sci-data.net/5/277/2013/essd-5-277-2013.pdf

Click to animate. The image has to be rather wide to take in an entire transect block of the fjord near the calving front  at 15 m resolution.
« Last Edit: June 19, 2015, 07:45:25 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #204 on: June 20, 2015, 04:50:15 PM »
The animation below, taken from the one above, shows motions in the Petermann ice shelf relative to the calving front and central drainage channel held fixed. The 2013 image has blue infill of morraine, the 2015 pink. Large displacements correspond to lagging absolute motion. The region with yellow inflow illustrates the gradient of motion -- it keeps up better with the central ice shelf. Note too deformation and rotation of some of the tributary blocks.

What should you look for in the animation:

(1) nothing is happening on the left side, that says even though the images were taken 740 days apart they can still be shifted into pixel-perfect alignment. That is only possible when the geometry of the central ice shelf is moving as a rigid block all the way to the calving front (barely visible dip in center).

(2) towards the center of the animation, to the right of the secondary drainage channel, there is a suggestion of the ice not quite keeping up with the main block which is moving at ~3 m per day. There could be a contribution from satellite viewing geometry to this even though the path,rows match and the nadirs flown differ only slightly. However the slippage is only a pixel or two which at 15 m Landsat resolution amounts to ~30 m out of a possible 3*740 = 2220 m so only a bit more than 1%.

(3) the real interest is nearer the fjord wall. Here the relative motion in the ice flows contributed by the Porsild and Sigurd Berg glaciers upstream shows dramatic slowing and even rotational distortion. These flows were once marked by clean lines of morraine grit but those has been block-faulted and shifted over time, a process that was just barely underway in year 2000 imagery (to be posted later).

(4) Along the fjord wall, especially in small embayments, the ice is stationary. Because the coordinate frame co-moves with the central block, ice that co-moves with the fjord walls (ie rapidly) is actually not moving at all in the fixed Greenland rock reference frame. Confusing? Well, it's even more confusing doing it the other way round.
« Last Edit: June 21, 2015, 03:15:28 AM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #205 on: June 21, 2015, 06:27:10 PM »
Below I tried to figure out a puzzling feature up-glacier on Petermann. The region is about 21 km southeast of the last rock on northwest channel (unnamed, shaped like an Native American arrowhead). It shows up in all Landsat-8 bands as well as that precious interferometric image of Petermann made in Radarsat2 emulation.

The later image was discussed here a year ago at #86 -#97. However coming upon an important image like this, it is imperative to reverse-search it by all three tools to find the highest available resolution, as explained over on Developers Corner / Re: Getting hi-res Landsat Images. This will also uncover all previous scientific uses of the image. In this case, an image with much better resolution of the fringes, 1484 x 1716 pixels, was available along with other useful goodies.

Note as the glacier passes over a bedrock mound or trough, the velocity field obtains a slight upward component which otherwise follows the glacier surface. This can show up in the insar line-of-sight motion as an area of wider/tigher fringes that could be misinterpreted as slowing/speeding (which would cause havoc with glacier continuum mechanics, not observed).

http://forum.arctic-sea-ice.net/index.php/topic,638.msg54112.html#msg54112
http://www.esa.int/spaceinimages/Images/2014/03/Petermann_on_the_move click on hi-res link
https://earth.esa.int/documents/10174/134665/S1-TN-ESA-SY-0452_RSAT_TOPS_InSAR_Scenes_Report expert technical note
http://rs.tudelft.nl/~rlindenbergh/education/Thesis_Lennert_van_den_Berg.pdf U Delft van den Berg MS thesis Chap.4

The feature under investigation has a inexplicable wave pattern to its south and leaves an peculiar ice track for many km downstream of its head. By great good fortune, there was an east-west Cresis radar track right through the middle of it, 20100324_01_036, which shows ice thickness, an upheaval feature, and unusual flares reaching the surface.

This is shown as an overlay along with coordinates and elevation transects provided by Google Earth which establish that the feature is a elevated surface bump within a depression lying at the base of the descending ridge separating Petermann and Humboldt and the overall descending ice sheet. Note two seasonal melt channels curve around the periphery (ie do not descend into it — it is a bump.)

Overall, an oblique airplane photograph would be very beneficial here. Alternatively, the hill-shaded DEMs being made with 0.5 m WorldView imagery are photorealistic but not yet available for northwest Greenland. The inset shows Russell Glacier at just half the available resolution.
« Last Edit: June 21, 2015, 06:36:40 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #206 on: June 21, 2015, 11:58:13 PM »
This contradicts a recent determination of velocities by a very experienced radar interferometry group (2nd image). This is troubling to say the least because of the number of glaciers where we are trusting the latter type of analysis. http://web.stanford.edu/group/radar/group.html
Perhaps they used InSAR to derive only the range-component of velocity?

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #207 on: June 22, 2015, 03:37:17 PM »
Quote
Perhaps they used InSAR to derive only the range-component of velocity?
No, this was quite the fancy study ... of Ryder Glacier to the north. The Petermann image is not in the dissertation and i  not described further on their web site.

Here is a more recent study that seems to better capture the horizontal velocity gradient seen on Landsat today. I've added the 2 km width checkerboard to their distance tape ... as Espen notes, the narrowing and turning of the channel has to have some effect on the ice sheet. The image does not have sufficient resolution to really capture what is going on with tributary blocks.

That paper concluded that the velocity response of Petermann to unbuttressing from large calving events was minimal (free full http://ftp.vub.ac.be/~fpattyn/papers/Nick2012_JGLAC.pdf).

I find these muddy colors make for an ineffectual display (and one that cannot be inverted to recover the original x,y data grid). It might be better to contour. Another option is to animate the cross-sectional velocity graph, along the lines of MacGregor's animation of Greenland ice thickness by age (1:36 into https://svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=4249).

The second image below is from "Unprecedented Retreat in a 50-Year Observational Record for Petermann Glacier, North Greenland"  by OM  Johannessen et al (free full http://159.226.119.58/aosl/CN/article/downloadArticleFile.do?attachType=PDF&id=383). It shows retreat history over a highly distorted ERS-1 SAR base map. 

They also determined a velocity history of Petermann, concluding no statistically significant trend exists for 1993-2011. However 2011-2015 are seeing consistent reports on the high end, pushing 1400 m/yr in July (see #203 GPS graphic). It's quite difficult to get consistent (ie 1-15 July) remote sensing imagery over a long time frame.

Petermann velocity m/yr
1993  1030
1994   960
1995   930
1996  1075
1997  1110
1998  1070
1999  1175
2000   965
2001  1095
2002  1045
2003  1100
2004  1050
2005   985
2006  1250
2007  1000
2008  1040
2009  1050
2010  1100
2011  1175


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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #208 on: June 22, 2015, 04:18:27 PM »
Below is the 20 Jun 15 sequel to #199, adding five days to 'is the east side fracture tip growing?' While the answer in my view is yes, there is nothing dramatic to see here, move along. The confirmable portion of the tip has perhaps advanced 39 pixels in the 5 m image below or 200 m (or 40 m/day); widening rather than extending per se is what makes rift tips initially perceptable at our resolution.

This fracture is a more likely origin of the next significant tabular iceberg than the other growing tip midway down the NW side. The three reasons for this: the history of fracturing at Petermann, stress distributional relief (the NE fracture has a lower pair but that has frozen up, the NW fracture is but 3rd in a series), and the velocity disparity necessary for rifting is more pronounced on the NE.

Petermann rides too low in the water to have a Jakobshavn-type calving front. According to thickness and assumption of hydrostatic equilibrium, it has ~7 m of freeboard in some average sense but look below as researchers on the Arctic Sunrise step ashore to find an ice bollard for their hawser.

The third image added below, an oblique shot of Belgrave, Hubert and the calving front, is an IceBridge plane shot from March 2010. It is a beauty -- click to open to full size. Note how tributary melange bulges up from the ice sheet -- that is not so apparent from Landsat view.
« Last Edit: June 23, 2015, 10:04:06 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #209 on: June 23, 2015, 11:11:17 PM »
There is another beautiful Petermann panorama, too wide for us here so link below. The location is not documented but I am thinking it shows the northeast side from Hubert Glacier down to Sigurd Berg. This would be a good one to add to Espen's google map online project at tps://www.google.com/maps/d/viewer?mid=zbdKKg4fRHYo.kjkUmepfXc9A

Along those lines, note a duplicative  Petermann forum called "Petermann Fjord / North Greenland" ran for a few months back in 2013. Espen and Andreas Muenchow made a great effort there to nail down a comprehensive set of place names for positions along this glacier (a topic that we return to frequently!). It would be worth stripping that fossil forum for its goodies and appending it here.

I found a really neat oblique Icebridge shot of Petermann spilling into Romer Søer (lakes) at the NW corner of Kap Schoubye -- it was unlabelled as such (deplorable practice) and I cannot relocate it at the moment.

http://forum.arctic-sea-ice.net/index.php?topic=270.0
https://docs.google.com/file/d/0B_a0Tecpig0jRm9GbnlLVGFvZDQ/edit?pli=1 Kap Agnes
http://asapdata.arc.nasa.gov/asf/gallery/icebridge/1000205.petermann_glacier_panorama.jpg

Meanwhile, today's fracture extension. If just my imagination, then a prediction for future fracture extension.

I came across some contrasting quotes from three prominent Petermann researchers after the 2012 event: http://www.samoanews.com/node/6907

Quote
"It's dramatic. It's disturbing," said University of Delaware professor Andreas Muenchow, who was one of the first researchers to notice the break. "We have data for 150 years and we see changes that we have not seen before." Glaciers do calve icebergs naturally, but what's happened in the last three years to Petermann is unprecedented, Muenchow and other scientists say.

Quote
"This is not part of natural variations anymore," said NASA glaciologist Eric Rignot, who camped on Petermann 10 years ago.

Quote
Ohio State University ice scientist Ian Howat said there is still a chance it could be normal calving, like losing a fingernail that has grown too long, but any further loss would show it's not natural: "We're still in the phase of scratching our heads and figuring out how big a deal this really is."

Normally in a scientific situation like this, with a bimodal probability distribution (year of calving event, surface area calved off) estimable using data back to the 1876 map and decades necessary for the front to recover from an event filling in for years missing maps/imagery, monte carlo sampling would be employed (see wiki).

That might show that the probability of a distribution as extreme or more than observed in terms of date and extent could arise from natural (random) variability is less than p =0.0000001 or some such. That would be as close to a proof as it gets in science. However that doesn't say when, where, or why the next rifting will occur, my objectives with this series of posts.

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #210 on: June 24, 2015, 05:52:45 PM »
Petermann has changed quite a bit over the satellite record, not just the much-studied retreat of the ice shelf but also in the interaction of the main with tributary glaciers, which has gotten much more intense possibly because the main glacier is moving relatively faster and beyond the ductile limit of tributary ice.

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #211 on: June 24, 2015, 06:08:30 PM »
Nice work A-Team, good to have an engaged and dedicated "sailor" on board ;)
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #212 on: June 28, 2015, 03:50:14 AM »
Well, watching these rift tips elongate is like watching paint dry. Today's image does show some changes from 10 days ago. Note with widening of the crack coming down from the top and the extension of the one coming in diagonally. It's not clear that the two fractures 'know' about each other and have plans to meet.

Even if this did result in a calving this season, the area involved is minor and it would not destabilize the rest of the ice shelf, much less cause the main glacier to measurably accelerate from so little reduced buttressing.

This ice shelf has a very interesting channelized underside that has been extensively studied, most recently in two papers from Andreas Muenchow and coworkers. The bedrock topography too is quite inexplicable, rather lopsided and not at all the classic U-shaped fjord profile.

http://muenchow.ceoe.udel.edu/papers/Muenchow2014-JGlac.pdf
https://www.researchgate.net/publication/274695487_Bathymetry_in_Petermann_fjord_from_Operation_IceBridge_aerogravity

It's not clear that ice shelf surface features reflect either of them to any extent, which implies departure from hydrostatic equilibrium, and raises the question how stable that can be over the long term. Petermann has some commonalities with Antarctic ice shelves but I'm skeptical about any lessons in either direction.

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #213 on: June 28, 2015, 05:47:36 PM »
At the mouth of Petermann Fjord, just behind Offley Ø and between
Kap Mary Cleland and Kap Tyson can you find this beautiful natural wonder lets call it "Offley Deltaet"

It reminds me of Edward Munch?
« Last Edit: June 28, 2015, 06:42:58 PM by Espen »
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #214 on: June 28, 2015, 06:03:15 PM »
Quote
At the mouth of Petermann Fjord, just behind Offley Ø
Are you at liberty to disclose the source of this image? Interestingly, it is not known to reverse-search at  https://www.imageraider.com/

The image below shows an oblique view of Humboldt and Petermann that I took from the updated NASA visualization of Greenland ice layer ages (at ~50x vertical exaggeration of the DEM) http://www.nasa.gov/content/goddard/nasa-data-peers-into-greenlands-ice-sheet

One has to wonder why Petermann was 'needed' to drain the ice sheet -- it seems an afterthought. Humboldt had a broad outlet yet that glacier is still sleeping in terms of output. If Petermann is fairly new, that could perhaps explain the 'immaturity' of its bedrock profile.

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #215 on: June 28, 2015, 06:13:00 PM »
Have a ice day!

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #216 on: June 28, 2015, 06:25:13 PM »
And I not sure Humboldt discharge less ice than Petermann, Humboldt just dont brag about it the same way Petermann does.
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #217 on: June 29, 2015, 03:13:30 AM »
Quote
Humboldt just don't brag about it like Petermann does.
.
Hmmm, that might account for some of the difference. Though Petermann doesn't talk much about its dominant ice shelf bottom melt nor enhanced inland velocity and isn't really responsible for those hyperventilated headlines on calving.

This 2015 Humboldt paper is talking trough but pleading pinning points, free full at google scholar but oddly paywalled at the journal itself and not yet on university sites or ResearchGate. They re-determined basal topography to much better resolution ... fig.2 is a great step forward. I'm moving the discussion of it over to the Humboldt forum though.

Quote
Discharge from marine-terminating outlet glaciers accounts for up to half the recent mass loss from the Greenland ice sheet, yet the causal factors are not fully understood. Here we assess the factors controlling the behaviour of Humboldt Glacier (HG), allowing us to evaluate the influence of basal topography on outlet glacier response to external forcing since part of HG’s terminus occupies a large overdeepening.

HG’s retreat accelerated dramatically after 1999, coinciding with summer atmospheric warming of up to 0.19°Ca–1 and sea-ice decline. Retreat was an order of magnitude greater in the northern section of the terminus, underlain by a major basal trough, than in the southern section, where the bedrock is comparatively shallow. Velocity change following retreat was spatially non-uniform, potentially due to a pinning point near HG’s northern lateral margin.

Consistent with observations, numerical modelling demonstrates an order-of-magnitude greater sensitivity to sea-ice buttressing and crevasse depth (used as a proxy for atmospheric warming) in the northern section. The trough extends up to 72 km inland, so it is likely to facilitate sustained retreat and ice loss from HG during the 21st century.

JR Carr et al Basal topographic controls on rapid retreat of Humboldt Glacier, northern Greenland
Journal of Glaciology, Vol. 61, No. 225, 2015 doi: 10.3189/2015JoG14J128
http://www.igsoc.org:8080/journal/61/225/j14j128.pdf
« Last Edit: June 29, 2015, 03:35:03 AM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #218 on: June 29, 2015, 08:11:34 PM »
My immediate estimate (just watching for a few years), without any proof or documentation is, both Zachariae and Humboldt are in the same league as Petermann when it comes to ice discharge, but I may be wrong, I am not an expert!
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #219 on: June 29, 2015, 08:51:08 PM »
Just a reminder of what my thoughts are about the "Majors":

https://forum.arctic-sea-ice.net/index.php/topic,400.msg28208.html#msg28208
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #220 on: June 30, 2015, 04:54:23 PM »
The issue for me is not just which marine glacier is discharging how much now -- for that we have the 2014 Enderlin study of 178 glaciers -- but which ones are accelerating today, how mass budgets (discharge + melt - accumulation) are changing, and whether there are any runaway situations in terms of sea level rise contributors in the near or medium term future.

I tend to think you have correctly identified the two hot button issues, Jakobshavn and Zach/79, methods may leave something to be desired?

Of the 178 glaciers studied, 4 accounted for roughly half and the top 15 accounted for 77% of the 739 ± 29 Gt of ice lost due to acceleration since 2000  Helheim Glacier dropped from third to fifth place and newbie SE Greenland glaciers Koge Bugt and Ikertivaq South joined the list.

Note in 2000 Petermann edged out Zach but Humboldt was not too far behind. (I'm skeptical about '79fjorden' discharge.) But with cumulative anomalies to 2012 added in, Zach gets ahead as best I could tell from the sketchy article info. Considering basin sizes really puts the spotlight on Zach/79/NEGIS.

http://dspace.library.uu.nl/bitstream/handle/1874/296062/grl51346.pdf?sequence=1
http://onlinelibrary.wiley.com/doi/10.1002/2013GL059010/full for supplemental

"The relative contribution of ice discharge to total loss decreased from 58% before 2005 to 32% between 2009 and 2012. As such, 84% of the increase in mass loss after 2009 was due to increased surface runoff. These observations support recent model projections that surface mass balance, rather than ice dynamics, will dominate the ice sheet's contribution to 21st century sea level rise."
« Last Edit: June 30, 2015, 05:12:57 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #221 on: July 05, 2015, 05:48:30 PM »
Petermann has been moving at a record pace of 1403 ± 20 m/yr according to a centerline measurement I made between today's cloud-free day and that of a Landsat8 16 days earlier (same orbital path, row necessary for alignable geometry). The error estimate comes from looking at sensitivity to a bump pixel measurement, sub-pixel rock alignment on both sides, scene center time differences, more accurate number of days in year, and so forth.

The velocity of Petermann has been measured numerous times since the 1950's with a variety of methods, notably photogrammetry and GPS devices fixed in the ice (see earlier post). These measurements are not made at an agreed-upon spot (here due west of Sigurd Berg), nor necessarily averaged over the same time span (here 16 days), or date in season.

The previously reported record for a quite similar location was ~1350 m/yr for a brief peak in early July 2013, post #203, suggesting the 04-20 July 2015 average may be higher still. (However any speedup over the last two years would only borderline detectable since methods differ.)

One bit of confusion has arisen from posting as km/yr rather than in meters per year. The former is easy to just brush off as unchanging ~1 km/yr whereas the pronounced acceleration is apparent expressing in m/yr where relative error shows up better. While this speed-up is quite respectable for ~81º N, it is nothing like the ~300% at Jakobshavn, 1050 km to the south.

The first image below gestures at the methods that work best for measurement. The tool in Gimp can be positioned precisely at higher magnification: it figures the pythagorean length on the pixel displacements. Enlargement is better done with bicubic rescaling than by linear bumping of monitor resolution.

None of the fractures have progressed measurably over the last five days; the rift coming from the NE corner is most active right now. Melt on the ice shelf is most notable in the hinge line region off Porsild Glacier; melt on the ice shelf per se does not affect speed via basal lubrication as it is floating. It's not clear what if anything the different shades of blue portend.  The cold anomaly shows up in the same spot again in Landsat band 10 in the glasbey palette but does not appear correlated. Colors in the lower right corner are clouds.
« Last Edit: July 05, 2015, 06:57:13 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #222 on: July 08, 2015, 02:47:56 AM »
Things have warmed on the weather maps but I am yet to see any dramatic changes in terms of ice shelf melt features on Petermann or east of Humboldt ... maybe something will emerge in the next days. It does seem like the west side of north Greenland is experiencing more of this high than the east.

Note too the lower Petermann melange has fractured as the Nares breaks up -- this could release pressure holding rifting back.

The large melt feature below Porsild Glacier forms every year in a low spot (lower right corner). There is very little glacial 'conflict' at the confluence so the origin of this depression may have more to do with compressional forces as the glacier ends its dive off higher elevation at the grounding line here. 

Melt on the ice shelf obviously doesn't contribute to basal sliding lubrication since the shelf is already floating. Freshwater discharge under the ice shelf may however significantly affect basal access and influence of relatively warm ocean water. It's not at all clear how much meltwater is being released below the ice shelf at the grounding line by the glacier itself .

I've looked at various schemes for measuring surface melt area and comparing locations year to year. If some combination of channels should result in a consistent separation of ice and melt in some color space, then the melt could be lifted off by specifying a point and radius in the color picker. Transitional areas make this difficult to do with any accuracy but see ...

Spaceborne derivation and validation of supraglacial lake volume along the western margin of the Greenland Ice Sheet
http://www.the-cryosphere-discuss.net/9/3257/2015/tcd-9-3257-2015.pdf

Estimating supraglacial lake depth in western Greenland using Landsat 8 and comparison with other multispectral methods
www.the-cryosphere-discuss.net/9/3257/2015/tcd-9-3257-2015.pdf
« Last Edit: July 08, 2015, 04:47:20 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #223 on: July 08, 2015, 08:11:14 AM »
An interesting expedition and research this summer at Petermann - http://polar.se/en/expedition/petermann-2015/

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #224 on: July 09, 2015, 04:40:43 PM »
Quote
Petermann Glacier is connected to the inland ice through an ancient (perhaps pre-glacial) channel system, which extends from Petermann Fjord, deep into the inland ice along a pathway near the NEEM and NGRIP ice cores. This implies that the expedition with icebreaker Oden, targeting the longer-term marine history of the Petermann system, is constrained near its inland-ice source by well published deep ice-core data.

I sincerely hope they find someone familiar with the scientific literature on interior Greenland to write up the land portion of expedition (which going by the many publications of the distinguished PIs will really be about moraine sediment cores and foraminifera as Holocene temperature recorders).

Ice from NEEM nor NGRIP does not flow towards Petermann and never has over the last 115 kyr, minimally. Ice does not flow down the "ancient channel system" but instead obliquely across it. Radar stratigraphy establishes beyond any doubt that the ice has not flowed down the channel trough the Last Glacial and has not contributed any ice present in the Petermann fjord at any time during the Holocene.

Now they could have said something like:

Quote
"The stratigraphic ages of all ice cores in Greenland are cross-correlated today, so to the extent these layers extend towards Petermann (see image below), the ice there can be dated despite the immense basal upheavals that dominate the apron of the fjord. Thus the extreme folding of the nearest core, NEEM, does not present issues since stratigraphy from any other drill site can replace problematic layers.

"From this we know that 300 m of ice underlies the 60 kyr triple and reaches Kap Schoubye and that Holocene stratigraphy, at least on the northwest side of the fjord, extends to the grounding line and beyond.

"However the longer-term marine history of the Petermann system is not well-constrained by ice layer chemistry and chronology data alone because ice flow in northern Greenland is extremely complicated even on the barely moving summit ridge and the region just above the fjord has been subject to unknown thermal flow processes on a massive scale resulting in extreme basal upheavals that nearly reach the fjord entrance itself on the northeast."
« Last Edit: July 09, 2015, 06:02:38 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #225 on: July 10, 2015, 04:58:14 PM »
Where to place the 5 drill holes through the floating section of Petermann to observe ocean temperature and salinity for the next few years?
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #226 on: July 10, 2015, 09:59:36 PM »
That is a very good question for us forumites to opinionate/bloviate over. First though, a round of applause for those out in the field arduously and riskily obtaining important field data that cannot be had from armchair remote sensing!

The significance of ocean waters warming and eroding ice shelf underside is not limited to Petermann as similar processes may be operative at Nioghalvfjerdsfjorden and indeed under Antarctic ice shelves. Petermann, while not a tour destination, is relatively more accessible and could provide early model guidance to the others.

Both aspects of ocean interaction with ice shelf are non-intuitive: it is not a matter of waiting centuries for local water to warm. Oceans have indeed warmed slightly but the issue here is pre-existing warmer currents advancing to new places, already possessing ample heat to melt ice sheet undersides.

Secondly, the induced bottom melting is not uniformly planar but by all accounts quite pronounced just down from the grounding line and elsewhere channelizing the ice underside in a complex fashion driven by daily and seasonal fjord circulation and intertwined temperature, salinity and density profiles.

Five drill holes is a decent but not extravagant number. These could conceivably be guided in their placement or augmented by one-time casts through 'whirlpools' and other direct routes through the ice which we could conceivably locate beforehand. These holes have some interest in that they collect regional melt water and inject it locally on top of the subsurface water column. However one-off casts at random tidal stages and season pale in comparison to steady time series and the effort could be a huge resource waster.

I'm supposing hot water drilling (no ice cores, no gravity sediment) with an instrumented wire frozen in (rather than moorings), periodically reporting from a top package to a satellite, not betting the farm on relocation and one-time retrieval. However placed, these strings will drift down their flow line some 3-4 km over the lifetime of the project. Thus, to the extent the bathymetry is known to resolution, these could be placed over bedrock channels or set instead to drift over troughs and sills.

Thermisters and conductivity give density and perhaps changes in ice depth overhead. I don't have a sense whether currents will be measured or are even disentanglable from positional drift via topside GPS (or more simply, corner radar reflectors).

Given five drill sites, it seems like the first decision is allocation between horizontal versus flowline transects. I would discard both edges as not representative nor safe to drill; these might add up to 7-8% of the width. It wouldn't make sense to drill too close to the calving front either.

The benefits of a staggered hole line escapes me, so that leaves a 3+3 configuration of horizontal plus flowline, either T or +. For now, I would say 3 across fairly near the grounding line and 2 down the flowline from the northeasternmost hole, subject to revisiting the bedrock radar and what is known already about shelf bottom channeling and fjord circuation

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #227 on: July 11, 2015, 06:42:55 PM »
The DMI Sentinel scenes of Petermann are only at moderate resolution but very convenient for making a time series. The filmstrip below shows the daily pattern from 25 Jun 15 to 10 Jul 15. Sentinel consistently detects certain features and their development.

The physical significance of these is not clear, other than flowlines, rifts and melt progression. There is an intriguing central feature between Porsild and Kap Schoubye, just below or maybe part of the grounding line, so part of the glacier rather than its floating ice shelf. Note too the melt region between Porsild and Sigurd Berg.

DMI is not the optimal place to monitor rifting or velocity -- that is best done with 15 m Landsat band 8 (or for those with access, WorldView 0.5 m) with full-on Sentinel resolution also useful. There are two active rifts right now, in the NE near the calving front and midway along the NE wall, the upper crack. These have progressed somewhat but not dramatically to date.

I've redone the images several times to get a more effective immediate display. Animation did not work because of too much jitter (Sentinel images have warped geometry), the filmstrip is too wide except for the fanciest monitors, so I ended up breaking up the 16 dates into 4 image of 4 days each. The latest date is on the left, ie July 10th in the left corner the first image, July 7th in right corner (file names are confusing).

To make these, I selected 'Sentinel only' at DMI and the default 16 images. These turned out to be a nice daily series without glitches. After uploading in gimp as 16 layers, cropping and so forth, I used 'filmstrip' in the filters -> combine submenu. This tiles up only the active layers. By adding an alpha channel, selecting and deleting the blank black followed by 'autocrop' cuts the product down cleanly to 4 tiled dates.

I did not enhance contrast or false-color etc because the original grayscale in Sentinel has a physical meaning (but what?).  Part of it is roughness/smoothness of the ice at scales commensurate with the probing radar wavelength.
« Last Edit: July 11, 2015, 07:34:07 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #228 on: July 11, 2015, 06:47:26 PM »
The physical significance of these is not clear.

Watch the cracks on the right?
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #229 on: July 11, 2015, 08:11:32 PM »
Getting back to developing resources for optimal placement of Andreas M's 5 drill sites later this month, here is a matched set of locally enhanced contrast viewed in the 'glow' false color palette. This sometimes can draw out features and trends that would be visible in grayscale but not necessarily jump out there.

I'll put together a reference Landsat image shorlty where people can place their 5 dot grid as well as something for bedrock topography and ice shelf bottom channeling.
« Last Edit: July 11, 2015, 08:26:26 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #230 on: July 12, 2015, 08:31:13 PM »
Thanks for the help and suggestions, everyone; here is what I got so far as one (of several) plans. Please note that this one oceanographer's perspective what good locations are. There are other oceanographer's plans which may even be better, and then there are people interested in sediments to core that have a different set of questions in mind that are as or even more important. There is also bottom topography under the ice-shelf (very poorly known) that will impact both sediment cores and ocean pathways of warm waters. So, there is always a give and take, but it is good fun. [I am also working off LandSat channel-8 imagery as the drilling requires a source of fresh water in the forms of some of the many ponds.]

EDIT: Grounding line is thick black line near km-0; open circles are sensor locations with the underside of the ice from laser altimeter flights shown as blue track lines to the east and west of the central channel of the floating glacier. AWS is a new automated weather station that shall support 5 sensors ocean sensors, all other locations support only 2. Looks like we may also have several very fancy differential GPS to resolve vertical (and horizontal) motion such as tides to within a centimeter or so for the 20-30 days that the ice drilling team works on the floating glacier.

EDIT-2: And yes, I am watching Espen's crack near km-30. All sensors shall be deployed to the south of it so that with another ice-island potentially forming there, our sensors are not drifting away with it.
« Last Edit: July 12, 2015, 08:44:00 PM by Andreas Muenchow »
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #231 on: July 12, 2015, 08:44:06 PM »
Hi Andreas, good idea to keep the holes behind the newly formed potential calving line.
How long do you expect it takes to drill those holes?

PS: Oden is now heading towards Faeroe Islands.
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #232 on: July 12, 2015, 10:20:24 PM »
If all goes well, then it takes about 5 days to drill each hole. So if only that bloody sea ice would flush out of Nares Strait between now and when Oden will arrives at Thule Air Force Base on July-29 ... [Writing a blog post on that as we speak, give me a few hours to finish it.]
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #233 on: July 13, 2015, 05:21:03 PM »
Still digesting above information. Meanwhile I looked to see what Cresis has for EW transects in this region: not too many, most are NS. They need to show another decimal point or two on their radargrams for lat,lon to allow better compatibility with Google Earth which goes to 6 decimal points.

I looked at a 2002 horizontal transect that actually went right up the Porsild Gletcher, meaning it captures the transition zone. There are two pronounced bedrock channels sections on the sides at depths 680 and 891 m; the asymmetry has been noticed previously (east side is 211 m deeper).

These depths need to be adjusted to WGS84 sea level via a track crossing from 2014 where this was first annotated by Cresis. The plane also made at 34º turn from oblique; I 'corrected' that flight segment to horizontal by cosine projection of image width.

These are actual 2 km wide channels, not just dips in one section (potholes) because they have profile-matching counterparts that extend for considerable distances in other sections. It is not clear when or how (ice or water erosion along faults?) these formed nor why they should be so close to the walls (no tributary glaciers on the west side).

The second two sections are from a somewhat oblique flight that doubled back. The eastern channel may be migrating more centrally. I did not try to second-guess Cresis here, just used their magenta and red surface picks. The vertical spacing bars are at 500 m intervals.

The eastern channel is quite close to the wall and may underlie chaotic tributary ice melange downstream. It is not clear if bedrock profile controls erosion of the ice sheet underside to any extent. Note the bedrock profile overall is quite rugged and has no resemblance to a classic U-shaped fjord valley.

Possibly two small glaciers formed the U-shaped  channels and only later was the intervening terrain reamed out during heavier glaciation, a work still in progress. This fits with the regional view of small narrow glaciers and an 'immature' landscape. Petermann flos was blocked by a huge glacier across Kane for a long period.
« Last Edit: July 13, 2015, 06:48:36 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #234 on: July 13, 2015, 05:55:01 PM »
Quote
working off LandSat channel-8 imagery as the drilling requires a source of fresh water in the forms of some of the many ponds.

Might be better off with their 731 product initially than band 8. The 731 can be pansharpened to 15 m with 8 with retention of colors (though this will gain little).  My impression is less meltwater now than two weeks ago (maybe some meltponds drained?) and it is mostly down south near or past the grounding line. And this despite clear weather.

I've attached the 12 July 15 taken at time 19:15:20.1278839Z which didn't cover any further south. Not really seeing ice in the fjord breaking up, indeed Kane ice outside is sort of in a holding pattern. Sometimes Sentinel GRDH imagery is better for ice status, the hv 002.tif image.

The fracture tips have not developed any further on band 8 as of today, but the middle one could very well over thenext 2-3 years.

Vertical GPS to a cm at five sites for three weeks, that could be very instructive. How long does it need to collect signal before making a measurement or is it continuous?
« Last Edit: July 13, 2015, 06:51:15 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #235 on: July 13, 2015, 06:40:03 PM »
The sea ice in the fjord will go soon after The Nares Express begins its journey to the south ;)

Andreas and his team will have no problems entering the fjord later in the month or August, maybe they will watch a minor calving as well, that piece in front of Belgrave Glacier could easily be sent of while they are there?
« Last Edit: July 13, 2015, 06:49:29 PM by Espen »
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #236 on: July 13, 2015, 07:10:07 PM »
The Belgrade rift tip has been progressing south which is not conducive to joining up with the rift coming down from the south. However removing or even softening of the melange would relieve back pressure on the Belgrade portion so your scenario could come to pass. As the ice shelf moves up now at 3 m per day, it has to push the whole fjord melange ahead which results in its own compression.

However removing that issue does not equate to extensional forces. Those are coming from friction with the slower moving adjacent tributary. This friction is not evenly distributed, causing rear parts of the ice shelf to be held back while forward section moves ahead at the bulk speed, resulting in the shear that gives rise to the rift and its widening and tip extension.

The midway rift was active earlier in the summer but has stagnated, though it would not be a good idea to set up a tent straddling the tip. The nearby melt ponds (which are seen in earlier years) have either frozen over or drained.

In past years, an interior rift developed inside the frozen-in Belgrade rift's NE corner to give an early warning. We might be able to see that even at 15 m width. The Greenpeace ship in 2011 moored more to the west, at the central channel. That does not appear to be flowing this year, much less kayak-able 35 km inland.

I wonder if Andreas or others can use the US license to Worldview3 to get current imagery, which is something like 0.3m.
« Last Edit: July 13, 2015, 07:26:52 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #237 on: July 14, 2015, 04:34:46 PM »
Here is a more northern transect of Petermann, this time from 2014 which is closer to current ice conditions than 2002 (bedrock won't have changed but the ice has moved ~ 17 km since). The transect is oblique, with its northeastern part past the nominal position of the grounding line.

It is 173 m from ice surface to interior bedrock at its highest. The western channel is still evident (and seemingly occupied by an upheaval: orange tinting) but the eastern one is ambiguous but regardless no longer deeper. Central basal topography again is rough, not that of a U-shaped fjord so might have younger sediment under the ice shelf.

The magenta and red lines represent Cresis' interpretation of the radar, which is challenging at places for the bedrock. The  image without markups can be viewed here: https://data.cresis.ku.edu/data/rds/2014_Greenland_P3/images/20140331_01/20140331_01_006_1echo.jpg

The second image is more oblique, intersecting the first on the west side and then angling up to just below Sigurd Berg. No sign of the eastern channel midway up the fjord; that on the west side is present but not a continuation (because it is below the intersection). The bedrock may appear to have a jagged surface but that is due to scale: only 185 pixels per 10 km -- each pixel represents 54 meters. Two layers of echoes, only partly shown below, is sometimes taken as characteristic of radar reaching reflective sea water instead of bedrock, in which case the red line represents a transect through channels on the underside of the ice shelf.
« Last Edit: July 14, 2015, 08:25:40 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #238 on: July 14, 2015, 05:14:52 PM »
Ohhh ... /me runs off to get the radar (and corresponding laser altimeter) data for this 2014 section across Petermann near the grounding line ...
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #239 on: July 14, 2015, 06:57:39 PM »
Thank you A-Team for being so helpful on "our" behalf.
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #240 on: July 14, 2015, 10:03:01 PM »
The Cresis radar collection is a fantastic public resource, right in there with Landsat, Sentinel and Modis for importance, intuitive interpretation and ease of use. I would say under-utilized though.

Not everyone archives their radar imagery though. Right now, I'm trying to get a hold of some secretive footage of Humboldt's northern pinning point.

The really smart groups like Renland send their people -- including their Photoshop whiz -- to Kansas for hands-on device building and advance training with custom sled radar data processing. So many cores drilled at the wrong place ... not going to happen at Renland. (Petermann ice shelf has different issues -- not looking for flat isochron stratigraphy because that's last seen 100 km back.)

Somewhere many posts ago I put up quite a few NS flowline radar tracks of which there are many (including near-exact repeats of different years) though none along the melange edges. I have some new processing tricks now so re-did a central 2014 flight line below -- be sure to view at full 1066 pixel width. To tile these tail-to-head, the second image had to be rescaled 82.2%, a bizarre aspect not expected (but the norm) of consecutive Cresis flight segments.

20140512_01_013_2echo_picks.jpg
20140512_01_014_2echo_picks.jpg

The second image shows all Cresis flight segments over Petermann for 1993-2014. These are really easy to load into Google Earth -- the only trick is to notice the big kml that does an entire year is at the bottom, out of initial sight. The way stations are provided at fabulous precision and can be extracted from kml viewed as plaintext. I don't recall any north of Humboldt available for 2015.

This jumble of tracks cannot be resolved at any Google Earth blow-up because they deliberately flew second and third flights in later years right on top of previous years to the best ability of plane GPS. They can be resolved though by just activating one year at a time in GE checkbox column.

The third images show two 2014 flowline tracks that resulted from the plane looping around at the end of the fjord and making another pass down a more westerly flowline (which controls for a number of technical radar issues). It is not so easy to align these (latidude? flowline normals?) but it does seem that the two do have some, but not all, features in common. Here the secondary and tertiary echoes are helpful.

Physically, features that extend horizontally may amount to ocean circulation erosive patterns on the underside of the ice sheet.

These region does not seem to have horizontal transects but those can be reconstructed using all the many verticals and kriging in between (left as exercise). Again, view image in a separate browser tab at full size (native Cresis resolution).
« Last Edit: July 14, 2015, 11:49:47 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #241 on: July 17, 2015, 04:52:43 PM »
Even the moderate resolution Sentinel provided at DMI provides quite an informative quick look. The image from July 16th shows Nares Strait with some open water but lots of large blocks of ice. It appears that an adjacent large block in the Petermann Fjord is becoming detached but currently has no place to exit.

Three pronounced white streaks on the ice shelf correspond to the longitudinal surface drainage channels which for some reason are better radar reflectors than intervening ice. There is also noteworthy reflections from melange along both sides.

The most interesting feature, seen consistently in recent Sentinel imagery, are the three 'plumes' coming in from tributary glaciers on the northeast side, most notably from the southernmost tributary, Porsild and Sigurd Berg.

The problem here is ice coming off these glaciers is immediately and completely deflected at their confluences with the main Petermann glacier, over much shorter distance scales. Only the 'plume' from Sigurd Berg is in the appropriate location. The others may have more to do with the grounding line / hinge line /equilibrium float line region ather than representing discharge, though this association too could be coincidental. These lines can shift over time; they have not been redetermined in recent years but may be visible in radar through some indirect effect.

Contrast has been adaptively enhanced in the image in the right half to bring out features.

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #242 on: July 17, 2015, 06:12:56 PM »
A-Team, I never thought about the grounding line marks at Petermann just below Sigurd Berg Gletscher, the same can be seen at Hagen Bræ: https://forum.arctic-sea-ice.net/index.php/topic,396.msg56639.html#msg56639

Things are not that complicated when you find the right thread in the ball of yarn ;)
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #243 on: July 17, 2015, 07:14:41 PM »
That is a really good idea, Espen. Rather than just staring at Petermann, find support from comparative glaciology. What others do we have where the grounding line is known, in addition to Hagen Dazs? This is where Mauri Pelto's knowledge comes in handy, it would be a lot of work to chase down otherwise. After that, what is the physical basis for visibility on non-visibility on Sentinel radar or 731 Landsat.

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #244 on: July 17, 2015, 07:52:10 PM »
A-Team actually we dont need all them flights
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #245 on: July 19, 2015, 06:48:50 PM »
Thirteen years ago, a comprehensive sample of 7 evenly spaced north-south tracks were flown over Petermann glacier and ice sheet. Three of these were re-flown in 2014. This provides an opportunity to compare lower surfaces of the ice sheet, provided the images can be aligned (since the calving front has retreated and the grounding line may have moved).

Since the ice sheet has moved about 10 km towards the sea over this time frame, it would seem that any fixed features on the lower surface of the ice sheet would be displaced by that distance, unless consistent erosion of the underneath by warm ocean water and refreezing of water onto displaced ice sheet conspire to keep a stationary pattern.

The first image establishes locations at Google Earth and aligns Cresis radar tracks 20020528_01_011 and 20140505_01_017-18 based on up-glacier features. Here the resolution (pixels provided per km of flight line) in 2002 was only half that of 2014 (18.7 pxl/km) which itself is not extravagant relative to plausible crevasse or channel dimensions, not quite 2 pixels per 100 m.

The crevasse region appears 'more developed' in the 2014 frame. At one time, E Rignot -- who camped on Petermann for three field seasons -- suggested that the ice sheet might break off at the hinge line rather than piecewise down the ice sheet. Ice depth here in these image is about 500 m.

Another alignment option is to anchor each north-south track (which follow ice flow lines) to the conveniently orthogonal flight segment from the same year, 20020528_01_019, which even more conveniently corresponds quite well to the grounding line.

These intersections are somewhat a nuisance to determine since every flight segment image uses different horizontal and vertical scales and is inadequately marked up for latitude and longitude. However these can be interpolated fairly easily provided the plane is not flying a curving path; additionally, there can be a good match between features as seen in NS and EW views. Below are the 2002 images not yet with this registration -- click to view at full width.

The 3rd image provides the last of the 2014 tracks (the other two are above). It has been enlarged to the point of diminishing returns in the region downstream of the grounding line. The magenta and red horizons were drawn on at Cresis; there may be a slight advantage to also display the unannotated companion image (1echo).
« Last Edit: July 19, 2015, 07:50:51 PM by A-Team »

A-Team

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #246 on: July 21, 2015, 04:32:43 PM »
Whoa ... the seasonal ice in front of the Petermann ice shelf is breaking up on an overnight time scale. The movement is attributable to tides and currents, not to glacier push. (I did last 3 days 20-19-18 July 15 side-by-side below because Sentinel images are not regionally alignable due to geometric distortion.)

Stand by ... I am looking for higher resolution imagery for today. There may be some slight calving of the ice sheet itself. Landsat is clouded over for the 20th; Sentinel has not yet posted for today.
« Last Edit: July 21, 2015, 04:43:45 PM by A-Team »

Tor Bejnar

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #247 on: July 21, 2015, 04:50:12 PM »
Wow!
from Polar View 2015-07-20 full resolution:
(edit:  the bluish mask is sure skewed!)
« Last Edit: July 21, 2015, 04:55:46 PM by Tor Bejnar »
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Tor Bejnar

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #248 on: July 21, 2015, 05:28:17 PM »
I see many (what I think are) cracks - red ovals show where calving fronts may occur (if I'm not 'seeing things').
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #249 on: July 21, 2015, 05:53:33 PM »
Might want to look at the GRDH instead of GRDM (H for high, M for medium) at the Sentinel hub https://scihub.esa.int/dhus/  Interesting how in going from Landsat to Sentinel we are merely swapping sun shadows for ascending oblique pushbroom view of walls blocking the radar.

Some of those cracks have been there for 5-7 years without progressing. None of the Sentinel image types are adequate for tracking progression compared to 15 m Landsat (or WorldView which is on-demand plus we don't have access to). But let's see what shows up today, the 21st.