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Author Topic: Zachariae Isstrøm / Nioghalvfjerdsfjorden / NE Greenland  (Read 367069 times)

Espen

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #500 on: August 01, 2015, 11:58:00 PM »
2nd calving this season at Nioghalvfjerdsfjorden this time larger than the previous:

Please click on image for animation!
Have a ice day!

Wipneus

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #501 on: August 03, 2015, 02:31:07 PM »
Detail sequence of Landsat 8 images from 17th July and 2nd Aug (one cycle = 16 days apart).

I used the panchromatic (band 8) 15m/pix data, over-sampled at 5m/pix and aligned the images just above the new calving line. From the displacement needed for the alignment the speed of the glacier can be calculated. This time it is 25x1 pixels, which gives 7.8 m/day, range 7.2 - 8.4 assuming uncertainty of two pixels.

See the shadow in the last frame (RH side, above the middle), something seems to be sticking its nose far above the surrounding bergs.

(click to start the animation)



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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #502 on: August 03, 2015, 02:56:16 PM »
My first impression was this was a patch of water that had opened up without melange. Then I realized that must be a shadow, similar to the one being cast just a little bit south but this shadow is much longer. It has to be an iceberg, no?????

Wipneus

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #503 on: August 03, 2015, 03:43:03 PM »
Now you say it, it has the look of a patch of open water as well.

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #504 on: August 04, 2015, 02:25:39 AM »
It sometimes helps to 'invert' these Landsats ...

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #505 on: August 04, 2015, 05:52:39 AM »
Quote
IW velocity-mapping by cross-correlation is preferable to manually picking features to track

Good point. Getting at the velocity vector field manually over any kind of area would drive a person to distraction. Repeating over many image dates would finish them off. However there is some value in identifying hotspots, anticipating what magnitudes to expect and determining whether acceleration is plausibly within reach of IW Sentinel within what sort of time frame and what level of error.

In the presence of contrast changes, normalised mutual information is a much better alignment measure that cross-correlation
Correlation matches absolute brightness values, where as mi roughy corresponds to comparing histograms.

The extreme example is 'match a white square on a black background with a black square on a white background' any corr. Based measure will return ' don't overlap the squares at all' any mi based measure will return 'overlap the squares as much as possible'.
Mi is also relatively insensitive to multiplicative noise, as long as the underlying 'texture'is still there.

Another alternative approach is to preprocess the data with x, and y gradient operators, and match vs offset is based on  abs (corr x) + abs(corr y)

You should be able to co register Landsat and sentinel data that way, given that edges in one field are also edges in the other, and we know the precise scaling factor between the 2 data sets.

(I have done post research looking at how to compare and align medical images)

Iceismylife

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #506 on: August 06, 2015, 06:29:18 PM »
2nd calving this season at Nioghalvfjerdsfjorden this time larger than the previous:

Please click on image for animation!

Any one else note the lack of melt water on the surface in the later pix rather than the first one?

Did it all go to bed rock?  The ones that got smaller all look to be interconnected.  The ones that got bigger don't look to be interconnected.

That is a lot of water to go somewhere.  Could it start moving soon?

Wipneus

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #507 on: August 08, 2015, 05:11:30 PM »
In this sequence a few of the calving's break and/or topple over. Among them is the piece of ice - in the orange circle - that has my "hexagonal" feature, that I have used to track the glacier movement for years. It is now finally gone (under).

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #508 on: August 08, 2015, 06:59:29 PM »
Seems like there are 3-4 others flipping over. With a careful study of before and after surface areas, it seems like those that are completely rolling over (to expose the underside of the glacier) could be distinguished from those just flipping on their sides. The former might reveal meltwater channels or ocean water erosion.

Espen

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #509 on: August 08, 2015, 07:48:54 PM »
I can count 7 places?
Have a ice day!

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #510 on: August 09, 2015, 07:03:07 AM »
Interesting ... seems like a lot. Note the orangish color around the very freshest turned ice. That would be bluish in the original (just subtract the %s provided from 100%) which I believe Wipneus made as fairly close to natural color. It is moderately difficult to exaggerate the orange (to isolate it) while leaving the rest undisturbed; what worked best is channel remixing. Probably principle components would work better.

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #511 on: August 09, 2015, 08:12:41 AM »
Gents,

please observe that those bergs, which flip over, all cast a longer shadow after their flip. This may indicate that they either had a keel before they flipped, or that they were more porous than surrounding icebergs.

Cheers P

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #512 on: August 13, 2015, 06:59:33 AM »
Gents,

please observe that those bergs, which flip over, all cast a longer shadow after their flip. This may indicate that they either had a keel before they flipped, or that they were more porous than surrounding icebergs.

Cheers P
A simpler explanation lies in the shape of the bergs. The tabular bergs have an even height above water and an even depth below. The volume giving it its buoyancy is area x average depth which in this case is the depth at any point, The same applies to  the height. After breaking into an irregular shape and turning it can have greater height and greater depth in some places despite the same ratio of volume above and below water.

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #513 on: August 13, 2015, 11:02:21 AM »
A simpler explanation lies in the shape of the bergs. The tabular bergs have an even height above water and an even depth below.
Not sure about the even depth as that should depend on the shape of the fjord-bottom. At Jakobshavn it seemed like the center of the trunk was thinker in the middle of the flow.

Iceismylife

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #514 on: August 13, 2015, 05:47:23 PM »
A simpler explanation lies in the shape of the bergs. The tabular bergs have an even height above water and an even depth below.
Not sure about the even depth as that should depend on the shape of the fjord-bottom. At Jakobshavn it seemed like the center of the trunk was thinker in the middle of the flow.
The free floating ice that has been that way for a bit would be more than less flat bottomed.  Give or take a bit.

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #515 on: August 13, 2015, 10:54:52 PM »
the row of bergs which turned sideways at calving a few weeks ago show flat bottoms and a constant thickness at least in that middle part of the glacier

Iceismylife

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #516 on: August 14, 2015, 07:19:39 PM »
Detail sequence of Landsat 8 images from 17th July and 2nd Aug (one cycle = 16 days apart).

I used the panchromatic (band 8) 15m/pix data, over-sampled at 5m/pix and aligned the images just above the new calving line. From the displacement needed for the alignment the speed of the glacier can be calculated. This time it is 25x1 pixels, which gives 7.8 m/day, range 7.2 - 8.4 assuming uncertainty of two pixels.

See the shadow in the last frame (RH side, above the middle), something seems to be sticking its nose far above the surrounding bergs.

(click to start the animation)
It looks to me to be something pushed up out of the water.  It is at the approximate point of contact between the advancing calving event and the prior events left overs.  Is the new one starting to role over?

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #517 on: August 15, 2015, 12:27:31 AM »
Today's Terra image shows some cracks in the Northeast Water Polynya.  The breakup is happening a little later than in the last two years.

Wipneus

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #518 on: August 16, 2015, 10:05:51 AM »
Follow up of the sequence presented on 8th August. Since these Landsat images are not from the same orbital position they cannot be perfectly aligned, I aligned -best effort- just above calving front.
More calvings do break or topple or flip over (the non-exclusive "or" is meant). The dark shadow identified some weeks ago is gone leaving a somewhat darkened patch of ice melange.

(click on the picture)

oren

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #519 on: August 16, 2015, 12:28:53 PM »
Somehow it feels that the rate of calvings has slowed down (gut feeling, not verified).
I'm anxiously waiting for the clearing of the sea ice, which could bring about a significant retreat. It's running late this year.

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #520 on: August 16, 2015, 03:33:56 PM »
Below is a close-up of a berg breaking in half, with one half rotating 45º or so about a horizontal axis exposing a cleavage plane. The other smaller berg has only a small eastward piece rotating, not clear why it is so white (= dark in original).
« Last Edit: August 16, 2015, 03:42:47 PM by A-Team »

Iceismylife

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #521 on: August 18, 2015, 06:43:05 PM »
Below is a close-up of a berg breaking in half, with one half rotating 45º or so about a horizontal axis exposing a cleavage plane. The other smaller berg has only a small eastward piece rotating, not clear why it is so white (= dark in original).
looks like shadows to me.

Iceismylife

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #522 on: August 18, 2015, 06:50:05 PM »
That is rotten ice and is just disintegrating.

Wipneus

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #523 on: August 22, 2015, 09:15:19 AM »
Detail of the sequence 14-20 August. The images are from different orbital positions, so some distortions and shadowing are from that. A thin sliver off the "new" expanding calving front shows it is unstable and collapses.

Wipneus

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #524 on: August 22, 2015, 06:10:14 PM »
It is barely visible in the animation above, but rendering the image to 5m/pixel shows the beginning of a future calving front a bit clearer. Added an arrow, just to be sure.

oren

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #525 on: August 22, 2015, 10:16:08 PM »
It is barely visible in the animation above, but rendering the image to 5m/pixel shows the beginning of a future calving front a bit clearer. Added an arrow, just to be sure.

Genius observation.
I'd never have noticed it myself, but now that you pointed it out it's very clearly shown in the animation.

Wipneus

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #526 on: August 25, 2015, 06:06:05 PM »
On today's hi-res (IW) sentinel 1 image that developing future calving front is easy visible.
 

Wipneus

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #527 on: August 28, 2015, 06:53:04 PM »
Animation of the two 2015 calvings of Nioghalvfjerdsfjorden in natural color (45m/pixel).

(click to start)

Espen

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #528 on: September 27, 2015, 12:07:41 AM »
Zachariae is still moving:
Have a ice day!

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #529 on: September 27, 2015, 04:07:56 PM »
Yearly review, not much change in calving front position, but the amount of ice debris in front of Zachariae Isstrøm is substantially larger:
Have a ice day!

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #530 on: September 29, 2015, 12:07:05 AM »
Yearly review, not much change in calving front position, but the amount of ice debris in front of Zachariae Isstrøm is substantially larger:

I think the main difference this year is that the sea ice in front of ZI never went away, which prevented the big flushing and retreat that happened last year in August.

Wipneus

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #531 on: October 20, 2015, 09:35:17 AM »
Animation from Sentinel 1A 23rd Sept-17th Oct, 24 days or 2 S1A cycles. Estimated speed measured at the calving front #3 (#1 is the front that has fully calved) is 6.2-7.2 [m/day], a bit slower than in the beginning of this summer. Measuring using the visually alignment method using the Gimp is far more difficult anyway: less tiny details are visible on the glacier surface.

About calving fronts: 4 more features are visible that could be future fronts 4-7.

(click to start animation)

Wipneus

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #532 on: October 20, 2015, 09:38:04 AM »
Detail of the same sequence: one of the upside down icebergs (the one with my hexagonal feature down now) breaks up again.

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #533 on: October 20, 2015, 12:45:09 PM »
What is causing the even spacing of future bands 4-7?

Espen

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #534 on: November 04, 2015, 10:06:05 PM »
Zachariae Isstrøm still alive

Here is a youtube of the animation:

« Last Edit: November 04, 2015, 10:56:14 PM by Espen »
Have a ice day!

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #535 on: November 09, 2015, 12:37:49 PM »
S-1 IW HH 5.11.2015 in UTM zone 27 at 15 pixel-size:


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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #536 on: November 09, 2015, 05:25:56 PM »
Awesome image.

Might as well be grayscale? It is coming up as RGB, that would cut file size down from 49 MB to 12 MB. With two dates and grayscale, we can make a movement RGB in ImageJ.

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #537 on: November 12, 2015, 09:41:03 PM »
The linked reference (and associated articles) discuss how the grounding line of Zachariæ Isstrøm became detached from a submarine sill and has been losing ice mass at an accelerating rate since 2012:

J. Mouginot, E. Rignot, B. Scheuchl, I. Fenty, A. Khazendar, M. Morlighem, A. Buzzi and J. Paden (12 Nov 2015), "Fast retreat of Zachariæ Isstrøm, northeast Greenland", Science DOI: 10.1126/science.aac7111


http://www.sciencemag.org/content/early/2015/11/11/science.aac7111.abstract


Abstract: "After 8 years of decay of its ice shelf, Zachariæ Isstrøm, a major glacier of northeast Greenland that holds a 0.5-meter sea-level rise equivalent, entered a phase of accelerated retreat in fall 2012. The acceleration rate of its ice velocity tripled, melting of its residual ice shelf and thinning of its grounded portion doubled, and calving is now occurring at its grounding line. Warmer air and ocean temperatures have caused the glacier to detach from a stabilizing sill and retreat rapidly along a downward-sloping, marine-based bed. Its equal-ice-volume neighbor, Nioghalvfjerdsfjorden, is also melting rapidly but retreating slowly along an upward-sloping bed. The destabilization of this marine-based sector will increase sea-level rise from the Greenland Ice Sheet for decades to come."

See also:
http://news.uci.edu/research/massive-northeast-greenland-glacier-is-rapidly-melting-uci-led-team-finds/

Extract: "A glacier in northeast Greenland that holds enough water to raise global sea levels by more than 18 inches has come unmoored from a stabilizing sill and is crumbling into the North Atlantic Ocean. Losing mass at a rate of 5 billion tons per year, glacier Zachariae Isstrom entered a phase of accelerated retreat in 2012, according to findings published in the current issue of Science."


http://www.nytimes.com/2015/11/15/magazine/the-secrets-in-greenlands-ice-sheets.html?_r=0

Also see:


Carolyn Gramling (2015) "How warming oceans unleashed an ice stream", Science

http://www.sciencemag.org/content/350/6262/728.summary

Summary: "Beneath the calm, white surface of Greenland, rivers of ice are flowing into the ocean—and some are moving very fast indeed. The speedy glaciers on the island's warmer west coast, shedding kilometers of ice into the sea each year as warm ocean waters undermine them, have raised the most alarm about potential sea level rise. But now, a vulnerable glacier on the other side of the island, part of a massive flow of ice known as the Northeast Greenland Ice Stream, shows that yet another region of Greenland is feeling the effects of warming oceans. And a new study suggests it's likely to continue its rush to the sea for decades to come."

Edit: See also:
http://climate.nasa.gov/news/2366/
« Last Edit: November 12, 2015, 11:25:47 PM by AbruptSLR »
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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #538 on: November 12, 2015, 10:43:41 PM »
The Guardian has an article:

Collapsing Greenland glacier could raise sea levels by half a metre, say scientists

Huge Zachariae Isstrom glacier has begun to break up, starting a rapid retreat that could continue to raise sea levels for decades to come



A major glacier in Greenland that holds enough water to raise global sea levels by half a metre has begun to crumble into the North Atlantic Ocean, scientists say.

The huge Zachariae Isstrom glacier in northeast Greenland started to melt rapidly in 2012 and is now breaking up into large icebergs where the glacier meets the sea, monitoring has revealed.

The calving of the glacier into chunks of floating ice will set in train a rise in sea levels that will continue for decades to come, the US team warns.

“Even if we have some really cool years ahead, we think the glacier is now unstable,” said Jeremie Mouginot at the University of California, Irvine. “Now this has started, it will continue until it retreats to a ridge about 30km back which could stabilise it and perhaps slow that retreat down.”

Mouginot and his colleagues drew on 40 years of satellite data and aerial surveys to show that the enormous Zachariae Isstrom glacier began to recede three times faster from 2012, with its retreat speeding up by 125 metres per year every year until the most recent measurements in 2015.

The same records revealed that from 2002 to 2014 the area of the glacier’s floating shelf shrank by a massive 95%, according to a report in the journal Science. The glacier has now become detached from a stabilising sill and is losing ice at a rate of 4.5bn tonnes a year.

Eric Rignot, professor of Earth system science at the University of California, Irvine, said that the glacier was “being hit from above and below”, with rising air temperatures driving melting at the top of the glacier, and its underside being eroded away by ocean currents that are warmer now than in the past.

“The glacier is now breaking into bits and pieces and retreating into deeper ground,” he said. The rapid retreat is expected to continue for 20 to 30 more years, until the glacier reaches another natural ledge that slows it down.

The scientists recreated the history of the glacier from aerial radar, gravitational measurements and laser profiles, and from radar and optical images taken from space. The combined data reveal the changing shape, size and position of Greenland glaciers over the past four decades.

To the north of Zachariae Isstrom, the scientists studied a second large glacier called Nioghalvfjerdsfjorden. Together, the two glaciers drain a region of nearly 200,000 sq km, amounting to 12% of the Greenland ice sheet. Were both to melt, they would contribute a full metre to global sea levels.

The monitoring showed that Nioghalvfjerdsfjorden glacier was also melting rapidly, but retreating more slowly than Zachariae Isstrom along uphill terrain. If the thinning continues at today’s pace, the scientists believe the ice shelf will become vulnerable to break up in the near future.

The bleak assessment of the glaciers’ retreat comes only months after Nasa launched an urgent six year project called Oceans Melting Greenland, aptly contracted to OMG, to understand the processes that drive the loss of Greenland ice.

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #539 on: November 13, 2015, 12:53:44 AM »
This is an absolutely stunning article, both from the sea level rise perspective and as satellite imagery tour de force. It is free full text with fabulous displays in the supplemental.

Looking at the carefully constructed flowlines though, I have to wonder whether Storstrømmen to the south shouldn't be a bigger part of the mix. And the bottom upheavals have to brought in as well. See #262 on the 6th page of this forum.

http://www.sciencemag.org/content/early/2015/11/11/science.aac7111.full
http://www.sciencemag.org/cgi/content/full/science.aac7111/DC1

Eric Rignot, a principal co-author, was also featured today in a very decent NY Times interview on Jakobshavn, balanced off by more cautious quotes from Joughin. (I snuck an http:// link to our forum in the comment section to see if it would draw traffic.) The WaPo obtained an interesting assessment of the new paper by J Box and astonishingly sourced the  accession number of the Landsat (if only we could get the journals to do this!).

http://www.nytimes.com/2015/11/15/magazine/the-secrets-in-greenlands-ice-sheets.html
https://www.washingtonpost.com/news/energy-environment/wp/2015/11/12/scientists-say-greenland-just-opened-up-a-major-new-floodgate-of-ice-into-the-ocean/

A round of applause to Espen for seeing the significance of Zachariae so far back? We have a very respectable forum on this already and this article is supplying a much improved baseline on sills and speeds for our future coverage.

Quote
On ZI, we find that the grounding line in year 1996 (5) was positioned 450-m below sea level (bsl), on a previously unknown sill that crosses the entire glacier width. Seaward of the sill, the seafloor drops to 800-m bsl (Fig. 1D). Inland of the sill, the glacier bed remains between 400 and 700-m bsl for 30 km. The bed then rises to reach a ridge at sea level. The ridge is cut across by a 300-m deep channel that connects with interior regions where the bed remains 300-m bsl for another 150 km.

On NG, the 1996 grounding line was 600-m bsl. We find no sill and the bed is sloping upward until 45 km inland. Seismic data collected in the 1990s (6) indicate that the ice shelf floats on a 900-m bsl cavity. The seafloor rises to 200-m bsl to the east where the ice front is anchored by islands and ice rises and 600-m bsl to the north into Dijmphna Sund.

We map the glacier grounding lines from 1992 to 2015 (Fig. 1C and figs. S2 and S3) using differential satellite radar interferometry (DInSAR). The grounding line of ZI retreated by 3.5 km at its center between 1996 and 2010, and 3.5 km in 2011-2015 (Fig. 1C). The mean rate of grounding line retreat therefore quadrupled from 230 m/yr to 875 m/yr before and after 2011. On NG, the grounding line retreated 1 km between 1992 and 2011, and has remained stable since. The DInSAR observations reveal a downward tilting of the ice front surface of ZI by 75 cm between Dec. 16-20 and Dec. 20-24, 2014 in a section 1-km wide by 7-km long (Fig. 1C). We attribute this deformation to a buoyancy-driven rotation of the terminus depressed below floatation and facilitated by the propagation of basal crevasses to the water line (8).

We project that ZI may continue retreating rapidly for another 20-30 yr. Its ice front will progressively widen from 19 km at present to 50 km about 30 km upstream, thereby increasing ice discharge. The height of the calving cliff will increase from its current 75 m to enhance the risk of ice fracture (11). With the formation of a calving cliff, the ocean-induced melt rates will increase significantly because buoyant melt water plumes rise faster along a vertical face than along a near-horizontal ice shelf bottom (5, 12). Beyond 30 km, the retreat will be slowed down by a rising bed topography but submarine channels will maintain the contact with the ocean into the deep interior.
« Last Edit: November 13, 2015, 11:34:02 AM by A-Team »

Neven

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #540 on: November 13, 2015, 01:01:30 AM »
Quote
A round of applause to Espen for seeing the significance of ZI so far back?

Absolutely! Espen should be thrilled, because he's been saying for a long time that Zachariae doesn't get the attention it deserves.

Well done, Espen!  :-*
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A-Team

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #541 on: November 13, 2015, 03:00:20 AM »
Quote
Espen's been saying for a long time that Zachariae doesn't get the attention it deserves.
Indeed his first post along those lines was on June 25, 2013 though Peak Indignation was not attained until a year later (post #175) as Jakobshavn and Petermann continued to hog all the attention. Please note that wipneus, sidd, nukefix and others have made important contributions to this forum in the areas of panchromatic sharpening, netCDF presentations, sentinel radar reprojection and IceBridge ice penetrating radar.

Flipping through the older posts, I'd say this has been a very respectable forum scientifically, considering it's done in real time. The Rignot group has introduced a few new techniques like mass conservation, difSAR and airgrav inversions that raise the bar for remote sensing bloggers but it's possible we can add them to our bag of tricks.

The prospects for new data on Zachariae in 2016 are excellent. The NEEM dome has been dragged over to middle NEGIS for a deep ice core and the critical near-shore bathymetry and physical oceanography may also get done. This is a very curious ice stream overall but one that we are getting a predictive grip on, once rheology temperatures and ocean circulation are in hand.

Quote
If the current trend continues, the northeast corner will become a significant contributor, or perhaps the largest contributor, in the near future -- Shfaqat Khan

http://www.scientificamerican.com/article/once-stable-greenland-glaciers-face-rapid-melt/
http://www.climatecentral.org/news/new-greenland-ice-melt-fuels-sea-level-rise-concerns-17187
« Last Edit: November 13, 2015, 12:24:09 PM by A-Team »

nukefix

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #542 on: November 13, 2015, 04:57:03 PM »
What is causing the even spacing of future bands 4-7?
Good question, I think the bands must be snow-covered surface depressions on the radar image. Are they also visible on this radar echogram or what is causing the banding upwards from the GL?


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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #543 on: November 13, 2015, 05:58:42 PM »
Right now there's massive discharge of fresh water at Z.I. http://www7320.nrlssc.navy.mil/hycomARC/navo/arcticsss_nowcast_anim30d.gif

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #544 on: November 13, 2015, 06:55:30 PM »
Quote
on this radar echogram what is causing the banding upwards from the grounding line?

The scale on these radargrams is 10 pxls/km horizontally vs 200 vertically, ie there's a 20:1 compression of the horizontal. This results in exaggeration of banding. Basically as the radar passes over bedrock with steep topography, there is attenuation of backscatter by the steep walls. The Cresis radars typically collect from about 100 m x 100 m so data is not point-like.

I'm of the opinion banding artifacts can be removed by masking prior to local contrast adjustment. In ImageJ this amounts to turning down the CLAHE filter action proportionally to overly bright or dark regions (which is an allowed operation). Typically people blast a copy of the original image with gaussian blur to create the companion image mask.

There's been hesitation about doing this systemically because the dB of attenuation has a physical meaning which is then lost (or warped beyond recognition). It's similar to tweaking Sentinel images for purposes other than for which it was designed eg surface roughness. However if the interest is in cleaning up the radargrams so that layering can be traced across the ice sheet, it makes sense. (It's not as if the originals get destroyed in the process.)

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #545 on: November 13, 2015, 07:47:19 PM »
Right now there's massive discharge of fresh water at Z.I. http://www7320.nrlssc.navy.mil/hycomARC/navo/arcticsss_nowcast_anim30d.gif

This is a re-post from another thread, but I provide it here to emphasize the importance of water drainage from coastal glaciers throughout Greenland, including for Z.I.:

Shuanggen Jin and Fang Zou (2015), "Re-estimation of glacier mass loss in Greenland from GRACE with correction of land-ocean leakage effects", Global and Planetary Change, doi: 10.1016j.gloplacha.2015.11.002


http://www.sciencedirect.com/science/article/pii/S0921818115301168

Abstract
The Gravity Recovery and Climate Experiment (GRACE) satellites can estimate the high-precision time-varying gravity field and the changes of Earth's surface mass, which have been widely used in water cycle and glacier mass balance. However, one of larger errors in GRACE measurements, land-ocean leakage effects, restricts high precision retrieval of ocean mass and terrestrial water storage variations along the coasts, particularly estimation of mass loss in Greenland. The land-ocean leakage effect along the coasts in Greenland will contaminate the mass loss signals with significant signal attenuation. In this paper, the precise glacier mass loss in Greenland from GRACE is re-estimated with correction of land-ocean leakage effects using the forward gravity modeling. The loss of Greenland ice-sheets is − 100.56 ± 8.86Gt/a without removing leakage effect, but − 171.56 ± 19.24Gt/a after removing the leakage effect from September 2003 to March 2008, which has a good agreement with ICESat results of − 184.8 ± 28.2 Gt/a. From January 2003 to December 2013, the total Greenland ice-sheets is losing at − 254.10 ± 6.90Gt/a from GRACE measurements with removing the leakage effect by 43.15%, while two-thirds of total glacier melting in Greenland is occurred in southern Greenland for the past 11 years. The secular leakage effects on glacier melting estimate mainly locate in the coastal areas, where larger glacier signals are significantly attenuated due to leaking out into the ocean. Furthermore, the leakage signals also have remarkable effects on seasonal and acceleration variations of glacier mass loss in Greenland. More significantly accelerated loss of glacier mass in Greenland is found by − 12.11 Gt/a2 after correcting leakage effects.
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nukefix

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #546 on: November 13, 2015, 08:41:19 PM »
Quote
on this radar echogram what is causing the banding upwards from the grounding line?

The scale on these radargrams is 10 pxls/km horizontally vs 200 vertically, ie there's a 20:1 compression of the horizontal. This results in exaggeration of banding. Basically as the radar passes over bedrock with steep topography, there is attenuation of backscatter by the steep walls. The Cresis radars typically collect from about 100 m x 100 m so data is not point-like.
So you're saying that if the fjord-bottom is very slanted the radar-return is smaller? In any case the previous two flights didn't show the same banding (the image is from the very recent Zacharias-paper).

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #547 on: November 13, 2015, 08:56:38 PM »
Better late than never ;)
I remember when Zachariae Isstrøm was more less Google unknown and that is only a few years ago.

My best moment wíth this glacier was when I informed a grand something son of Georg Hugh Robert Zachariae [1850-1937] a Danish naval officer, that a very important and huge glacier had his family name, this happened 2 - 3 years ago.

Thanks guys!

(there a several more "unknown" glaciers out there)

Lets drop the filters that prohibit viewers to watch what we are doing!
« Last Edit: November 13, 2015, 09:10:59 PM by Espen »
Have a ice day!

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #548 on: November 13, 2015, 10:35:33 PM »
Quote
the previous two flights didn't show the same banding
Probably didn't follow exactly the same flight path, so not over the same steep bedrock terrain. They seldom do, by intent, since the objective is to cover the ground with a grid. I didn't notice any accession numbers accompanying these images.  There is a case however over at Petermann where they did deliberately re-fly three lines over the ice shelf.

However the radar design changes each season, sometimes drastically. The height above the ice can also make a big difference, ie sled vs 1500 m vs way up in the sky. To my knowledge, the Cresis ice penetrating radar never shows any surface features beyond various horizontal echoing artefacts. Firn radar is something else again.

The image below shows the only repeated track at Zachariae.  Blow up the scale and the tracks will diverge. So it depends on this divergence vs the size of the sloped bedrock feature as to whether it is actually a repeat. I have no idea if these are the ones used in the paper.

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My best moment wíth this glacier was when I informed a Danish naval officer
You dress up in a uniform like that while writing your forum posts?

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Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #549 on: November 13, 2015, 11:39:04 PM »
Hi Johnm33 & ASLR

Good to know that one of my observations on the ASIB ( see http://neven1.typepad.com/blog/2012/12/piomas-december-2012.html?cid=6a0133f03a1e37970b017c34bfc063970b#comment-6a0133f03a1e37970b017c34bfc063970b ) turned out be really important.

A 12 GT/year  increase in volume loss from Greenland is also worth a note.

From time to time I see a clear line of thinking: Ideas brought to market on this site do indeed have a chance to survive immediate peer criticism. Soon, some enthusiastic researchers go off on their own in order to document ideas proposed here. Within a year or two, they come back with solid science, robust estimates and enhanced credibility. This is the way it should work. I now only hope, that the turn-over time from ideas to solid science can be reduced quite a bit. We do not have all the time in  the world to get every little detail in the cryosphere confirmed by trial and error.