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Author Topic: Petermann Gletscher / Petermann Fjord / North West Greenland  (Read 312431 times)

BornFromTheVoid

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #750 on: August 18, 2020, 11:19:57 AM »
Hmm, IDK, Samuel. Is it sea ice thawing or crack opening what we see in your GIF?

In the zoomed in direct comparison i don't see the crack growing really.

I aligned the southern part so as to highlight the gap growth.
Doesn't seem like much, but it's close to 20 m growth on the northern section here
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blumenkraft

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #751 on: August 18, 2020, 11:48:11 AM »
Right, the gap on that side widened a bit. I agree.

(Great alignment job there, BTW ;) )

Tor Bejnar

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #752 on: August 26, 2020, 06:57:04 PM »
Seasonal evolution of supraglacial lakes on a floating ice tongue, Petermann Glacier, Greenland

by Grant J. Macdonald, Alison F. Banwell and Douglas R. MacAyeal
 Annals of Glaciology, Volume 59, Issue 76pt1  -  July 2018
Quote
Abstract

Supraglacial lakes [SGLs] are known to trigger Antarctic ice-shelf instability and break-up. However, to date, no study has focused on lakes on Greenland's floating termini. Here, we apply lake boundary/area and depth algorithms to Landsat 8 imagery to analyse the inter- and intraseasonal evolution of supraglacial lakes across Petermann Glacier's (81°N) floating tongue from 2014 to 2016, while also comparing these lakes to those on the grounded ice. Lakes start to fill in June and quickly peak in total number, volume and area in late June/early July in response to increases in air temperatures. However, through July and August, total lake number, volume and area all decline, despite sustained high temperatures. These observations may be explained by the transportation of meltwater into the ocean by a river, and by lake drainage events on the floating tongue. Further, as mean lake depth remains relatively constant during this time, we suggest that a large proportion of the lakes that drain, do so completely, likely by rapid hydrofracture. The mean areas of lakes on the tongue are only ~20% of those on the grounded ice and exhibit lower variability in maximum and mean depth, differences likely attributable to the contrasting formation processes of lakes in each environment.
The paper's conclusions end with
Quote
...
Based on previous studies of SGLs on Antarctic ice shelves, the presence of SGLs on Petermann's floating tongue may be indicative of its vulnerability to instability and potential collapse (e.g. Scambos and others, 2000, 2003; Banwell and others, 2013). We find lakes to cover <2.8% of the total surface area of Petermann's tongue, compared with the 5.3% of Larsen B's area that was covered prior to its collapse in 2002 (Banwell and others, 2014). Predicted future rises in air temperature (Kirtman and others, 2013) could enable a higher density of lakes, with larger volumes, to develop from earlier in the season, possibly leading to increased ice tongue instability. However, the decline of SGLs through July and August in each year studied, despite sustained high temperatures during those months, suggests that evacuation of meltwater from the tongue (e.g. by a river cf. Bell and others, 2017) may limit the total volumes of meltwater storage on Petermann's floating tongue, thereby mitigating the risk of instability and break-up.

(I found the article because I was looking for natural "hydrofracking" events being reported.  I had mis-read an ASLR comment a few days ago...)

I know this paper is a couple years old, but I don't recall reading about it before ... 

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Espen

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #753 on: November 09, 2020, 08:11:05 AM »
Petermann Gletscher:
The crack we have been following for some time now seems to have extended by a few hundred meters from point 2 in the animation and the distance from point 1 to point 2 could now be down to 1.600 meters or 8 % of the width of the glacier.
Notice a new crack branch is developing into the glacier, close to point 2.
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Espen

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #754 on: November 11, 2020, 10:36:57 AM »
Here is a 8 months study showing the crack expansion at Petermann Gletscher:

Please click on image to enlarge and animate!
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Espen

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #755 on: November 17, 2020, 08:33:46 PM »
Here is small study to what happened prior to the extremely large calving (+/- 260 km2) that took place on August 10 2010.

Due to difficulties picking up relevant data from Nasa / Landsat I have chosen 3 dates June 30 2010 (about 1,5 month before the calving) July 28 2010 (about 2 weeks before the calving) and August 11 2010 (a day after the calving event).

What really surprised me when I looked into the data, was how close the crack came from starboard to the port side (July 30 2010) before Petermann Gletscher actually calved August 10 2010.

There are many similarities between the situation now and then, the 2020 crack was only 1.600 meters away from the closest crack at port side last time I measured it.
 
But although the 2010 crack was almost perfect (100%), the Glacier chose to take several shortcuts when the glacier actually calved.

I would like to hear your comments to what you see?

Please click on image to enlarge and animate!
« Last Edit: November 18, 2020, 06:03:36 PM by Espen »
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Tor Bejnar

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #756 on: November 18, 2020, 05:35:17 PM »
Thanks, Espen.

Here is some of my thinking:
We often think of ice as being brittle.  Well it is - it cracks!  But it is also elastic.  Even as the middle of the tongue advances slightly faster than the edges (at least, this is typical of a glacier - the ice drags on the side walls), the cracks on the edges shows that forward motion there happens in a jerky motion (little or no motion, then something snaps) and does not only ooze [steady elastic movement], therefore the cracks.  In a perfectly symmetrical glacier, the left (facing 'down stream') side cracks and right side cracks would meet (eventually) in the middle.  In Petermann Glacier, for the currently watched cracks, they will be meeting (we predict) well to the left of center.  The middle (or left of middle, anyway) has more even flow, so the ice continues to ooze there, rather than crack. 

The compression forces in the "1.6 km" wide area (at least) must be strong, as the tongue 'slowly' flows northwestward down the fjord.  Given the tongue is floating, it moves forward by being pushed from behind (where gravity causes the glacier to flow downhill).  Espen's GIF shows the cracks on both sides of the floating tongue are slowly widening at the surface.  Why aren't the cracks closing, given the compression?  I wonder if well-below-the-surface parts of the tongue are still connected.  Might some of the tongues motion be some sort of lateral ridging (buckling), where the crest breaks open, creating a crevasse open at the surface but definitely compressed at the bottom?
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Espen

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #757 on: November 18, 2020, 09:42:06 PM »
Petermann Gletscher:
Here is an animation containing the period 1976 to 2020.
Included in the animation is the first Landsat documented Petermann calving between 1976 and 1977 and the big 260 km2 August 10 2010 and the latest big calving July 16 2012.

Please click on image to enlarge and animate!
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oren

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #758 on: November 19, 2020, 06:25:06 AM »
I would like to hear your comments to what you see?
I cannot elaborate as much as Tor, but, it seems to me the tributaries on the right play a role in determining the calving position by exerting pressure on the ice shelf, and I also wonder why then and now the last holdout is the middle left of the glacier. I also note the longitudinal channels (on the left) play a small role in the outline of the breakup.

Espen

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #759 on: November 20, 2020, 11:59:11 AM »
I am surprised with the low numbers of comments, so I will add some more data I find relevant in my shot at what I think is / will be happening next.
As an example I picked the Ryder Gletscher calving on July 20 2016 *), it is a very similar glacier althoug Petermann is bigger but the physics are the same in my opinion:

*) First documented here : https://forum.arctic-sea-ice.net/index.php/topic,886.msg84398.html#msg84398

Click on the image to enlarge and animate!
« Last Edit: November 20, 2020, 12:06:52 PM by Espen »
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SimonF92

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #760 on: November 20, 2020, 01:40:57 PM »
I am surprised with the low numbers of comments, so I will add some more data I find relevant in my shot at what I think is / will be happening next.
As an example I picked the Ryder Gletscher calving on July 20 2016 *), it is a very similar glacier althoug Petermann is bigger but the physics are the same in my opinion:

*) First documented here : https://forum.arctic-sea-ice.net/index.php/topic,886.msg84398.html#msg84398

Click on the image to enlarge and animate!

Espen, I do not feel informed enough to contribute but I always follow these threads and highly enjoy the material you post
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gerontocrat

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #761 on: November 20, 2020, 01:47:05 PM »
I am surprised with the low numbers of comments, so I will add some more data I find relevant in my shot at what I think is / will be happening next.
As an example I picked the Ryder Gletscher calving on July 20 2016 *), it is a very similar glacier althoug Petermann is bigger but the physics are the same in my opinion:

*) First documented here : https://forum.arctic-sea-ice.net/index.php/topic,886.msg84398.html#msg84398

Click on the image to enlarge and animate!

Espen, I do not feel informed enough to contribute but I always follow these threads and highly enjoy the material you post
Same with me...not sure that "Cor, blimey" and "Bloody hell" add much science to the discussion.

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Tor Bejnar

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #762 on: November 20, 2020, 09:07:59 PM »
Thanks for that important (and very possibly controlling) addition, Oren.
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Espen

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #763 on: November 21, 2020, 12:03:28 AM »
Hello guys,

I appreciate all your comments, I expected more though from you citizen sciencetist, some of you are heading in the same direction that will be my conclusions, but let's see  if more is willing to contribute to this interesting discussion, you all know Petermann will make headlines again.
And the professionals will not show their cards and and try to make profits from it, a well known exercise in this business, but we can afford to loose in that game!
« Last Edit: November 21, 2020, 12:27:11 AM by Espen »
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oren

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #764 on: November 21, 2020, 12:14:07 AM »
I will add one more observation. After the crack is seen widening significantly, even though it's not yet completed across the glacier, the calving comes along very soon, within days or a few weeks at most. So I am guessing we are nearing the end.

SimonF92

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Re: Petermann Gletscher / Petermann Fjord / North West Greenland
« Reply #765 on: Today at 12:26:40 PM »
Not really sure if this helps, but the underwater topography might play a role in calving, the bedrock is significantly elevated to the 'left'.

I suspect the calving face is thicker on the left due to the ocean being colder on that side due to less upwelling of warmer currents
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