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Author Topic: Petermann Gletscher / Petermann Fjord / North West Greenland  (Read 144696 times)

Thomas Barlow

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #500 on: June 19, 2017, 09:31:43 PM »
Oh, ok, Thanks!
Glad that's all it is.
Where you getting that image from?

Tor Bejnar

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #501 on: June 19, 2017, 09:37:37 PM »
Here's today's DMI Sentinel - shows same curved lineation.  My guess was "a river".  The "air photograph" (I presume) certainly shows the melt ponds!
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ghoti

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #502 on: June 20, 2017, 03:56:50 PM »
The Petermann ice expedition tweeted and animated GIF which shows the progression of the melt ponding.

https://twitter.com/StefLhermitte/status/876836364855259136

VeliAlbertKallio

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #503 on: June 23, 2017, 03:31:35 PM »
Andreas Muenchow's report suggests me hydrofracturing occurring on a newly-forming crack that is propagating inversely from the centre towards the Petermann glacier's shear zone at the edge. I would see this as evidence of meltwater river induced hydrofracturing in progress which may also be supported that the pressurised water filling the crevasse when it escapes stirs the basal sea water to increase temperature variance of turbulent water. As the meltwater filling crack and the river supply are both getting bigger and bigger, water will exponentially widen and fill this crevasse that acts as a conduit of meltwater discharge as the holding base also gets thinner. It happens just this way, inversely, because the hydrofracturing has not reached yet glacier's base on its bottom and the water builds up presumably on the crevasse.

There's a new IcySeas blog post up about temperatures rising under the Petermann Glacier.

https://icyseas.org/2017/06/16/is-petermann-gletscher-breaking-apart-this-summer/

Tor Bejnar

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #504 on: June 23, 2017, 05:51:34 PM »
The June 21 Polar View image shows a crack crossing the mid-glacial stream (about 1 red bar's width distance above the red bar) and the string of melt ponds that (on less distinct imagery) looked like a massive crack (or something). (Screen print has the brightness adjusted.)
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johnm33

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Tor Bejnar

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #506 on: June 23, 2017, 08:27:48 PM »
Thank you johnm33!  Scrolling up on your image, I see more clearly than ever before what I believe will be the next calving event (right [east] side of the glacier's tongue).  (just above my added red line).  (Ignore the eyes on what appears to be someone from the land of Point.)  At least from this view, there appears to be a 30 or 40 m gap between the hook on the left (west) crack and the straighter (south) crack, and because the two cracks are approaching parallel at the hook's end, there can't be much strength in this 'bridge'.  I'll guess the fast ice in the fjord is holding this would-be (2 x 4 km) ice island in place. (Slight enlargement doesn't have added red marks.) (Edit: image is actually June 22, not 15)
« Last Edit: June 23, 2017, 08:36:33 PM by Tor Bejnar »
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Tor Bejnar

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #507 on: June 23, 2017, 08:38:16 PM »
Fast ice at the end of Petermann Fjord is beginning to crack. (Sentinel Playground image 2017-06-22)
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oren

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #508 on: June 23, 2017, 11:42:21 PM »
Fast ice at the end of Petermann Fjord is beginning to crack.
Small wonder, with so much meltwater on the ice, and air temps reaching 5-8oC in the vicinity.

A-Team

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #509 on: June 29, 2017, 05:00:27 PM »
Petermann is having an exceptional meltwater year and calving events, large and small, would come as no surprise. However it's been incorrectluy stated up-forum and elsewhere that the new central crack is an unprecedented crevasse and likely to lead to a massive tabular iceberg.

Actually there is no stress on the ice shelf here, either horizontally or from local buoyancy forces; the alignment angle cuts across meltwater drainages, and there's been no distortion of features on either side. It appeared last year in early July but has largely been stable since (2nd image, 01 Jun 17). This has nothing in common with the physics of hydro-fracturing of Antarctic glaciers at their grounded calving fronts.

The Petermann ice shelf has all sorts of deeply channeled bottom topography as seen by ice-penetrating radar; one of these has merely been eroded up to the surface and re-frozen at the waterline (~30m below ice shelf surface). There's another much older example of this below the eastern calving front that's been dormant for a decade or more.

The animation looks at TorB's terminal pair of cracks over the last 12 months. The first two frames are co-registered to sidewall rocks, ie as provided straight off the Sentinel-1AB site (best by whole-window screenshots). The third frame has the June 2016 ice advanced ~1200m so fixed features west (left) of the drainage channel co-register with the June 2017.

The fourth frame shows by differencing that that the left-central half of the ice sheet indeed moves as a rigid block. Geometric changes on the west side, largely induced by the ongoing collision with the incoming tributary glacier, are then effectively displayed relative to this moving lagrangian reference frame.

What happens next? Last year, after the annual ice in the upper fjord went away, a small piece in the NW corner broke off. There is a similar piece developing this year right at the junction of main ice shelf with tributary ice from much further south (2nd frame); that crack extends into the annual ice.

Alternatively, the much larger piece that Tor has highlighted could come apart, though the lower crack seems to have over-shot the vertical one instead of joining it.

At least 3 other 'traditional' cracks have developed over the last 12 months as the ice shelf has moved past stress-inducing shore features (that induce bends in the channel). These relieve stress on older fractures. Thus a big tabular event may happen at one of these, rather than rapid propagation at a now-dormant feature. The 2nd animation shows changes mid-sheet; note the much slower moving tributary ice along the shore.

While the main fracture is still growing, the direction of propagation is up-glacier rather than than across-glacier. Yet this can literally change overnight.

Whatever, big tabular bergs from floating ice shelves do not raise sea level and, unlike in Antarctica, will not significantly un-buttress Petermann nor speed up discharge of grounded glacier ice from the mainland.
« Last Edit: June 29, 2017, 05:29:50 PM by A-Team »

Espen

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #510 on: June 29, 2017, 05:15:01 PM »
"Hear Hear"!
Have a ice day!

A-Team

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #511 on: June 30, 2017, 10:36:16 PM »
There have been some issues up-forum and elsewhere distinguishing features on the ice surface from intervening clouds, contrails, fog, changing light, melt status and satellite viewing angle.

To be safe, look at more than one date and more than one wavelength. This is best done in the initial post. It just takes a click at either LandViewer or WorldView.

Clouds come and go, ice moves very slowly (up about 3 pixels a week at Petermann at Sentinel-1AB resolution). Ice features are persistent and go with the flow. Compare the visual channel (RGB) with infrared admixtures.

The 3rd frame of the upper animation shows how to make transparency mask for cloud cover: split the 367 into its RGB grayscales, take the B. Then use the mask to preferentially strip off clouds via localized proportionate contrast adjustment.
« Last Edit: July 01, 2017, 05:01:55 AM by A-Team »

Espen

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #512 on: July 01, 2017, 11:49:15 AM »
But adding up to what we have seen until now, I do not expect a calving at Petermann this year, maybe a small piece will leave the glacier up front at starboard side.
Have a ice day!

A-Team

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #513 on: July 01, 2017, 04:35:33 PM »
Maybe we'll get another clear day?

While this appears to be a fairly good surface melt year, that's not really what drives Petermann calving -- the real story is going on underneath the floating ice shelf as warm water swirls underneath. Buttressing annual ice in the upper fjord has not yet melted out.

I looked to see if A Hubbard and J Box ever did anything with the radar data from the 2009 kayak adventure. It seems not. In particular, I could not find the lat,lon of that scary moulin in the central drainage channel. It may have calved off in the last big event, mooting the value of their down-shelf ice thickness soundings.

"Ocean warming currents are circulating around the fjord and eroding the underbelly of Petermann glacier at an incredible rate," says Hubbard. Melting at the surface of the ice forms huge whirlpools of relatively warm fresh water that bore holes into the floating sheet.

The scientists believe this process is accelerating the ice's demise. In places, the meltwater bores holes through the ice right down to the bottom of the ice tongue. Surfacing seals are proof that some of the holes called moulins pierce to bottom of the ice."




If similar features exist today, could we identify them with Sentinel-1AB imagery? Possibly, by scanning pairs of dates and looking for disappearance of melt water. That could be done synoptically from loss of blue meltwater color. So it might be worth doing later in season.

Another thing we could do is follow up on previous high-resolution triangulation to map geometric distortion (indicative of strain). It appears to me that the Sentinel-1AB in Land Viewer have pixel-perfect registration on rock features on both sides yet east-central ice features are changing in relative position (on a monthly scale). Ice, even of several hundred meters thickness, is still deformable, especially if it has warmed a bit.
« Last Edit: July 01, 2017, 05:41:31 PM by A-Team »

Thomas Barlow

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #514 on: July 12, 2017, 10:25:37 PM »
Does anything think there could be a relationship (conjoining) of these 3 features in the future?
This is from July 8. That seems to be a crack cutting partly across the main central drainage channel, not just another stream flowing into the drain.
(maybe get a better view next time the clouds clear)

Tor Bejnar

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #515 on: July 14, 2017, 04:12:04 PM »
I'm curious about the crack in Petermann Glacier that appears to have caused a crack in the abutting fast ice (red arrows) (July 13 Sentinel Playground).  I wonder if the crack (purple arrow) that is collinear with the dirty streak suggests an unseen crack in Petermann.  And of course, I regularly look for crack growth (or connection) within the red circled area.  Finally, interesting crack propogation in the purple circle area (with enlargement).
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