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blumenkraft

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Re: PIG has calved
« Reply #1200 on: November 24, 2019, 12:42:57 PM »
The cork 03. vs. 23. Nov.

Damn clouds...
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blumenkraft

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Re: PIG has calved
« Reply #1201 on: November 24, 2019, 12:47:18 PM »
And a zoom-out!
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blumenkraft

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Re: PIG has calved
« Reply #1202 on: November 24, 2019, 12:50:50 PM »
And the middle.
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blumenkraft

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Re: PIG has calved
« Reply #1203 on: November 24, 2019, 12:59:25 PM »
And that area that caused the whole angles debate recently...

No alignment was done.
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blumenkraft

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Re: PIG has calved
« Reply #1204 on: November 24, 2019, 01:14:39 PM »
Last but not least, the side we started with as a zoom-out.

Thank you, ladies and gentlemen, for taking the tour today. Come back and tell your friends.
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Stephan

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Re: PIG has calved
« Reply #1205 on: November 24, 2019, 08:50:00 PM »
And the middle.
If you adjust the pictures to the southern rim's movement you can see the widening of both big cracks. If my eyes don't misguide me, I see lines of open waters in the cracks at some places. Some of the smaller bergs within the southern crack have tilted or tumbled. The big cracks haven't yet met the WSW (the "cork") and NE part of the PIIS. Generally the cracks have widened a little bit and lengthened a little bit, but there was no fundamental change in the last ten days. [Which means I am still waiting for the big calving event that should happen soon].

Thank you for the animations, blumenkraft!
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blumenkraft

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Re: PIG has calved
« Reply #1206 on: November 24, 2019, 09:15:40 PM »
Most welcome, Stephan! :)

A minor correction, it's 20 days in between. The 13th sadly was a cloudy day.
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blumenkraft

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Re: PIG has calved
« Reply #1207 on: November 24, 2019, 09:22:31 PM »
within the southern crack have tilted or tumbled.

You think this here is open water?
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Re: PIG has calved
« Reply #1208 on: November 24, 2019, 09:54:18 PM »
I marked the black "lines" with red arrows which I think they could be open water. The sun is shining from ENE (almost parallel to the crack) and it is hard to believe that these lines represent shadows from icebergs in the crack.
But ... I am no eye-witness so maybe I am wrong.
These black "lines" are completely absent on Nov 03 picture with a very comparable sun's illumination.

See attached picture.
« Last Edit: November 24, 2019, 10:08:47 PM by Stephan »
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crandles

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Re: PIG has calved
« Reply #1209 on: November 24, 2019, 11:01:51 PM »
My non-expert completely amateur view:

Top right arrow: I think I see signs of ice collapsing from top of cliff. While scattering into a slope seems more likely, I doubt we can rule out a slump that stays together and creates steep side which casts a shadow.

Bottom left could also be explained in that way, but others particularly second arrow from left is more difficult to explain in that way. But why not icebergs in the crack? They look like the cliff shadows rather than looking different to them.

Also what shape is the crevice? Is it a very clear U shape with very steep sides and almost flat floating bottom or can the sides collapse leading to sloping sides at the bottom of the cliffs? Could that stay for a while or would it quickly float away to level out the bottom?

wdmn

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Re: PIG has calved
« Reply #1210 on: November 25, 2019, 12:10:20 AM »
Looking at past years, is there a certain width that we would expect before seeing open water, or before calving?


oren

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Re: PIG has calved
« Reply #1211 on: November 25, 2019, 01:59:19 AM »
And a zoom-out!
Thanks for this.
The little piece corking the cork has now calved, and the cork itself will calve soon I think. It cannot rotate forever.

baking

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Re: PIG has calved
« Reply #1212 on: November 25, 2019, 05:56:28 AM »
Also what shape is the crevice? Is it a very clear U shape with very steep sides and almost flat floating bottom or can the sides collapse leading to sloping sides at the bottom of the cliffs? Could that stay for a while or would it quickly float away to level out the bottom?

Pine Island Glacier is made of very old ice.  By the time it reaches this close to the front it has spread out and melted to about 400 meters thick.  Only about 40 meters are above water.  The space inside the crack is filled with a mixture of sea ice, which is water that has frozen recently on the surface, and smaller chunks of icebergs that have fallen off of the walls.

As the crack widens, the sea ice does not form quickly enough which leaves gaps where the the water can be seen through the ice.  There are some shadows visible on the right side of the crack, but shadows are fairly easy to distinguish because some details can be seen in the shadows while the water is almost pure black.  Also, there cannot be shadows on both sides of the crack.

The walls of icebergs 40 meters high are fairly stable.  Most of the large chunks that have broken off were probably weakened in the process of the original crack formation.  Before it split there may have been forks in the crack from the stress that led up to the break.

There has been some discussion about the stability of very high ice cliffs, over 100 meters above the water surface, but it most icebergs breakoff in thicknesses of 300-400 meters of 30-40 meters high and survive until they get out into the open ocean.  They often break into smaller pieces before then, but the larger pieces still maintain the tabular or flat-topped shape.

Edit:  Perhaps what you are looking for is that the chunks of ice will spread out and not pile up.  Basically, they are floating and will eventually find a point of lowest energy which is where every chunk of ice is floating at its own level.
« Last Edit: November 25, 2019, 06:04:02 AM by baking »

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Re: PIG has calved
« Reply #1213 on: November 25, 2019, 11:22:11 AM »
Attached a gif with the days of 24/10, 03/11 and 23/11 (damage on the day of 13/11 was unusable)

It is obvious that the amalgam of icebergs and ice detritus is tearing up as a result of the widening of the rift (typical fractures more or less parallel).

I agree with Baking, the icebergs in the rift are typically formed by the ice between two parallels fractures connected at the beginning to both sides of the rift and torn up by the enlargement of rift.

Anecdotal: In the image of 23/11 we can notice the fracking of the first iceberg on the right (top left corner). It can also be noted that the second, which had not fully turned around, is currently completely returned

(click to move)
« Last Edit: November 25, 2019, 12:03:30 PM by paolo »

crandles

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Re: PIG has calved
« Reply #1214 on: November 25, 2019, 12:57:33 PM »
OK thanks, you have convinced me.

paolo

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Re: PIG has calved
« Reply #1215 on: November 25, 2019, 03:50:19 PM »
Before its next end I wanted to retrace the history of the "Cork", its childhood and its old age (its maturity being hidden by the Antarctic night), it seemed to me an honor that belonged to him.

It's not nice, but I'd point out that he got fatter with age and he shrink.

Images of 18/11/2018, 28/12/2018, 06/02/2019, 14/09/2019, 24/10/2019 and 23/11/2019

In the first picture a round signals the point of view (alignment point)

(click to move)

paolo

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Re: PIG has calved
« Reply #1216 on: November 25, 2019, 04:40:10 PM »
To look a little SWT side, I wanted to have a unique and consistent document giving information on speeds, elevations (and thus thicknesses for the floating part) and bathymetry.

The sources were very varied: pdf, word, image, the images each had its scale and direction and areas represented were always different ...

So, I built a gif from this motley set.

Minimal information on these images

First image: MacGregor et al. 2013 (figure 9b) Surface velocity (Rignot and others, 2011a) and recent coastline history (MacGregor and others, 2012a)
Second image: Shean et al. 2017 (figure 1) 2006–2016 median surface velocity (Christianson et al., 2016; Joughin et al.,2010) over a shaded relief map from October–December 2012 DEM mosaic
Third image: Shean et al. 2019 (figure 3) October–December 2012 WorldView/GeoEye DEM mosaic of the PIG ice shelf. White outline shows∼2011 grounding line. Elevation values are the corrected surface height (Eq. 1) above the EGM2008 geoid.
Fourth image: Millan et al. 2017 (Supporting Information, figure S2g) Bed elevation from BEDMAP-2 [Fretwell et al., 2013]


(click to move) delay between images 3s
« Last Edit: November 25, 2019, 05:36:50 PM by paolo »

blumenkraft

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Re: PIG has calved
« Reply #1217 on: November 25, 2019, 04:41:00 PM »
I played around with the Gamma and Gain on both, L1C and L2C.

I found the feature Stephan thinks is seawater surely has a different colour than the shadows seen in the picture which makes me think he is right.

I think there is a current (or perhaps upwelling) going on there; driving the mushy stuff (the melange of sea ice and iceberg pieces described by baking above) apart.
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blumenkraft

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Re: PIG has calved
« Reply #1218 on: November 25, 2019, 04:46:12 PM »
BTW, a very low Gain and a high Gamma gives you the most beautiful effect! <3

Click to enlarge!
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paolo

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Re: PIG has calved
« Reply #1219 on: November 25, 2019, 05:07:15 PM »
Blumenkraft,

If you look closely at my gif (post 1213), and especially for example the last fracture on the right you will notice that the small icebergs at the top of this fracture remain attached to the corresponding rift walls at the top of the image and so the small icebergs at the bottom of the fracture remain attached to the rift wall at the bottom of the image.

There are no currents in the rift orthogonal to the rift and it simply means that the mixture has a certain rigidity: it is frozen and the movements between the two edges of the rift are important.
« Last Edit: November 25, 2019, 05:13:07 PM by paolo »

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Re: PIG has calved
« Reply #1220 on: November 25, 2019, 06:09:59 PM »
@ all:
Thank you for your further analyses, information and gifs.
Greetings from Germany Stephan
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blumenkraft

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Re: PIG has calved
« Reply #1221 on: November 25, 2019, 07:37:19 PM »
There are no currents in the rift orthogonal to the rift

How do you mean, Paolo?

The open water lines are rather parallel to the crack.
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blumenkraft

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Re: PIG has calved
« Reply #1222 on: November 25, 2019, 07:38:32 PM »
Greetings from Germany Stephan

Grüsse aus dem Pfälzer Wald zurück! ;)
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paolo

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Re: PIG has calved
« Reply #1223 on: November 25, 2019, 08:28:10 PM »
Blumenkraft,

1.   If the rift widens "quickly" and the middle surface is sufficiently rigid being frozen, then inevitably there are open waters parallel to the edges of the rift (what shows the gif, post 1213)


2.   On the other hand, a current that creates open water tends normally to create them orthogonal to the movement of water

Stephan

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Re: PIG has calved
« Reply #1224 on: November 25, 2019, 09:42:36 PM »
If the rift widens "quickly" and the middle surface is sufficiently rigid being frozen, then inevitably there are open waters parallel to the edges of the rift (what shows the gif, post 1213)

This sounds very reasonable to me. The rift is widening faster than the ice mélange that it is filled with can expand and cover the whole surface.
And, the rift is also widening faster than a re-freeze of any open water surface happens.
I see for many weeks now open waters in front of the PIIS, although further north, where the air should be warmer, a lot of sea ice is still waiting for the summer thaw. I guess that warmer waters from below upwell in front of the PIIS. Why shouldn't this upwelling also happen below PIIS and keep the waters there warmer as they would be → with the effect of a slower re-freezing ability ?!?
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baking

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Re: PIG has calved
« Reply #1225 on: November 26, 2019, 01:00:38 AM »
Why shouldn't this upwelling also happen below PIIS and keep the waters there warmer as they would be

These are called polynyas, or persistent areas of open water.  The main reason you don't see it in cracks is that the water needs to well up to the surface and there is not enough area for upwelling to occur.  Remember that 90% of the ice is below the water line so these cracks are very large and very steep and  room to circulate very little current will upwell in the cracks, or not enough to melt all the ice that is collected there.

Also, winds can blow floating ice away from polyanas, but in a crack there is nowhere for it to go.

blumenkraft

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Re: PIG has calved
« Reply #1226 on: November 26, 2019, 07:32:12 PM »
Do we know they are persistent though?
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baking

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Re: PIG has calved
« Reply #1227 on: November 27, 2019, 12:51:15 AM »
Do we know they are persistent though?
(Karen) Alley 2019 "Troughs developed in ice-stream shear margins precondition ice shelves for ocean-driven breakup"
https://advances.sciencemag.org/content/5/10/eaax2215

and especially the supplementary materials will teach you more than you would ever want to know about persistent polynyas.

blumenkraft

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Re: PIG has calved
« Reply #1228 on: November 30, 2019, 06:28:29 PM »
Oh ...
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baking

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Re: PIG has calved
« Reply #1229 on: December 02, 2019, 08:23:57 PM »
The "hook" at the end of the main cravasse that blumenkraft discovered on Saturday is significant because it aligns with the marginal cravasses down stream.  This is evidence of a possible connecting crack through to the margin which would lead to a major calving event.  (This is the only area where the leading edge is still solidly connected to the rest of the glacier.)

Yesterday's high-resolution Sentinel-1 image below actually shows a possible crack.  Of course it could also be an artifact of the speckle filtering, but we need to keep an eye on it.

IceConcerned

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Re: PIG has calved
« Reply #1230 on: December 03, 2019, 04:18:25 PM »
On top of Baking's remarks, we can note on the eastern side many small cracks (inside the "square"
Besides, the main (putative) iceberg has fractured a lot, its lifetime after the calving might be quite reduced as a consequence
On the crack on the SW tributary is far more apparent than on previous images : is it an effect of the post treatment on an aggravation ?

paolo

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Re: PIG has calved
« Reply #1231 on: December 03, 2019, 09:19:26 PM »
With regard to the PIG it is important to remember that there are three types of rifts:

Rifts transversal, more or less orthogonal to stream and opening up to the center of the stream, their origin being basal crevasses created at the grounding zone. These rifts will spread to margins

Rifts marginal, more or less orthogonal to stream, in the shear margins, they can extend to the center of the stream (but this is not currently the case), their origin being the shear between the shelf and the flow

Rifts, more or less parallel to streams in the shear margins, their origin being the basal channel who form in the shear margin: thinning and melt that create more fragile areas where the shelf can break. In the case of Pig often contribute to the detachment final of the icebergs

In our case I think that once broken the SW join, the NE join will break by opening a rift parallel to the flow and that the marginal rifts will instead contribute to the cleaning of shear margin, during and/or after the break

baking

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Re: PIG has calved
« Reply #1232 on: December 04, 2019, 08:26:35 PM »
There is currently a small scientific team camped in tents within sight of Pine Island Glacier.  Anyone who thinks the calving will be within the next month should be watching this twitter feed:

https://twitter.com/geologicalJo/status/1198916665808961536

paolo

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Re: PIG has calved
« Reply #1233 on: December 04, 2019, 08:40:52 PM »
Thanks for the url
 8)

blumenkraft

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Re: PIG has calved
« Reply #1234 on: December 04, 2019, 09:10:07 PM »
New growing crack upstream (also above the grounding line).

Click GIF to play.
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Re: PIG has calved
« Reply #1235 on: December 05, 2019, 03:54:53 PM »
End of November, all the Sentinel 1 images of the PIG area (and far beyond that) are turning a darker shade of grey. Happens every year, surface melting is cause no doubt. The reduced scattering makes some previously undetected  cracks  more visible, see the arrow drawn in the second frame.
For the overview I scaled the image to 50m/pix (from 10m/pix in the original). I will show some hi-res images in another post.

blumenkraft

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Re: PIG has calved
« Reply #1236 on: December 05, 2019, 04:07:34 PM »
surface melting is cause no doubt.

OMG that makes so much sense. I was wondering what happened. Thanks for pointing this out, Wipneus!
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Re: PIG has calved
« Reply #1237 on: December 05, 2019, 04:13:44 PM »
Two more details of the crack discussed in the previous post. The first is the Sentinel 1 SAR image in native 10m/pix resolution. The second is from an earlier Sentinel 2 optical image, scaled to 5m/pixel (from native 10m). Here the contrast was enhanced to bring out the linear feature, that I had missed if not for the Sentinel 1 image.

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Re: PIG has calved
« Reply #1238 on: December 05, 2019, 07:07:37 PM »
I carefully compared the Sentinel pictures from Dec 03 with Nov 23, 2019.
Almost all cracks have widened and lengthened a little bit, and the "cork" has further turned counterclockwise. No microcalving and no fundamental change of the situation.
Everybody seems to be waiting for the big calving...
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Re: PIG has calved
« Reply #1239 on: December 05, 2019, 07:20:52 PM »
New big crack on SW Tributary.
What has already been mentioned by other posters in this thread the "signature" in the SW Tributary has now opened into a crack. Unfortunately the sky had been covered by thin high clouds. So I marked the new crack with a red line.

See attached picture.
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paolo

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Re: PIG has calved
« Reply #1240 on: December 05, 2019, 10:03:59 PM »
Indeed, as shows the master of all us: Wipneus  :), we must use both: Sentinel 1 and Sentinel 2 and sometimes you have to use Sentinel 2 by zooming in as much as possible.

As an example, a little anecdotal, a fact that intrigues me: I thought that the rift R1 (the northern) would have quickly joined the rift R2 rift (the southern).
The image of 03/12, with the zoom that I usually use, did not give much, but, intrigued by the image Sentinel 2, I looked more carefully and currently, in place of an extension of R1, we are witnessing the opening of several parallel cracks shifted to northwest.
In other words, the tensions are still there, but the structure of the glacier being weaker downstream we see the opening of parallel crevasses in this area, crevasses that will finish to connect between them and R1.
And for now, R1 and R2 do not seem to have to meet.

Another anecdotal fact: there is a small current in R2

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Re: PIG has calved
« Reply #1241 on: December 06, 2019, 02:31:33 AM »
The PIG definitely seems like it's undergoing a slow-motion collapse, with the only consolation being the super-high level of science in this thread. Thanks to all the posters keeping us updated with images, animations and commentary.

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Re: PIG has calved
« Reply #1242 on: December 06, 2019, 05:54:22 PM »
... and the great discussions. Love that! :)
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Re: PIG has calved
« Reply #1243 on: December 06, 2019, 07:37:36 PM »
The PIG definitely seems like it's undergoing a slow-motion collapse, with the only consolation being the super-high level of science in this thread. Thanks to all the posters keeping us updated with images, animations and commentary.
you're welcome  :)
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paolo

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Re: PIG has calved
« Reply #1244 on: December 06, 2019, 10:52:01 PM »
While waiting for the crack (the cork doesn't want to give in), and always to show the complementarity between Sentinel 1 and Sentinel 2, I wanted to watch the rift opening at the NW in the SWT.

Attached are the commented images: Sentinel1, Sentinel 2 and two zooms of the Sentinel 2 image

I found them interesting, hence this post
« Last Edit: December 06, 2019, 11:08:09 PM by paolo »

Stephan

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Re: PIG has calved
« Reply #1245 on: December 07, 2019, 09:09:54 AM »
Latest development on SW Tributary.
The calving front has moved around 100 m in the last five weeks. I identified new or deepened cracks, marked by red lines. The "nose" turned clockwise - I ask myself how long will it be connected to the ice shelf. Grounded icebergs are circled in pale magenta.

The same as with PIIS - waiting for a major calving event.

See attached picture.
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paolo

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Re: PIG has calved
« Reply #1246 on: December 07, 2019, 11:56:38 AM »
At your posts the crack movie begins

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Re: PIG has calved
« Reply #1247 on: December 07, 2019, 12:22:22 PM »
Indeed, very little support left on this side left. The darkening has reversed again (Sentinel 1 SAR, at least for the PIG) and we can make a decent animation again.

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Re: PIG has calved
« Reply #1248 on: December 07, 2019, 12:24:31 PM »
Sure enough, enough people are having an eye on this!!  ;D

(Deleted my post since Wipneus' GIF is so much better. But i want to point out that i was first! :P)
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Stephan

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Re: PIG has calved
« Reply #1249 on: December 07, 2019, 02:26:34 PM »
Thank you for posting this Wipneus and for having posted this, Blumenkraft.
From the latest Sentinel picture and the actual mini-calving I calculated that the distance between the actual calving front and the "zone of destruction" on the SW edge of PIIS has reduced now to only 1600 m, protected somewhat by the rotating "cork" which has lost another bit of support from today's calving event. And, please have a look at the current! How fast the two new icebergs leave the calving zone as if they want to say "we have been locked in here for too long, let's escape now".
I guess this is the beginning of a massive destruction of PIIS, even if the "big calving event" may be delayed due to the still intact ice shelf on the NE edge of PIIS.
It is too late just to be concerned about Climate Change