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Author Topic: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland  (Read 575418 times)

Shared Humanity

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #250 on: June 11, 2014, 03:37:05 PM »
Many thanks for all the nice topography maps everyone. I kind of share your concern SH, perhaps first and foremost because of the developments we have seen in the Northern branch during recent years, a branch which also seems to have a quite limited depth. My assumption after seeing the newst maps, is that it wouldn't be a very big surprise to see a third branch slowly emerge.


However, what really frighten me is this.


Hopefully, and probably, it is going take several decades before Jakobshavn splits into multiple branches that each are capable of penetrating several hundred kilometers into the Greenland ice sheet, but the fact that all these canyons exist would suggest pretty strongly that there have been numerous fast moving glacial outlets right in the middle of Greenland during previous meltdowns. It strikes me though, that most of this vast network of canoyns is excluded from more detailed topography maps of Jakobshavn and doesn't seems to be a videly discussed issue. Is it just too far into the future?

I guess my reason for focusing on the ice sheet that is northeast of the Jakobshavn calving face is it is already moving fast, has been unzipped by the retreating Jakobshavn, and, based on the animations by Espen,  is speeding up towards the fjord as I type. The impact of this sheet being exposed to the sea, causing melt and retreat of the grounding line means it will contribute to ice mass loss this year and into the near future. It will be fun to watch. I will be looking at every image Espen posts here.

Laurent

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #251 on: June 11, 2014, 04:48:05 PM »
I don't think it is full speed already, there is a few kilometers before it is moving in his deep bed (If I see it correctly).

LRC1962

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #252 on: June 11, 2014, 08:28:17 PM »
Very interesting talk about studying movement of  Greenland glaciers from an engineers point of veiw.
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Espen

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #253 on: June 11, 2014, 08:30:04 PM »
Cross post from Zachariae Isstrøm thread:

What is all the fuss about?

https://forum.arctic-sea-ice.net/index.php/topic,400.msg28208.html#msg28208
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Laurent

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #254 on: June 11, 2014, 09:14:21 PM »
It seems to me that the potential collecting area is higher for Jakobson !? On the geological map of Zachariae it is not clear if it is linked to the inside of Greenland, can you tell us what are the volume potential of each of them ?
« Last Edit: June 11, 2014, 09:43:32 PM by Laurent »

Hans

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #255 on: June 11, 2014, 09:41:02 PM »

Espen

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #256 on: June 11, 2014, 09:48:29 PM »
It seems to me that the potential collecting area is higher for Jakobson !? On the geological map of Zachaeria it is not clear if it is linked to the inside of Greenland, can you tell us what are the volume potential of each of them ?

Laurent,

It seems so, and since Jakobshavn is in the "tropical" and the more accessible part of Greenland, that is where the focus is, but I think there is just as much "raw material" (ice) around Zachariae and not so many "natural outlets" Zachariae is wise to be open for business, when the conditions are fair (climate change).

Just to put it an economical, and not only in an engineering perspective?
« Last Edit: June 11, 2014, 10:16:54 PM by Espen »
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solartim27

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #257 on: June 12, 2014, 02:00:45 AM »
I had posted this on the Greenland melt page:
Any idea what the line to the northand inland of Jacobshavn is?  Melt water channel?  Seemed to form in a week, last clear day there was 6/2.
http://lance-modis.eosdis.nasa.gov/imagery/subsets/?subset=Arctic_r02c02.2014162.terra
 
FNORD

icefest

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #258 on: June 12, 2014, 03:18:48 AM »

Any idea what the line to the northand inland of Jacobshavn is?  Melt water channel?  Seemed to form in a week, last clear day there was 6/2.


Clouds.
You can see the shadow in 3-7-6:


Furthermore if you compare Aqua and Terra true color shots, it visibly moves to the east.
Attachment is animated.
Open other end.

Lennart van der Linde

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Espen

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #260 on: June 14, 2014, 10:39:44 AM »
Further calving at Jakobshavn, although the documentation is vague and based on Nasa Modis images, this calving could be beyond September 2013 maximum retreat point:
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Shared Humanity

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #261 on: June 14, 2014, 05:42:18 PM »
With the current condition of the Baffin sea ice, if we get a high established over Greenland with its characteristic flows from the south along the western edge of Greenland, the Jakoshavn may be one of the big stories in this melt season. Not only will the calving face retreat dramatically but I believe we will see significant and unprecedented calving from the ice sheet that is grounded below sea level and is being exposed by the retreat of Jakobshavn.

Espen

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #262 on: June 14, 2014, 06:35:53 PM »
Current conditions are not so important when talking about glaciers, calving and more important retreats are results of what happened years ago, there is time lack. Therefore the current sea ice situation cannot be compared to what is happening with the glaciers.
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LRC1962

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #263 on: June 14, 2014, 10:54:50 PM »
Although I have very little direct knowledge, think of it this way. The glaciers lakes are happening at the top. That is where the ice is originally weakened. Now think how long it will take to get that weakened ice down to the end. Granted, what happens at the bottom end certainly influences the amount of stress farther up, but if the ice has been weakened farther in the first place then the quicker it will break up in the end.
The same as with CO2 and temps. Current temps are results partly of CO2 forces 30-40 years ago. Take CO2 levels then then think of what they are now an project temps 30-40 years down the road.
On the other hand when as in a case of a full glass of water, when things become too much then one thing starts impacting other things that were not impacted before. Are we there yet. I think not, but I also think we are far closer then anyone of us would wish.
The sad thing is all of this could have been avoided because we knew the basics 200 years ago.
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Espen

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #264 on: June 15, 2014, 08:32:43 PM »
I am now pretty convinced, on Friday June 13 2014, of all dates? Jakobshavn went into retreat land with another large calving, better documentation will hopefully be available soon, cant figure out Landsat release schedules and priorities? ???

Please click on image to start animation!
« Last Edit: June 15, 2014, 08:44:00 PM by Espen »
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nukefix

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #265 on: June 23, 2014, 05:43:24 PM »
Here is a SAR-image of Jakobshaven taken with Sentinel-1 on 7.6.2014. The images are uncalibrated, polarisation is HH,HH,HV for the RGB-channels, they are both zooms from the same 250-km wide scene. The calving front is very visible in these conditions and in the future we will be getting two images like this in every 12-day period, through weather and clouds!

ps. image-processing done with the open source ESA NEST-toolbox
pps. this is some other ice-stream close to Disko Bay and not Jakobshaven. anyway, the potential of this satellite data is evident for calving-front location detection.
« Last Edit: June 23, 2014, 05:54:23 PM by nukefix »

nukefix

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #266 on: June 23, 2014, 05:51:54 PM »
Oops, wrong ice stream, I'll keep you posted once ESA releases an image that covers Jakobshaven.

Espen

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #267 on: June 23, 2014, 08:52:32 PM »
Really good images for calving and retreat documentation, I thought the intervals (12days) would be shorter when it is a so called Supersite?
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Sonia

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #268 on: June 24, 2014, 03:05:04 AM »
Don't know the name of the glacier but it's easily seen a bit north of the Jakobshavn.

Espen

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #269 on: June 24, 2014, 06:41:46 AM »
Jakobshavn update:

Due to growth since June 8 the loss since don't look that bad.

Please click on image to start animation!
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Yuha

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #270 on: June 24, 2014, 07:15:16 AM »
Espen, could you post the June 24 image on its own and without overlapping titles?
It shows the shapes of the ice surface with spectacular clarity.

nukefix

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #271 on: June 24, 2014, 09:27:00 AM »
Really good images for calving and retreat documentation, I thought the intervals (12days) would be shorter when it is a so called Supersite?
It will be imaged from both ascending and descending orbits every 12 days, it's too early to tell what will be the separation in days between the ascending and descending acquisitions.

Espen

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #272 on: June 24, 2014, 04:08:44 PM »
Espen, could you post the June 24 image on its own and without overlapping titles?
It shows the shapes of the ice surface with spectacular clarity.

NB! This is a cut-out of the original image 4MB.
« Last Edit: June 24, 2014, 04:42:47 PM by Espen »
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Espen

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #273 on: June 24, 2014, 09:18:53 PM »
Calving front update:

The image below show where the calving front was on September 27 2013 (red line) and June 24 2014 (yellow line).
At some points Jakobshavn retreated beyond the Sep. 27 2013 calving front, but still behind 2013 at the eastern main gate.
« Last Edit: June 24, 2014, 09:29:35 PM by Espen »
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Yuha

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #274 on: June 25, 2014, 04:34:16 PM »
Espen, could you post the June 24 image on its own and without overlapping titles?
It shows the shapes of the ice surface with spectacular clarity.

NB! This is a cut-out of the original image 4MB.

Thanks!

A-Team

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #275 on: July 01, 2014, 09:49:55 PM »
The 01 Jul 14 Landsat-8 image was released today. Skies were clear over the calving front. Unfortunately LC80842322014182LGN00 being orbital path 84, row 232 just nicks the calving zone, providing the triangular overlap below. Image quality is not the best at the edge of its frame either.

Modis Aqua/Terra on 30 Jun 13 show more of the event but they are a couple of bricks short of a wall in terms of adequate resolution.

The bottom image in the 15 m slide show below has been painted to display different origins of ice arriving at the calving front, based on shear lines. This ice likely has different properties (temperature, thickness, velocity) depending on its origin which presumably correlate with where the calving front calves.

I'm very skeptical of journal displays indicating uniform velocity transects. We have an event here each fall where the students line up on a bridge at 1 m intervals, dropping rubber duckies into the river. As you can imagine, these hardly arrive at the next bridge downstream at the same time. I will put this matter to rest for Jakoshavn Isbrae the day we get our first pair of supersite radar images this fall.

It's very difficult to get a sense of volume lost, which after all is the main interest (increasing contribution to sea level rise). Rather than modeling the glacier, I think it would be better just to make accurate lidar measurements of the downstream fjord (noting tide level). Heights there imply depths since this is mainly ungrounded freshwater ice. Volume is then proportional to the surface integral of the DEM.


Shared Humanity

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #276 on: July 03, 2014, 02:36:45 PM »
Calving front update:

The image below show where the calving front was on September 27 2013 (red line) and June 24 2014 (yellow line).
At some points Jakobshavn retreated beyond the Sep. 27 2013 calving front, but still behind 2013 at the eastern main gate.

I believe this image is clearly showing the new calving face is already contributing ice loss into the fjord. The yellow line is retreating northeast. How quickly will the main calving face retreat or will the speed of the glacier prevent this calving face to retreat quickly and allow this new calving  face to march toward the fjord?

Espen

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #277 on: July 03, 2014, 04:41:38 PM »
SH;

That is a very good question, there is bedrock threshold in the near vicinity below where the eastern calving front is at the moment, and I think no one really knows what will happen next, but at one point it will retreat in that direction, if current conditions are not changed.   
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Espen

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #278 on: July 03, 2014, 08:09:44 PM »
Still waiting for Landsat, but here is another calving and potentially a retreat:

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A-Team

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #279 on: July 04, 2014, 12:45:43 AM »
Landsat imagery is now available for download at EarthExplorer. The big one is LC80090112014184LGN00_B8. I adjusted all the contrasts within the 16 bit framework of ImageJ as tests show it does make difference.

The most curious thing about this calving event is its edge at the fast-moving main ice stream. The slower ice coming off the hill to the south has not yet participated. The image below is 30m. I upsampled the 15m panchromatic to 7.5 m to see what the details of the calving front looked like ... a work in progress it appears.

Espen

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #280 on: July 04, 2014, 05:34:05 AM »
Here is the Landsat proof of the calving events between June 8 and July 3 2014:

Please click on image to start animation!
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Espen

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #281 on: July 04, 2014, 06:03:21 AM »
I would say we are into retreat land already, judge by yourself?
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A-Team

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #282 on: July 04, 2014, 07:50:54 AM »
This Landsat-8 is a path,row 9,11 which is always followed by a 10,11 the next of which should appear on July 7th, or if not then July 11. It has a few puffies but otherwise is an exceedingly sharp image.

I noticed a peculiar thing about Band 8 panchromatic. It looks a lot better with its grayscale inverted (x --> 255-x). This could have something to do with the lighting. According to the metadata file:

SCENE_CENTER_TIME = 15:00:24.4130118Z
CLOUD_COVER = 1.49
IMAGE_QUALITY_OLI = 9
SUN_AZIMUTH = 173.30855079
SUN_ELEVATION = 43.33909208

It looks like a couple of bergs at the two extremes of the calving front will be calving off sideways. Otherwise, lots of crevices (and even patterns to them) but nothing that indicates a next calving front.

ablair

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #283 on: July 04, 2014, 11:45:17 AM »
I've been watching a developing dark spot above the northern branch and it has suddenly grown into a large hole/ nunatek which indicates how much the level must be lowering. Will be interesting to see this develope into an ice stream divide.

A-Team

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #284 on: July 04, 2014, 04:38:59 PM »
Good observation. The north branch is quite interesting in its own right. In the late 1980's, it joined the south fork in an extended down-fjord feature called 'the zipper' (the thicknesses were markedly different). Now that the south fork has retreated up its own channel, there is less back pressure on the north and middle fork calving fronts.

The black streaking on discharged ice below the nunatak probably represents rock on the bottom of calving pieces (as in 'Chasing Ice') rather than moraine streaking from the nunatak.

Note the north fork does not overlie a deep channel nor does it have a large drainage. Thus it does not have potential to be a major contributor to sea level rise.

Below is the panchromatic 7.5m from yesterday of this nunatak. We won't be seeing similar pinning points emerge on the south fork because its channel is thousands of feet below sea level for 125 km inland.

In terms of developing a time series, I looked through the Landsat-8 imagery database posted on the other forum http://forum.arctic-sea-ice.net/index.php/topic,909.0.html, both for other path 9, row 11 cloud-free imagery (which will have the most similar platform geometry if you want a stereo pair or count interometric movement fringes) and also for the most recent regardless of path, row. There is no counterpart to July 3 imagery in the 2013 archive.

Espen

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #285 on: July 04, 2014, 05:36:22 PM »
Nice observation Ablair, yes we will probably see several Nunataks popping up here and there over the years, as we observe at several glaciers around Greenland.
I myself thought it was a melt pond :-[
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Rubikscube

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #286 on: July 04, 2014, 08:26:09 PM »
I thought this was just a melt pond as well and that it would rather be caused by an emerging nunatak. Either way it is kind of strange looking.

A-Team

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #287 on: July 05, 2014, 03:20:54 AM »
Right, strange flow pattern in the vicinity, topography is a bit of a puzzle. However ordinary google map has a great series of zooms here (once you're sure you're zooming on the right object, the north fork has retreated a lot since this imagery. The melt lake is very black in Landsat today but by clicking on it with the color wand tool, other lakes prove just as black (while most are classic blue). Click if the animation doesn't start.

A-Team

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #288 on: July 05, 2014, 03:33:25 AM »
Here we go, perspective DEM from TerraSar. May or may not be some degree of vertical exaggeration. Not sure of the date either. North branch is clear locatable; more extreme topography than i would have guessed from the nadir view of Landsat. It's not so easy to be sure the melt lake is in the pocket though.

TerryM

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #289 on: July 05, 2014, 08:11:23 AM »
Amazing images A-Team !
Terry

ablair

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #290 on: July 05, 2014, 01:09:12 PM »
Right, strange flow pattern in the vicinity, topography is a bit of a puzzle. However ordinary google map has a great series of zooms here (once you're sure you're zooming on the right object, the north fork has retreated a lot since this imagery. The melt lake is very black in Landsat today but by clicking on it with the color wand tool, other lakes prove just as black (while most are classic blue). Click if the animation doesn't start.
[ Wow - good detective work there, thanks - it's a melt lake then!. that would explain why it grew so quickly. As you say, the topography a bit unusual there/quote]

Shared Humanity

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #291 on: July 05, 2014, 02:15:57 PM »
Here we go, perspective DEM from TerraSar. May or may not be some degree of vertical exaggeration. Not sure of the date either. North branch is clear locatable; more extreme topography than i would have guessed from the nadir view of Landsat. It's not so easy to be sure the melt lake is in the pocket though.

I don't think this is vertical exaggeration. I believe what we are seeing is a suggestion of the grounding line and the lowering of the ice surface is evidence of bottom melt. This picture shows this on the North Branch but also shows a similar condition on the new calving face that is emerging due to the retreat of the Jakobshavn up the deep gorge. I think this emerging new calving face has the potential of dumping far more ice into the fjord than we might expect.

As the Jakobshavn calving face continues its retreat, we are going to see the very high ice north of the fjord behave in a very similar fashion as it is also grounded below sea level. The area that should have aggressive bottom melt and calving will be the heavily crevassed regions closest to the fjord.

Oh! I also agree. This is quite simply the most amazing image I have ever seen. Can you secure such images of other glaciers of interest?
« Last Edit: July 05, 2014, 03:06:28 PM by Shared Humanity »

Shared Humanity

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #292 on: July 05, 2014, 02:44:09 PM »
I thought this was just a melt pond as well and that it would rather be caused by an emerging nunatak. Either way it is kind of strange looking.

I absolutely do not believe this is an emerging nunatak. The most accurate topography maps to date (see image below) show no land above sea level in this area of the north branch. On the ice speed image on the left, we also see no pinning effect that should be present if such a nunatak existed. This north branch of the glacier is moving uniformly fast where the shallow basin of water sits beneath it. We do, however, see the effect of the large island that sits just east of the north branch. If you look at the ice speed image on the left, the ice sheet here is moving much slower.

This could be a melt pond but I do not believe this is the case either. I think what we are seeing is open sea, the actual surface water of the shallow basin that sits underneath this portion of the ice sheet. The ice sheet on this particular portion of the north branch has completely melted away. This is why this area is as dark as the water in the open fjord. We are not looking at a shallow melt pond or lake. We are looking at open sea. While it is certainly shallower then the fjord, the topography map suggests this water is perhaps 100 meters deep, maybe deeper.
« Last Edit: July 05, 2014, 03:11:15 PM by Shared Humanity »

crandles

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #293 on: July 05, 2014, 04:17:06 PM »
I thought this was just a melt pond as well and that it would rather be caused by an emerging nunatak. Either way it is kind of strange looking.

This could be a melt pond but I do not believe this is the case either. I think what we are seeing is open sea, the actual surface water of the shallow basin that sits underneath this portion of the ice sheet. The ice sheet on this particular portion of the north branch has completely melted away. This is why this area is as dark as the water in the open fjord. We are not looking at a shallow melt pond or lake. We are looking at open sea. While it is certainly shallower then the fjord, the topography map suggests this water is perhaps 100 meters deep, maybe deeper.

Why wouldn't ice be pushed into this below sea level area? Perhaps it is but it keeps melting due to warm water is the only reason I could suggest. This seems unlikely to me without there being known geothermal activity but then I wouldn't really trust my intuition given my lack of knowledge.

Shared Humanity

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #294 on: July 05, 2014, 04:24:51 PM »
I have been reviewing and comparing these 2 images daily for the past 2 weeks. It is simply remarkable how the ice speed image matches precisely the underlying topography. Every time I review these images (again I do this daily) I begin to understand more about the existing behavior of this glacier but believe I can see how this glacier will behave in the future. Of course, I could be fooling myself. I've had a career in manufacturing management so what makes me an expert?

I have wanted to begin a review of Zacharia Istrom (sic) using the same images because Espen is certain that this glacier has far more sea level rise potential. If Espen feels this way, I am certain I do as well. I simply cannot tear myself away from this monster.

Shared Humanity

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #295 on: July 05, 2014, 04:29:20 PM »
I thought this was just a melt pond as well and that it would rather be caused by an emerging nunatak. Either way it is kind of strange looking.

This could be a melt pond but I do not believe this is the case either. I think what we are seeing is open sea, the actual surface water of the shallow basin that sits underneath this portion of the ice sheet. The ice sheet on this particular portion of the north branch has completely melted away. This is why this area is as dark as the water in the open fjord. We are not looking at a shallow melt pond or lake. We are looking at open sea. While it is certainly shallower then the fjord, the topography map suggests this water is perhaps 100 meters deep, maybe deeper.

Why wouldn't ice be pushed into this below sea level area? Perhaps it is but it keeps melting due to warm water is the only reason I could suggest. This seems unlikely to me without there being known geothermal activity but then I wouldn't really trust my intuition given my lack of knowledge.

You could be right. I have no more knowledge than you. It could very well be a melt pond or lake. I am fairly certain it is not a nunatak or evidence of an emerging nunatak.
« Last Edit: July 05, 2014, 04:40:01 PM by Shared Humanity »

Espen

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #296 on: July 05, 2014, 05:09:37 PM »
I have wanted to begin a review of Zacharia Istrom (sic) using the same images because Espen is certain that this glacier has far more sea level rise potential. If Espen feels this way, I am certain I do as well. I simply cannot tear myself away from this monster.

Compared to Zachariae Isstrøm, Jakobshavn is "just a Mickey Mouse" machine, the image below shows the difference (same scale).
« Last Edit: July 05, 2014, 05:36:50 PM by Espen »
Have a ice day!

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #297 on: July 05, 2014, 06:22:38 PM »
I'm going with freshwater melt lake.

The TanDEM-X DEMs provide 5 m/px resolution; DigitalGlobe WorldView 0.5 m/px stereo pairs. Surface rock observation IceBridge ATM/LVIS, ICESat GLAS, and GPS give absolute control on elevation data. A forthcoming paper from the Joughin group has a time series of sub-meter horizontal and vertical absolute accuracy, fully rectified photogrammetry. This feature is well above the error bars for sea level.

Jakobshavn has not been a floating ice shelf situation cf Antarctica since 1998 or so (except ephemerally in winter). The ice here is thousands of feet thick; it goes all the way down to the bedrock (modulo basal meltwater) and retains stratifications formed during the early and mid Holocene down 1500 m below sea level.

On ice-penetrating radar (bedrock), far fewer flight lines exist for the north branch. Here you want to look at adjacent raw radar profiles, look at their spacing and ask yourself how the bedrock surface in between is filled in.

In the late 1980's, there was quite a bit of discussion of a pinning point out in the main south bay of the main fjord. Although way below sea level, it had a very noticeable effect on movement of  ice (which was grounded out there then). It might better be called a seamount than a nunatak.

I don't recall any experimental data on the geothermal gradient for the Jakobshavn region. The expectation is way lower than central Greenland where the weight of the ice has thinned the viscous mantle, thickening it under the island margins and so slowing heat flow upward. There's only been some shallow steam drilling on the icestream and three bore holes (from the 1990's) way up the main branch that did not go to bedrock.

Now the Disko Bugt suture could change the geothermal picture. Although ancient -- archaean micro-continent pushed against paleoproterozoic -- anomalous heat flow could occur along the suture or a re-activated fault. It could also explain why the main Jakobshavn channel is located where it is. However the geology map is showing the suture well to the north, more like Epiq.

Shared Humanity

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #298 on: July 05, 2014, 07:57:57 PM »
Thanks for the explanation A-Team. Everything you said makes sense. Melt lake it is. I would not call it a pond. It is too large and too deep. Were you able to confirm whether this lake is centered in the depression that you can see in that gorgeous image that you posted above?

Espen

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Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #299 on: July 06, 2014, 10:11:06 AM »
Here is another reminder animation the first image is almost 42 years old compared to 2014, what a change?

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