Support the Arctic Sea Ice Forum and Blog

Author Topic: Jakobshavn Isbræ / Ilulissat Isfjord / West Greenland  (Read 575412 times)

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #500 on: November 07, 2014, 06:13:42 PM »
Remember all that rubbish from the '70's about nuclear-powered tugs towing icebergs to Los Angeles, providing water too cheap to meter?

How about some nonsense of our own -- floating the whole of Greenland off its bedrock and towing it to the Arctic Ocean to replace the vanishing sea ice?!?

I believe this would be fully credible to the general public if only we could hoax up a viral visualization. However there a number of technical issues in doing this responsibly. 

For Greenland as iceberg, the newly combined bedrock and surface DEMs in Bamber 2013 provide a 3D ice thickness model, the bottom of the berg appearing as the reverse mold of the bedrock. The absolute thickest spot is 3367 m with NGRIP core only 3085 m in length. The volume of ice has been re-estimated at 2.93 million km3 and the mass as 2.67x 10^18 kg.

If we neglect bubbles, impurities, firn density, temperate ice, ablation, melt lake, moulins and tipping issues, where is the freeboard line on Greenland as it stands? Taking the density of icesheet ice at 911 kg/m3 and that of seawater as 1025 kg kg/m3, it follows that  ~89% of the iceberg's mass is below water.

To find the giant berg's water line, it seems necessary to slice the thickness DEM into say 1 m horizontal sections and determine their area, volume and finally mass contribution. These would have to be summed until the 89% was reached. As byproduct, the center of mass could be located (first moment).

I have no idea where these would be, maybe 1500 m above sea level on present Greenland and midway under the center ridge. It may well be the case that the equilibrium position is upside down yet the initial flotation metastable.

Sleepy

  • Guest
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #501 on: November 07, 2014, 07:08:52 PM »
 ;D
Too much information won't sell.
Just tow it up a bit, and let it get stuck between Ellesmere and Svalbard. ;)

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #502 on: November 08, 2014, 02:47:08 PM »
That's a good start, Sleepy. Perhaps someone could make a paper cut-out of Greenland ice, slide it over a large globe to see if it squeezes through. It docks very nicely to the western shore of Siberia. They have an app for this in plate tectonics (euler angles and all that).

I determined the ocean is far too shallow to permit a full roll-over at half-width. However it is top-heavy so floation collars would be necessary to prevent tilting.

The data (and incoming revisions) to actually make these calculations are not online; it is necessary to email Bamber for the netCDF/GeoTiff file. If it is gridded to 1 km, that would amount to 2.2 million spreadsheet lines with 5 columns (lat,lon, bedrock,surface, ice thickness or 11 million cells.

Set up 300 bins of 10 m height increments and populate them as 1,0 from a spreadsheet logic operation such as IF (if-expression, if-true, if-false) applied to 3rd and 4th columns, sum bin columns, then sum the sum row until 89% is reached.

I would try this first gridded to 25 km for a 625-fold reduction in file size, or better, take one east-west cross section at a time as waterlines of the sections average to the waterline of the whole ice mass.  Easier still, Greenland's surface looks like a small chord of a very large circle in cross section and the bedrock could be similarly approximated. Both waterline and center of gravity then follow from symmetry considerations without any calculation.

So yes, if sea level was 1500 m higher (or following massive, rapid melt-off that outpaced isostatic rebound), nothing  would "... keep sea water from rushing in and the whole mountain of ice bobbing up".

Similar considerations apply to the first 100 km of Jakobshavn Isbrae. Not going to see dynamic thinning at the calving front to the point waves are rolling inland over an underwater  icestream.
« Last Edit: November 08, 2014, 02:56:11 PM by A-Team »

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #503 on: November 08, 2014, 02:53:49 PM »
Espen asks if the southern branch has a 'doorstep or threshold' pausing further retreat for the next few years.

The calving front has about reached the main sill below a major overdeepening (latest maps from Gogenini paper in post #492). Being much higher, the bedrock of that sill prevents warm sea water from reaching as much of the underwater part of the calving front. That would favor slow-down. Notice that the resolution of the south channel floor is still not great -- the oblique cross section 20130404_02_032 shown in post #493 has 1 pixel per 56x56 sq m of channel (soccer field resolution).

Perhaps there are some clues from overall history of the fjord. Notice it is a straight deep shot all the way from the bay (confluence of north and south channels) all the way out to the mouth at Disko Bay, where detailed bathymetry is available (fig.1,2). In the past, the glacier has ground a channel into bedrock all the way out to the continental shelf (fig.3). Morraine studies have clarified Holocene glacier advances and retreats though the history is complex having been overwritten so many times.

Upstream of the current calving front, we could ask why the icestream has not gouged out a straight shot as well, despite 2-3 million years of erosion. Instead it resembles a meandering river with tributaries (which indeed may have defined the original course in a balmier era).  I am not recalling anything like this in Norwegian fjords however.

Paleo-drainage reconstruction studies of JI have to undo isostatic depression over time to recover the original topography. Note only the upper of the three main tributaries seems very active today though the lower ones once reamed out a major overdeepening at their confluence.

Espen

  • ASIF Governor
  • Posts: 3078
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #504 on: November 08, 2014, 04:11:51 PM »
A-Team so we will have to wait a few years for the the grinding process of the sill to take place, before we enter into further retreat of the southern branch?
Have a ice day!

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #505 on: November 09, 2014, 02:23:48 PM »
Espen asks if we will have to wait a few more years before JI becomes unglued.
 
The bedrock is hard unfaulted metamorphic precambrian, the sill has withstood a much larger glacier, one that carved a channel out to Baffin Bay. But we may have *already* waited 'a few more years' --  next year could be a biggie.

The question is what sort of year was 2014, did the acceleration of ice in JI's collection basin fade, continue, or get worse relative to 2010-2013 acceleration years? The position of the calving front, while easy to monitor, does not really address this. The last published data from the Joughin group only cover mid-summer 2013 http://www.the-cryosphere.net/8/209/2014/tc-8-209-2014.html

With a bit more effort, it turns out (if you believe abstract C24B-02 AGU 2014), we could have measured the 2014 velocity field ourselves from Landsat:

Fahnestock, Scambos, Kinger: The high radiometric resolution [12 bit] of Landsat 8 enables one to track subtle patterns on the surface of the ice sheet, unique at spatial scales of a few hundred meters, between images separated by multiple orbit cycles. In areas with significant dynamic topography generated by ice flow, this requires use of simple spatial filtering techniques first applied by Scambos 1992 [free full http://tinyurl.com/loveunb].

The result is densely sampled maps of surface motion that begin to rival the coverage available from SAR speckle tracking and interferometry. Displacement accuracy can approach one tenth of a pixel for reasonable chip sizes [small chunk of large image] using conventional normalized cross-correlation; this can exceed the geolocation accuracy of the scenes involved, but coverage is sufficient to allow correction strategies based on very slow moving ice.

I could not find any ppts, posters or publication with actual results. Rather than wait for a paper, we might try the ImageJ-SURF plug-in, http://labun.com/imagej-surf/ which can provide cross-correlation on matched pairs of cloud-free Landsats (ie same row,path,center). The longer the separation in dates, the better the resolution for the slower moving regions.

Flipping through the 7 articles citing Joughin 2014 at GoogScholar for an update, it turns out T Moon et al just put up a new free/full article that updates JI velocity, detrended velocity, run-off (calving volume), and calving front up to 1 Aug 2013: http://onlinelibrary.wiley.com/doi/10.1002/2014GL061836/full

Here Jakobshavn is glacier #26 among the 55 studied. Using a pdf extractor on supplemental, the graphical data is provided at a very satisfactory resolution 1800 x 1200 for measuring areas-under-the-curve etc. This shows 2013 continued the remarkable speedup of this glacier despite unremarkable weather (measured as the proxy, runoff), recalling the whole summit melted in 2012, run-off reaching its peak.

Daily ice sheet runoff data comes from KNMI's regional atmospheric climate model RACMO2.3; it is used to identify  subglacial water  and the timing of the seasonal melt cycle. Modeled runoff is defined as unretained melt + rain presumably available for drainage to the bed of the ice sheet, that presumably lubricates it some of the time, facilitating sliding.

If that data is available up through Aug 2014, we might estimate the other lines for 2014 from correlation.
« Last Edit: November 10, 2014, 02:09:40 AM by A-Team »

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #506 on: November 10, 2014, 04:15:10 PM »
Here are the 35 cloud-free Landsat-8 scenes of Jakobshavn Isbrae for 2014. They are presented in blocks of matching path,row as this best corresponds to distortion-free geometry (same nadir lat,lon).

The 4th column gives the gap in days between successive cloud-free scenes (a multiple of the 16 day orbit). That gap can be as high as 96 days -- it can be cloudy in west central Greenland.

To measure 2014 velocities with the ImageJ SURF plugin (see post above), the choice of block depends on where you wish to measure it. In late July, the icestream might be moving 50 m/day near the calving front, or ~3 pixels at 15m panchromatic resolution so even the shortest interval (16 days) gives a very generous 48 pixels of feature displacement.

The velocity farther upstream at 5 m/day could still be measured fairly accurately at this temporal gap. However velocity mearurement off to the sides or way upstream would benefit from more time between frames. That might raise other issues: mis-matching of sun angle or azimuth causing different illumination conditions. Note too some path,row combinations do not extend very far to the east.

I've attached a piece of the remarkable scene of 08 Nov 14. Late season to be sure but Landsat with 12 bit sensitivity may be able to continue even later into November. LC80090122014312LGN00 just barely covers the eastern Jakobshavn Fjord but that is enough to show it is not yet frozen into a buttress resisting calving. The shadows are very dramatic and quite useful for the topography of glacial surfaces to the south.

Scene                  Date       Day  Gap Path Row Cent Lat  Cent Lon   sunº  azimu  time
LC80100112014303LGN00  30 Oct 14  303  32  10   11  69.60623  -50.88680  6.63  179.9  15:06:41
LC80100112014271LGN00  28 Sep 14  271  32  10   11  69.60641  -50.87276  18.4  178.1  15:06:55
LC80100112014239LGN00  27 Aug 14  239  16  10   11  69.60655  -50.86838  33.9  174.8  15:07:07
LC80100112014223LGN00  11 Aug 14  223  16  10   11  69.60636  -50.87335  35.6  173.5  15:06:34
LC80100112014207LGN00  26 Jul 14  207  16  10   11  69.60629  -50.85146  39.7  172.9  15:06:41
LC80100112014191LGN00  10 Jul 14  191  32  10   11  69.60648  -50.86015  42.6  173.0  15:06:38
LC80100112014159LGN00  08 Jun 14  159  32  10   11  69.60660  -50.86458  43.2  174.9  15:06:11
LC80100112014127LGN00  07 May 14  127  16  10   11  69.60623  -51.01040  37.4  175.7  15:06:09
LC80100112014111LGN00  21 Apr 14  111  --  10   11  69.60624  -51.00720  32.4  175.4  15:06:25
                   
LC80080112014305LGN00  01 Nov 14  305  96   8   11  69.60646  -47.80095  06.0  179.9  14:54:21
LC80080112014209LGN00  28 Jul 14  209  16   8   11  69.60651  -47.76666  32.3  172.9  14:54:21
LC80080112014193LGN01  12 Jul 14  193  32   8   11  69.60641  -47.76994  42.3  172.9  14:54:17
LC80080112014161LGN00  10 Jun 14  161  32   8   11  69.60660  -47.77320  43.5  174.8  14:53:51
LC80080112014129LGN00  09 May 14  129  32   8   11  69.60666  -47.92350  37.9  175.7  14:53:46
LC80080112014097LGN00  07 Apr 14  097  16   8   11  69.60652  -47.91558  27.4  174.6  14:54:18
LC80080112014081LGN00  22 Mar 14  081  --   8   11  69.60650  -47.90940  21.2  173.7  14:54:32
                   
LC80090112014280LGN00  07 Oct 14  280  64   9   11  69.60638  -49.34008  14.9  178.9  15:00:32
LC80090112014216LGN00  04 Aug 14  216  16   9   11  69.60667  -49.32530  37.5  173.1  15:00:36
LC80090112014200LGN00  19 Jul 14  200  16   9   11  69.60648  -49.31127  41.2  172.8  15:00:29
LC80090112014184LGN00  03 Jul 14  184  32   9   11  69.60638  -49.31424  43.3  173.3  15:00:24
LC80090112014152LGN00  01 Jun 14  152  48   9   11  69.60662  -49.31691  42.6  175.3  14:59:57
LC80090112014104LGN00  14 Apr 14  104  48   9   11  69.60643  -49.45147  30.0  175.0  15:00:20
LC80090112014056LGN01  25 Feb 14  056  16   9   11  69.60626  -49.43474  11.4  172.6  15:01:03
LC80090112014040LGN00  09 Feb 14  040  --   9   11  69.60632  -49.42945  05.8  172.6  15:01:16
                   
LC80080122014305LGN00  01 Nov 14  305  96   8   12  68.27964  -49.49216  07.3  178.4  14:54:45
LC80080122014209LGN00  28 Jul 14  209  16   8   12  68.27976  -49.46022  40.5  170.8  14:54:45
LC80080122014193LGN01  12 Jul 14  193  32   8   12  68.27971  -49.46386  43.6  170.8  14:54:41
LC80080122014161LGN00  10 Jun 14  161  32   8   12  68.27988  -49.46752  44.7  172.6  14:54:15
LC80080122014129LGN00  09 May 14  129  32   8   12  68.27992  -49.60601  39.2  173.7  14:54:10
LC80080122014097LGN00  07 Apr 14  097  16   8   12  68.27955  -49.59887  28.7  172.8  14:54:42
LC80080122014081LGN00  22 Mar 14  081  --   8   12  68.27951  -49.59312  22.4  171.9  14:54:56
                   
LC80812332014208LGN00  27 Jul 14  208  16  81  233  69.60660  -48.19174  05.7  -47.9  23:54:03
LC80812332014192LGN00  11 Jul 14  192  --  81  233  69.60659  -48.20579  08.4  -46.8  23:53:59
                   
LC80822332014200LGN00  19 Jul 14  200  --  82  233  69.60623  -49.73775  07.4  -47.3  00:00:11
                   
LC80832332014175LGN00  24 Jun 14  175  --  83  233  69.60667  -51.29052  09.6  -45.7  00:06:12
« Last Edit: November 10, 2014, 09:04:54 PM by A-Team »

nukefix

  • ASIF Citizen
  • Posts: 407
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #507 on: November 12, 2014, 01:22:22 PM »
S-1 image from 4.11.2014 (large jpeg)

nukefix

  • ASIF Citizen
  • Posts: 407
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #508 on: November 12, 2014, 01:50:57 PM »
Here's my best guesstimate of the current location of the Calving Front. There's a quite large uncertainty on the CF location on the northern edge of the southern branch as it's difficult to tell whether some areas area still stuck to the ice-stream or are just ice melange. A time-series would make this easier to do..

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #509 on: November 13, 2014, 01:58:45 PM »
Those are very impressive at full resolution. That area (red tint below) is indeed ambiguous: early start on an ice shelf, melange forming, just passive ice on a relatively quiet zone, or ice cascading down the flattening middle branch terminating in the fjord.

This Sentinel is going to provide an impressive winter time series. Did you not say earlier that the return time between scenes was ~11 days? That will make berg tracking a snap. Those move west only by being pushed out by new calving so are a proxy for that.

The velocity of the icestream will also be easy to get at, either from speckle tracking or interferometry. For the latter, 3 consecutive dates from the same satellite position are needed. (I've tried 2, doubling one to make RGB, with so-so outcome. Here we have data for the past five winters that provide a context for 2014-15.

At some point, we may want to rectify the images from perspective to nadir view for comparison to Landsat. Do you recall the Sentinel viewing angle?

nukefix

  • ASIF Citizen
  • Posts: 407
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #510 on: November 13, 2014, 08:09:13 PM »
The area is currently imaged every 12 days and in the future it should be done twice every 12 days from different directions (ascending and descending orbits). Once scihub allows for scripted downloads it would be relatively easy to make a processing-graph in the S-1 Toolbox to add (coregister) the new scene onto the existing stack automatically, making the generation of animations very easy.

Speckle-tracking is the easier and more robust method for getting the ice-stream velocity, IMO, but InSAR (and Multi-Aperture-InSAR) is impressive when it works.

The incidence-angle is between 29 and 46 degrees in IW-mode, next time I'll generate the incidence-angle band too to get exact values at this location. I've been using just ellipsoid-geocoding (as I have no DEM of the area) into WGS84 . Do you know what the projection of the Landsat-imagery is? Using the same projection should help the comparisons..

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #511 on: November 14, 2014, 05:33:11 PM »
Nice work, Nuke. I am looking forward to how this comes out.

LandSat shut down on 01 Nov for the year at least around the calving front. The metadata file provides endless detail on image geometry. From the table above, it appears that path,row set 9,11 is best centered over the current calving front at lat 69.135, lon -49.570.

GROUP = PROJECTION_PARAMETERS
 MAP_PROJECTION = "UTM"
 DATUM = "WGS84"
 ELLIPSOID = "WGS84"
 UTM_ZONE = 22
 GRID_CELL_SIZE_PANCHROMATIC = 15.00
 GRID_CELL_SIZE_REFLECTIVE = 30.00
 GRID_CELL_SIZE_THERMAL = 30.00
 ORIENTATION = "NORTH_UP"
 RESAMPLING_OPTION = "CUBIC_CONVOLUTION"
END_GROUP = PROJECTION_PARAMETERS

NADIR_OFFNADIR = "NADIR"
SCENE_CENTER_TIME = 14:55:00.8997918Z
ROLL_ANGLE = -0.001
SUN_AZIMUTH = 178.40216329
SUN_ELEVATION = 7.31615968
GROUND_CONTROL_POINTS_MODEL = 291
GEOMETRIC_RMSE_MODEL = 13.438
GEOMETRIC_RMSE_MODEL_Y = 9.108
GEOMETRIC_RMSE_MODEL_X = 9.881


I revisited the "55 Glaciers" paper for Jakobshavn velocity to get some idea of what we could expect for Nov 2014 Sentinel velocities. The paper did not provide the actual data points underlying the graphs nor the method for curve-fitting (which was barely exposed under the fat data triangles).

The authors provided a link to the data repository at NSIDC but this proved to be a wild goose chase. Not only did this not contain the 2013 data used in the graph, instead of storing the Vx,Vy data as plain text csv array (comma separated values, excel friendly), they went for a novel 'binary' format for which Matlab/IDL must be purchased for k$$ (users still have to write code!), the explanation being "data files are in 4-byte big-endian IEEE floating point format with the same byte ordering used by non-Intel Macs, not the standard PC byte order." Non-intel macs are of 2005 vintage, so this page hasn't been updated in 9 years.

Here's what you get in your browser opening on the NSIDC ftp link, dozens of screens like this:

(Îîk(Îîk(Îîk(Îîk(Îîk(Îîk(Îîk(Îîk(Îîk(Îîk(Îîk(Îîk(Îîk(Îîk(Îîk(Îîk(Îîk((Îîk(Îîk(Îîk(?ï­ ?Ø"Á?Îþ»?ÇK?¼ÁP?¸õ­?¸¼?Á4â?âì??öé/@…É@½@fˆ?ýš"@U‘@Nm@JÂ@v¦@}?î?o?Ïãh?¼ Õ?ºxg?Á *?¾:{?­ï?«I%?¨2I? lþ? Ws?£Wþ?¢À#?ŸæY?£´0?¤3˜?¬ÔÆ

Most velocity fields we see simply plot velocity magnitude by color; a simple rectangular png could have been provided here so magnitudes need not be computed by each user from components.

So I just re-processed the graph image. It could not be sub-gridded by month or velocity because the pixel intervals were prime numbers, necessitating rescaling. I extended the trend line into 2015 and replaced the heavy triangles with a smooth hand-drawn curve.

The trend line, probably a least square fit heavily influenced by the big melt year 2012, can't keep going up forever; it shows icestream velocity even in the dead of winter is still quite high. I shadowed in 2013 under 2012 to facilitate comparison. Click on the image to see it at the proper scale.

sidd

  • ASIF Upper Class
  • Posts: 1710
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #512 on: November 14, 2014, 10:30:04 PM »
Somewhere, long ago, i wrote a FORTRAN program that read IEEE bigendian files. I believe GNU fortran had a specifier like CONVERT that did something like this

sidd

nukefix

  • ASIF Citizen
  • Posts: 407
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #513 on: December 09, 2014, 04:56:46 PM »
S-1 6.12.2014 in UTM22/WGS84, full-resolution jpeg. Looks like the southern side of the southern branch has retreated some more.
« Last Edit: December 09, 2014, 05:10:41 PM by nukefix »

Espen

  • ASIF Governor
  • Posts: 3078
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #514 on: December 09, 2014, 06:09:18 PM »
S-1 6.12.2014 in UTM22/WGS84, full-resolution jpeg. Looks like the southern side of the southern branch has retreated some more.

Not really, in the animation below is the 2014 minimum (September 28) compared to December 6 2014:
Have a ice day!

nukefix

  • ASIF Citizen
  • Posts: 407
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #515 on: December 10, 2014, 02:49:24 PM »
Thanks Espen! I can keep producing S-1 in UTM22 as they come in order to make a time-series..

Espen

  • ASIF Governor
  • Posts: 3078
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #516 on: December 10, 2014, 05:18:28 PM »
Thanks Espen! I can keep producing S-1 in UTM22 as they come in order to make a time-series..

Good idea Nukefix! :)
Have a ice day!

nukefix

  • ASIF Citizen
  • Posts: 407
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #517 on: December 19, 2014, 04:41:53 PM »
I have trouble getting the forum to accept my attachments...perhaps they are too big? Does anyone know what the limits are?

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #518 on: December 19, 2014, 05:01:33 PM »
I have trouble getting the forum to accept my attachments...perhaps they are too big? Does anyone know what the limits are?

It's an enduring mystery. Still, I hope you can continue to post this imagery with degradation of resolution or clarity.

Meanwhile, the 0.5m DigiGlobe panchromatic is now in the public domain and is being processed by Noh and Howat into 2m hillside shaded DEMs, quite spectacular. Hmmm, I wonder how and whether this could be combined with Sentinel. See images at http://forum.arctic-sea-ice.net/index.php/topic,1090.msg41740.html#msg41740



« Last Edit: December 19, 2014, 07:41:00 PM by A-Team »

nukefix

  • ASIF Citizen
  • Posts: 407
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #519 on: December 19, 2014, 05:22:57 PM »
S-1 18.12.2014 - practically no calving in the last 12 days

Neven

  • Administrator
  • ASIF Governor
  • *****
  • Posts: 4151
    • View Profile
    • Arctic Sea Ice Blog
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #520 on: December 20, 2014, 10:52:18 PM »
I have trouble getting the forum to accept my attachments...perhaps they are too big? Does anyone know what the limits are?

How big are your attachments, nukefix? I don't know what the limits are, but I've asked the ASIF Dungeonmaster. He might know.
Il faut cultiver notre jardin

nukefix

  • ASIF Citizen
  • Posts: 407
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #521 on: December 22, 2014, 02:40:24 PM »
I've had trouble with attachments that are ~4.5MB jpegs, roughly 6000x6000 pixels. They seem to upload fine but then the interface freezes with a blank page and they don't get posted. It would be good to know what the limit is, i.e. is it the number of pixels or the filesize..

Lennart van der Linde

  • ASIF Middle Class
  • Posts: 697
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #522 on: December 22, 2014, 07:04:27 PM »
Wipneus pointed to Messerli et al on GIS-glacier velocities:
http://www.the-cryosphere-discuss.net/8/6235/2014/tcd-8-6235-2014.html

So it seems Jakobshavn was over 10% faster this year than the previous record in 2012.
« Last Edit: December 22, 2014, 10:17:29 PM by Lennart van der Linde »

Lennart van der Linde

  • ASIF Middle Class
  • Posts: 697
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #523 on: December 22, 2014, 10:16:52 PM »
I checked with co-author Aslak Grinsted, who says the average over 199 days of observations was about 30 m/day, so circa 11 km/yr, if this can be extrapolated like that:
https://twitter.com/AGrinsted/status/547105147567230976

I think the record average speed was 11.6 km/yr in 2012, according to Joughin et al 2014:
http://www.the-cryosphere.net/8/209/2014/tc-8-209-2014.pdf

This is almost as fast as the maximum observed temporary speed for Jakobshavn of 12.6 km/yr  in Pfeffer et al 2008:
ftp://soest.hawaii.edu/coastal/Climate%20Articles/Pfeffer%20sea%20level%20calving%202008.pdf

So how much faster can this and other GIS-glaciers go? Pfeffer et al assumed an average speed of all GIS marine glacier of 12.6 km/yr between 2020 and 2100 for their extreme 2m by 2100 SLR-scenario. Would that be possible?

And if not, how about WAIS and EAIS glacier contributions this century? Could they lose more ice than the 62cm by 2100 that Pfeffer et al assumed, with over 39cm from WAIS alone? How much of a worst-case was their AIS-scenario?

Rignot recently said he thinks WAIS could lose about one third of its ice in 100-200 years, so that could be almost 1 meter from WAIS alone by 2100, I suppose. So that would add almost 60 cm to Pfeffer et al's worst-case.

So even if GIS would lose less than the 54cm in the worst-case of Pfeffer et al, we could possibly still end up worse than Pfeffer et al assumed, if Rignot is right with his worst-case judgement. Or would he be too pessimistic?

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #524 on: December 28, 2014, 06:34:00 PM »
we could possibly still end up worse than Pfeffer et al assumed, if Rignot is right with his worst-case judgement. Or would he be too pessimistic?
Seems right on-target to me, given the adverse developments lately in Greenland and need to apply the Precautionary Principle in any event.

I located some interesting new papers calculating (yet again) the Greenland mass loss and its allocation to surface and deep interior processes. B Csatho has a most excellent interactive graphic for the PARCA flux gate data around the perimeter of Greenland (http://rsl.geology.buffalo.edu/data/Pages/Greenland_MB_Data.html). There was a very useful ePoster at AGU 2014 from A Sommers et al as well as our first 2015 paper (from ML Andersen et al.) which compares eight different studies (over different data ranges) in Fig.3.

https://agu.confex.com/data/handout/agu/fm14/Paper_26038_handout_952_0.pdf
Thermo-­mechanically coupled modeling of high elevation regions of the Greenland Ice Sheet
A Sommers et al
http://www.williamcolgan.net/pubs/S0012821X14006360.pdf
Basin-scale partitioning of Greenland ice sheet mass balance components (2007–2011)
M.L. Andersen et al
« Last Edit: December 28, 2014, 06:39:40 PM by A-Team »

sidd

  • ASIF Upper Class
  • Posts: 1710
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #525 on: December 28, 2014, 10:25:39 PM »
That Andersen paper confirms Enderlin et al,doi:10.5194/tc-7-1007-2013
 in that SMB now domiantes in Greenland.

sidd

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #526 on: December 29, 2014, 02:23:16 PM »
Andersen paper confirms Enderlin et al, doi:10.5194/tc-7-1007-2013 in that surface mass balance now dominates in Greenland
That doi is an interesting paper in its own right but you may have intended her Feb 2014 doi:10.1002/2013GL059010; note the Nov 2014 B Csatho PNAS paper (free full, reviewed on another forum) has SMB at about half. There's yet more coming per AGU 2014.

Is it time for a worldwide moratorium on Greenland mass loss papers? Perhaps we should lock them all in a room until the various approaches are reconciled. In other disciplines, people do meta-analyses or write expert review articles. Greenland is admittedly a moving target.

I tend to measure progress in terms of observational advances (or not). At this time, there is very little experimental data on the geothermal gradient, roughness of bedrock, properties of till, regions of basal melt and basal stick-slip, churning in the EFZ (radar echo free zone), bedrock deflection of stratigraphy, thermal upheaval and englacial temperature profiles. There's only so far we can go just with surface observations.

The bottom line is indeed sea level rise -- we have a baseline now and the start on the trend; by 2025, maybe a grip on the noise in the trend line and a sense of its change in slope (acceleration). That would amount to descriptive phenomenology supplemented by many a theoretical paper lacking validated initial conditions while pooh-poohing sensitivity to them. However I'm optimistic that the englacial experimental side will improve markedly within the next few years.

Enderlin, E. M., I. M. Howat, S. Jeong, M.-J., Noh, J. H. van Angelen, & M. R. van den
Broeke, 2014. An improved mass budget for the Greenland ice sheet. Geophys. Res. Lett.,
866-872, doi:10.1002/2013GL059010

Annual measurements for 178 outlet glaciers reveal that, despite widespread acceleration, only 15 glaciers accounted for 77% of the 739 ± 29 Gt of ice lost due to acceleration since 2000 and four accounted for ~50%. Among the top sources of loss are several glaciers that have received little scientific attention. The relative contribution of ice discharge to total loss decreased from 58% before 2005 to 32% between 2009 and 2012. As such, 84% of the increase in mass loss after 2009 was due to increased surface runoff. These observations support recent model projections that surface mass balance, rather than ice dynamics, will dominate the ice sheet's contribution to 21st century sea level rise.

Laser altimetry reveals complex pattern of Greenland Ice Sheet dynamics
BM Csatho et al www.pnas.org/cgi/doi/10.1073/pnas.1411680112

We estimate a mean annual GrIS mass loss of 243 ± 18 Gt·y−1, equivalent to 0.68 mm·y−1 sea level rise (SLR) for 2003– 2009. Dynamic thinning contributed 48%, with the largest rates occurring in 2004–2006, followed by a gradual decrease balanced by accelerating SMB loss...

High sensitivity of tidewater outlet glacier dynamics to shape
EM Enderlin et al doi:10.5194/tc-7-1007-2013  www.the-cryosphere.net/7/1007/2013/
http://cci.siteturbine.com/facultystorm/profile/cv.php?profileId=933

... for glaciers with similar discharge, the trunks of wider glaciers and those grounded over deeper basal depressions tend to be closer to flotation, so that less dynamically induced thinning results in rapid, unstable retreat following a perturbation.... varying the bed topography within the range of observational uncertainty can result in either stable or unstable retreat due to the same perturbation.

nukefix

  • ASIF Citizen
  • Posts: 407
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #527 on: December 29, 2014, 08:15:47 PM »
Hmmm box-plots, I think more research is needed to produce plots with mass-balance each year.

nukefix

  • ASIF Citizen
  • Posts: 407
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #528 on: December 30, 2014, 05:49:26 PM »
Argh again I cannot get the attachments through :(

Laurent

  • ASIF Governor
  • Posts: 2525
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #529 on: December 30, 2014, 06:46:15 PM »
Try to reduce the picture (700x700 pixel is enough)?
Change the extension ?

nukefix

  • ASIF Citizen
  • Posts: 407
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #530 on: January 02, 2015, 04:14:36 PM »
Try to reduce the picture (700x700 pixel is enough)?
Change the extension ?
Well, they are already crops of ~20000*20000 pixel images so going to that extent would be really unsatisfactory..

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #531 on: January 03, 2015, 11:19:49 PM »
already cropped way down from 20000 x 20000 pixels
nuke, could you conveniently post some of those orange quicklooks like Wipneus has been posting? Way lower resolution but still possibly of interest.  http://forum.arctic-sea-ice.net/index.php/topic,176.msg42626.html#msg42626

It seems like Sentinel is missing the Jakobshavn glacier on about half the days, according to the Danish perimeter series. Better if it went 100 km farther to the east. Is this temporary malfunction, shake-down of routine, or irrevocable orbital consequence? http://ocean.dmi.dk/arctic/morrisjessup.uk.php

Wipneus

  • ASIF Governor
  • Posts: 3641
    • View Profile
    • Arctische Pinguin
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #532 on: January 04, 2015, 08:22:18 AM »
Jacobshavn is further south so the orbits are wider apart. Further the preferred acquisition mode is higher resolution IW (Interferometric Width) rather than the EW (Extra Width) products I use for Nares. Better resolution->narrower swath->longer return times. Perhaps the DMI images can only use EW products?

From the following list we can deduce that at the moment data is missing for 2014-12-30 and perhaps 2015-01-03. The datahub servers seem to be very stressed at the moment, so that may cause the delay.

S1A_IW_GRDH_1SSH_20141019T095957_20141019T100022_002897_003480_C464
S1A_IW_GRDH_1SSH_20141023T204643_20141023T204712_002962_0035DA_DD2A
S1A_IW_GRDH_1SSH_20141031T095957_20141031T100022_003072_003842_4CA7
S1A_IW_GRDH_1SSH_20141104T204643_20141104T204711_003137_0039AD_E786
S1A_IW_GRDH_1SSH_20141112T095957_20141112T100022_003247_003C08_EA1B
S1A_IW_GRDH_1SSH_20141116T204642_20141116T204711_003312_003D5F_79EF
S1A_IW_GRDH_1SSH_20141124T095957_20141124T100022_003422_003FE8_3FAF
S1A_IW_GRDH_1SSH_20141128T204642_20141128T204711_003487_00416D_ABE0
S1A_IW_GRDH_1SSH_20141206T095956_20141206T100021_003597_004401_1CDA
S1A_IW_GRDH_1SSH_20141210T204641_20141210T204710_003662_004568_80DD
S1A_IW_GRDH_1SSH_20141218T095956_20141218T100021_003772_004800_0405
S1A_IW_GRDH_1SSH_20141222T204641_20141222T204710_003837_00496D_02C5

The EW, dual polarization images listed (Nov 29 and later) suggsts a new product today.

S1A_EW_GRDM_1SDH_20141129T100720_20141129T100824_003495_00419D_4E85
S1A_EW_GRDM_1SDH_20141130T203015_20141130T203119_003516_00421B_DCEF
S1A_EW_GRDM_1SDH_20141201T095119_20141201T095223_003524_004252_5179
S1A_EW_GRDM_1SDH_20141203T205450_20141203T205550_003560_004324_4775
S1A_EW_GRDM_1SDH_20141204T101623_20141204T101723_003568_004359_5E8F
S1A_EW_GRDM_1SDH_20141205T203830_20141205T203930_003589_0043CC_E0CA
S1A_EW_GRDM_1SDH_20141211T100719_20141211T100823_003670_004597_FCF3
S1A_EW_GRDM_1SDH_20141212T203014_20141212T203118_003691_004611_338A
S1A_EW_GRDM_1SDH_20141213T095118_20141213T095222_003699_004643_263C
S1A_EW_GRDM_1SDH_20141215T205450_20141215T205550_003735_004724_8AE4
S1A_EW_GRDM_1SDH_20141216T101623_20141216T101723_003743_00475A_6D69
S1A_EW_GRDM_1SDH_20141217T203830_20141217T203930_003764_0047D4_29B4
S1A_EW_GRDM_1SDH_20141223T100719_20141223T100823_003845_004998_2EB3
S1A_EW_GRDM_1SDH_20141224T203013_20141224T203118_003866_004A12_093A
S1A_EW_GRDM_1SDH_20141225T095118_20141225T095222_003874_004A48_6B69
S1A_EW_GRDM_1SDH_20141227T205450_20141227T205550_003910_004B1C_5A9D
S1A_EW_GRDM_1SDH_20141228T101623_20141228T101723_003918_004B4F_F257
S1A_EW_GRDM_1SDH_20141229T203830_20141229T203930_003939_004BBE_D964

I will have a look at the preview images when the servers start responding again.

PS. for the single polarization mode (in the fourth field, SH means "Single HH polarization available") the quick look images are in B&W

Wipneus

  • ASIF Governor
  • Posts: 3641
    • View Profile
    • Arctische Pinguin
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #533 on: January 04, 2015, 10:00:41 AM »
Quick-look image of the last EW product.

Wipneus

  • ASIF Governor
  • Posts: 3641
    • View Profile
    • Arctische Pinguin
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #534 on: January 04, 2015, 10:03:28 AM »
Quick-look image of the latest IW product.

Wipneus

  • ASIF Governor
  • Posts: 3641
    • View Profile
    • Arctische Pinguin
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #535 on: January 04, 2015, 10:28:56 AM »
Hot from the press, the expected 2015-01-03 IW product just came in.

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #536 on: January 05, 2015, 11:18:04 AM »
Wip, thanks for the Sentinel info, very helpful.

The image below shows the location of the Parca stakes in the vicinity to Jakobshavn. The elevation contours are taken from sidd's site after Bamber 2013. Because the flux gates are about 30 km wide and the velocities were read in 1996, it is less than ideal for Jakobshavn details. None of the 3 stakes shown are among the six for which ePoster data from Sommers AGU2014 is available; the nearest is n32 to the north which is not in the current Jakobshavn drainage basin.

The flux across gates cd64-cd87 sees 7.3 cubic km per year go by at a velocity of 168 m/yer from a 16144 sq km drainage basin; that across cd38-cd64 has 5.8 at 128 m/yr from a 11664 drainage basin, per (per B Csatho web page
http://rsl.geology.buffalo.edu/data/Pages/Greenland_MB_GateJ.html#

It is getting quite complicated to collect and display all of the relevant data layers, here Landsat-8, Sentinel, elevation, ice thickness, ice surface velocity, flow lines to the summit ridge, ice velocity with depth, ice temperature with depth and isotherm blocks, radar isochronal stratigraphy layers, strain rate, bedrock topography, geothermal flux, temporal variation of all these and so forth.

Greenland 3D representation will tilt software away from gimp towards ImageJ2 which has more advanced features for higher dimensional processing and display.

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #537 on: January 26, 2015, 11:52:26 PM »
It's quite interesting to follow the peer-review comments on the ImGRAFT algorithm paper -- I wish more journals would do this transparently. Recall the paper proposed yet another way of measuring feature movement across pairs of satellite photos, maybe better, maybe not (3 such algorithms come bundled with ImageJ2 alone!).

http://www.the-cryosphere-discuss.net/8/6235/2014/tcd-8-6235-2014-discussion.html

Along the way, the authors asserted a record velocity for Jakobshavn of 52 m/day during July 2014 (suggesting a speedup). However today, Joughin writes that they measure back from the calving terminus itself to avoid crevasse artefacts.

"Nonetheless, we do measure velocities >50 m/day at times in the summer of 2012 (not shown in our paper). We have extended the TSX [TerraSAR-X satellite] record through 2014 and find that, when comparing the speeds at the same point, peaks speeds in July 2014 are considerably slower than the 2012 peak by about 2-3 km/yr [~17% slower] . So any references to faster speeds in 2014 should be removed (this in no way detracts from the quality of the results)."

We're left wondering why this should be the case, what caused the acceleration bubble in the first place, what we can predict for 2015, and what this means long-term for sea level rise from this major contributor. It does seem that runaway acceleration is not a concern for the immediate future.

Espen

  • ASIF Governor
  • Posts: 3078
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #538 on: February 12, 2015, 09:13:34 PM »
Just from the "oven". As can be seen in the animation below, Jakobshavn grew during the winter since the low in September 28 2014. The growth is especially seen at the southern branch:
« Last Edit: February 13, 2015, 06:42:07 AM by Espen »
Have a ice day!

icefest

  • ASIF Citizen
  • Posts: 258
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #539 on: February 12, 2015, 10:02:31 PM »
 :o
Open other end.

A-Team

  • ASIF Upper Class
  • Posts: 1914
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #540 on: February 13, 2015, 12:42:15 PM »
So we have a rather good match year on year on path row, sun elevation and azimuth. That means good co-registration can be obtained; however the colors on the greenish preview pdfs behave quite differently. the 15 m seems to show major ongoing crevassing and calving in the 2015 (or a shot of them frozen in time).

09 Feb 15 LC80090112015043LGN00 9 11 172º 6.67º
    
09 Feb 14 LC80090112014040LGN00 9 11 173º 5.78º
25 Feb 14 LC80090112014056LGN01 9 11 173º 11.4º

As noted on a Feb 15 Sentinel image at another imagery forum, the calving front of Jakobshavn Isbrae recovers somewhat each winter because the ice mélange in the fjord provides buttressing that tamps down calving retreat. I have not looked into the date and position of maximal annual recovery to see if there is a trend.

http://forum.arctic-sea-ice.net/index.php/topic,638.msg45094.html#msg45094

We know that the Joughin group, which has been providing the best quantitative coverage of Jakobshavn, extended their analysis through 2014. This raises an interesting question as it may not be possible to keep publishing 'Brief Reports' each year -- they may have to bundle it into something bigger (or just put it online).

We also know from AGU 2014 that T Scambos has performed ultra-sensitive velocity measurements using pairs of Landsat-8's in 16-bit mode but that seems not to have come out yet.

Here is something along those lines from U Herzfelt et al in Nov 14: $36 paywalled doi: 10.3189/2014JoG13J129

Our objective is to map dynamic provinces and investigate dynamic changes in Jakobshavn Isbrae. We use an approach that combines structural glaciology and remote-sensing data analysis, facilitated by mathematical characterization of generalized spatial surface roughness that provides parameters related to ice dynamics, deformation and interaction of the ice with bed topography.

The approach is applied to derive time series of elevation and roughness changes and to attribute changes during rapid retreat. Different dynamic types of fast- and slow-moving ice can be mapped from ICESat (2003–09) and ATM data using spatial roughness characterization, validated with ASTER and bed-topographic data.

Results of comparative analysis of elevation changes and roughness changes indicate surface lowering of 10–15 m/yr between 2004 and 2009 but no change in surface roughness and dynamic types. These findings are consistent with a front retreat as part of a fjord-glacier cycle or following warming of fjord water and with climatic warming, but not with an internal dynamic acceleration as a cause of rapid retreat. Relationships to changes in basal water pressure are discussed. All glaciodynamic changes appear to have initiated near the front and propagated up-glacier.
http://www.ingentaconnect.com/content/igsoc/jog/2014/00000060/00000223/art00002

This is nice enough Nov 2013 article on melt lakes and water channels specific to Jakobshavn, not sure why it took 6 years to analyze 2007 data but it might serve as a template for an update with today's satellites.

Supraglacial melt channel networks in the Jakobshavn Isbræ region during the 2007 melt season
DJ Lampkin and J VanderBerg
free full: http://onlinelibrary.wiley.com/doi/10.1002/hyp.10085/full

Supraglacial channels are an important mechanism for surface water transport over the ablation zone of western Greenland. The first assessment of the spatio-temporal distribution of surface melt channels and their relationship to supraglacial lakes over the Jakobshavn Isbræ region of Western Greenland was analysed using Landsat during the 2007 melt season.

A total of 1188 melt channels were delineated and show an increase in the number of melt channels throughout the season, reaching a peak on 9 August. Water-filled melt channels advanced to a maximum elevation of 1647 m on 9 August and attained a minimum average slope of 0.009 on 8 July. The ablation zone demonstrates two hydrologic modes, where crevasse and moulin terminating channels dominate at elevations <800 m and higher-order channel networks >800 m. Development of higher-order networks is interrupted by flow divergence due to partitioning of melt water into vertical infiltration through moulins and crevasse fields prevalent at lower elevations. Tributary and connector networks between 800 and 1200 m in elevation are correlated with fewer lake occurrences, lower surface velocities (~50 m a−1), and ice flow dominated by internal deformation over basal sliding...
« Last Edit: February 13, 2015, 02:23:45 PM by A-Team »

Shared Humanity

  • ASIF Upper Class
  • Posts: 2315
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #541 on: February 14, 2015, 03:47:31 PM »
Amazing images!

Shared Humanity

  • ASIF Upper Class
  • Posts: 2315
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #542 on: February 14, 2015, 04:07:05 PM »
I continue to be fascinated by the calving front on the north side of the main glacier that is right at the main calving front and that is exposed to the fjord that can be seen on the last image. This calving front appears to be draining ice from a portion of the ice sheet that is grounded below sea level and situated north of the main channel. It is actually a fairly large feature.

(See topographical map)

If you look at the ice surface on the red image you have posted, it seems you can see the outline of this shallow bay? Is this visible feature actually surface ice that is lower than the ice adjacent?

Portions of this bedrock are 500 meters below sea level.

Espen

  • ASIF Governor
  • Posts: 3078
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #543 on: February 19, 2015, 11:39:16 PM »
Believe it or not! Massive calving seen at the southern branch of Jakobshavn Isbræ:
« Last Edit: February 19, 2015, 11:44:35 PM by Espen »
Have a ice day!

AbruptSLR

  • ASIF Emperor
  • Posts: 13283
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #544 on: February 20, 2015, 12:11:25 AM »
A major calving in the middle of Winter!  I can't wait to see what happens this Summer.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

sidd

  • ASIF Upper Class
  • Posts: 1710
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #545 on: February 20, 2015, 03:58:05 AM »
any guesses as to how long b4 JI  southern branch grounding line retreats behind the lip in Gogineni Gorge ?

Jim Hunt

  • ASIF Governor
  • Posts: 3306
    • View Profile
    • The Arctic sea ice Great White Con
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #546 on: February 20, 2015, 01:24:47 PM »
Believe it or not! Massive calving seen at the southern branch of Jakobshavn Isbræ:

Does this Sentinel "PolarView" from the 15th help throw additional light on matters?

The second one, from the 14th, is even less "crystal clear"!
« Last Edit: February 20, 2015, 01:39:52 PM by Jim Hunt »
Reality is merely an illusion, albeit a very persistent one - Albert Einstein

nukefix

  • ASIF Citizen
  • Posts: 407
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #547 on: February 20, 2015, 03:03:44 PM »
The image from scihub reveals that a calving-event has apparently taken place.

Jim Hunt

  • ASIF Governor
  • Posts: 3306
    • View Profile
    • The Arctic sea ice Great White Con
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #548 on: February 20, 2015, 03:18:37 PM »
The image from scihub reveals that a calving-event has apparently taken place.

Thanks nukefix. I've incorporated that image into my blog post at:

http://GreatWhiteCon.info/2015/02/shock-news-massive-calving-of-jakobshavn-isbrae/
Reality is merely an illusion, albeit a very persistent one - Albert Einstein

solartim27

  • ASIF Citizen
  • Posts: 438
    • View Profile
Re: Jakobshavn Isbræ / Sermeq Kujalleq / Ilulissat Icefjord
« Reply #549 on: February 20, 2015, 05:48:05 PM »
A major calving in the middle of Winter!  I can't wait to see what happens this Summer.
Could it be that this is routine, and that we are just noticing it, or just now have the ability to see it?  Helheim had a calving event in early January, and has also retreated substantially over the winter in past years.

Be aware, Helheim is fast moving monster (probably why it is named so), the calving line seen late 2013 moved some 2 -  3 km during the winter, we will follow it during the season to see where it ends this season.
 
FNORD