Support the Arctic Sea Ice Forum and Blog

Author Topic: Zachariae Isstrøm / Nioghalvfjerdsfjorden / NE Greenland  (Read 364268 times)

Espen

  • Young ice
  • Posts: 3251
    • View Profile
  • Liked: 177
  • Likes Given: 0
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #450 on: June 14, 2015, 06:35:43 PM »
Calving front update, although Zachariae seems to be slowing down for the time being,

I am not seeing anything of a slow down.


Everything is relative?
Have a ice day!

oren

  • Young ice
  • Posts: 4482
    • View Profile
  • Liked: 873
  • Likes Given: 1287
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #451 on: June 14, 2015, 11:33:10 PM »
That's a great name for that island!

Seeing that the calving front has regressed less (on average) each year since 2010, is this proof that global warming is slowing down?  :P  (You know, one contrary cherry-picked bit of data overrides a world full of actual data.  ;D)

More Seriously, do you have ideas why 2010-11 and 2011-12 might have calved so much more than other years?  (I recall that 2012 was a big melt year for Greenland.  Would this be part of the picture?)

Since a lot of the 2014 retreat happened during August and September when the sea ice cleared away, and since 2015 saw quite a retreat during winter and spring, My expectation is that 2015 will finish above 2014 in terms of retreated area. I am working on some calculations to compare the years, still learning the ropes with all these heavy images.

btw my layman's thinking is that Zacharia is currently at a bottleneck of sorts, where the glacier has to squeeze at the relatively narrow pass between Kap Zacharia and Kap Zach. I believe that once it retreats a little further inland it might lose more buttressing and speed up (and possibly retreat faster).

A-Team

  • Nilas ice
  • Posts: 2403
    • View Profile
  • Liked: 282
  • Likes Given: 19
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #452 on: June 15, 2015, 02:41:03 AM »
Quote
#449  animation  rendered at 7.5 m/pix. The images are shifted so the main glacier is aligned. Even allowing for 2 pixels error, that is still at minimum 6.1 m/day.

I think those were about optimal decisions. To expand a bit on the method for Oren and others following along:

  • two landsat-8s were downloaded with same path,row to avoid relative distortion
  • the dates had enough temporal separation given the ~ velocity so fixed features will be sufficiently separated by pixel displacement
  • Landsat-8 has 12-bit resolution but is bicubic-interpolated to 16-bit already
  • initial operations are done at 16-bit depth, starting with 'normalization' to full dynamic range
  • the images are blown up from 15 m nominal resolution to 7.5 m, again with bicubic
  • processing can be shifted now to 8-bit tools like gimp with convenient features
  • a sensible stationary reference frame is chosen in a scene where everything but rocks is moving
  • vertical thinning is ignored as probably a negligible contributor error over this time frame and season
  • pixel and angle displacements are determined with the 'measure' tool
  • relative velocity (to a different pair of dates) is easier than absolute velocity as the same processing error cancels

Could the final velocity estimate be improved by enlarging to 3.75 m? By using a different interpolation method? By tweaking contrast more? I have no idea. What we need here really is access to two fixed GPS stations on poles drilled into the ice. Then we would have an accurate velocity calibration standard against which to compare potential improvements in image processing.
« Last Edit: June 15, 2015, 03:34:42 AM by A-Team »

oren

  • Young ice
  • Posts: 4482
    • View Profile
  • Liked: 873
  • Likes Given: 1287
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #453 on: June 15, 2015, 07:38:56 AM »
Quote
#449  animation  rendered at 7.5 m/pix. The images are shifted so the main glacier is aligned. Even allowing for 2 pixels error, that is still at minimum 6.1 m/day.

I think those were about optimal decisions. To expand a bit on the method for Oren and others following along:

  • two landsat-8s were downloaded with same path,row to avoid relative distortion
  • the dates had enough temporal separation given the ~ velocity so fixed features will be sufficiently separated by pixel displacement
  • Landsat-8 has 12-bit resolution but is bicubic-interpolated to 16-bit already
  • initial operations are done at 16-bit depth, starting with 'normalization' to full dynamic range
  • the images are blown up from 15 m nominal resolution to 7.5 m, again with bicubic
  • processing can be shifted now to 8-bit tools like gimp with convenient features
  • a sensible stationary reference frame is chosen in a scene where everything but rocks is moving
  • vertical thinning is ignored as probably a negligible contributor error over this time frame and season
  • pixel and angle displacements are determined with the 'measure' tool
  • relative velocity (to a different pair of dates) is easier than absolute velocity as the same processing error cancels

Could the final velocity estimate be improved by enlarging to 3.75 m? By using a different interpolation method? By tweaking contrast more? I have no idea. What we need here really is access to two fixed GPS stations on poles drilled into the ice. Then we would have an accurate velocity calibration standard against which to compare potential improvements in image processing.

A-Team thanks for all the detailed pointers. Are you using ImageMagick for the 16-bit stages?
I am trying to calculate some other stuff in my slow crude way, not the glacier speed as I have no chance on improving anything already calculated.

Espen

  • Young ice
  • Posts: 3251
    • View Profile
  • Liked: 177
  • Likes Given: 0
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #454 on: June 17, 2015, 08:37:35 PM »
More calving activity at Zachariae:
Have a ice day!

Wipneus

  • Citizen scientist
  • Young ice
  • Posts: 3996
    • View Profile
    • Arctische Pinguin
  • Liked: 619
  • Likes Given: 0
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #455 on: June 18, 2015, 01:53:49 PM »
Here is a detail animation (taken from different orbit positions, lights and shadows may differ, some distortion possible).
Visible are: old calving breaks in two, small calving flips and a crack getting longer. Quite active for a 4 day difference.

Andreas T

  • Nilas ice
  • Posts: 1126
    • View Profile
  • Liked: 11
  • Likes Given: 2
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #456 on: June 18, 2015, 10:20:58 PM »
Might add some cracking and albedo changes in the sea ice on the southern part of the image not showing up on the lower Worldview resolution

Espen

  • Young ice
  • Posts: 3251
    • View Profile
  • Liked: 177
  • Likes Given: 0
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #457 on: June 21, 2015, 07:24:51 PM »
Zachariae Isstrøm update:

Zachariae is still moving ahead.
Have a ice day!

Espen

  • Young ice
  • Posts: 3251
    • View Profile
  • Liked: 177
  • Likes Given: 0
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #458 on: June 22, 2015, 09:10:36 PM »
Latest movement at Zachariae:
Have a ice day!

Espen

  • Young ice
  • Posts: 3251
    • View Profile
  • Liked: 177
  • Likes Given: 0
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #459 on: June 24, 2015, 09:21:42 PM »
Ka-pow! Suddenly out of nowhere, the solstice celebrations around Zachariae showed some action:

Just imagine the sound level?
Have a ice day!

A-Team

  • Nilas ice
  • Posts: 2403
    • View Profile
  • Liked: 282
  • Likes Given: 19
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #460 on: June 24, 2015, 10:27:36 PM »
Very impressive. Here is the 15 m if someone wants to measure areas ...

oren

  • Young ice
  • Posts: 4482
    • View Profile
  • Liked: 873
  • Likes Given: 1287
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #461 on: June 24, 2015, 11:17:33 PM »
Wow! Looks like a very strong tide went out or something.
This could ease the local buttressing and possibly lead to more calving.

Shared Humanity

  • Young ice
  • Posts: 3965
    • View Profile
  • Liked: 416
  • Likes Given: 48
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #462 on: June 24, 2015, 11:26:06 PM »
Am I seeing what I think I am seeing? Zach usually sends off tabular icebergs but this latest calving seems to have rolled over.

Tor Bejnar

  • Young ice
  • Posts: 3055
    • View Profile
  • Liked: 369
  • Likes Given: 189
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #463 on: June 24, 2015, 11:34:44 PM »
Hmmm, that could explain how much movement there was.
Arctic ice is healthy for children and other living things.

Yuha

  • Frazil ice
  • Posts: 254
    • View Profile
  • Liked: 17
  • Likes Given: 12
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #464 on: June 25, 2015, 12:56:32 AM »
I think that particular iceberg was too narrow, having width smaller than height, causing it to roll over.

A-Team can you estimate the thickness of the ice from your image?

A-Team

  • Nilas ice
  • Posts: 2403
    • View Profile
  • Liked: 282
  • Likes Given: 19
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #465 on: June 25, 2015, 02:08:24 AM »
Quote
that could explain how much movement there was.that particular iceberg was too narrow, having width smaller than height, causing it to roll over.
The magenta bergs are about 30 pixels thick or 450 m. Not so easy to measure accurately because of shadowing vs rock accretion on the bottom and angled top maybe still showing. The sun angle was 34.8º at an azimuth of -162.7º on this particular Landsat.

Right, instability of a tall but narrow calving product makes sense given the center of gravity and what we saw so graphically in 'Chasing Ice.' Roll-over then creates a massive wave that pushes away the older, stable tabular bergs.

Espen

  • Young ice
  • Posts: 3251
    • View Profile
  • Liked: 177
  • Likes Given: 0
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #466 on: June 25, 2015, 06:26:34 AM »
Am I seeing what I think I am seeing? Zach usually sends off tabular icebergs but this latest calving seems to have rolled over.

In the earlier days Jakobshavn produced tabular icebergs too, maybe we will see a shift at Zachariae in the future, the next pre-calving line is also relatively narrow.
Have a ice day!

mspelto

  • New ice
  • Posts: 25
    • View Profile
  • Liked: 0
  • Likes Given: 0
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #467 on: June 25, 2015, 01:16:54 PM »
It was the rapid rift extension illustrated between 6/6 and 6/17 that set this up.  No surprise.  The rift development behind the front is now poorer than typical it seems to me.

A-Team

  • Nilas ice
  • Posts: 2403
    • View Profile
  • Liked: 282
  • Likes Given: 19
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #468 on: June 25, 2015, 04:38:09 PM »
Here is 104 km of an ice-penetrating radar transect (red and following blue) taken from the set available (pink inset). The dotted magenta line is Cresis' take on the surface, the dotted red line bedrock resp. ice shelf surface. The depth indicators are relative (ie not set to WGS84 sea level). The scheduled 2015 over-flight had to be cancelled because of weather. https://data.cresis.ku.edu/data/rds/
« Last Edit: June 25, 2015, 04:52:50 PM by A-Team »

Shared Humanity

  • Young ice
  • Posts: 3965
    • View Profile
  • Liked: 416
  • Likes Given: 48
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #469 on: June 25, 2015, 07:50:53 PM »
Could a speeding up of Zachariae Isstrom be causing thicker ice to be pushed into deep water? If the glacier is moving slow, it might allow sufficient bottom melt that the glacier will be more prone to tabular calving.

Yuha

  • Frazil ice
  • Posts: 254
    • View Profile
  • Liked: 17
  • Likes Given: 12
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #470 on: June 26, 2015, 10:05:37 AM »
Could a speeding up of Zachariae Isstrom be causing thicker ice to be pushed into deep water? If the glacier is moving slow, it might allow sufficient bottom melt that the glacier will be more prone to tabular calving.

The height of the calving front is probably growing because the bed is getting deeper and the ice surface higher as the calving front retreats. This can be seen in the following image posted by sidd last year.


Shared Humanity

  • Young ice
  • Posts: 3965
    • View Profile
  • Liked: 416
  • Likes Given: 48
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #471 on: June 26, 2015, 05:02:25 PM »
So we could be seeing a major  shift in the behavior of this glacier with thick bergs calving and rolling over. Could this trigger a much faster retreat of the calving front?

Espen

  • Young ice
  • Posts: 3251
    • View Profile
  • Liked: 177
  • Likes Given: 0
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #472 on: June 26, 2015, 06:53:17 PM »
So we could be seeing a major  shift in the behavior of this glacier with thick bergs calving and rolling over. Could this trigger a much faster retreat of the calving front?

If so we will see the same behavior as seen at Jakobshavn and Helheim, but that said, Zachariae is relatively speaking on the move anyway, but what happens at Zachariae should make any decision maker worried!
« Last Edit: June 26, 2015, 06:58:19 PM by Espen »
Have a ice day!

A-Team

  • Nilas ice
  • Posts: 2403
    • View Profile
  • Liked: 282
  • Likes Given: 19
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #473 on: June 27, 2015, 03:51:42 PM »
Big news ... EGRIP, a new core to bedrock will be drilled in the ice stream leading to Zachariae! The 46 metric ton dome at NEEM and sledges of equipment have already been dragged on skis 465 km to the new site (with a Pistonbully 300).

Before stripping out NEEM, its 2550 m deep antifreeze-filled borehole at NEEM, undisturbed since 2012, was logged for temperature (precision 0.01 °C), inclination, azimuth, borehole diameter and pressure.  The temperature data can help reconstruct past surface temperatures and monitori meltwater at the bed. Diameters tracl pressure balance of the liquid in the borehole and putative changes around the folded zones of the Eemian ice. The inclination and azimuth  monitor the deformation of the borehole. The surface velocity  at NEEM has been ~5 m/yr for five years allowing adequate accuracy.

The strain net around the NEEM drill hole will also be remeasured. The motion of the ice sheet is critical to interpretation of ice layers; curiously neither direction nor speed has been constant (1st image below).

Various science projects were conducted en route to EGRIP such as firn and depth radars, even continuing them to Summit. There is a most excellent 96 page pdf that covers everything from scientific priorities to the bamboo pee pole at camp. The project was funded to the tune of 25 million DKK by a Danish charitable foundation AP Møller with historic ties to Maersk shipping.
 
The biggest ice stream in Greenland passes through the EGRIP site. It is not really possible to anticipate future changes in the marine outlets Zachariae, Nioghalvfjerdsfjorden and Storstrømmen without a good grip on NEGIS itself, which supposedly starts over a geothermal anomaly. That will be checked with seismic studies. The ice reaches velocities over 100 m/yr some 200 km from the ice divide yet still 500 km from the coast.

It proved possible to find a site on the upper ice stream without crevasses. The ice at the chosen site is 2550 m to bedrock and flows at 65 m/yr (see attached images). Previous deep cores have all been along the summit ridge; this one would allow study of ice flow dynamics via rheology and deformation below the surface as well as basal sliding, borehole deformation, and basal water processes.

They totally botched the logarithmic color key on the high resolution velocity map (which seems to overlie a hillshaded DEM) and it is not possible read out velocities at specific points nor contour them. The low resolution velocity map did have sufficient detail to let speed profiles to be put on Cresis radar profiles.

Over 50 kyr of radar stratigraphy at EGRIP has been mapped back to NGRIP which in turn is tied in to all the other cores including Antarctica's. A core here will enable climatic studies of this period and especially the Holocene where high resolution records of greenhouse gasses, water isotopes and impurities are not available for East Greenland.

Drilling is a little tricky as borehole will slant 195 meters downhill (7.6% off-vertical) before the three year project is finished in 2020. The drill cores themselves will be ~13 cm in diameter. They have not yet disclosed the precise lat,lon of the drilling site but the ice stratigraphy appears slightly concave upward rather than flat as advertised.

http://www.isogklima.nbi.ku.dk/nyhedsfolder/danske_nyheder/egrip-2015-feltplan/Renland-EGRIP2015FieldPlan.pdf
« Last Edit: June 28, 2015, 06:14:13 PM by A-Team »

Espen

  • Young ice
  • Posts: 3251
    • View Profile
  • Liked: 177
  • Likes Given: 0
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #474 on: June 28, 2015, 11:02:06 AM »
A new updated retreat version (please note the background image is from September 2014, click for enlargement):
« Last Edit: June 28, 2015, 11:32:36 AM by Espen »
Have a ice day!

A-Team

  • Nilas ice
  • Posts: 2403
    • View Profile
  • Liked: 282
  • Likes Given: 19
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #475 on: June 28, 2015, 06:44:11 PM »
The vertically exaggerated oblique image below of the NEGIS ice stream feeding Zachariae is one of the most peculiar features in Greenland. Shear boundaries are already visible just from the ice surface DEM yet there are no real traces in the bedrock DEM corresponding to these surface features and indeed the ice stream flows uphill in places. I took this oblique image from Nasa visualizations http://www.nasa.gov/content/goddard/nasa-data-peers-into-greenlands-ice-sheet

That possibly fits with the putative upstream geothermal anomaly providing lubrication to sliding (rather than glacial grinding). The anomaly is model-based and lacks direct observational support. I'm skeptical because it seems improbable that the only proposed heat anomaly in Greenland should lie right along the summit ridge, too much of a coincidence considering the small percentage area.

Zachariae etc are very important to get right because the huge iceshed -- 16% of Greenland -- could potentially be a big contributor to future sea level rise. Without knowing the interior physics of NEGIS, it is difficult to predict its acceleration in respnse to removal of marine buttressing. (For comparison, Jakobshavn drains 8% and Petermann just ~4%.)

EGrip is drilling fairly high up though below the anomaly. Temperature profiles with depth are very sparse in Greenland so the experimental data here will be of great interest. Recent efforts to get at temperatures using radar and layer dating have made some progress but overall have not produced a stellar outcome. http://www.ig.utexas.edu/people/staff/joemac/pdf/MacGregor_2015_JGR_2.pdf
« Last Edit: June 28, 2015, 11:14:00 PM by A-Team »

Rubikscube

  • Frazil ice
  • Posts: 247
    • View Profile
  • Liked: 0
  • Likes Given: 0
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #476 on: June 28, 2015, 11:34:12 PM »
Zachariae etc are very important to get right because the huge iceshed -- 16% of Greenland -- could potentially be a big contributor to future sea level rise. Without knowing the interior physics of NEGIS, it is difficult to predict its acceleration in respnse to removal of marine buttressing. (For comparison, Jakobshavn drains 8% and Petermann just ~4%.)
Is this an estimate exclusively based on current flow rates, and the respective ice thickness, with little or no regard for the underlying topography? 8% to Jakobshavn seems like a terribly low estimate.

A-Team

  • Nilas ice
  • Posts: 2403
    • View Profile
  • Liked: 282
  • Likes Given: 19
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #477 on: June 29, 2015, 02:48:48 AM »
Quote
Is this an estimate exclusively based on current flow rates, and the respective ice thickness, with little or no regard for the underlying topography? 8% to Jakobshavn seems like a terribly low estimate.
No, it is just an area. Each outlet glacier gets to count a spot of area above it if a snowflake falling there would eventually exit through it (assuming unvarying current conditions), that is to say, flow lines or trace-forwards of trajectories on surface velocity maps.

In other words, it's just like dividing a mountain range into its watershed basins. These areas were all quantitated years ago as part of the PARCA circumferential stake line at 2000 m and revisited more recently by a geography expert group in Wales.

Perpendiculars to elevation contours are quite troublesome to calculate from digital data and some landscapes give rise to vexatious anomalies (eg flowlines crossing or disappearing). It is better here to fit a differentiable surface to the DEM.

B Csatho maintains a most excellent interactive tool for this data at http://rsl.geology.buffalo.edu/data/Pages/Greenland_MB_AllGates.html which I've posted several times on various forums. Jakobshavn there is given 116,401 km2 above 2000 m. It pinches out at the coast, especially to the south.

The ~1% slope of central Greenland means the difference between using on-the-ground area vs projected area is negligible compared to the error associated with determining iceshed boundaries which are difficult to position because ice is nearly stagnant on dividing ridges and an ambiguous nearly horizontal cylindrical surface on the west slope.

Sub-surface flow need not match surface flow direction but in Greenland it does for most practical purposes. And despite all the hulabaloo about sills and canyons, the elevation of bedrock interior in central and northern Greenland actually has a rather small variance around sea level. So, given symmetry about the central NS surface ridge, bedrock topography has little effect on the volume of ice associated with particular snowshed -- you could get a decent ice volume estimate just integrating over each basin's elevation above sea level (or more crudely as the fraction of total Greenland area x total Greenland ice volume)

None of this gives volumes of ice that will be eventually discharged by each glacier. For one thing, annual snow accumulation varies greatly, as does melt and run-off with latitude. For another, the rapidly thinning glaciers could capture iceshed area from slower moving neighbors. And flow rates of Greenland glaciers are accelerating very unevenly.

In summary, area isn't a great proxy but it suggests the relative importance of basins and the numbers in my previous post are the ones that Greenland glaciologists use (especially if writing up a grant proposal for a remote/expensive region like NE Greenland.)

Rubikscube

  • Frazil ice
  • Posts: 247
    • View Profile
  • Liked: 0
  • Likes Given: 0
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #478 on: June 29, 2015, 09:55:28 PM »
Really appreciated with such extensive answers. I was foremostly thinking about topography because it tells about the potential for future acceleration in speed and volume discharge, but I recon it will be extremely difficult to make a more precise definition of iceshed (based on volume) especially when the timescale of a GIS collapse in unknown.

A-Team

  • Nilas ice
  • Posts: 2403
    • View Profile
  • Liked: 282
  • Likes Given: 19
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #479 on: June 29, 2015, 11:11:34 PM »
Right, that would get into modelling a whole ice sheet whose basal friction and internal temperature profile -- the determinant of the key rheology parameter in Glen's flow low -- are scarcely constrained by observation. It’s tough to make predictions especially about the future, according to baseball coach Yogi Berra. So people just toss out these areas which at least are in the right ballpark.

The attached image shows a typical basin in NW Greenland -- pinched out at the summit ridge, as are its neighbors. Here Humboldt was assigned 3% of the total area in a 2006 study. While Zachariae jumps out as a big number, Greenland-wide you end up having to sum a lot of small numbers, each of which is a special case on its own in terms of topography etc.

It is easier -- and maybe more accurate -- just to watch GRACE mass loss for trend development.

Rignot E and Kanagaratnam P (2006) Changes in the velocity structure of the Greenland Ice Sheet. Science, 311(5673), 986–990 (doi: 10.1126/science.1121381)

oren

  • Young ice
  • Posts: 4482
    • View Profile
  • Liked: 873
  • Likes Given: 1287
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #480 on: July 04, 2015, 04:46:22 PM »
A new updated retreat version (please note the background image is from September 2014, click for enlargement):

For the last month I have been trying to assess Zachariae's retreat pattern and whether it is slowing down. So having gone through all the process of downloading, unzipping, converting and processing, following A-Team's and Wipneus' detailed guidelines, I finally have some crude output on the subject.

Using Espen's material as a basis, I compared the calving front movement over the 10 cold months of the year, roughly Sep 1 to Jul 1, in both 2013-2104 and 2014-2015. And the result is that in 2013-2014 the calving front advanced during that period (red to yellow line), while in 2014-2015 the calving front actually retreated even though the sea ice still remains. I find that quite interesting. Perhaps it points to accelerated calving front retreat?

p.s. apologies for the grayscale image for 2014, some stuff is still beyond me.

Espen

  • Young ice
  • Posts: 3251
    • View Profile
  • Liked: 177
  • Likes Given: 0
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #481 on: July 11, 2015, 09:01:02 AM »
Zachariae update. A new calving line is seen and further developing:
Have a ice day!

oren

  • Young ice
  • Posts: 4482
    • View Profile
  • Liked: 873
  • Likes Given: 1287
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #482 on: July 11, 2015, 10:27:50 AM »
To satisfy my curiosity, I went back one more year and dug up Jul 1 2013. As can be seen, 2012 to July 2013 did show a retreat during the cold months, somewhat similar in magnitude to 2014-2015 and opposed to 2013-2014. So this behavior is not new. Perhaps it does however predict a bigger retreat year.

« Last Edit: July 24, 2015, 09:29:52 AM by oren »

Shared Humanity

  • Young ice
  • Posts: 3965
    • View Profile
  • Liked: 416
  • Likes Given: 48
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #483 on: July 11, 2015, 03:18:48 PM »
Zachariae update. A new calving line is seen and further developing:

It will be interesting to see if this calving rolls over or is tabular as the calvings usually are.

Espen

  • Young ice
  • Posts: 3251
    • View Profile
  • Liked: 177
  • Likes Given: 0
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #484 on: July 12, 2015, 08:17:45 PM »
The latest calving line at Zachariae is moving in a big way in the northern part of the glacier front, sorry about the weather conditions :(
Have a ice day!

Espen

  • Young ice
  • Posts: 3251
    • View Profile
  • Liked: 177
  • Likes Given: 0
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #485 on: July 19, 2015, 09:01:44 PM »
Here a rare calving at Nioghalvfjerdsfjorden (79 Fjord), I haven't seen this for quite some time now:

Sorry about the double s in the animation ;)

Please click for animation!
Have a ice day!

Andreas T

  • Nilas ice
  • Posts: 1126
    • View Profile
  • Liked: 11
  • Likes Given: 2
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #486 on: July 23, 2015, 08:29:08 PM »
This clear worldview image from 20rd July seems to show the surface structure produced by the movement shown in the velocity chart. I thought it would be nice to compare.
The blue seems to be freshly uncovered ice, it seems to become whiter later, can someone say more about what we see there?
on the 21st the icebergs in front of Zachariae had a sudden push possibly from wind coming down the glacier.
« Last Edit: July 24, 2015, 04:33:22 PM by Andreas T »

Wipneus

  • Citizen scientist
  • Young ice
  • Posts: 3996
    • View Profile
    • Arctische Pinguin
  • Liked: 619
  • Likes Given: 0
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #487 on: July 24, 2015, 10:16:55 AM »
Time for a new natural color animation of the Zachariae, July 4 - 20. I kept it low resolution (45m/pix) to be able to show the overview. The development of melt ponds is striking, also have a look at the old tongue in the NE corner of the image. Most melt ponds can be seen appearing or growing, but there are some (look in the SW corner) that drain between the two snapshots.

For measuring the average speed I used the Gimp image editor with both images stacked and displaying their difference:
- boost the resolution to 5m/pix.
- align the images using the various rocks.
- align the images near the centre of the glacier front, about one calving away from the latest developing calve;

The latter alignment required a shift of (x,y)=(26,2) pixels, that works out to 8.1 m/day for speed. Assuming a 2 pixel uncertainty in the procedure that would be a range of 7.5-8.7 m/day.

That is high compared with previous measurements (upto until now about 6-7 m/day). Until confirmed with newer images I'd suggest a possible speed-up.

(can't escape the click)

« Last Edit: July 24, 2015, 10:41:09 AM by Wipneus »

Tor Bejnar

  • Young ice
  • Posts: 3055
    • View Profile
  • Liked: 369
  • Likes Given: 189
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #488 on: July 24, 2015, 01:45:47 PM »
Whitish light blue ice next to the July 4 melt ponds are generally covered by melt pond water on the 20th.  I suspect these were melt ponds last year that froze, and are now covered by new melt water.  (I have no expertise in this, however.  My air photography class [remember pocket stereoscopes?] 40 years ago did not include any images with ice.)
Arctic ice is healthy for children and other living things.

Andreas T

  • Nilas ice
  • Posts: 1126
    • View Profile
  • Liked: 11
  • Likes Given: 2
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #489 on: July 25, 2015, 10:50:55 AM »
An alternative explanation for the whitish areas Tor has spotted would be this: I have noticed in photos by Jason Box (dark snow project) that the darkened surfaces of the ice can be cleared of the accumulated dust, soot and algae which make it so dark by meltwater flowing over it. even more so by a draining meltpond. If the whitish area was a meltlake last year it is likely that draining of that lake flushed the "dirt" away.

Shared Humanity

  • Young ice
  • Posts: 3965
    • View Profile
  • Liked: 416
  • Likes Given: 48
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #490 on: July 25, 2015, 04:31:39 PM »
I have to admit, I am drawn to the Greenland glaciers threads every day, mainly for the beautiful images that Espen posts and now, more frequently, by the personally challenging explanations provided by A Team. I would like to thank both of you.

I really got interested when I stumbled across this research which develops a much more detailed map of the underlying topography.......

http://www.nature.com/ngeo/journal/v7/n6/extref/ngeo2167-s1.pdf

As always, I am challenged to explain their approach.

I have always been fascinated by Jacobshavn (visit that thread often) and Espen always expresses that he is more concerned about Zachariae. I decided to go back and look at the detailed topography maps that have been developed, using the  mass conservation technique. (See below)

I now understand the concern. This glacier is a monster. There is an enormous section of the ice  sheet, well inland from the calving face, which is grounded many hundreds of meters below sea level.

Shared Humanity

  • Young ice
  • Posts: 3965
    • View Profile
  • Liked: 416
  • Likes Given: 48
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #491 on: July 25, 2015, 04:42:39 PM »
This clear worldview image from 20rd July seems to show the surface structure produced by the movement shown in the velocity chart. I thought it would be nice to compare.
The blue seems to be freshly uncovered ice, it seems to become whiter later, can someone say more about what we see there?
on the 21st the icebergs in front of Zachariae had a sudden push possibly from wind coming down the glacier.


When you look at obvious surface melt occurring here and align this melt with the detailed topography map I have just posted, it seems the location of the melt is concentrated over where the ice sheet is grounded well below sea level. As this melt water drains, it would seem to have to pool here.

I don't believe surface winds can move glaciers so quickly. Could we be seeing the effects of sub glacial melt water pulses?

Andreas T

  • Nilas ice
  • Posts: 1126
    • View Profile
  • Liked: 11
  • Likes Given: 2
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #492 on: July 25, 2015, 08:59:14 PM »
sorry if I have not been very clear, its the icebergs in the sea in front of the glacier which moved. They do rise quite high above the water possibly enough to be moved by wind a few hundred meters in a day. But increased flow of meltwater is also something one can expect, its just the suddenness comparing 14th - 20th with 20th - 21st. which caught my eye.

P-maker

  • Frazil ice
  • Posts: 238
    • View Profile
  • Liked: 14
  • Likes Given: 0
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #493 on: July 25, 2015, 11:21:42 PM »
Andreas T,

Quote
“The blue seems to be freshly uncovered ice, it seems to become whiter later, can someone say more about what we see there?”

Let me have a go at interpreting your image:

Starting in the left hand side, the first ¼ is a white melting snow surface gently sloping towards the sea. Near the foot of this slope you see the light bluish zone, which is most likely slush ice near the base of the slope. Please observe the clear blue drainage channels on the eastern side of this zone draining water from the slush ice. Once the water has drained off, the snow turns white again. The large homogenous white areas in the centre of the image  is most likely superimposed ice formed during initial melt this year or during earlier melting seasons. The easternmost 1/3 of the image consist of greyish bare ice with plenty of melt lakes. The glacier tongue itself is clear of melt lakes, since they have all drained through cracks in the ice.

Sorry to say Andreas, but in my view the front of the glacier and all the newly produced icebergs in front of it are moving eastwards at roughly the same speed between 4th and 20th July. The mélange between the icebergs seems to be rather compact, so the suggested movement by winds is highly unlikely.

Wipneus

  • Citizen scientist
  • Young ice
  • Posts: 3996
    • View Profile
    • Arctische Pinguin
  • Liked: 619
  • Likes Given: 0
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #494 on: July 29, 2015, 01:37:08 PM »
A Sentinel 1A hi-res pair of images that can be compared with the Landsat pair I posted a few days ago. Differences:

- radar vs optical
- 10m vs 15 m resolution
- 12 days cycle vs 16 days.

A hi-res S1A image seems now to become available every 12 days. Due to the speckle noise in the radar images the method that I use is much harder to employ and even useless at longer intervals than 12 days (at the current surface conditions).

Using the same method as in the Landsat case,  I measure the speed at the center of the calving front of 8.3 m/day (range 7.4 - 9.2). A little larger error, but confirmation that this is definitely beyond the 6-7 m/day measured during the past year.

A-Team

  • Nilas ice
  • Posts: 2403
    • View Profile
  • Liked: 282
  • Likes Given: 19
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #495 on: July 29, 2015, 04:38:31 PM »
Quote
I measure the speed at the center of the calving front of 8.3 m/day  but definitely beyond the 6-7 m/day measured during the past year.

That’s of great interest if it is a non-seasonal increase, ie matching late July dates with 2013 ~ 2014 and 2015 a jump. The % increase looks to be very large, 30%? I’m not sure what the best way of estimating error in this percentage increase; the usual way of combining independent gaussian errors during division may not be applicable here.

I’m thinking Landsat-8 would have 3 full years of July/Augest data whereas Sentinel started posting this October or so. (It would be very problematic anyway to combine a Sentinel with a Landsat for a velocity measurement.) However cloudy days can frustrate plans to get closely matched Landsat date pairs at peak summer velocities. In theory Earth Explorer allows filtration by cloud content but that last 5% can still sit exactly where you don’t want it.

It might work better to script a download of just the blue preview and Band 10 cirrus. Someone has mirrored Landsat-8; this may allow an escape from whole package downloads at EExp. Though really since the time window is fairly small and the returns so far apart, it might be faster just to do this visually on the preview to the preview.

Quote
Sentinel is too speckled.

I have looked at many procedures for cleaning up Sentinel images, none of which notably improve ice feature resolution. I could try a few more things with hv modulating vv  in gimp ‘modes’ or in ImageJ ‘math’ on stack but this is one where a physics-based rational procedure could be more effective than monkey at the keyboard.

oren

  • Young ice
  • Posts: 4482
    • View Profile
  • Liked: 873
  • Likes Given: 1287
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #496 on: July 29, 2015, 10:50:24 PM »
Up-thread I can find measurements by Wipneus of (m/day):
6.1 in April-May 2014
7.5 (6.7 further from the front) in August 2014
6 during the 2014-2015 winter (Oct-Mar?)
7.5 in early June 2015.
8.3 in late July 2015.
To compare to a more distant past, Rignot 2006 shows Zacharia's speed as 1600 m/y (4.4 m/day) in 2005, and about 1400 m/y (3.8 m/day) in both 2000 and 1996.

I believe this is an acceleration in top speed of about 10-20% year over year, and 50-70% in 10 years, up from 0% acceleration in the preceding period.
Of course, comparison of matching dates over both winter and summer will give a better estimate.

A-Team

  • Nilas ice
  • Posts: 2403
    • View Profile
  • Liked: 282
  • Likes Given: 19
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #497 on: July 30, 2015, 12:11:18 AM »
Nice work, chasing this history down. Goes to show measuring velocity is one thing, measuring changes in velocity (subtracting similar numbers from each other) takes it to a whole new level of error issues.

At 10% increase in velocity per year, it would take Zach 14 years to catch up to Jakobshavn's 28 m/d in late July 2015; 10 years at 15%; and 8 years at 20%. Assuming JI itself is no longer accelerating as fast as it has been. Petermann, the only other ice stream with noteworthy velocity roots in the interior, is a ways back at 3 m/day with no drama on the horizon.

nukefix

  • Frazil ice
  • Posts: 461
    • View Profile
  • Liked: 9
  • Likes Given: 1
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #498 on: July 30, 2015, 04:15:32 PM »
I have looked at many procedures for cleaning up Sentinel images, none of which notably improve ice feature resolution. I could try a few more things with hv modulating vv  in gimp ‘modes’ or in ImageJ ‘math’ on stack but this is one where a physics-based rational procedure could be more effective than monkey at the keyboard.
Speckle-noise is multiplicative and cannot be cleaned out with a *loss* of resolution. I  think the resolution of the EW-mode of S-1 is simply too coarse for detailed mapping - better to wait for IW-data that should be more available in the future. In any case velocity-mapping by cross-correlation is preferable to manually picking features to track..

A-Team

  • Nilas ice
  • Posts: 2403
    • View Profile
  • Liked: 282
  • Likes Given: 19
Re: Zachariae Isstrøm / Nioghalvfjerdsfjorden / North East Greenland
« Reply #499 on: July 30, 2015, 11:27:01 PM »
Quote
IW velocity-mapping by cross-correlation is preferable to manually picking features to track

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