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Messages - nukefix

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1
So experimenting around with sentinel hub playground I discovered that although I could get the day, I couldn't figure out the precise image acquisition time...
That should be available here to the necessary degree:

https://sentinel.esa.int/web/sentinel/missions/sentinel-2/acquisition-plans

2
The word of the street seems to be that not all scientists are fully convinced that marine cliff instability will play out as current models predict. It would help if it could be observed to happen in nature.

3
johnm33

There is no change in the flow-path of the main trunk as it is confined to a narrow canyon. The "paths" plotted in the image are the flight-paths NASA did with their Airborne Topographic Mapper (ATM):

https://icebridge.gsfc.nasa.gov/?page_id=118

The Sentinel-1 images you posted are from ascending and descending orbits so their geometry differs. Also the presence of wet snow darkens the image by a great deal and I assume PolarView does some adaptive contrast stretching that complicated side-by-side comparisons when large contrast differences are present.

I still have no reason to believe salt water penetrates very far above the grounding zone but if you find a study arguing so I'd be delighted to read it!

4
Policy and solutions / Re: Becoming Vegan.
« on: May 05, 2019, 06:19:01 PM »
Unfermented soy is potentially harmful...better to use seitan etc. instead of tofu.

5
The GRACE satellite was an amazing tool from April 2002 until June 2017 & I look forward to seeing even better results from GRACE-FO:
Unfortunately there are serious problems on-board GRACE-FO, so the data-quality is likely to be a lot worse than for the original GRACE  :(

6
Antarctica / Re: PIG has calved
« on: May 04, 2019, 03:25:51 PM »
Also, what is your secret to getting such gorgeous images from SAR?  I assume your tool chain is SNAP and Gimp, but I'm struggling getting anything comparable.
Personally I like to view SAR data in SNAP using contrast stretched dB-scale. The steps to generate this are:
1) Use the calibrate-operator and calibrate the image into sigma0
2) In the calibrated image right-click on the sigma0-band and select 'linear to/from dB'
3) Adjust contrast stretching in the Colour Manipulation tool-window if needed. I find that the default 95% stretch works well in most cases.

7
my take is that it then took three years for sufficient seawater ingress to float the basal ice in the first trough and have that break through the overburden of glacial discharge.
The glacier is too thick to float and both GPR & seismic indicate it's solid ice until water-permeated glacial till at the bottom. The meltwater is fresh and it's being dumped into the fjord continuously. Can you find a scientific publication where the authors argue that the saltwater gets to penetrate upstream?

8
According to this paper:

https://www.cambridge.org/core/journals/annals-of-glaciology/article/characteristics-of-meltwater-export-from-jakobshavn-isbrae-and-ilulissat-icefjord/1DEDDDC0AAD910F83213AEBAFD7A31E6/core-reader

...the salinity of the Subglacial discharge, Submarine meltwater and Basal Submarine meltwater is zero, which would not be possible if saline waters were penetrating into and under the ice-stream. Case closed?


9
Greenland and Arctic Circle / Re: What's new in Greenland?
« on: April 18, 2019, 01:01:10 PM »
ps: GRACE Follow-On - where are you? No info from NASA or Germany since late 2018. Is it in trouble as data was promised by now.
I heard that the accelerometer on one of the satellites is kaputt. That is not good at all and will quite drastically lower the data quality compared with fully functioning GRACE  :( :( :'(

10
Well, since (virtually) all rivers near coasts have salt wedges running up them as the tide comes in, I would expect similar tide-synchronised intrusions of denser salt water below the outflowing meltwater beneath the fragmented glacier by the cil.  Why wouldn't it?
I don't think the ice is fragmented as the pressure is around 100bar at 1km depth, but there are drainage channels. Tides increase back-pressure and can be detected far upstream but how far does the salt-water intrude? dunno

11
I found nothing in it that confirmed or challenged my view, apart from the step change of melange height above the cill [fig2] which, to me, suggests a deep logjam of melange, and if so then voids enough for tidal seawater/meltwater exchange.
johnm33 it's not "melange" but solid deforming ice all the way to the glacial till according to both seismic and ground penetrating radar studies. There is a changing network of glacial drainage but are there studies arguing seawater can be found upstream of the grounding-zone?

12
There is no water under Jakobshavn upstream from the grounding zone, which is very close to the calving front (part of the year they are at the same spot, so no floating tongue).

As far as I know there is zero evidence of sea-water penetration under Jakobshavn. Tides influence the back-pressure in the glacial plumbing system that should be full of glacial meltwater.

This thesis says

Quote
For this investigation I re-processed a 10 km-long high-resolution reflection seismic line at Jakobshavn Isbræ, Greenland, using an iterative velocity determination approach for optimizing sub-glacier imaging. The resultant line imaged a sub-glacier sediment layer ranging in thickness between 35 and 200 meters. I interpret three distinct seismic facies based on the geometry of the reflectors as a basal till layer, accreted sediments and re-worked till. The basal till and accreted sediments vary in thickness between 4 and 93 meters and are thought to be water-saturated actively-deforming sub-glacier sediments. A polarity reversal observed at one location along the ice-sediment interface suggests the presence of water saturated sediments or water ponding 2-4 m thick spanning approximately 240 m across.

https://kuscholarworks.ku.edu/handle/1808/19476
https://kuscholarworks.ku.edu/bitstream/handle/1808/19476/VelezGonzalez_ku_0099D_13964_DATA_1.pdf

A seismic image plus legend are attached.



13
this is why I formed the view that seawater penetration once past the first cill, at 5K, was more or less unstoppable, the more saline water will always pass over the inner cills and move upstream before it becomes dilute enough to move back towards Disko.
Above the grounding-zone (thai is close to the calving front in this case) the ice-stream is grounded, in other words not floating but sliding on bedrock/till. So your theory is that despite this there is sea-water penetration far inland? I guess that is possible and there are these deep blue icebergs coming out of Jakobshaven that could be a sign of ice made from freezing water (instead of compaction of snow).

Quote
The recorded calvings of giant icebergs I suspect are the result of the ice in the deepest part of the trough being floated and breaking through the weight of ice above freeing that space for seawater penetration.
No. If they were floating there would be a clear tidal signal there, and there isn't.

Quote
Is it correct to call it a grounding line if the ice is a composite of melange and seawater? if so ok.
The grounded ice is too thick to float, so no...

15
I think Espen means that the ice flow became noticeable after 2000, not because lack of coverage but because of actual changes.
Loss of buttressing sea-ice?

16
I can see from old satelite data that it started as a noticeable stream in the early 2000s, but you may have more to add?
Do you mean that before 2000 the satellite-data was so scarce the existence of the stream was not noticed from space? Today it's being mapped every 6 days by Sentinel-1...times have certainly changed  8)

17
So I'm guessing the build up of ice at the calving front is just a logjam of bergs waiting to be melted/lifted above the cill hence the pulsed release of bergs associated with tidal extremes.
AFAIK the grounding line of Jakobshavn is at the calving front, or very close to it. That would mean that the fjord waters are not penetrating under it. If they did, the tidal signal would be detectable far upstream from the calving front, and I don't think that is the case.

The following paper states:
Quote
We hypothesize that Jakobshavn Isbræ maintains a short floating tongue from winter to early summer, when ice flow exceeds ice loss by calving and the glacier front advances. In summer, iceberg calving surpasses ice flow, and the glacier front retreats, becoming nearly grounded by late summer.
https://search.proquest.com/openview/2b991b9bb1ea4dbf97777598550b275e/

18
I have long suspected that sea water has penetrated as far as the round feature upstream,
...not sure if there is any evidence of that:

https://kuscholarworks.ku.edu/bitstream/handle/1808/21964/Braaten_2014.pdf

19
There's also a feature midstream at the bottom of the image which is what I imagine a giant berg would look like moving in the stream.
The channel is full of ice upstream from from the grounding-line, so certainly in that position. Thick ice acts like a viscous fluid (like honey).

20
Glacier-ice is malleable at depths of 30m and deeper. Therefore the ice-stream channel must be full of ice until the grounding-line (which for Jakobshavn is estimated to reside within ~1km of the calving front depending on the season) even if the depth of the channel varies. That being said, meltwater channels do form under the ice-stream.

Quote
Surface meltwater-induced velocity variation is a quasi-diurnal signal. Podrasky et al. (2012) detected an amplitude of up to 0.1 m d−1 diurnal signal 20–50 km upstream from the terminus of Jakobshavn Isbræ

https://search.proquest.com/openview/2b991b9bb1ea4dbf97777598550b275e/1?pq-origsite=gscholar&cbl=105732

21
Petermann is EXACTLY where it was last year on this date (area wise).  The shape of the terminus and it's position relative to the valleys north and south of it is unchanged. There are no major fractures sen either.  This summer has been the coldest in at least 7 years in NW Greenland as well.
It is still flowing 3+ m/day and the past summer looks "normal" to me. The graph is from Enveo Cryoportal.

22
Greenland and Arctic Circle / Re: How quickly could Greenland melt?
« on: November 11, 2018, 04:01:55 PM »
Ok, better to keep the terminology straight. For example Austfonna is an ice cap (= glacier), and sea ice is sea ice.

23
Greenland and Arctic Circle / Re: How quickly could Greenland melt?
« on: November 11, 2018, 02:44:55 PM »
Over the last many decades, the Arctic ice sheet has annually melted ~15,200 (circa 1980) -17,900 (now) km3/year averaged over many years. Individual years have lost more or less than this. The annual melt volume rate is increasing.
Those numbers are grossly incorrect. The Greenland Ice Sheet has lost about 3000km3 if ice from 1991-2011, so almost two orders of magnitude less ice loss than the quoted numbers.

http://imbie.org/imbie-2012/result.


24
Nice example of iceberg collapse at the mouth of the Ilulissat icefjord. 3MB animated GIF attached.

26
Here's the speed along the central flowline from Enveo's Cryoportal. Red is most recent (early July 2018) and shows some of the customary summer speedup:

27
In Ilulissat the big icebergs frequently get stuck on a reef at the exit from the fjord and it takes them weeks or longer to free themselves...so we should have plenty of time to admire them in the webcam.

...if I had the time I'd keep tracking the big ones with S-1 as post results here..

28
Since the S-1 radar is higher up in the sky than the Sun the shadowing-effect is clearer...in the image posted by johnm33 one could assess the height of the whole calving-front.

29
Any way of telling how thick they are from the freeboard ?

sidd
Yes by measuring the shadow...requires calculating the elevation-angle of the Sun at the time of the acquisition.

30
Radar altimetry cannot resolve something as narrow, deep and complex and Jakobshaven calving front. Measuring shadow-length from VHR optical/SAR imagery does work but the data is scarce.

31
Jacobshavn during the dark season in 16 Sentinel 1B images.

Is it possible to estimate the flow rate from those images?
Yes, for example with the ESA SNAP toolbox.

33
Greenland and Arctic Circle / Re: Greenland 2017 melt season
« on: November 06, 2017, 12:10:12 AM »
We could assume that since the next loss has been averaging 200 Gt/yr, and this past season gain was 200 Gt above average, that the net balance was near zero.  However, without GRACE data, we do not know for sure.
GRACE is not the only game in town, satellite altimetry and the input-output methods still work.

34
Hint: Now that winter is coming start using S-1 SAR, it's great for glacier monitoring... ;)

35
Arctic sea ice / Re: The 2017 melting season
« on: September 18, 2017, 12:47:55 PM »

36
Arctic background / Re: Whose data is wrong?
« on: September 13, 2017, 10:46:10 AM »
I am really puzzled by the assertion in the cited article that the collapse of the western ice shelf in Antarctica would raise sea levels by 6 meters. I understand that nuance is not the forte of the media, but what the article should have said is that if the ice shelf were to collapse, the conditions required for this to happen would be accompanied by rapid flow of glaciers into the ocean, which would be the actual cause of sea level rise. Ice shelves melting per se cannot raise sea level since they are floating and, according to Archimedes principle, already displace a volume of water equal to their mass.
You've misunderstood. The part of the West Antarctic Ice Sheet (WAIS) that is unstable and likely to raise sea-levels is the ice-sheet itself, not the floating ice shelves. WAIS is up to 2000m thick and laying on bedrock way below current sea-level, which makes it inherently unstable. It has collapsed before in warmer climates and is likely to do that again if warming continues.



37
I think there has been meltwater exiting Zach that has been melting sea-ice close to the calving front. The feature 1st appears on the 26.8. S-2 image:

http://apps.sentinel-hub.com/sentinel-playground/?lat=78.98372644988251&lng=-20.759353637695312&zoom=11&preset=1_NATURAL_COL0R&layers=B04,B03,B02&maxcc=100&gain=0.3&gamma=1.0&time=2015-01-01|2017-08-26&cloudCorrection=none&atmFilter=ATMCOR&showDates=true&evalscript=

38
Greenland and Arctic Circle / Re: Greenland 2017 melt season
« on: August 31, 2017, 02:27:48 PM »
DMI say net mass loss p.a. is circa 200 gt due to peripheral melting and glacial loss.
That's in a normal year I reckon.
Quote
QUESTIONS:-
- What will the net mass loss be in 2017? Depends on snowfall now to December ? (see next question) Will have to wait for NASA's GRACE system to tell us next year?
I wonder if zero net mass-loss or even net gain is possible this year. BTW GRACE is about to die and the data is getting more and more patchy. We must hope that the GRACE Follow-On launches and works without a glitch (godspeed!).

39
Arctic background / Re: Whose data is wrong?
« on: August 30, 2017, 09:52:21 AM »
...
You have not provided any references asserting that the Greenland ice sheet can produce Heindrich-style events during this interglacial. You also made unsubstantiated claims about catastrophic subsidence that can soon take place in the Melville-bay sector. Sorry to say but this smells like FAKE THREATS that are without basis.  :o >:( :(

40
Arctic background / Re: Whose data is wrong?
« on: August 29, 2017, 05:26:01 PM »
Nevertheless his work, with all it's faults, shows the consistency of the ancients stories and warnings about when the earth stopped spinning, some experiencing prolonged day others night.
Nice fairy-tale. Or where do you reckon the angular momentum and the associated ginormous amount of energy went, and how did it return?

41
Arctic background / Re: Whose data is wrong?
« on: August 29, 2017, 09:48:37 AM »
As per the isostatic rebound across the Melville Bay front, note the image of 2010 anomaly how sensitive that isostatic rise is to weather events like snow or rain accumulation v. off-loading by melt.
Yes, this is normal. On top of the "slow" ground uplift happening due to removal of huge ice masses at the end of the last ice age, there's also a "fast" component of uplift that happens very rapidly (~immediately) when ice-mass is removed. Therefore the "anomaly"-plot shows the anomaly against average "slow" and "fast" uplift. As indicated in the figure I posted, the Melville-bay area is rebounding very rapidly at about 1-2cm per year (average "slow" + average "fast" over the GPS time-series).

Quote
There will be a tipping point where this reverses as slushy, water-infested ice begins to pack against the Melville Bay.
That does not appear to be possible.

42
Arctic background / Re: Whose data is wrong?
« on: August 28, 2017, 04:52:09 PM »
I hope I'm not off topic here, but there is a possibility, a strong possibility in my mind, that an asteroid strike on the Laurentide ice sheet brought on the Younger Dryas, shattered the ice sheet, and ended the Rancholabrean mammals reign in North America. A boundary layer referred to as the "Black Mat" that is rife with nano diamonds, iridium, and fused carbon is evidence of the event.
If this did occur it makes the ending of the last ice age a unique event, and comparisons of it to modern warming inconsequential.
Terry
This hypothesis does not seem to be very popular today:

https://en.wikipedia.org/wiki/Younger_Dryas_impact_hypothesis

43
Arctic background / Re: Whose data is wrong?
« on: August 28, 2017, 04:16:47 PM »
Please post the references/citations here please, of course only published research will do. No need to have secret papers on shady hard disks :P

There was a lot more ice around at the end of the last ice age than now, including now extinct ice sheets. Therefore it's not surprising to me that bigger icebergs were being created. If you think there's a threat please show the references...


44
Arctic background / Re: Whose data is wrong?
« on: August 28, 2017, 09:33:42 AM »
The ice dome not only causes perimeter's vertical uplift to form an elevated shield around the ice dome's edges, but also a spatial tightening of the continental edges when the ice pushes the interior down the edges get squashed tighter. Similarly, the Weischelian Ice Sheet's outlet that crossed across the middle of Sweden has almost completely disappeared by now. (This palaeo-strait let ice out from the Baltic Sea Ice Dome which sat at the centre of the Weischelian Ice Sheet). Even as recently as in the Middle Ages, the Viking's Capital, Birka, was at the seaside in the middle of Central Sweden. Today it is far inland and only the deepest part of that palaeo ice stream carved strait remains water-covered as a freshwater lake.

The above process will cause Melville Bay initially to subside catastrophically by water infested slushy ice (post-sea summer ice era), in the process causing turbidic rockfalls and pulling flat shoreline into a tight neck (like currently in Ilulissat).
Subside catastrophically? What are you talking about? Reality-check: Melville-bay is rebounding fast (see Fig. 2) and this is only going to increase if and when the ice-sheet thins out:

http://www.pnas.org/content/109/30/11944.full.pdf

Quote
Once the ice is off-loaded, the wet solidus damage and the nucleation of gases in asthenosphere fluids drives isostatic uplift. This produces either a tight Ilulissat-type narrow passage surrounded by mountains on both sides. Or, if the discharge of rock is large enough to produce a sufficient 4th rapid erosion force (planing), enough abrasion can occur to remove rocks to get the Hudson Strait type wide opening to form there.
You have not posted any references that point this being a glaciological concern in the near future of GIS.

45
Arctic background / Re: Whose data is wrong?
« on: August 26, 2017, 12:16:02 PM »
Note that I have not said what Nukefix must have mis-read. We fear the failure occurs in future after the Arctic Ocean is summer-time ice free from midseason and it requires EXHAUSTIVE surface melting. I never said such situation is there already, nor anyone worried nuclear reactors say so.
I don't think that is glaciologically possible, according to present understanding the Greenland ice-sheet just isn't unstable the way WAIS is. Or can you find recent research articles arguing so?

BTW this is why I called the claim "alarmism" - scientists could not invent threats and present them to the parlament/media as their reputation would be seriously damaged.

46
Arctic background / Re: Whose data is wrong?
« on: August 25, 2017, 11:34:14 AM »
The situation now is very much different with that at the end of the last ice age with regards to land ice and glacial lake configuration. WAIS-collapse is discussed a lot in scientific literature, but I haven't seen glaciologists arguing that anything similar could happen in Greenland, even in theory. There is no underwater-channel into the interior of the Greenland ice-sheet, except a very narrow one at Jakobshaven. In particular NEGIS is being blocked by an inland-ridge several hundred meters above sea-level (see bedmap2), and that is before any rebound that takes place when the overlaying ice-sheet thins due to warming.

47
Arctic background / Re: Whose data is wrong?
« on: August 24, 2017, 10:28:21 AM »
(3) Heindric Ice Berg Armada (= Ice Debris Flows + Slip-Slide Ice Discharges + Hydrofracturing of North Greenland Ice Sheet on land + Rapid Erosion Forces + Perimeter and Continental Slope Failures), producing the job finished:
This is 100% a glaciological prediction - can you find peer reviewed publications by glaciologists predicting this mode of ice-sheet failure in Greenland? I've spent quite a bit of time talking about the Greenland ice sheet with professional glaciologists and I don't recall it ever been mentioned. References please.
Hey VAK care to comment on the above? Have you invented a new failure-mode for the Greenland ice sheet or is this already known in the glaciological community?
These are the only pieces I could find on Heidrich events in the future of Greenland, not very convincing and lacking proper references:

https://robertscribbler.com/2013/08/08/climate-monsters-we-want-to-keep-in-the-closet-heinrich-events-superstorms-and-warming-the-deep-ocean/

https://robertscribbler.com/2014/03/17/nature-human-warming-now-pushing-entire-greenland-ice-sheet-into-the-ocean/

Are there any credible papers that claim parts of Greenland are unstable so that they can trigger "catastrophic" events? Let's see them.

48
Arctic sea ice / Re: Latest PIOMAS update (August mid monthly update)
« on: August 23, 2017, 11:10:24 AM »
I do not believe there is any major sea ice rebound until there is next Heindrich Ice Berg Calving event from Greenland that pumps large amounts of meltwater and slushy ice into the ocean.
Hey VAK, I have to ask you again, what is this?? If I'm not mistaken you've written about it into report for UK parlament(!). I hope this is not something you've just made up, but is actually a concern in the glaciological community.

I know this is off-topic, I propose the discussion should be moved here where I've already raised this question a couple of times:

http://forum.arctic-sea-ice.net/index.php/topic,635.msg125647.html#msg125647

50
Arctic background / Re: Whose data is wrong?
« on: August 17, 2017, 05:15:27 PM »
(3) Heindric Ice Berg Armada (= Ice Debris Flows + Slip-Slide Ice Discharges + Hydrofracturing of North Greenland Ice Sheet on land + Rapid Erosion Forces + Perimeter and Continental Slope Failures), producing the job finished:
This is 100% a glaciological prediction - can you find peer reviewed publications by glaciologists predicting this mode of ice-sheet failure in Greenland? I've spent quite a bit of time talking about the Greenland ice sheet with professional glaciologists and I don't recall it ever been mentioned. References please.
Hey VAK care to comment on the above? Have you invented a new failure-mode for the Greenland ice sheet or is this already known in the glaciological community?

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