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Author Topic: Petermann Gletscher / Petermann Fjord / North West Greenland  (Read 212744 times)

Espen

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #150 on: February 12, 2015, 02:00:43 PM »
Nice image, it is easy to see where the next potential calving point is.
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #151 on: February 13, 2015, 12:17:33 AM »
Anreas Muenchow has an interesting new post on Petermann over at his blog - http://icyseas.org/2015/02/08/lab-notes-of-a-physical-oceanographer/ - he is hoping to install some instruments on Petermann this summer - hoping to crowdfund this project (see https://experiment.com/projects/ocean-warming-under-a-greenland-glacier )

sidd

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #152 on: February 27, 2015, 08:00:23 AM »
Contours from Morlinghem

A-Team

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #153 on: February 28, 2015, 04:05:47 PM »
Nice. It's been a puzzle how the Grand Canyon actually connects up to exit through Petermann, especially how it gets past that 600 m knob. The canyon has left little trace in the glacier outlet proper, that has all been over-written by glacial erosion and differential isostatic depression. Even though the canyon is quite broad with gently sloped walls, the Petermann channel is several times wider and deeper, so clearly cannot be attributed to canyon water erosion.
« Last Edit: March 01, 2015, 12:52:31 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #154 on: March 01, 2015, 02:04:13 PM »
The Morlighem .nc bedrock files only use radar sounding data up through 2013. This provides the opportunity to use the 2014 flight segments in the Petermann area to test the accuracy of bottom topography and its interpolation.

The images below show two radar segments from 2014 that go down the lower region of Petermann Glacier proper, an area below sea level on the contour map above. They would have to be co-registered with the contour map to probe its accuracy.

Note that 2014 is the very first year that Cresis has labelled the vertical axis with WGS-84 geoid elevations, ie zero is zero (sea level). This key reference level can be carried transitively to recalibrate depth from flight segment from other years that a 2014 path intersects, and so on to all earlier years.

The radar squiggles have been interpreted by Muenchow 2014 as bottom crevasses in the Petermann ice sheet. That paper called attention to an area of dramatic crevasses but less striking ones occur across the whole channel (extrapolating from the five along-channel overflights). These may contribute to current melting and future calving.
« Last Edit: March 02, 2015, 11:51:21 AM by A-Team »

A-Team

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #155 on: March 03, 2015, 11:59:30 AM »
Here is an annotated version of the flexure hinge that marks the start of Petermann's bottom crevasse zone. Click on the first image to see full size. The along-glacier flight is on the west side of the glacier (red track in GooglE, second image); all 8 north-south tracks show the same pattern: the 17 km long crevasse zone extends east-west across the entire glacier.

The hillshaded surface elevation map shows ice has piled up just south of the hinge line, as expected from the abrupt change in surface slope. It is 48 km from the start of the hinge line to the glacier's calving front.
« Last Edit: March 03, 2015, 12:11:00 PM by A-Team »

A-Team

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #156 on: March 03, 2015, 03:19:07 PM »
Here is the full set of available north-south radargrams through the crevasse zone of Petermann glacier. These thumbnails allow for a quick comparison of this zone to show how it varies from east to west. There are no flight segments directly perpendicular to the glacier channel but the next post will provide several diagonal transects, some of which are nearly orthogonal.

Short of a program specifically targeted to the crevasse zone, this ensemble provides the best possible sense of the structure and variability of the bottom crevasse field (which has no surface manifestations). It may be possible to track individual crevasses through pattern matching -- some may extend from wall to wall -- though the radar resolution may not be enough.

Other than a brief mention in Muenchow 2014, this crevasse zone does not seem to have a literature even though it was clearly documented in 1997 overflights.

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #157 on: March 04, 2015, 01:08:58 PM »
The images below show 4 transects of the Petermann glacier in the crevasse zone. Vertical lines show where they meet the previously described along-channel slice through the zone. As with the upheaval region to the south, these not-quite orthogonal sections help in reconstructing a three-dimensional view of the crevasse area.

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #158 on: March 05, 2015, 12:22:07 PM »
Petermann glacier, being so far north, is very favorable for Landsat8, with almost daily coverage. The scene below, from Sep 2014, shows cross-hatched surface features in the crevasse zone. It's not clear what these represent nor whether they have any connection to the crevasse features shown above.

A number of melt lake feature can be seen, these peak much later in the season than say Jakobshavn. The melange in front of the calving front is also much later to melt out.

It is not so easy to place a lat,lon grid on EarthExplorer Landsat images. One method is to use the lat,lon of the four corners that are provided in metadata and interpolate under the assumption of a regular mercator grid. The other is to view the preview image as a 'street map' as more lat,lon grid numbers are visible (instead of being white on white as in satellite view).

Unlike Jakobshaven, the underlying Google Earth map has only a fuzzy worldview image at high zoom levels (even though incredibly clear 15 m Landsat images are in the public domain). The second image below provides a coordinate system for the crevasse system and some key fixed rock features seen in almost all scenes that allows for correlation of visible and ice-penetrating radar imagery.

The final image shows the calving front in mid-August of 2014 quite a distance from the crevasse zone.

A-Team

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #159 on: March 06, 2015, 02:05:59 PM »
I consider it 'cheating' to actually look at prior publications on Petermann before analyzing the data first myself. Some of these papers are excellent but others look nutty today (in light of data that became available later). Petermann mostly gets in the news for its mammoth big calving events but not so much for 'awakening' or sea level rise (to which floating ice shelves don't contribute).

There are a couple more things to do here such as put the glacier's grounding line to images above, rescale some to less spikey 1:1, and add more Greenland place names, but more importantly look for possible connections between the floating shelf, hinge line, subglacial melt channels, crevasse zone, termination of the upheaval region, and entrance of the not-so-Grand Canyon to the main glacial channel.

Looking now at two ice penetrating radar images from last summer 20140505_01_036, 20140502_01_010 and their earlier low resolution connector 20030514_01_011, it is clear that a small relic of the upheaval zone continues into the confined region of Petermann glacier.

« Last Edit: March 06, 2015, 03:00:16 PM by A-Team »

Espen

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #160 on: March 08, 2015, 09:24:38 PM »
First image from Petermann by Modis:
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #161 on: March 09, 2015, 03:37:21 PM »
Below, the first image tracks back the only upheaval to enter the glacier outlet proper. This is quite complicated to do because Cresis does not use a consistent depth scale except for 2014 where elevations are finally given sensibly (ie relative to WGS84 geodetic datum). I used the 2014 segments wherever possible. Otherwise the meter scale is arbitrarily offset on every flight segment, of every flight with 0 having no fixed meaning relative to sea level.

It is a real mystery why no one from the glaciological community intervened during the first 21 years of the program. No use has ever been made of these radar images other than bed topography.

The second image shows 7 consecutive segments of this upheaval feature at 10 km width and 5:1 vertical exaggeration. Click to see it at the (2014's) original resolution. The path spans 126 km but is by no means the end of this upheaval ridge (see earlier Petermann Grid tracebacks in the subglacial topography forum) . By the two southernmost scenes, the ridge has joined the main massive upheaval zone.

The third image shows a 10 km section of the fourth and tallest segment (~700 m) at 1:1 scale. This has the effect of rounding off an impossibly steep upheaval to a rounded hill that represents a section through a long ridge. There is no known explanation for the radar structure; the dark bands bear no relationship to overall ice isochron stratigraphy but may reflect density contrasts or impurities.

The final image shows a continuous down-glacier flight path from 1997 for purposes of providing a uniform depiction of bedrock and surface elevations. It passes slightly to the west of the upheaval at 80.266º. Note once again convergence at the crevasses zone of gradually shoaling bedrock and rapidly plunging ice surface elevation, which would be more subtle using something less than provided 29.9:1 scale.
« Last Edit: March 09, 2015, 05:26:21 PM by A-Team »

A-Team

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #162 on: March 12, 2015, 12:57:39 PM »
I've been looking at five concepts for the Petermann-Humboldt sector using ice penetrating radar data from 1995-2014:

-- the old ice layer cake stratigraphy continues right up to and even into Petermann proper on its western wall indicting slow movement and unexceptional history for ~125 kyr, applicable to all of Humboldt as well yet only Petermann has the massive upheaval features.

-- Humboldt is the broadest marine outlet glacier in Greenland but this is an accident of geography, not the result of past glacial excavation; Petermann has classical steep fjord walls but no glacial overdeepenings (like Jakobshavn) but instead a bedrock hump indicating it has never been a large volume ice conveyance.

-- the radar data itself, discarding kriging artifacts connecting adjacent basins, does not support the not-so-Grand Canyon making the published big bend westward into the entrance to Petermann Glacier (ie south of Kap Egedesminde) but instead a route north and east of the rock ridge east of Petermann never entering it.

-- the grounding line is where it is because of a paleo debris wedge partly excavated by Porsild glacier entering from the east

Seismic Architecture and Geometry of Glacial Grounding-Zone Wedges from Greenland and North Africa
C Decalf, E Fugelli, J Dowdeswell
AAPG 3P Oct 2013
http://www.searchanddiscovery.com/documents/2013/30310decalf/ndx_decalf.pdf

Ice-sheet grounding-zone wedges (GZWs) on high-latitude continental margins
CL Batchelor, JA Dowdeswell
Marine Geology 363 65–92 2015
https://www.sciencedirect.com/science/article/pii/S0025322715000304?np=y

Seismic architecture and geometry of grounding-zone wedges formed at the marine margins of past ice sheets
JA Dowdeswell, E Fugelli
GSA Bulletin 124 1750-1761 2012
http://gsabulletin.gsapubs.org/content/early/2012/09/13/B30628.1

-- Petermann, currently moving 3 m/day (a tenth the speed of Jakobshavn), will likewise speed up once the last of its floating ice shelf disintegrates this decade. However the curvature of its smaller basin defined by two summit ridge forks and its narrowing exit funnel geometry (noted earlier here by Espen) requiring flow convergence have major implications for near-term future ice flow given its incompressibility of ice and the need for mass conservation.

None of these considerations seems to explain the association of the upheaval zone and the long ridge described above with Petermann's velocity field, nor the observed similarity with the upheaval zone of Zachariae on the low accumulation side of the summit ridge.

The 2014 radar imagery below, not available at the time of the Bamber 2013 Grand Canyon paper, show the canyon does not now, nor did it ever, drain into the entrance of Petermann fjord. This is established beyond any doubt by placement of 20140505_01_036, 20140502_01_011, 20140505_01_038 etc of Cresis 'good_kml' coordinates on Google Earth.

The topography of the former drainage has been greatly rearranged from what we see today by compression of bedrock by the weight of overlying ice. The gentle swale called the Grand Canyon, 10:1 vertical scale exaggeration below, has little relevance to ice flow in Greenland today.
« Last Edit: March 12, 2015, 10:41:05 PM by A-Team »

nukefix

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #163 on: March 12, 2015, 04:46:24 PM »
-- Petermann, currently moving 3 m/day (a tenth the speed of Jakobshavn), will likewise speed up once the last of its floating ice shelf disintegrates this decade.
Why do you prophesize ice shelf disintegration, is the ocean warming up at a relevant depth? As far as I can remember the grounding line is stable and there's not much thinning or speed-up so I'm wondering why you think the ice-shelf will be gone in an instant?

Espen

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #164 on: March 12, 2015, 05:01:17 PM »
As A-Team mentioned above Petermann is definitely moving, this animation show how much in 6 months: 
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Espen

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #165 on: March 12, 2015, 06:21:34 PM »
-- Petermann, currently moving 3 m/day (a tenth the speed of Jakobshavn), will likewise speed up once the last of its floating ice shelf disintegrates this decade.

That would be a surprise?
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Espen

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #166 on: March 12, 2015, 08:49:37 PM »
As A-Team mentioned above Petermann is definitely moving, this animation show how much in 6 months:

It is interesting to watch that the tongue of Belgrave Gletcher *) is withstanding the pressure of Petermann?

*) The glacier at the front to the right of Petermann.
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A-Team

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #167 on: March 13, 2015, 12:17:31 AM »
Quote
Why prophesize ice shelf disintegration, is the ocean warming up at a relevant depth?  The grounding line is stable and there's not much thinning or speed-up
It doesn't take long when 25% of the ice tongue can be lost in a single year so yes I expect all the post-hinge line ice that you see today will have gone out to sea, as happened at Jakobshavn (attributed there to warming ocean). It's easy to find and follow distinctive fixed markers in the ice in either Landsat or Sentinel; five years out is a testable prediction, unlike climate 200 years out, safely beyond accountability.

However, just as a small tongue at Jakobshavn gets replenished each winter, new thin floating ice will replace calved ice at Petermann indefinitely. The issue with Greenland and Petermann in particular, is whether it will ever be a significant contributor to sea level rise. Pick your favorite flux gate here, it doesn't matter where up-channel of the grounding line. The reality is, if ice flux doesn't change at the Parca gates, we are not even talking a mm of sea level rise from here.

Petermann is a totally one-off situation that is not productively compared to other Greenland or Antarctic glaciers:

Quote
Interannual changes of the floating ice shelf of Petermann Gletscher from 2000 to 2012
A Muenchow et al
Journal of Glaciology, Vol. 60, No. 221, 2014 doi: 10.3189/2014JoG13J13
http://muenchow.cms.udel.edu/papers/Muenchow2014-JGlac.pdf

Petermann Gletscher, northwest Greenland, drains 4% of the Greenland ice sheet into Nares Strait. Its floating ice shelf retreated from 81 to 48km in length during two large calving events in 2010 and 2012. We document changes in the three-dimensional ice-shelf structure from 2000 to 2012 using repeated tracks of airborne laser altimetry and ice radio-echo sounding, laser altimetry and visible imagery.

The recent ice-shelf velocity measured by tracking surface features between flights in 2010 and 2011 s 1.25km/yr, 15–30% faster than estimates made before 2010. The steady-state along-flow ice divergence represents 6.3 Gt/yr mass loss through basal melting (5 Gt/yr) and surface melting and sublimation (1.0 Gt/yr).

Airborne laser altimeter data reveal thinning, both along a thin central channel and on the thicker ambient ice shelf. From 2007 to 2010 the ice shelf thinned by 5m/yr, which represents a non-steady mass loss of 4.1 Gt/yr . We suggest that thinning in the basal channels structurally weakened the ice shelf and may have played a role in the recent calving events.
My sense is far too much attention is being paid to nostalgic topics such as iceberg hazards where the deeper issues with Petermann's future have not so much do with attention-getting calving events (aka loss of buttressing). While the ocean is indeed warming (not as fast as the Zachariae side), there is a surprising amount of melt above and below for 80º N and even visible side melange.

However Petermann has gone through warm millennia before, both in the Holocene and Eemian, and even very rapidly ramped warming. On these, we don't have to opinionate on short-term trends because the past is right there in 125 kyr of ice penetrating radar record, once we get past some rubbish and neglect.

As noted earlier, after mopping up a few more things on the remote sensing record for the Humboldt/Petermann basin, a transition to literature review is definitely in order, probably starting with the 2012 paper below and the 21 that cite it (eg the Muenchow paper above). 

The response of Petermann Glacier to large calving events and its future stability in the context of atmospheric andoceanic warming
FM Nick et al
Journal of Glaciology, Vol. 58, No. 208, 2012 doi: 10.3189/2012JoG11J242
http://ftp.vub.ac.be/~fpattyn/papers/Nick2012_JGLAC.pdf

Quote
Despite some publicity to the contrary, neither the observations nor the model results point to a significant effect
from the loss of 25% of the floating ice tongue on the flow of Petermann Glacier. The cause lies in the low level of back-
stress provided by the ice shelf at or near the glacier front.

From our numerical modelling, we conclude that marine outlet glaciers with a long and relatively thin ice shelf are
not sensitive to changes in their terminus region.

From combined velocity observations and model results, we conclude that the seasonal variations in the velocity of Petermann Glacier are mainly controlled by surface melt and consequently enhanced basal lubrication, and to a lesser extent by frontal back pressure caused by ice melange or sea ice.

Our results further show a dominating influence of sub-shelf ocean melt on future glacier stability. This emphasizes
the urgent need for more observations of fjord temperatures and heat circulation.
« Last Edit: March 13, 2015, 12:24:39 AM by A-Team »

A-Team

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #168 on: March 13, 2015, 12:44:59 AM »
Quote
note the tongue of Belgrave Gletcher, the northeastern-most contributing glacier, is still discharging despite the pressure of Petermann unlike several of the tributatary glaciers to the south

Place names in the interior are hard to come by -- note Espen's nice earlier compilation on this forum. I am still looking for a proper name for 'Arrowhead Island' at the southwest entrance. Pilespids ø?

Here is a brand new compilation of 733 Greenland glacier names. It comes as an excel table with lat,lon but no accompanying map (!!!) with the glacier names attached. The ones for NW Greenland (listed under Qaanaaq) are extracted below. It would be feasible to load these into Google Earth -- surely this was the job of the authors, not the readers

Getting Greenland’s glaciers right – a new dataset of all official Greenlandic glacier names
AA Bjørk et al
http://www.the-cryosphere-discuss.net/9/1593/2015/tcd-9-1593-2015.pdf

This site has quite a few obscure Kaps like Schoubye and glaciers like Porsild, all slightly misplaced: http://mapcarta.com/19187754

Espen has also grabbed the very first Landsat-8 of 2015 Petermann. These will soon pile on because of the polar orbit, the azimuth being rather oblique if accustomed to ~180º of Jakobshavn. Long shadows are cast by steep high walls on the upper west end of the channel on much of the Petermann imagery which otherwise can be extraordinarily sharp.

LC80380022015070LGN00
Sun Elevation 5.45403859
Sun Azimuth -154.85800992

Academy Gletsjer   77.2345   -65.3164
Bamse Gletsjer   78.0224   -72.1669
Berlingske Bræ   77.0607   -69.4712
Bowdoin Gletsjer   77.6934   -68.5829
Brother John Gletsjer   78.2834   -72.2654
Bryant Gletsjer   77.5257   -69.3009
Bu Gletsjer   78.0598   -72.2383
Cecil Gletsjer   80.8651   -61.7795
Chamberlin Gletsjer   76.719   -68.4795
Child Gletsjer   78.0717   -72.4301
Clements Markham Gletsjer   77.9475   -71.8501
Cluett Gletsjer   76.1741   -68.3171
Constance Gletsjer   81.0778   -62.4186
Diebitsch Gletsjer   77.9521   -71.5907
Döcker Smith Gletsjer   76.3   -61.7666
Dodge Gletsjer   78.1761   -72.6796
East Branch Gletsjer   77.7336   -68.3524
Edith Gletsjer   81.058   -62.19
Faith Gletsjer   80.9504   -61.8693
Fan Gletsjer   77.5629   -69.6524
Farquhar Gletsjer   77.7   -66.233
Gable Gletsjer   77.6167   -68.1828
Gade Gletsjer   76.3833   -62.7665
Gnom Gletsjer   77.6059   -68.7698
Harald Moltke Bræ   76.5797   -67.626
Hart Gletsjer   77.6991   -67.1759
Heilprin Gletsjer   77.5167   -65.6664
Helland Gletsjer   76.2533   -64.8682
Hermiarrupaluk   77.4312   -70.4834
Hermiarrupaluk   77.5009   -68.7533
Hermiarrurruaq   77.4494   -70.6344
Hermipaluk   75.9394   -66.5167
Hermipaluk   76.0473   -65.5518
Hermipaluk   77.2065   -70.4837
Hiawatha Gletsjer   78.8203   -67.0518
Hubbard Gletsjer   77.6025   -67.7419
Humboldt Gletsjer   79.4462   -63.3957
Hundetunge Gletsjer   76.8413   -67.2557
Hurlbut Gletsjer   77.3618   -67.9872
Illuluarsuit Gletsjer   77.8108   -70.9613
Ivar Berendsen Gletsjer   76.9406   -70.0698
Kiatak Gletsjer   77.3403   -71.6945
Kissel Gletsjer   77.4323   -72.2135
Knud Rasmussen Gletsjer   76.7578   -67.7971
Kong Oscar Gletsjer   75.989   -59.8575
Kumait Hermiat   76.0655   -65.6468
Leidy Gletsjer   77.2562   -65.977
Marie Gletsjer   77.2069   -66.0029
Meehan Gletsjer   77.88   -70.2973
Melville Gletsjer   77.7315   -66.6444
Mirror Gletsjer   77.6777   -68.3207
Mohn Gletsjer   76.2667   -63.7998
Morell Gletsjer   76.3167   -62.4999
Morris Jesup Gletsjer   77.907   -71.0812
Nassersorfik; Naherhorvik   76.0941   -67.7789
Neptune Gletsjer   76.1555   -68.2844
Nordre Tvillinggletsjer   76.7275   -67.0014
Østgletsjer   77.5764   -68.2681
Parish Gletsjer   77.4199   -72.0338
Peary Gletsjer   76.1167   -60.3999
Pituffik GGletsjerletscher   76.2774   -68.5855
Politiken Bræ   77.1602   -69.6024
Qaanaaq Gletsjer   77.4999   -69.1936
Rampen   76.773   -67.0206
Rink Gletsjer   76.2167   -60.9999
Robins Gletsjer   77.4072   -71.8968
Salisbury Gletsjer   76.7229   -68.6739
Savage Gletsjer   77.1858   -69.2548
Savissuaq Gletscher   76.2397   -65.2432
Scarlet Heart Gletsjer   77.6545   -69.4092
Sermipaluk   76.0912   -68.2097
Sharp Gletsjer   77.7167   -66.9496
Siorarsuaq Gletsjer   77.854   -70.6716
Skjulgletsjer   76.85   -66.8329
Søndre Tvillinggletsjer   76.6988   -67.0432
Store Landgletsjer   76.5001   -68.2366
Storm Gletsjer   78.1415   -72.7544
Sun Gletsjer   77.779   -69.4287
Sydgletsjer   77.5602   -68.7785
Tracy Gletsjer   77.6564   -65.9679
Tunnelgletsjer   76.8303   -66.8866
Twin Gletsjer   77.5262   -68.6944
Tyndall Gletsjer   77.1299   -70.7566
Ujarahuhhuarruit Itivtiat   77.3783   -72.2667
Verhoeff Gletsjer   77.8757   -69.8513
Vestgletsjer   77.4072   -72.4525
Witzansky Gletsjer   76.8887   -70.1684
Yngvar Nielsen Gletsjer   76.3334   -64.0831
« Last Edit: March 13, 2015, 12:15:27 PM by A-Team »

sidd

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #169 on: March 13, 2015, 04:02:26 AM »
Mr. A-Team remarked: "The gentle swale called the Grand Canyon ... has little relevance to ice flow in Greenland today."

Well, we can see the kink in surface contours above the canyon even in the Bamber data (see my reply 117 on this thread)

I haven't tried this with the Morlinghem data yet

sidd

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #170 on: March 13, 2015, 01:45:17 PM »
Quote
Prof. A-Team remarked: "The gentle swale called the Grand Canyon ... has little relevance to ice flow in Greenland today." Well, we can see the kink in surface contours above the canyon even in the Bamber data (see my reply 117 on this thread)I haven't tried this with the Morlinghem data yet
It should not then be a problem to provide notable correlation statistics for people like me who cannot see any particular relationship between surface elevation and swale bedrock topography kinks at the #117 imagery cited. http://membrane.com/sidd/greenland-2013/walkback.html.

Note the swale is filled its entire length with undisturbed stratigraphy right down to Eemian, does not follow the gravitational gradient, nor connect to the sea via Petermann, nor correlate at all with ice upheavals, nor track maximal surface velocity: it has been inactive in transport the last 125 kyr, minimally.

Where are the other channels -- in some cases deeper -- that radar shows are there? The swale is only one feature of  bedrock -- there are many comparable excursions. The same false positive/false negative issues come up in Bell 2014, reading surface elevation tea leaves for upheaval detection.

If surface contours could reveal the swale (considered at 1:1 true scale), why is the Grand Canyon paper dated 2013 when the surface elevation has been known for decades?

Why would the radar bedrock program be funded for 22 years (and counting) if bedrock topographic details could be read off from readily measured surface elevation? There had to have been very broad support for it in both the Greenland and Antarctic research communities: when radar is funded, something else isn't.

Note Morlighem is fill-in refinement, not ab initio bedrock. Models require ice thickness at the outset, severe uniqueness issue arise from just current surface elevation and velocity field. These methods alone have not obtained the bedrock profile, the experimental data from radar was essential.

Looking at low angle shadowed Landsat imagery of the surface, it is surprisingly complicated with many features not yet explained -- large patches of rough surface sitting over large patches of smooth bedrock, and conversely. This shows the level of noise in the surface elevation system relevant to predicting bedrock profile -- which will get a whole lot worse when Howat reaches the Petermann basin with 2 m worldview DEM.

Instead of using heavily kriged interpolation of bedrock (data gaps can be 10,000 km2), it is preferable to directly examine radar transects across the swale looking for consistent dips at the surface. In fact, older stratigraphy does sometimes track bedrock bumps and hollows (after high pass filtering) but this is rapidly damped out during times of epic melt and Holocene ice horizons, themselves overlain by upper layers of unconsolidated firn and drifted snowfall. The problem comes elsewhere -- the surface dips but the bedrock doesn't.

Using the recent Sentinel, the 15 m shadowed Landsat of lower Petermann surface of 11 Mar 15 below and the surface velocity map above, how might I reproduce the bedrock profiles shown in #163 and #162?

I am seeing a vast array of surface features below the Petermann channel proper not readily explained by surface, bedrock nor anything in between -- no surprise because local features cannot account for ice dynamics which needs consideration of the whole basin (as well as temperature profiles and other ice properties and interaction with the atmosphere etc). The surprise come in other marine-terminating glaciers in Greenland that don't exhibit this overt surface complexity.
« Last Edit: March 13, 2015, 02:33:07 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #171 on: March 13, 2015, 04:21:48 PM »
A-Team

What do you mean ?:
"I am still looking for a proper name for 'Arrowhead Island' at the southwest entrance. Pilespids ø?"
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #172 on: March 13, 2015, 05:00:01 PM »
Quote
Why prophesize ice shelf disintegration, is the ocean warming up at a relevant depth?  The grounding line is stable and there's not much thinning or speed-up
It doesn't take long when 25% of the ice tongue can be lost in a single year so yes I expect all the post-hinge line ice that you see today will have gone out to sea, as happened at Jakobshavn (attributed there to warming ocean). It's easy to find and follow distinctive fixed markers in the ice in either Landsat or Sentinel; five years out is a testable prediction, unlike climate 200 years out, safely beyond accountability.
The loss of a large part of the ice tongue might be completely normal. How much thinning is happening at the grounding line and is there evidence of grounding-line migration? Hopefully S-1 allows for grounding line detection with InSAR once the 2nd satellite is up.

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #173 on: April 12, 2015, 12:05:26 PM »
The Kap Bemerton crack got a sister, which can be seen in the animation below, if you doubt it, you can check Wipneus "very-high" resolution image from 2014 here:

https://forum.arctic-sea-ice.net/index.php/topic,53.msg22935.html#msg22935
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #174 on: April 12, 2015, 12:27:00 PM »
And maybe we have a mother crack for a future calving here, the location is seen in the image below the animation:
« Last Edit: April 12, 2015, 09:04:44 PM by Espen »
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #175 on: May 16, 2015, 08:56:39 AM »
A potential calving crack is expanding further:
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #176 on: May 17, 2015, 12:00:45 PM »
Petermann Gletscher is preparing for a series of minor calvings at the front of Belgrave Gletscher:

A high resolution image of the area is seen below the animation.
« Last Edit: May 17, 2015, 04:03:38 PM by Espen »
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #177 on: May 17, 2015, 10:31:09 PM »
Maybe better seen in this Sentinel Image:
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #178 on: June 02, 2015, 07:22:54 PM »
On Petermann (but not lower Jakobshavn or Zachariae), you can zoom way in on Google Earth to see some really extraordinary DigiGlobe WorldView2 imagery. This is a great step forward from 15 m resolution Landsat-8 if you are interesting in locating fracture tips and monitoring their progression, though at this point I don't know of free access to a 2015 time series.

The images below look at the northeastern-most rift of the nine fractures currently operative on Petermann. The digiglobe has several rounds of zoom left but the resolution provided below already makes the calving boundaries clear, if not the timing (fractures can remain dormant for years).

The two cracks are only ~300 m from joining up (bottom image). As explained over at Jakobshavn #654, even the digiglobe benefits from CLAHE contrast enhancement.

Losing this minor piece of the ice shelf would have not have the same significance as fracturing of the two largest active fractures midway south. I’m finding Patrick Lockerby’s old posts here provide much better insight into the physics driving Petermann calving than purely descriptive journal articles. These latter however are very important for this peculiarly asymmetric ice shelf and the dominant operative process here, bottom melting.

Here is a link to a 2010 Lockerby post which links to the rest in the series, including calving prediction. The most interesting parts are naming/describing individual tributary glacier sidestreams and analogizing their interaction with the main ice shelf to mechanical 'chatter'. This amounts to brittle failure rather than ductile response (continuum mechanics being the main tool of glaciology).
 
http://www.science20.com/chatter_box/petermann_ice_island_revisited_0

« Last Edit: June 04, 2015, 05:33:34 AM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #179 on: June 03, 2015, 09:03:59 PM »
Quote
Espen:  I have some difficulties interpreting the Sentinel images from June 1, maybe Wipneus can help?
Wipneus: EW med-res image not entirely useless. Wait for a IW mode. more frequent for Jakobshann than Zachariae (waiting for months now).

The extended EW mode lacking any detail, I placed a box in central Petermann at Sentinel search https://scihub.esa.int/dhus/ and under 'advanced' required sensor mode to be IW, finding 28 images. As far as I can tell, the SLC and RAW categories are fairly worthless from our perspective. That leaves IW_GRDH_1SSH which means 10 m pixels on the ground. There were 9 of these.

The image comes as a 16-bit 865 MB file that opens as black in ImageJ. However, looking at the histogram, there is a very tight peak in the near-black. The best first step is to normalize the histogram -- then crop and try CLAHE or Equalize to improve contrast.

The image arrives upside-down but not oriented to south, which is not indicated either. I flip it vertically but do not rotate to north which would be needed, for starters, to co-register with Landsat-8. Next, a 90º CCW rotation which for Petermann's geometry fits nicely on my monitor which is wider than tall. It is still 60 MB after cropping so certain processing steps are still slow.

The attached image is cropped from S1A_IW_GRDH_1SSH_20150527 at maximal resolution. Not totally thrilled with it; Landsat band 8 at 15 m is a lot clearer. Sentinel is not suited for monitoring rift growth in the Petermann ice shelf.

S1A_IW_GRDH_1SSH_20150204
S1A_IW_GRDH_1SSH_20150205
S1A_IW_GRDH_1SSH_20150208
S1A_IW_GRDH_1SSH_20150216
S1A_IW_GRDH_1SSH_20150217
S1A_IW_GRDH_1SSH_20150220
S1A_IW_GRDH_1SSH_20150523
S1A_IW_GRDH_1SSH_20150523
S1A_IW_GRDH_1SSH_20150527*

S1A_IW_SLC__1SSH_20150123
S1A_IW_SLC__1SSH_20150124
S1A_IW_SLC__1SSH_20150127
S1A_IW_SLC__1SSH_20150204
S1A_IW_SLC__1SSH_20150205
S1A_IW_SLC__1SSH_20150208
S1A_IW_SLC__1SSH_20150216
S1A_IW_SLC__1SSH_20150217
S1A_IW_SLC__1SSH_20150217
S1A_IW_SLC__1SSH_20150220
S1A_IW_SLC__1SSH_20150523
S1A_IW_SLC__1SSH_20150523
S1A_IW_SLC__1SSH_20150527

S1A_IW_RAW__0SSH_20150204
S1A_IW_RAW__0SSH_20150205
S1A_IW_RAW__0SSH_20150208
S1A_IW_RAW__0SSH_20150523
S1A_IW_RAW__0SSH_20150523
S1A_IW_RAW__0SSH_20150527

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #180 on: June 04, 2015, 03:54:17 PM »
On these forums, we have used almost exclusively bands 3-5 and 8 of Landsat-8, ie the visible RGB channels and the panchromatic. And it is not just us. I got to wondering whether any of the other bands were worth following, maybe band 10, which measures thermal infra-red ground emission (rather than reflected sunlight), could give us ice surface temperature.

Here we know the emission spectrum is that of a blackbody (or rather graybody); the peak in that would give the temperature via Planck's Law except that the satellite records at the top of the atmosphere so the spectrum needs adjusting for absorption in the intervening atmosphere at the time band 10 is measured. As that is beyond the scope of what we can do, I settled for relative temperature.

Band 10 over the Petermann ice shelf displays as indistinct grays even after enhancement of contrast. However there is a special false color palette developed by Glasbey that is designed exactly for this purpose: maximal separation in color space of adjacent grays. (ImageJ will provide this to 8-bit files via Image --> Lookup Tables --> Glasbey.)

This produces rather suggestive bands on Petermann, with temperatures cooling upstream and a pronounced cold spot anomaly on the west-central grounding line which coincidentally (?) has long caused gaps in SAR velocity measurement (yellow arrow). This effect may need a really cloud-free day to be reproducible.

This temperature gradient may or may not have significance to rifting propensity. There has been an automatic weather stations on the ice shelf since 2002 possibly providing ground calibration but it seems not to be working, try 4th link.

http://fiji.sc/Glasbey
https://github.com/fiji/fiji/blob/master/luts/glasbey.lut
http://www.bioss.ac.uk/people/chris/colorpaper.pdf
http://cires1.colorado.edu/science/groups/steffen/gcnet/mapSelect.php
« Last Edit: June 04, 2015, 04:18:55 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #181 on: June 04, 2015, 08:28:34 PM »
A-Team, please see my post in Jakobshaven-thread about conversion of S-1 images to log-intensity for human viewing.

Also, SAR-images are not photographs and the top-left pixel of the image is the first sensed pixel, so the images are "flipped" depending on ascending/descending acquisition geometry. S-1 Toolbox can be used to convert from imaging geometry to map coordinates, I could write short instructions in case anyone is interested..

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #182 on: June 05, 2015, 05:03:16 PM »
Sentinel is not producing photographs? True, no film or lenses involved but the end product is indistinguishable from one once amplitude and phase are tossed. We have seen many examples of the best that the Danish Meteorological Institute could do with Sentinel. http://ocean.dmi.dk/arctic/kennedy.uk.php.

DMI does appears to have fixed one great aggravation with Sentinel, its re-projection to standard Greenland coordinates (if I recall EPSG 3408, NSIDC EASE-Grid, polar stereographic 70º N 45ºE). It is absolutely imperative to bring Sentinel images into layers that co-register with all the other GIS data types, even if it slightly degrades the resolution.

Could you please list the steps in the S-1 Toolbox for re-projecting a IW Sentinel image for Petermann and attach an example of the product image at 10 m?

However the question asked above is how do we best monitor crack propagation at Petermann. The answer is obviously with 0.3 m visible WorldView3 satellite to which we do not seem to have  access, despite the US scientific license. However we do have a superb baseline for August 2014 due to Google Earth having just put up two WorldView2 images covering most of Petermann.

These photos prove that 15m Landsat-8 is incapable of reaching fracture tips and that Sentinel 10 m IW is considerably worse. Both are kms away from imaging the 2014 tips, much less any progression in 2015. This is because they do not have sufficient resolution/contrast. Consequently, we are reduced to monitoring crack width far from the propagation zone. But here too it takes a gross change to give 2-3 pixels of change. And lighting angle and melt condition can complicate width change measurement.

Fracture origins and gross extensions can be dated using glorious 2000-08 NASA imagery which I'll post shortly; some have frozen in with marine ice and are dormant; others, as Espen noted a page back, are much newer and seem to be active.

The first image below shows the fracture tip on the longest rupture on Petermann in the upper right corner, at about 1 m. What has happened here since August 2014? We have no idea.
« Last Edit: June 05, 2015, 07:53:57 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #183 on: June 13, 2015, 02:50:17 PM »
Summer is arriving at Petermann, several melt ponds are showing up on top of the glacier:
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #184 on: June 13, 2015, 04:39:58 PM »
Yes, suddenly soggy. If leading a trip there, Espen, please don't forget to take the kayaks out of the water *before* reaching the moulin.

http://www.telegraph.co.uk/news/earth/earthpicturegalleries/5851014/Greenpeace-in-Greenland-The-Arctic-Sunrise-surveys-the-Petermann-glacier.html?image=11

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #185 on: June 13, 2015, 05:18:27 PM »
WOW!

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #186 on: June 13, 2015, 06:28:32 PM »
Several as in hundreds?

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #187 on: June 13, 2015, 06:57:41 PM »
Several as in hundreds?
Why exaggerate?  ;)
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #188 on: June 13, 2015, 09:03:51 PM »
Espen, you crack me up. Can we meet halfway? It's 2.4 km from the new tip to the center drain.

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #189 on: June 13, 2015, 09:58:37 PM »
Not much change the last 3 weeks on the west side. Today's image is displaced a few pixels downward and is halfway transparent, allowing the older image to show through, for purposes of crack propagation comparison.

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #190 on: June 14, 2015, 04:14:16 PM »
The graphic below shows hours of sunshine at the latitude of the growing tip shown two posts above. This is related to, but not the same, as ground level insolation (energy impingent) adjust for albedo (energy received by the surface).

The nearest weather station may be Hans Island. It is showing 3.17° C currently and above zero air temperatures the last three days. This is not necessarily a good proxy for the central Petermann ice sheet so may not explain the abrupt onset of melting there observed by Espen. http://dalriada.sams.ac.uk/aws_hans/

The polar orbit of Landsat-8 is such that coverage of Petermann is really extraordinary, with 17 images for the first 14 days of June, many of them clear. Here Zachariae had 10 and Jakobshavn calving front only 4 during this same period.

The downside of Petermann coverage is that these images don't often have the same or even similar path,row. Thus some allowance has to made for different viewing geometries if images are to be aligned. However the bigger issues for fracture comparison are change in illumination angle and melt status.

For example, if two images a few days apart could be perfectly aligned, subtracting the earlier from the more recent would yield the newly melt areas. However the slightest distortion would cause some issues with this. Central Petermann moves ~3 m/day or a pixel every 5 days at 15 m resolution; motion on the side tributaries tapers off to fixed ice.

June path rows for Petermann ice sheet:

LC80 30 248 2015 158 LGN00
LC80 33 248 2015 163 LGN00
LC80 34 248 2015 154 LGN00
LC80 35 248 2015 161 LGN00
LC80 36 002 2015 152 LGN00
LC80 36 247 2015 152 LGN00
LC80 36 248 2015 152 LGN00
LC80 37 002 2015 159 LGN00
LC80 39 001 2015 157 LGN00
LC80 39 002 2015 157 LGN00
LC80 40 001 2015 164 LGN00 today
LC80 41 001 2015 155 LGN00
LC80 42 001 2015 162 LGN00
LC80 43 001 2015 153 LGN00
LC80 44 001 2015 160 LGN00
LC80 46 001 2015 158 LGN00
« Last Edit: June 14, 2015, 05:35:01 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #191 on: June 15, 2015, 06:29:29 PM »
The animation below shows three years of Landsat-8 images, mid-June of 2013-15, of rifts in the northwest corner of Petermann's floating ice shelf.

It is 'challenging' to properly align these given that the glacier moves NNW about a km per year (67 pixels at 15 m resolution if a year apart) while new rifts open and old ones widen, in addition to compressive deformation and path,row viewing geometry issues.

For the purposes of seeing changes, it does not work so well to align to fixed rock along the channel because the 'same' feature is not in the same place from year to year. However feature pairs often remain in the same relative position as this ice shelf is not dynamically thinning. The animation therefore is in co-moving Lagrangian coordinates rather than Eulerian. This causes Faith Glacier (FG) to 'move' down from year to year.

2015 164 40 001 LC80400012015164LGN00 June 13
2014 168 41 001 LC80410012014168LGN00 June 17
2013 163 43 001 LC80430012013163LGN00 June 12

Click to view animation. Frames are at 800 ms except for 2015 which is 1600 ms. The lower image is a fixed side-by-side at the 700 pixel width allowed.
« Last Edit: June 15, 2015, 07:01:03 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #192 on: June 15, 2015, 06:45:14 PM »
The animation below shows three years of Landsat-8 images, mid-June of 2013-15, of rifts in the northwest corner of Petermann's floating ice shelf.

It is 'challenging' to properly align these given that the glacier moves NNW about a km per year (67 pixels at 15 m resolution if a year apart) while new rifts open and old ones widen, in addition to compressive deformation and path,row viewing geometry issues.

For the purposes of seeing changes, it does not work so well to align to fixed rock along the channel because the 'same' feature is not in the same place from year to year. However feature pairs often remain in the same relative position as this ice shelf is not dynamically thinning. The animation therefore is in co-moving Lagrangian coordinates rather than Eulerian. This causes Faith Glacier (FG) to 'move' down from year to year.

2015 164 40 001 LC80400012015164LGN00 June 13
2014 168 41 001 LC80410012014168LGN00 June 17
2013 163 43 001 LC80430012013163LGN00 June 12

Click to view animation. Frames are at 800 ms except for 2015 which is 1600 ms.

Nice work A-Team, try the same with the cracks around Kap Bemerton?
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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #193 on: June 15, 2015, 07:22:39 PM »
Quote
try the same with the cracks around Kap Bremerton?

Here is what I have so far on the east side, same three June dates and years. The central ice shelf is sliding past an ice stream contributed by a tributary glacier. As it does so, large brittle blocks of the middle tributary ice shear and slowly pivot as they move seaward at a slightly lower velocity. The medial moraine is shown in colors in the animation.

This process began in earnest sometime prior to the year 2000 and may be associated with rift initiation on the NE side of the ice shelf. (The NW side has no significant tributary glaciers and indeed Petermann spills into valleys there, eg above Kap Schoubye.)

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #194 on: June 16, 2015, 04:55:40 PM »
Quote
try the same with the cracks around Kap Bemerton?

The triple rift of today has developed fairly recently. The northernmost member is not visible in 2009 but fairly well developed (but not as extensive) in 2012. It has stalled for the last three years as the strain was taken up by a rift below it that developed in 2013. That rift too stalled as the strain was taken up by a major new rift, again up-glacier, that arose in mid-summer 2014 but has extended in 2015 (though not in recent weeks -- see #189 ).
« Last Edit: June 16, 2015, 05:58:59 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #195 on: June 16, 2015, 05:34:40 PM »
The first image shows an enlargement of the elbow in the oldest rift in the Kap Bemerton area. The rift is 130 m in width (from top to bottom on left edge of image). These rifts go clear through the ice shelf to the sea water underneath (as determined in some cases by CTD casts). Over time, a stagnant rift can fill in with marine ice, captured meltwater, avalanching ice from the sides, and snowfall (though that is minimal according to several years of AWS measurement).

The second image shows a detail of the main rift on the other side of the fjord. The 'intrusive ice dike' is 10 m across and squeezed up (by tributary block brittle fracture and subsequent rotation) about the same amount going by the shadow lengths.

The third image shows the south lateral moraine of Sigurd Berg glacier just to the east of the dike, which becomes a prominent medial moraine as this glacier joins Petermann and undergoes severe shearing as it moves downstream (due to its lesser velocity). Over time, the grit comprising the moraine can be spread by meltwater into channels.

These high resolution Worldview2 images suggest that the 15 m scale of Landsat is not sufficient to resolve the physical forces operating on this ice shelf.
« Last Edit: June 16, 2015, 06:06:48 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #196 on: June 17, 2015, 10:03:39 AM »
Wait a minute . . . what's that watermark-like facial image doing there in the corner of the last image (Petermann moraine shear.jpg)???  :o

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #197 on: June 17, 2015, 11:06:45 AM »
It is not August Petermann?
Have a ice day!

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #198 on: June 17, 2015, 12:47:27 PM »
Artistic license? Petermann was a distinguished German cartographer of the 19th century. Never traveled to Greenland, much less this glacier or his 2940 m berg. Similarly, Alexander von Humboldt never visited Humboldt Glacier.

Petermann made some mighty fine maps of Greenland in 1865 and 1869, below. He proposed that warm currents from the Atlantic and Bering Sea could lead to open water in the Central arctic. More details are cross-posted at http://forum.arctic-sea-ice.net/index.php/topic,417.msg54320.html#msg54320
« Last Edit: June 17, 2015, 11:51:08 PM by A-Team »

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #199 on: June 17, 2015, 01:30:18 PM »
The animation below pushes the limit -- and then some -- on resolution of Landsat-8 band 8. The most active rift at the moment on the Petermann ice shelf has almost reached the midline.

Although comparing June 10th to June 15 seems to show unambiguous change, the widespread melting, growth in the size of melt ponds and water flow in tributaries can be confused with tip propagation in comparing June 13th to June 15th.

The resolution has been bumped up to 5 m. It's debatable whether this helps very much; a week from now the situation will be clearer. Note detecting a linear feature is easier than point features because of spatial correlation.

Unless someone feels up to purchasing WorldView3 imagery, this seems to be the end of the road in terms of tip propagation monitoring -- it  does not help particularly to apply de-speckling, sharpening, or shadow exagerating.

This is a fairly respectable crack, 15 feet, in terms of jumping over. There is no guarantee that it will continue growing, nor that it will lead to a calving rift, this summer or ever.

If it does, I estimate (from google earth pixel counting, assuming a semi-circular route extension) that the area calved off will be ~220 km2, similar to the 2010 event and maybe twice that of 2012. As the ice is already floating, calving would not contribute directly tp sea level rise, nor indirectly via glacier acceleration as modeling suggests very limited importance to buttressing.
« Last Edit: June 17, 2015, 01:50:20 PM by A-Team »