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Author Topic: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland  (Read 517642 times)

nukefix

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1750 on: August 31, 2016, 09:42:59 AM »
There's a lower-resolution S-1A EW from 30.8.2016 available from PolarView. The bumps are visible even at 40m pixel-size. Looks like no big calving took place between the 25th and the 30th.

Tealight

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1751 on: September 01, 2016, 10:00:38 PM »
Yup, I scan the amazon cloud every day. Tail of current output:

dag : 2016/8/17 ( R025 , 100.0 , 0.95)
dag : 2016/8/20 ( R068 , 100.0 , 95.42)
dag : 2016/8/23 ( R111 , 77.13 , 97.68)
dag : 2016/8/24 ( R125 , 45.73 , 96.34)
dag : 2016/8/26 ( R011 , 1.89 , 100.0)
dag : 2016/8/27 ( R025 , 100.0 , 98.16)

Last two numbers are coverage and cloud percentage. None of the latest was of interest for ice watchers.

Scanning Amazon tiles is a great idea! Thanks for that.

It saves a lot of time compared to going through all major glacier tiles individually and checking the preview. Do you use the R000 identifier in any way or is it just a reference of the actual productname?

For the last three days I get these results:
Day   Glacier   Cloud   Data
28   Jakobshavn   Null   Null
28   Petermann   73.77   100
28   Zachariae Isstrøm   Null   Null
29   Jakobshavn   Null   Null
29   Petermann   99.35   6.61
29   Zachariae Isstrøm   67.94   44.66
30   Jakobshavn   0.01   100
30   Petermann   19.76   91.88
30   Zachariae Isstrøm   100   100


Wipneus

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1752 on: September 02, 2016, 10:05:18 AM »


Scanning Amazon tiles is a great idea! Thanks for that.

It saves a lot of time compared to going through all major glacier tiles individually and checking the preview. Do you use the R000 identifier in any way or is it just a reference of the actual productname?


You need the relative orbit number when there are more images matching the tile id. in one day. They do not come in order (eg. 0 has a higher orbitnr. than 1). 

Wipneus

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1753 on: September 02, 2016, 10:08:20 AM »
In the mean time 2016/8/30 ( R068 ) came in, 0.01% clouds. S2A_R068_V20160830T151912.22WEB.B04.tiff has been uploaded.

The animation: we are lucky, not much has happened during the clouded days.
« Last Edit: September 02, 2016, 12:34:16 PM by Wipneus »

oren

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1754 on: September 02, 2016, 11:54:50 AM »
It seems JH failed to retreat this year in spite of other signs of AGW progressing. I wonder.

johnm33

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1755 on: September 02, 2016, 02:09:54 PM »
"I wonder."
 My thinking is that the sheer size and number of bergs getting stuck on the inner and outer cills has slowed everything down. That said Baffin is warm, all that melt has to percolate through, and the seasons not yet done.

A-Team

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1756 on: September 02, 2016, 05:12:58 PM »
It seems
JH failed to retreat this year in spite of other signs of AGW progressing
Further retreat of JI, now that it has completely lost its floating ice shelf, is not a prediction of AGW nor a proxy for it. (Further thinning is and we've seen evidence for that this year in newly exposed rock.)

The position of the calving front is a delicate and ever-changing balance between the rate of calving vs rate of forward advancement which are both highly specific to individual glaciers and their basal topography, till hydration and melt status. (The Rignot paper discusses these in the case of Eqip Glacier to the north which has not retreated in the last 70 years.)

The main reason Wip is providing the archive of clear, high contrast, high resolution tiffs is so we can, in the leisure of the off-season, determine whether the dramatic forward acceleration of ~2012 has continued, increased, or abated using the vast improvements in precision possible with S2A.

Because of the unusual S1A found by nfx and the continuation of the main calving season noted by j33, a major event may still be in the works. These we like to bracket with all the applicable imagery to unravel the cascade of events. It is quite possible to snooze through big events if the clear imagery is too infrequent.

The Landsat LC80100112016245LGN00 of 01-SEP-16 is completed clouded out.

oren

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1757 on: September 02, 2016, 06:31:12 PM »
Further retreat of JI, now that it has completely lost its floating ice shelf, is not a prediction of AGW nor a proxy for it.
That is understood (and thanks for the detailed reply), yet I still expected it to retreat further given the ongoing loss of buttressing at the north wall area, plus warmer globe in general, plus most (all?) recent years did produce a retreat. I am guessing the calving front has reached an area that is difficult to "cross".

nukefix

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1758 on: September 07, 2016, 04:54:06 PM »
The bumps are moving: S1A 25.8.2016-6.9.2016. Should animate..

edit: The GIF I download from the Forum does not want to animate...I don't understand why. >:(
« Last Edit: September 07, 2016, 05:16:26 PM by nukefix »

nukefix

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1759 on: September 07, 2016, 05:24:30 PM »
2nd try with small GIF, undulations are moving with the ice-stream. 25.8.2016-6.9.2016

oren

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1760 on: September 07, 2016, 06:04:29 PM »
Maybe it's just my wild imagination, but it seems the cracks in the bump have widened slightly. Calving soon?

Jim Hunt

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1761 on: September 12, 2016, 09:15:20 PM »
Reality is merely an illusion, albeit a very persistent one - Albert Einstein

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1762 on: September 18, 2016, 06:15:56 PM »
The yacht Northabout is currently moored in Ilulissat, on her way back to Bristol:

http://GreatWhiteCon.info/2016/09/northabout-heads-for-home/#Sep-18

She reports:

Air 5.4C  water 1.4C


and sends a selfie, plus a picture of a "Grand Hotel" sized iceberg:
Reality is merely an illusion, albeit a very persistent one - Albert Einstein

Wipneus

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1763 on: September 27, 2016, 06:24:59 PM »
I have uploaded S2A_R025_V20160906T150912.22WEB.B04.tiff and S2A_R025_V20160926T150952.22WEB.B04.tiff.

Attached an animation of the south calving front, with hardly any calving in the 40 days span. Quite unusual.

johnm33

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1764 on: October 03, 2016, 11:15:02 AM »

Wipneus

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1765 on: October 05, 2016, 12:59:45 PM »
2016/10/2 ( R111) is in, cloud percentage 1.14%. I Have uploaded S2A_R111_V20161002T153052.22WEB.B04.tiff.

Now nearly 2 months of no major calving of the south branch.

A-Team

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1766 on: October 05, 2016, 03:25:33 PM »
hardly any calving in the 60 days span. Quite unusual.
Most peculiar. The velocity of glacier advance has been surprisingly steady at 32.5 meters per day (from S2A at 5 m). In the past, it has markedly peaked earlier in the season. It's been so cloudy that it does not make for good animations.

The north branch seems to have a section calving in, perhaps driven by a faster drop in elevation to the east of the area encircled by yellow.

A-Team

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1767 on: October 05, 2016, 09:58:53 PM »
Hmm, calving a wedge-shaped piece in the north corner of the south branch?

S2A_R111_V20161002T153052.22WEB.B04

nukefix

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1768 on: October 06, 2016, 10:20:17 AM »
I believe the "wedge" is still attached, see this lower resolution S-1A from 5.10 (the corresponding GeoTIFF would clearer as no land-mask has been applied):

http://www.polarview.aq/images/105_S1jpgfull/S1A_EW_GRDM_1SDH_20161005T205513_E454_N_1.final.jpg

..the big wavelike structures on the north edge of the southern branch are curious, I wonder if they are caused by thinning, increased flux from the north edge or something else..


Tealight

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1769 on: October 08, 2016, 09:07:35 PM »
Could a change in ocean currents be responsible for the calving inactivity? By comparing the motion of icebergs in the fjord and the calving front I noticed that they move at exactly the same speed. The large iceberg moved 318m and the calving front advanced 300-320m depending on location. This could mean that all forward motion is caused by pressure of the glacier and ocean currents aren't tearing ice away from the front.

johnm33

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1770 on: October 10, 2016, 11:42:20 AM »
"Could a change in ocean currents be responsible for the calving inactivity?"
With huge icebergs stuck on both cills, and the fjord full of ice,the tidal exchange of waters is extremely limited and much of it's heat energy is lost melting bottom ice on it's way up the fjord.  The recent breakout at the outer cill will allow a huge amount more water to penetrate and we may yet see a clearout of the fjord before winter sets in. There's no shortage of warm Irminger water in Baffin.
An alternative may be that the ice flowing down into the fjord will have to be slowed by the winter freeze before the tides can make any impression. Time will tell.

Iceismylife

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1771 on: October 11, 2016, 04:09:48 AM »
We've had a calving event.  Movement in the ice in front of the glacier.

edit= http://go.nasa.gov/2dXXbMV http://go.nasa.gov/2eaK3Ur

Those two pix show a big berg and a shadow.  The shadow is just over two miles from where the berg was and is there several days in a row.
« Last Edit: October 13, 2016, 03:47:17 AM by Iceismylife »

prokaryotes

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1772 on: October 18, 2016, 09:20:45 PM »
We've had a calving event.  Movement in the ice in front of the glacier.

edit= http://go.nasa.gov/2dXXbMV http://go.nasa.gov/2eaK3Ur

Those two pix show a big berg and a shadow.  The shadow is just over two miles from where the berg was and is there several days in a row.


johnm33

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1773 on: October 19, 2016, 11:39:38 AM »
From the 17th http://ocean.dmi.dk/arctic/images/MODIS/Disko/20161017s01a.ASAR.jpg

The full moon just passed and it's very close to Earth just now so the tides are high and though I haven't checked this image looks like it's just after high tide with cold water being flushed out. http://ocean.dmi.dk/arctic/images/MODIS/Disko/20161018s01a.ASAR.jpg

And now some movement  http://go.nasa.gov/2egoEvy

I'll just add that the first 20K of ice now looks mobile, the fjord is about 5K wide and the ice was about 100m thick so now an extra 103Km will be flushing through twice a day. So I'd be surprised if there's no calving upstream soon.
http://ocean.dmi.dk/arctic/images/MODIS/Disko/20161019rs02.ASAR.jpg
« Last Edit: October 19, 2016, 04:00:06 PM by johnm33 »

Iceismylife

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1774 on: October 19, 2016, 05:16:50 PM »
http://go.nasa.gov/2etWM4E
http://go.nasa.gov/2etXmiX

Icebergs in different places and the calving face has retreated significantly.  I'd say we've had a calving event.\

It looks like it happened between the 7th and 8th.

nukefix

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1775 on: October 20, 2016, 11:35:28 AM »
It looks like it happened between the 7th and 8th.
Yes, 2/3 of the Southern branch has calved between 06.10 & 12.10...I'm waiting for Scihub to get back online in order to download the S-1B scene acquired on the 18th. Meanwhile here's a lower-quality snapshot downloaded from polarview.
« Last Edit: October 20, 2016, 12:31:29 PM by nukefix »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1776 on: October 20, 2016, 01:44:05 PM »
I'm waiting for Scihub to get back online in order to download the S-1B scene acquired on the 18th.  2/3 of the Southern branch has calved between 06.10 & 12.10.

This would be the first fractional calving event in the record. EarthExplorer continues to have issues both in search, processing backlog, and erratic download connections. The site has been rearranged, burying the #1 most popular product (recent Landsat-8s) into something called a pre-collection http://landsat.usgs.gov/landsatcollections.php

The Sentinel 1A above is marked up provisionally below as we await sharper imagery.

Entity ID:LC80100112016293LGN00 ... refuses to download, months into this problem
Coordinates:69.60634,-50.90001
Acquisition Date:19-OCT-16

Entity ID:LC80090112016286LGN00 ... too cloudy
Coordinates:69.60667,-49.34388
Acquisition Date:12-OCT-16

Entity ID:LC80090112016270LGN00 ... clear but too early
Coordinates:69.60642,-49.32456
Acquisition Date:26-SEP-16
« Last Edit: October 21, 2016, 01:12:42 PM by A-Team »

nukefix

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1777 on: October 20, 2016, 04:10:18 PM »
Hey A-team you are way better with animations than I am so here's the raw material.

ps. All hail S-1A/B!!
pps. The funky large waves are the cause of this...but what is causing the waves?
ppps. Looks like something introduced small shifts between the frames...in any case they should align perfectly if the small integer pixel shift is corrected
« Last Edit: October 20, 2016, 04:42:07 PM by nukefix »

A-Team

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1778 on: October 21, 2016, 02:40:30 PM »
here's the raw material. All hail S-1A/B!! The funky large waves are the cause of this...but what is causing the waves?
Fascinating series, a whole lot of analysis could be done with it.

The 06 Oct is a little weird in terms of contrast. However, looking at the set 'interferometrically' (four grayscales combined into CMYK color, not shown), the rocks aren't moving, ie they are co-registered to the pixel as provided. Perhaps snow/rain/clouds affected the Oct 6th?

Because the animation in the stationary coordinate frame was hard to grok, the images were rotated   to put the motion in the vertical direction, then adjusted to no motion in the lower part of the ice stream (ie Lagrangian co-moving coordinates), enlarged 2x to 5 m resolution, adjusted to improve contrast match, and cropped to the main action near the calving front.

This gives the peculiar animation below in which a lot of residual twisting and shifting remains near the calving front, unlike in the orderly forward progression normally seen. This would result in compressional and extensional effects that might, earlier, have given rise to the apparent waves of surface buckling.

The sides appears to be moving backwards because they are moving slower than the center, the left side especially. The second animation 'seasick' runs faster and without a pause on Oct 18th. This raises all manner of questions given the thickness and supposed rigidity of this ice, almost a mile deep (1400 m) here.

I wish the scientific community would get off their butts and start monitoring Jakobshavn. (Maybe they are but just aren't sharing.) This was another remarkable event but it seems the documentation of it will essentially be limited to 6-day Sentinel passes.
« Last Edit: October 21, 2016, 02:58:23 PM by A-Team »

Shared Humanity

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1779 on: October 22, 2016, 11:29:29 PM »
Hey A-team you are way better with animations than I am so here's the raw material.

ps. All hail S-1A/B!!
pps. The funky large waves are the cause of this...but what is causing the waves?
ppps. Looks like something introduced small shifts between the frames...in any case they should align perfectly if the small integer pixel shift is corrected

I believe this entirely new behavior of the main ice stream is due to the rapid transformation of the ice sheet, north of the main ice stream. It is thinning and retreating which is serving to unpin the ice along the north. When I look at these large waves in the ice, it seems that the ice is pivoting away from the north (moving faster along the north wall than the south), causing these shallow troughs and then these sections are fracturing and collapsing in large events. That second wave, shaded in yellow, will go next.

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1780 on: October 23, 2016, 12:10:28 AM »
I agree. The 2nd North Branch (can we call it that already?) is changing the balance of forces in the main channel, and my unsubstantiated impression has been for a while now that the calving front is skewed, with the south side holding on longer while the north side is calving.
Note: I did try going over old images, but couldn't verify my gut feeling.

A-Team

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1781 on: October 23, 2016, 03:51:45 AM »
Looking for Sentinel 2A visible images bracketing the calving date, mostly not: the satellite is now passing more over Disko Island and when it does go over Jakobshavn calving front, the cloud cover has been 60% or more, with only another ten days of sufficient daylight.

However the calving front was miraculously visible on Oct 6th It's shown below at 10 and 5 m scale after some cloud removal processing. The northeast corner is quite interesting.

Some additional ideas to kick around:

(1) the surface waves reflect bottom topography, in the sense of a stream passing over cobbles. The ice is not likely to be softened enough for this and is in any case far too thick (per paper found by sidd).

(2) The central area is getting torqued by differential resistance from the sides, the left being slower and the right faster. Consistent with this, the waves are angled up at about 45º as expected from faster side influence.

(3) google 'Jakobshavn' to see if anything has surfaced on twitter etc from people with more information; search AGU 2016 meeting abstracts for JI research; though this particular event is too recent, Jakobshavn is mentioned in 12 of the Dec 16 abstracts. It takes many steps of drilling through the AGU site to locate the abstract search tool.
« Last Edit: October 23, 2016, 12:51:31 PM by A-Team »

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1782 on: October 23, 2016, 12:44:14 PM »
Here are four noteworthy Jakobshavn talks to be presented at AGU 2016 in December, including 3 of 17 (!) from the E Rignot group. None provided supplemental material, draft documents on researchgate, or title matches at google scholar. (However gs does not index articles under open review.)

Progress seems to have been made near the calving front for ocean bathymetry, thickness of ice and bed topography; if so, earlier numbers we've used here are wrong if the depth right at the calving front is only 900 m. However 500 m resolution is less than thrilling as it amounts to 10-12 pixels at the width of Jakobshavn's calving front.

It does not appear that radar altimetry will have captured the height of the October ice stream waves above mean ice stream surface. They don't seem to have cast shadows in oblique S2A's, being too gradual but possibly nukefix can get at that from the S1AB perspective.

Bed Topography of Jakobshavn Isbræ and Helheim Glacier
Lu An, E Rignot, M Morlighem, JD Paden,D Holland, D Holland
https://agu.confex.com/agu/fm16/meetingapp.cgi/Paper/136824

It is essential to know their ice thickness and bed topography as well as the bathymetry in the fjords. Here we infer the glacier bed topography, ice thickness and sea floor bathymetry near the grounding line using high-resolution airborne gravity data from AIRGrav collected in August 2012 with a helicopter at 500 m spacing grid, 50 knots ground speed, 80 m ground clearance and sub-milligal accuracy, which improves on OIB's 5.2 km resolution, 290 knots, and 450 m clearance.

We use a 3D inversion of the gravity data combining our observations and a forward modeling of the surrounding gravity field with point measurements of the bathymetry at the ice-ocean boundary and a reconstruction of the glacier bed topography upstream using a mass conservation method combining re-analyzed airborne radar-derived ice thickness data from CReSIS with ice flow motion vectors from satellite radar interferometry.

The results provide a more accurate view of the bed topography of these glaciers and resolve major uncertainties from past attempts to probe the deepest part of the bed near the ice front from radio echo sounding data alone. The results reveal that Jakobshavn is now retreating into an even deeper bed, from 600 m in 1996 to 900 m at present and 1,400 m in the next 25 km. The glacier will continue to retreat at an increasing rate (0.6 km/yr at present) along its retrograde bed into thicker ice.


On the impact of ice-ocean interaction on Greenland glaciers versus calving speed
E Rignot et al
https://agu.confex.com/agu/fm16/meetingapp.cgi/Paper/199909

Glacier retreat from frontal ablation is a delicate balance between subaqueous melt, calving processes and bed geometry. Here, we model subaqueous melt from a large number of Greenland tidewater glaciers using generalized 3D, high resolution simulations of ice melt from the MITgcm ocean model constrained by subglacial melt from RACMO2.3 and ISSM, ocean temperature from ECCO2-4km Arctic, and bed topography from OMG and MC for 1992–2015. The results are analyzed in combination with ice-front retreat and glacier speed from Landsat and imaging radar data since the 1990s.

We find that subaqueous melt is 2–3 times greater in summer than in winter and doubled in magnitude since the 1990s because of enhanced ice sheet runoff and warmer ocean temperature. Glaciers that retreated rapidly are characterized by subaqueous melt rates comparable to their calving speed and favorable bed geometry. Glaciers dominated by calving processes are in contrast more resilient to thermal forcing from the ocean, especially in the presence of stabilizing geometry.


Surveying Greenland’s marine terminating glaciers with an airborne radar altimeter
A Khazendar  IG Fente EJ Rignot JK Willis
https://agu.confex.com/agu/fm16/meetingapp.cgi/Paper/163483

Oceans Melting Greenland is a NASA suborbital mission to investigate the role of the oceans in ice loss around the margins of the Greenland Ice Sheet. A five-year airborne and ship-based campaign, the project will directly measure ocean temperatures and glacier changes around most of Greenland.

A main component of the airborne campaign is a once-per-year survey of glacier surface elevations of most of the marine terminating glaciers with GLISTIN-A. This radar is a topographic mapper that measures ice surface elevations with 50 cm vertical accuracy at 25 m horizontal resolution over a 10-12 km swath, independent of weather.

The first survey took place in March 2016 over 10 days and nearly 65 flight hours. We present here a first look at the measurements made during this campaign, how they compare with existing laser and radar altimetry data and a preliminary evaluation of their contribution in estimating ice thinning rates.


HF/VHF Dual-Frequency Sounding of Temperate and Fast-flowing Glaciers
E Arnold PS Gogineni S Yan F Rodriguez-Morales
https://agu.confex.com/agu/fm16/meetingapp.cgi/Paper/186981

One of the greatest challenges in airborne remote sensing of ice sheets is the sounding of temperate and fast-flowing glaciers. Temperate ice has a much higher liquid water content than “dry” ice which results in significantly greater attenuation and scattering of the radar signal. In addition, the surface of fast-flowing glaciers is highly crevassed and rough which also scatters the signal.

Radars designed to sound these glaciers must overcome the volumetric and surface scattering to detect the ice bed echo. While progress has been made in mapping fast-flowing glaciers with high-sensitivity Very High Frequency (VHF) radars, the significant signal attenuation at these frequencies results in a weak bed echo that is difficult to detect in the presence of strong clutter returns.

CReSIS has developed a dual-frequency High Frequency (HF)/VHF sounder for the purpose of sounding temperate and fast-flowing glaciers, particularly near their calving front, where surface clutter is the greatest and the effectiveness of existing microwave VHF radars is more limited. The longer wavelength of this system, which operates at 14 and 35 MHz, makes it much less sensitive to the temperate ice attenuation and scattering and crevassing.

In October of 2016, the HF/VHF sounder will be integrated onto a Twin Otter aircraft and flown over Jakobshavn Glacier. These flights will target the first 10 km from the calving front in an effort to sound and image the most challenging and significant part of this outlet glacier. We plan to present the results from this field campaign.
« Last Edit: October 23, 2016, 02:11:22 PM by A-Team »

johnm33

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1783 on: October 23, 2016, 01:59:18 PM »
I'm wondering if the collapse of the north face of the south branch was due to the trough adjacent to it, having been filled with ice, melting out, undermining support for the north face ice? Is it now filled with more saline water from tidal flows? Does that saline water now spill over into the upstream trough and how far along is it's melt. Images hereabouts
http://forum.arctic-sea-ice.net/index.php/topic,154.msg86358.html#msg86358 Perhaps we'll shortly see a similar collapse further along that face.
Despite some rather large breakouts from the fjord there have been no significant eruptions of fresh water into Disko, is that due to the saline  tidal flows being captured by the troughs?  Or just diluted by the prodigious amount of ice sliding into the fjord?
Also worth another look http://forum.arctic-sea-ice.net/index.php/topic,154.msg61021.html#msg61021

Shared Humanity

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1784 on: October 23, 2016, 07:26:36 PM »
On the attached image, I have drawn where I believe the real calving face (probably the wrong term) of the north wall is. This is where the ice sheet in this area slumps dramatically. What appears to be a portion of the ice sheet south of this slump is actually calved (highly fractured?) ice which is dragging (being pushed) along the shallow sea bed until it reaches the deep main channel where it quickly slips away. This ice has far less pinning effect on the main ice stream as it is not solid ice but fractured. The waves and deep fracturing of the main ice stream seem to form exactly where this calving front begins. The protruding section of this calving face is where the ice sheet is resting on a very small island which accounts for its higher elevation.
« Last Edit: October 23, 2016, 08:47:21 PM by Shared Humanity »

Shared Humanity

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1785 on: October 23, 2016, 08:25:04 PM »
I would now like to draw your attention to the large, wedge shaped section of ice sheet circled in yellow. If you look at the topography map, this solid piece of ice rests on the shallow sea bed, just west of the large island that runs along the north of the deep trough of the main ice stream after it turns east. This is a thick, minimally crevassed, relatively stable section of ice and serves to apply pressure and pin the main ice stream at this point. Note the condition of the main ice stream at this point, less crevassing, suggesting a higher integrity. If this wedged section of the ice sheet were to be compromised, destabilized, fractured and mobile, I believe the south branch could quickly retreat to the bend and the very deep trough that runs east into the interior of Greenland.
« Last Edit: October 24, 2016, 02:20:43 AM by Shared Humanity »

Shared Humanity

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1786 on: October 23, 2016, 08:46:20 PM »
By the way, I come to this thread daily and want to thank everyone for these wonderful satellite images and animations. They are fascinating. I love to compare these images to topography maps to tease out the effects of topography on ice sheet behavior. I would like to point out the area circled in red which is where the ice sheet rests on the high elevation, western section of that large island north of the main ice stream. I would also like to note that the ice stream south of this large island appears to be highly crevassed where it rests over the deep trough. Does this suggest the ice is more prone to calving here?

johnm33

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1787 on: October 25, 2016, 01:36:04 AM »
So if the collapse of the north wall was due to the trough ice melting out I figured at some point we'd have seen some huge bergs breaking out, http://forum.arctic-sea-ice.net/index.php/topic,154.msg56275.html#msg56275 693,698
If thats what that was then a huge reservoir of saline water will have replaced it, and it won't be going below 4c for some time.
 The 'wave' feature is close to orthogonal to the flow up the fjord, which suggests flow beneath, possibly in both directions, if thats the case I expect we'll see another collapse of the north face where S.H.  has his yellow wedge. Very likely next year although we may have to wait a bit longer for the next troughs ice to surface.

Shared Humanity

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1788 on: October 25, 2016, 01:50:43 AM »
So if the collapse of the north wall was due to the trough ice melting out I figured at some point we'd have seen some huge bergs breaking out, http://forum.arctic-sea-ice.net/index.php/topic,154.msg56275.html#msg56275 693,698
If thats what that was then a huge reservoir of saline water will have replaced it, and it won't be going below 4c for some time.
 The 'wave' feature is close to orthogonal to the flow up the fjord, which suggests flow beneath, possibly in both directions, if thats the case I expect we'll see another collapse of the north face where S.H.  has his yellow wedge. Very likely next year although we may have to wait a bit longer for the next troughs ice to surface.


I don't know. I am way out of my depth here. I just like the pretty pictures.    :)
« Last Edit: October 25, 2016, 04:57:39 AM by Shared Humanity »

abbottisgone

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1789 on: October 27, 2016, 07:48:42 AM »
Does this :..

"Eight thousand years ago, the Jakobshavn filled the fjord completely, all the way to the moraine. By the mid-nineteenth century, when the first observations were recorded, the position of the calving front had shifted inland by about ten miles. Over the next hundred and fifty years, the front’s position shifted again, by another twelve miles.

Then, suddenly, in the late nineteen-nineties, the Jakobshavn’s stately retreat turned into a rout. Between 2001 and 2006, the calving front withdrew nine miles. Just in the past fifteen years, it has given up more ground than it did in the previous century. The fjord extends for at least another forty miles and deepens as it moves inland. At this point, there doesn’t seem to be anything to prevent the calving front from withdrawing the entire way.

“It appears now that the retreat cannot be stopped,” David Holland, a professor at N.Y.U. who studies the Jakobshavn using seals equipped with electronic sensors, told me. (When the seals surface after a dive, the sensors transmit data about conditions in the fjord.)

Meanwhile, as the calving front has receded, the ice stream has sped up. This appears to be the result of yet another feedback loop. Since the nineties, the Jakobshavn has nearly tripled its pace. In the summer of 2012, it set what’s believed to be an ice-stream record, by flowing at the distinctly unglacial rate of a hundred and fifty feet per day, or more than six feet an hour. The Jakobshavn’s catchment area is smaller than the negis’s; still, there’s enough ice in it to raise global sea levels by two feet."

(link: http://www.newyorker.com/magazine/2016/10/24/greenland-is-melting)

 ..perhaps indicate that this glacier could be gone in 10 years?

 If not, then what is the current flow rate of this glacier?
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johnm33

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1790 on: November 01, 2016, 09:59:19 PM »
Following up on the notion I aired above, I looked back through the thread [what an amazing resource], and came up with the following as a possible for the loss of the basal ice from the first of the two basins hereabouts and the  first collapse of the north shore of the southern branch. 
 http://forum.arctic-sea-ice.net/index.php/topic,154.msg21016.html#msg21016 
complete with this animation [from Wipneus]

A-team posted an improved image of the two basins in this immediate area area which I'll find and post here when time permits.
Here we are from 586 above

and the next section

looking at these it seems possible that the bottom ice in the troughs would come out in 3 or more  pieces, each coincident with a collapse of the north 'shore' front.
If the next trough is going to empty out in a similar way [if this notion is correct] then it should be proceeded by the forming of a similar overturning wave/collapse feature on the north shore nearer the bend.
« Last Edit: November 06, 2016, 06:17:29 PM by johnm33 »

iwantatr8

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1791 on: December 14, 2016, 01:54:36 PM »
I'm not sure this really belongs here but given our discussions on monitoring glacier speed using image tracking this seems appropriate.

The EO blog post features the results of an AGU paper looking at Landsat 8 images for tracking.

http://earthobservatory.nasa.gov/IOTD/view.php?id=89261&src=eorss-iotd

and the paper here:

http://www.sciencedirect.com/science/article/pii/S003442571530211X

nukefix

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1792 on: December 15, 2016, 10:56:19 AM »
I'm not sure this really belongs here but given our discussions on monitoring glacier speed using image tracking this seems appropriate.
ESA SNAP toolbox does image tracking, it works at least for SAR images and perhaps also with optical.

Iceismylife

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1793 on: December 27, 2016, 05:32:07 PM »
Anyone care to check the radar?  It mint be worth a look.

Iceismylife

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1794 on: January 06, 2017, 05:15:57 PM »
http://ocean.dmi.dk/arctic/image_container.php

http://go.nasa.gov/2iJFi8G


It looks like another calving event has taken place.

solartim27

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1795 on: January 06, 2017, 09:06:17 PM »
I'm not seeing anything major, lots of winter advance at the south front, something on the north side goes, but I can't say when.  Oct 18 to Jan 5

http://www.polarview.aq/images/105_S1jpgfull/S1A_EW_GRDM_1SDH_20170105T100733_E29C_N_1.final.jpg

S1A_EW_GRDM_1SDH_20161018T101545_1B11_N_1.final.jpg
« Last Edit: January 06, 2017, 09:11:19 PM by solartim27 »
FNORD

johnm33

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1796 on: January 07, 2017, 12:08:24 AM »
More of a surge than a calving?

zxy

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1797 on: February 02, 2017, 06:55:13 AM »
Jacobshavn visible on terra / MODIS again.


zxy

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1798 on: February 10, 2017, 03:01:49 AM »
Comparison between 2016-02-10 and 2017-02-09. (Click to see)

Image with less ice in the fjord is 2017.


« Last Edit: February 10, 2017, 03:11:58 AM by zxy »

Iceismylife

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Re: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland
« Reply #1799 on: February 14, 2017, 01:07:36 AM »
http://go.nasa.gov/2kN1jE4

The fjord is clear of cloud cover some good high res photos would be nice.  See what the winter did.