Support the Arctic Sea Ice Forum and Blog

Author Topic: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland  (Read 575405 times)

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #350 on: July 19, 2014, 10:16:34 PM »
Espen, it is hard to pinpoint what exactly is casting the shadows. I did calculate the heights of two peaks and the saddle between them as 261, 205, and 220 meters above sea level. Here I had to triple check that Landsat-8 uses negative azimuths if ccw of true north before tracing shadow peaks back according to sun position.

The sun is above the horizon but the angle is low at 7.415º so the shadows are long. This has conspired with azimuth to shine through the pass while casting long but not too long shadows on sea ice. This method does not provide a height for the white calving front which must have horizontal extent because it would just project as a line if a vertical cliff because the scene is fully nadir.

I don't know what will be left of this by mid-September ... retreat has entered new territory. However the north branch is not currently a large contributor to ice volume discharge in the larger scheme of things (south branch). Its disappearance would perhaps give some general Greenland ice sheet a better route to the sea. The potential drainage basin is small though.

Shared Humanity

  • ASIF Upper Class
  • Posts: 2315
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #351 on: July 20, 2014, 12:39:58 AM »
A-Team....

Beautiful image of the north branch.

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #352 on: July 20, 2014, 02:12:48 AM »
Yes, these low angle shots on clear days are so photo-realistic that what I labelled the hypotenuses from shadow tip to top of mtn above really should have been the base line pixel measurements that continue at sea level underneath it. So should have used tangent as this is strictly a down-looking satellite (roll angle parameter negligible per metadata).

As luck would have it, sin/cos ~ tan for low angles as cos ~1. So it makes only a tiny difference, ~ another 1-2m in heights, the tallest now being 263 m (more impressive as 863 feet).

The comparison below with 03 Jul 14 (LC80090112014184LGN00_B8) -- which I'm pretty sure are correctly co-registered despite appearances -- shows most of the recession action in the last two weeks has been to the northwest corner of the bay, a more conventional undimpled, simply-crevassed glacier that creeps into the bay without a perceptible cliff.

It would be better here to find an earlier one with a similar sun angle -- I'm updating my Landsat database to include azimuth, sun angle and time of day columns now that I see their utility.

The north branch, after you get past everyone pooh-poohing its significance, is actually quite instructive.

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #353 on: July 20, 2014, 02:14:52 AM »
Grrrr! Click above to animate. I couldn't clip to 700 pixels without losing natural Landsat scale. The blog software won't animate above this width.

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #354 on: July 20, 2014, 03:21:02 AM »
This is a totally amazing image for its clarity and relief ... more to come for newly revealed ice wave structures to the east. Below, just the height of Big Berg out in the fjord:

100.5   m: height above sea level by shadow measurement
904.8   m: height below assuming 90% of height is below sea level (ie mass uniformly distributed)
1005.4 m: total height, again assuming first height is representative of mass above sea level

Espen

  • ASIF Governor
  • Posts: 3078
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #355 on: July 20, 2014, 04:51:00 AM »
As reported by A-Team above most action is now at the northern branch, the southern branch is in a healing mood:

Please click on the image to start the animation!
Have a ice day!

solartim27

  • ASIF Citizen
  • Posts: 438
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #356 on: July 20, 2014, 08:02:33 AM »
I think the rapid advancement of the calving face just means the bottom of the ice sheet upstream is well lubed by melt water.  I wouldn't call that healing. ;)
FNORD

Espen

  • ASIF Governor
  • Posts: 3078
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #357 on: July 20, 2014, 10:28:24 AM »
And I wonder what happened in that dimple/tarn was it emptied?
Have a ice day!

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #358 on: July 20, 2014, 03:15:47 PM »
Looking at the larger picture, there sure are a lot of mid-elevation melt lakes right now. As they fill and overflow to some extent, extensive linear water features are appearing, I suppose ending in draining moulins.

However it's not clear that very many of these drain to the bedrock bottom of Jakobshavn Icebrae, and if they do, whether that ends up as lubricating sheet flow or simply as water coursing down well-established tunnels.

Quite a few other mechanisms have been put forward to explain the moderate seasonality of terminal velocity, among them reduction in back pressure at the calving front from fjord melange (bergs in frozen sea ice).

The speedup is well underway by the first of June, even mid-May in some years, as shown in Fig.2 of Joachin 2014. Moulin meltwater may contribute later on but cannot really be a factor this early in the season.

The calving front position is not a proxy for the rate of ice volume discharge. That is determined solely by ice stream velocity and thickness because sooner or later ice that has surged forward will calve off. Sea level rise -- that is end game here, not JI as a kantian Ding an Sich.

However, as Steve B pointed out a month back, retreat of the calving position  is still significant because it reduces back pressure on the ice stream itself (especially at curves) and side pressure limiting Greenland ice sheet discharge into the ice stream track, the consequences of both being faster volumetric discharge.


Espen

  • ASIF Governor
  • Posts: 3078
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #359 on: July 20, 2014, 03:54:43 PM »
I will soon show you a giant melt water lake, that gets emptied now and then, just waiting for August and the right image. The lake is about 4 - 5 km2 and pretty deep, with a rocky bottom.
Have a ice day!

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #360 on: July 20, 2014, 04:08:49 PM »
That incredible low angle image yesterday with perfect shadows got me motivated to find the others. It is quite convenient to get Landsat metadata in bulk at http://landsat.usgs.gov/consumer.php and then sort through the nuisance xml (that nothing can parse) to find the low sun angle scenes. Discarding the cloudy ones over JI gives the table below for 2014. Snow cover can be an issue with earlier dates.

I've attached a region flanking the upper ice stream where shadowing contrast reveals adjacent ice sheet to the north being seriously distorted by the out-competing ice stream. Note too the subtle ribbing farther up. It looks to me like much older relic features are being over-written but not quite obliterated by the strain rate tensor operative in more recent years.

Shared Humanity

  • ASIF Upper Class
  • Posts: 2315
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #361 on: July 20, 2014, 04:10:24 PM »
It seems that the linear water features around the larger melt lakes are feeding water to the lakes rather than draining them. Is there a way to determine elevation in this area? My guess is the lakes are lower in elevation and  the linear features are melt streams that are feeding them.

Espen

  • ASIF Governor
  • Posts: 3078
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #362 on: July 20, 2014, 04:16:28 PM »
That incredible low angle image yesterday with perfect shadows got me motivated to find the others. It is quite convenient to get Landsat metadata in bulk at http://landsat.usgs.gov/consumer.php and then sort through the nuisance xml (that nothing can parse) to find the low sun angle scenes. Discarding the cloudy ones over JI gives the table below for 2014. Snow cover can be an issue with earlier dates.

I've attached a region flanking the upper ice stream where shadowing contrast reveals adjacent ice sheet to the north being seriously distorted by the out-competing ice stream. Note too the subtle ribbing farther up. It looks to me like much older relic features are being over-written but not quite obliterated by the strain rate tensor operative in more recent years.

Yes A-Team yes you got yourself stocked with a lot of work, be careful with the PCs energy consumption! ;)
Have a ice day!

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #363 on: July 20, 2014, 07:49:41 PM »
"Yes A-Team yes you got yourself stocked with a lot of work, be careful with the PCs energy consumption!"

I wouldn't be surprised if the internet police busted down the door for too many gigabyte Geotiff downloads.

"I will soon show you a giant melt water lake, that gets emptied now and then, just waiting for August and the right image. The lake is about 4 - 5 km2 and pretty deep, with a rocky bottom."

Excellent spotting. The Landsat-8's from last year were so spotty and August so cloudy, I couldn't find a good photo. Google map is showing bottomless pits and strange distributories to the west of JI (did not chase down photo date).

"It seems that the linear water features around the larger melt lakes are feeding water to the lakes rather than draining them. Is there a way to determine elevation in this area?"

Nice suggestion, though the lakes are forming weeks before the linear features. Could be some of both. The Greenland DEM is 'out there' to cm accuracy somewhere on the internet but not necessarily co-registered with our high resolution imagery.

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #364 on: July 20, 2014, 08:06:44 PM »
Here is another feature from digiglobe hi res, to the east and slightly to the north of the end of the south branch of the JI. The drainage here shows a couple of things going on, including streamlets seemingly confined to linear troughs of ice compression waves. The surface is so flat that the slightest change in elevation can give rise to extended, complex water features.

Espen

  • ASIF Governor
  • Posts: 3078
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #365 on: July 20, 2014, 08:16:00 PM »
A-Team that melt water lake I mentioned above, is far up north!
Have a ice day!

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #366 on: July 21, 2014, 02:11:35 AM »
See down to rocks in a glacial lake? Way up north on coast maybe, not at JI.

Here is transect through Jakobshavn south branch from the Greenland divide ridge down to the sea via the sole experimental channel borehole:1989, Iken B. It depicts bedrock depth (do I believe interior sea level bedrock depiction?), ice thickness, ice temperature profile, and if you look closely the layer of seriously less viscous temperate ice. 2011 ppt from Kristin Poinar, dissertation due out this Sept.

http://students.washington.edu/kpoinar/Presentations/jakobshavn_temperateice_GCC4.pptx

I also came across the original incredibly sharp image of the N and S calving area acquired by Worldview-2 on 16 Jul 11, apparently mislabeled at google earth as 10 Nov 10. The scale is not provided but may be in feet (1 pixel = 15') rather than meters. In any event, way nicer than anything being kicked out by Landsat. Looks better with grayscale inverted -- north branch calving area shown.

http://eoimages.gsfc.nasa.gov/images/imagerecords/76000/76590/jakobshavn_wv2_2011197_lrg.jpg

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #367 on: July 21, 2014, 02:14:52 AM »
Grrrr. Forum software took it upon itself to make a crummy compression of the image (why provide scroll bars then not really use?). Click to really see it.

Rubikscube

  • ASIF Citizen
  • Posts: 247
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #368 on: July 21, 2014, 01:09:34 PM »
(do I believe interior sea level bedrock depiction?)

No, you do not, or rather, you should not. The newest topography maps (posted by SH further up the thread) clearly shows that the southern branch continue below sea level for several hundred kilometers inland. Unfortunately all the detailed topography maps for JI that I know of (perhaps I have not looked hard enough) end shortly after the 1000+ overdeepening ends and where the southern branch splits into two (arguably three).

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #369 on: July 21, 2014, 03:09:41 PM »
Not to worry R, I don't plan to scrape surface and bedrock depths from Bamber 2013 when the whole intermediate ice stratification profile and its transects can be assembled from Cresis. But where did Poinar get the temperature profile? NEEM is way to the north, GISP, GRIP, and NGRIP are old hat and not at the divide on the 69º latitude of JI. Just heuristic I suppose, but what does that say about the modeling that comes off of it.

GISP     65°N 43°W  2037 m
GRIP     72°N 37°W  3028 m
NGRIP   75°N 42°W  2917 m
NEEM    77°N 51°W  2537 m

More not to worry for Espen about "stocking myself with a lot of work". The answer is multi-tasking. Consider 8 small researches @ 4 hours each = 32 hours/week. Fine, not allowed under EU work rules. However with multi-tasking, 8 x 4 still = 4. I think you see the savings?

Recent acceleration of Jakobshavn Isbrae is well-described but not understood. That situation won't last.

The sticking point is not so much the second invariant of a deviatoric strain tensor but rather Newton's first two laws of motion (resp. Euler's deformable rigid bodies, Cauchy's continuum mechanics).

That is, people largely bought into warming of Baffin Bay, turbulent warming at the calving front, loss of ice tongue and melange back pressure and unblocking of north branch fjord side pressure. That caused the south branch to accelerate. However that's a one-off effect yet the acceleration continues, seemingly without a force.

I am reminded of a recent trip getting stuck behind a carload of foreigners on the Grand Canyon south rim, who would repeatedly brake sharply to see a view or photograph a squirrel, then accelerate off abruptly as the driver recalled the midnight flight home from Vegas, with Bryce and Zion yet to do.

As car #2, I had to slow down and speed up the whole while because I could not pass nor reach the AK-47 in the back seat. Car #3 could not see car #1 so had to brake and speed according to car #2. And so on, back to the trading post back on the Navajo res.

Seen from a satellite, this amounts to a wave train moving backwards and dissipating as it goes. The counterpart at JI is then the seasonal cycle at Baffin Bay and fjord slowly propagating up the main ice stream, continuing even as the driving mechanism recedes into irrelevance.

Although dissipating from friction with the ice sheet and its walls, the ensuing heat and softer ice manifest as faster motion of the ice stream under the always-available gravitational force. Under this scenario, the fact that 2013 was actually observed by Joachin 2014 as slower than 2012 is no accident of weather or moulins: it predicts that 2014 etc, while still very fast, will be slower yet as Baffin Bay damps out, rather than be the new normal or a sign of catastrophic accelerations yet to come.

A pity that we cannot determine 2014 surface velocities from Landsat.
https://en.wikipedia.org/wiki/Digital_image_correlation

SteveMDFP

  • ASIF Citizen
  • Posts: 286
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #370 on: July 21, 2014, 04:30:52 PM »
....
As car #2, I had to slow down and speed up the whole while because I could not pass nor reach the AK-47 in the back seat. 
....

ROFL !!!!  I thoroughly enjoy your posts, as much for the lively language as the technical matters.

Rubikscube

  • ASIF Citizen
  • Posts: 247
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #371 on: July 23, 2014, 01:43:26 AM »
Sweet A-team. I applaud you, Espen and all the other contributing to this section of the forum and I would have loved to help out with the digestion of the massive amounts of data if I had the expertise to do so.

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #372 on: July 25, 2014, 03:47:12 PM »
Here is one of my better efforts at animation -- for once outdid the Nasa Visualization Lab, a site I mainly study in awe. File size could only be reduced so far (827 kb) without compromising the data so it will likely take a click below to get the animation going.

The data here -- maybe $100k of purchased SAR imagery carefully processed by Rignot 2012 doi:10.1029/2012GL051634 -- is the best we have on overall Greenland ice sheet surface velocities, though higher elevations are still unsatisfactory due to ionospheric disturbances on the same scale as motion and too-long satellite return times.

These velocities range from 1.5 m a year on the summit ridge to 17,000 near the Jakobshavn Isbrae calving front. The animation shows isovelocity contours moving down to the coast. (Technically these are just magnitudes but direction is implied.)

Frame delay is a uniform 100ms; if upper contour movements literally reflected their slow velocities, the animation would take thousands of years to play.

The drainage basins are displayed quite clearly. It looks to me like JI is going to seize catchment from its neighbors over time (for watersheds, called stream piracy). If so, its future contribution to sea level rise has been under-estimated.

If you recall the marching transect animations of lower JI above, those show ~12 km of icestream to be flushed out this  year into the fjord. Ice occupying km 12-20 will be calved off next year. Ice to be flushed in years 3-5 could also be worked out (from a double integral of Joachin's v(x,t) data) but with increasing uncertainty due to coalescence of multiple flows.

The animation here shows the other end of the ice turnover scale. All of Greenland's ice is on the move and always has been, it's just slumping faster now. That's not always apparent when it's replenished by snowfall. The US learned about deformation in a hurry, abandoning Camp Century by Thule in year two; more recently the NEEM drill team came across a doubled-over fold of stratified ice at depth.

DaddyBFree

  • ASIF Lurker
  • Posts: 30
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #373 on: July 25, 2014, 04:16:08 PM »
Beautiful work A-Team!

Shared Humanity

  • ASIF Upper Class
  • Posts: 2315
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #374 on: July 25, 2014, 04:23:23 PM »
Here is another feature from digiglobe hi res, to the east and slightly to the north of the end of the south branch of the JI. The drainage here shows a couple of things going on, including streamlets seemingly confined to linear troughs of ice compression waves. The surface is so flat that the slightest change in elevation can give rise to extended, complex water features.

It would seem that, once this kind of complex melt driven feature is established on the surface, it would tend to reappear each melt season in this location.

Shared Humanity

  • ASIF Upper Class
  • Posts: 2315
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #375 on: July 25, 2014, 04:33:32 PM »
"The drainage basins are displayed quite clearly. It looks to me like JI is going to seize catchment from its neighbors over time (for watersheds, called stream piracy). If so, its future contribution to sea level rise has been under-estimated."

Stream piracy....I love the image this term creates. It would seem that the underlying topography of Greenland  suggests why this piracy will  take place.

Espen

  • ASIF Governor
  • Posts: 3078
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #376 on: July 25, 2014, 06:16:15 PM »
Beautiful work A-Team!

Just proud I got him on the the team! ;) ;)
Have a ice day!

Espen

  • ASIF Governor
  • Posts: 3078
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #377 on: July 25, 2014, 06:18:20 PM »
I am studying the melt streams at the moment, but this time the rivers not glaciers!
Have a ice day!

Espen

  • ASIF Governor
  • Posts: 3078
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #378 on: July 26, 2014, 08:19:17 PM »
Seems like some serious calving since last Landsat 8 update July 19 2014, but be aware of Modis uncertainty:

 
Have a ice day!

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #379 on: July 26, 2014, 08:43:24 PM »
Below I've tiled eleven consecutive ground-penetrating radar scenes from 2013 that represent an east-west flight line just south of Jakobshavn Isbrae up and slightly over mid-island ridge. These are selected via the kml google earth files Cresis provides.

Quite a few stratifications are easily traceable over the entire 550 km profile. These will have distinct ice fabric and rheological properties and extend as surfaces over huge areas. As isochrons, these surfaces carry a huge amount of information about the deformational and melt history of the Greenland ice sheet.

It is a huge mistake to walk away from these files with just ice-thickness = ice sheet surface - bedrock and toss the stuff in between. The layer surfaces along with z height form the natural modeling coordinate system. Stratification surfaces have the essential physics baked in; coordinates with street smarts can do the heavy lifting. The  x,y,z alternative is just Ptolemy gone polar <url>https://en.wikipedia.org/wiki/Epicycles</url>

Tiling takes maybe an hour per 1000 km of flight path because Cresis images do not quite have a uniform width, vertical scale , or overlap even on a single flight. Although it is easily scriptable up to final adjustments, the cross-country files must first be distinguished within a heavy background of grid flights. It is feasible though to work out the whole 3D stratification onion if interpolation works out (left as exercise).

The most useful tracks would start at Jakobshavn and angle right up to a modern bedrock core like NEEM, GRIP, and GISP2. Milcent, Pakitsoq and Crete are lesser known drill sites closer to JI, probably with fewer attributes measured. The steam-drilled holes on JI itself anchor temperature and hydraulics but don't provide ice core stratifications.

The full 550 km profile at original Cresis resolution is rather wide at 10,672 x 656 pixels amd 17.8 Mb for the blog in Gimp xcf format. However as a jpeg at 95% of lossless compression, file size is a more reasonable 2 Mb. However all those pixels still work out to 12.4 feet (3.8 m) of screen width.

If the blog software chokes on this, the original image is online at <url>http://tinyurl.com/na7bxnx</url>. If it works, in-blog scroll bars will provide good panning of the wide image.

I've also attached 700 pixel wide snapshots in decreasing scale. The first is at natural scale but just the first 700 pixels out of the 10,672. The next shows the first 700 of a 2:1 downsampled image; the third 4:1, the fourth is at a 6.5% scale (plus vertical exaggeration) that gets in the whole scene  while maintaining the surface and bottom markups that Cresis annotators put in.

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #380 on: July 27, 2014, 01:15:23 AM »
Espen, a rather nice Landsat just came in, LC80100112014207LGN00, the path, row 10,11 on day 207 of 26 Jul 14. It's a 3 pm shot with unremarkably azimuths and solar angles, resp 172.9º and 39.7º. (The other one expected for this date, LC80100112014207LGN00 path,row 83,233 is too cloudy.)

I haven't had a chance to really analyze the good one but here it is below. It looks like the south branch is calving so much thick ice that it has completely squeezed out the north branch at its egress, though the discolored streamer (origin unknown) seems to have been entrained in the south branch melange current.

I'm not sure yet what happened to Mr. Big. Seems like it was just the other day that i was measuring his shadow. He may be well down the ford,b over-turned, disintegrated or unrecognizable. However Mr Big II may have taken his place close to the front.

There may be an end-of-season tilt in the balancing act between the glacier coming forward and calving cutting it back. Both are most active in the summer months but their strengths may be differently phased, depending on the year and position relative to the sill.

Espen

  • ASIF Governor
  • Posts: 3078
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #381 on: July 27, 2014, 05:25:37 AM »
As warned earlier (by Modis https://forum.arctic-sea-ice.net/index.php/topic,154.msg32498.html#msg32498) and indicated by A-Team, Jakobshavn did it again and almost reached the record retreat line set in September 27 2013 in the southern branch, but is now far beyond the prior record retreat line in the northern branch:
« Last Edit: July 27, 2014, 10:04:42 AM by Espen »
Have a ice day!

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #382 on: July 27, 2014, 02:56:23 PM »
A dozen Landsats expected between now and the first of September. Half or more will be too cloudy to see the calving front!

LC80 08012 2014 241 LGN00 Aug 29
LC80 08011 2014 241 LGN00 Aug 29
LC80 83233 2014 239 LGN00 Aug 27
LC80 10011 2014 239 LGN00 Aug 27
LC80 09011 2014 232 LGN00 Aug 20
LC80 08012 2014 225 LGN00 Aug 13
LC80 08011 2014 225 LGN00 Aug 13
LC80 83233 2014 223 LGN00 Aug 11
LC80 10011 2014 223 LGN00 Aug 11
LC80 09011 2014 216 LGN00 Aug 04
LC80 08012 2014 209 LGN00 Jul 28
LC80 08011 2014 209 LGN00 Jul 28

Very few clear days for August and September 2013. The animation below compares yesterday with a month later in 2013. Some curious parallels in regards to the north branch.

Espen

  • ASIF Governor
  • Posts: 3078
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #383 on: July 27, 2014, 03:03:22 PM »
A-Team, that repeating shade in the northern branch could probably be a underground (ice) river outlet?
Have a ice day!

mspelto

  • ASIF Lurker
  • Posts: 25
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #384 on: July 27, 2014, 05:43:26 PM »
A Team and Espen excellent work examining the changes in the calving front.  The northern branch cannot be done this summer, the current calving front is too high and the crevassing-rifting behind the front to extreme to be stable.  This area of visible instability does not seem to extend far behind the front at this point.

DaddyBFree

  • ASIF Lurker
  • Posts: 30
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #385 on: July 27, 2014, 07:49:39 PM »
Is it wildfire smoke that is visible across Greenland from Jakobshavn to Kangerlussuaq today? If so, (the resulting ash/soot) couldn't be good for albedo...

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #386 on: July 27, 2014, 09:48:35 PM »
"The northern branch cannot be done this summer, the current calving front is too high and the crevassing-rifting behind the front to extreme to be stable.  This area of visible instability does not seem to extend far behind the front at this point."

Thanks for that analysis. Something like 55 days left yet, relative to last year's maximal retreat of south branch. I've attached a 10 m view from yesterday's panchromatic.

Here are the prospects for animating the north branch for 2014 in terms of cloud-free Landsat. I wonder if a trend is recognizable or the final fall position just remains unpredictable.

1 Jul 26 207 10 11  good 172.9 39.7
1 Jul 19 200 9 11   good 172.8 41.2
1 Jul 19 200 82 233 good -47.3 7.4
1 Jul 12 193 8 11   good 172.9 42.3
1 Jul 11 192 81 233 good -46.8 8.4
1 Jul 10 191 10 11  good 173.0 42.6
1 Jul 03 184 9 11   good 173.3 43.3
1 Jun 24 175 83 233 good -45.7 9.6
1 Jun 10 161 8 12   good 172.6 44.7
1 Jun 10 161 8 11   good 174.8 43.5
1 Jun 08 159 10 11  good 174.9 43.2
0 Jun 01 152 9 11   good 175.3 42.6
0 May 09 129 8 12   good 173.7 39.2
0 May 09 129 8 11   good 175.7 37.9
0 May 07 127 10 11  good 175.7 37.4
0 Apr 21 111 10 11  good 175.4 32.4
0 Apr 14 104 9 11   good 175.0 30.0
0 Apr 07 097 8 12   good 172.8 28.7
0 Apr 07 097 8 11   good 174.6 27.4
0 Mar 22 081 8 12   good 171.9 22.4
0 Mar 22 081 8 11   good 173.7 21.2
1 Feb 25 056 9 11   good 172.6 11.4
1 Feb 09 040 9 11   good 172.6 5.78

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #387 on: July 27, 2014, 10:48:02 PM »
It turns out to be quite easy to tile Jakobshavn up to all the bigtime drill core sites, three flight lines do it for 2012. Those sites jump out just from kml flight line intersection mazes. Once tied into the grid, radar isochrons can be assigned actual dates and sometimes interpretations using all the work that has been done on the cores.

Those dark lines on radar represent higher radar dielectric returns and do not literally correspond to optically dark annual rings seen in ice cores. At one time, radar stratification was interpreted as volcanic ash layers, then ionic conductivity, oxygen isotope proxy, then Schmidt ice fabric diagram correlates, Eemian, Wisconsian, hard/soft ice, and more recently as bottom freeze-up deformations.

There's a very helpful ppt online entitled "Deformation and folds of the basal ice under the Greenland ice sheet" by Dahl-Jensen. Antarctica has more advanced analysis in some ways but check out the orientation tensor ellipsoid's eigenvalue profile of NEEM, slide 37.

CraigsIsland

  • ASIF Citizen
  • Posts: 196
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #388 on: July 28, 2014, 07:31:33 AM »
Is it wildfire smoke that is visible across Greenland from Jakobshavn to Kangerlussuaq today? If so, (the resulting ash/soot) couldn't be good for albedo...

Yeah appears to be from Canada wildfires. At least it's not methane? Looks so bad and I imagine will increase probabilities of more melt. Amazing looking pictures looking down at the area pic pocked with large melting ponds and grayish swatches of ice being transformed by sunlight and gravity.

Nice work lads (and ladies?)

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #389 on: July 28, 2014, 05:09:48 PM »
Interesting paper (free) entitled "Wintertime storage of water in buried supraglacial lakes across the Greenland Ice Sheet" that discusses features 90 km east of the Jakobshavn calving feature, notably melt lakes that neither drain through a moulin nor freeze solid during the winter.

Some correspond to surface features, others are only detectable by radar (and not all radar configurations at that). We generally have access to less favorable McCORDS so it would be difficult to locate any trend towards new/larger  supraglacial lakes.

http://www.the-cryosphere-discuss.net/8/3999/2014/tcd-8-3999-2014.pdf

CraigsIsland

  • ASIF Citizen
  • Posts: 196
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #390 on: July 28, 2014, 05:31:33 PM »
Thanks A-Team for the fascinating read. Great stuff.

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #391 on: July 28, 2014, 06:27:40 PM »
Whoa ... melting gone wild in the main ice streams feeding the south branch upstream. This is an oblique midnight Landsat with so little contrast that 16 bit processing could barely rescue it: LC80080122014209LGN00 28 July 2014 azimuth -47.9º and sun angle 5.7º.

There is a whole lot more going on than can be displayed in a reasonably sized file. The two images below show more or less the same region with the first at 15 m scale and the second 60 m.

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #392 on: July 28, 2014, 06:40:48 PM »
Here are the north and south branches from today. Some changes are evident but it may not warrant a full-on comparison with yesterday. The resolution has been slightly upsampled from the original panchromatic 15 m.

There is a new drone paper out from Jason Box and colleagues on the Store Glacier just to the north of JI. It has some interesting new techniques for monitoring calving fronts, among them extracting crevasses by subtracting an original image from its gaussian blur. I will try that for Jakobshavn with one of our better-lit obliques. The crevasses in their zone 4 were mostly filled with water, though that was not determinative of calving.

http://www.the-cryosphere-discuss.net/8/2243/2014/tcd-8-2243-2014.html

Espen

  • ASIF Governor
  • Posts: 3078
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #393 on: July 28, 2014, 08:25:59 PM »
Be aware it is Modis only, but it looks like some more southern branch calving between July 27 and 28:

False alarm Landsat just showed different!
« Last Edit: July 28, 2014, 11:01:25 PM by Espen »
Have a ice day!

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #394 on: July 29, 2014, 12:18:07 AM »
Somebody quick, Heimlich manoeuver -- I am choking on the crevasse job those guys did up on Store Glacier.

No, I am not too concerned about the crevasses in Fig.6A either not arriving at the bedrock or continuing well into it (leftmost slice below), though in situ processing won't produce disturbing artifact like this.

Nor am I greatly troubled by the 'difference between image and its gaussian blur' when grain merge would have been better. Subtracting defaults negative numbers to black, differencing takes absolute value to avoid negative grayscales conflating an important distinction; whereas grain merge offsets by 128 thereby centering the resultant histogram losslessly about neutral gray.

However we have to march to a different drummer at Jakobshavn given the large volume of ice discharged. Serious image processing of crevasse fields might start with a fourier transform to determine optimal azimuth for the ideal starting image (which should also be very low sun elevation; LC80822332014200LGN00 does both for JI) and to rationally support subsequent fourier band pass filtering (the high end to clean up speckle). ImageJ and Gimp have the tools, the latter high end donated by David Tschumperlé of CNRS.

After the gaussian blur step pulls out the crevasses from mean field, nothing further can be accomplished dinking with contrast. Instead the best enhancement is probably a bump map (aka height map) lit from the same azimuth (fifth strip below).

That is, we are interested in the orientations, torquing, and widths of crevasse fields relative both to overall and local glacier motions and also in their characteristic spacings (wavelength). Somewhere in there (after due consideration of ice temperature, terrain, velocity, etc etc) lies the explanation of why they are there in the first place.

Imaging processing alone can draw out features not apparent in the original image; at the end of the day, only an experienced glaciologist can read what the crevasse fields are spelling out.

On Jakobshavn Isbrae, the ultimate communicating illustration is perhaps a translucent coloring of each crevasse field with a mouse-over popup providing interpretation; in numerical modeling terms, components of rheology tensors.

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #395 on: July 30, 2014, 03:18:46 PM »
I looked at one of several approaches to automating internal ice penetrating radar striation finding, that of Patton. This paper  as its example processes an absolutely critical flight line, that between NGRIP and NEEM on 6 May 2011. Those two ice cores have been analyzed for every conceivable parameter, so not only provide precise dates for radar striations (which will not necessarily correspond to visible ice layers) but also clues to why the radar echo is there at all and what the lines mean.

The ice cores thus possess a rather subtle property not initially appreciated, one that forms a coherent hypersurface over a million sq km of Greenland ice sheet in the case of the WHT (Wisconson Holocene Transition). That follows from identification of the WHT down to sea level at FOXX and GULL steam holes (at 515 m and 600 m depths) along a flowline down from Swiss Camp, as well as DUCK right there on the Jakobshavn Isbrae channel.

The key point here is drill holes are points (expensive ones) but radar transects are planar sections whereas Greenland is volume (2.9 million cubic km of ice). We will never have enough drill holes to extrapolate to isochronal hypersurfaces of O18, pH, conductivity, sulfate, methane, beryllium, smoke, ash etc etc. However we do have enough flight lines. Thus correlation of radar striations with ice core dates and properties allows the transects to push out the core properties.

For lower Jakobshavn Isbrae south channel, striations are most evident in the adjacent ice sheet. However seismicity triggered by calving but propagating upstream has been associated with striation overthrusting at the big curve.

The Patton paper begins by clever sharpening of radar striations with a slant gaussian adapted to local slope (discovered by monte carlo sampling of angle to maximal contrast in the fourier transform). That is quite effective (Fig.2-3). The echograms are then processed with contour snakes (ImageJ plugin not used). I found the contiguous color picker in gimp pulls out the same lines with less effort.

Using this, I gave each striation a distinct color in the NGRIP to NEEM product. We'll want this later to color the island-wide hypersurfaces in voxel view and cutaways (plugin --> 3D --> volumeViewer in ImageJ, resp. PovRay). The dark blue line shows an extra just done with the color picker.

The striations on this 423 km track are all continuous so obviously isochrons, but Patton properly verifies this in view of the extreme importance of these two drill sites. Here snowfall and compaction had slightly different histories on NGRIP relative to NEEM so depths and dates had to be cross-correlated (inset below, far right corner), and striations shown to fall on that curve. Which they do, red dots. There is otherwise no interpretation of striation dates in this paper.

Once an overall major striation pattern has been established, its labels can be easily transferred to a new track that doesn't tie in conveniently to NGRIP to NEEM using methods lifted from dendrochronology, tree rings just being annual isochrons wrapped into circles. There, rings on an unknown tree core (~ radar striations on an isolated flight line) are dated by best match to a sliding window (ie convolution) with the fiducial tree ring chronology (itself tiled up like flight lines). That can also be done internally on radar echoes where a striation might be missing for part of the track.

Since only long straight flight lines are really worth processing and unique additions barely run to 10,000 km/yr, I see little point to automation since a given year can be done manually in 20 hours at 500 km/hour. Between the many glitches in data acquisition and bizarre deformations, there has to be a human supervising the process anyway.

I find it exceedingly implausible that glaciologists can add to a field having already undergone 80 years of intensive methodological development (dendrochronology). True,  academics wrote the book on the not-invented-here syndrome so glaciologists can write a new chapter if they choose.

<url>http://www.igsoc.org/annals/55/67/t67A048.pdf</url>
<url>http://www.igsoc.org/journal/60/222/t13J196.pdf</url>

The image below is at the original Cresis resolution. It takes 2047 pixels of width to tile up this 423 km flight line as an 877 kb jpeg.

Espen

  • ASIF Governor
  • Posts: 3078
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #396 on: July 30, 2014, 04:55:11 PM »
You are a tough cookie, A-Team ;)
Have a ice day!

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #397 on: July 31, 2014, 02:20:28 AM »
Espen, there is nothing quite as tasty as a large bowl of low-hanging fruit.

I pulled the raw Cresis radar profiles that go with the recent Swiss Camp drill site article about the Wisconsin-Holocene Transition ice at ~ 600 m into the ice in the vicinity of Jakobshavn north branch. As you can see, that appears associated with a dramatic radar return striation.

https://data.cresis.ku.edu/data/rds/2012_Greenland_P3/images/20120421_01/

So not only do we have a definite date for local ice, its hyperplane mesh can be determined from coast to NGRIP-NEEM summit ridge. For that Delauney (~ Voronoi) interpolation can be used, just as they did here for the bedrock grid.

This article "Sustained high basal motion of the Greenland ice sheet revealed by borehole deformation" by C Ryser et al (Journal of Glaciology, Vol. 60, No. 222, 2014 doi: 10.3189/2014JoG13J196) has a lot of interesting detail on the partitioning of surface velocity between deformation and basal sliding. Their study area lies just outside the area of direct dynamic influence of JI.

AbruptSLR

  • ASIF Emperor
  • Posts: 13283
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #398 on: July 31, 2014, 05:58:50 PM »
A-Team,
When you are finished with your bowl of low hanging fruit for the JI, you might be interested in looking at another bowl of low hanging fruit of data for the Thwaites Glacier in the West Antarctic Ice Sheet, WAIS.  In Reply #100 in the "Surge of WAIS Ice Mass Loss" thread (see link below) of the Antarctic folder I posted some of the data from their Antarctic 2012 IceBridge survey for the Thwaites Glacier from two websites that I can no longer access (see following links):

http://forum.arctic-sea-ice.net/index.php/topic,21.100.html


ftp://n4ftl01u.ecs.nasa.gov/SAN2/ICEBRIDGE_FTP/IRMCR1B_MCORDSxyEcho_v01/2012_AN_NASA/pdf/

ftp://n4ftl01u.ecs.nasa.gov/SAN2/ICEBRIDGE/ILATM2.001/

The IceBridge program also provides the following current links to survey data of the Thwaites Glacier:

http://nsidc.org/data/ilatm2

http://nsidc.org/data/icebridge/index.html

Best,
ASLR
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #399 on: August 01, 2014, 12:46:11 AM »
Antarctica makes me sick to the stomach.

Not the WAIS collapse, that had to be coming. However with Abrupt providing a truly superb forum resource, it becomes quite feasible to delve quickly into the details of why and when.

No, it's the graphics. Compare the two below. The first collates all the radar striations that exhibit the Bølling–Allerød transition 14,700 years ago. Fine, a start. But if the Danish government could only give me a krone for every mistake here, I'll not be on a short tourist overflight of the JI calving front -- I'll have enough to buy the helicopter and enjoy a catered ice camp.

Antarctica is totally kicking Greenland's bøtt. Not only did they swipe my idea of bump-mapping the striations a year before I even thought of it, they have miraculously fitted curved vertical radar flight lines onto an elegant bedrock DEM. Which you can rotate, look at upside down or whatever, all with freeware.

Denmark just 5.6 mm people, what can be expected? Well guess what, n.o.b.o.d.y lives in Antarctica.

http://www.antarctica.ac.uk/bas_research/data/access/res/data.php