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Author Topic: Thwaites Glacier Discussion  (Read 28005 times)

baking

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Re: Thwaites Glacier Discussion
« Reply #150 on: October 14, 2019, 09:49:23 PM »
baking,
I have to wait for clear pictures from Sentinel to do an evaluation of the things that are going on.

I get all of my images from polarview.org.  I have not had much success with other sources and I find Polar View to be "good enough" for what I'm doing.

baking

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Re: Thwaites Glacier Discussion
« Reply #151 on: October 19, 2019, 05:12:04 AM »
This will be the first in a short series of posts which will attempt to overlay published bathymetry and grounding line data on recent Sentinel-1 satellite images.

I will start with the bathymetry map found in Millan 2017 "Bathymetry of the Amundsen Sea Embayment sector of West Antarctica from Operation IceBridge gravity and other data"
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016GL072071

The first image below is box (b) "Thwaites/Haynes" from figure 2.

The second is the matching Sentinel-1 "Extra Wide Mode" image from October 16, as processed by Polar View.

The third is the bathymetry overlaid over the satellite image at 50% opacity.  Grounding line positions are red (year 1996), ice front positions (year 2008) are yellow. Bed elevation is color coded from light blue to dark blue (−1400 m), with light contours every 100 m and thick contours every 400 m (300, 700, 1100 m) although the light contour lines are very faint in the overlay.

The next step will be to update the grounding lines from Milillo 2019 "Heterogeneous retreat and ice melt of Thwaites Glacier, West Antarctica"
https://advances.sciencemag.org/content/5/1/eaau3433

For those that are interested, here is the process that I used to generate these images.  Anyone with minor proficiency in GIMP or any similar image processing software (Photoshop, etc.) should be able to duplicate this and verify my results.

Start by downloading this Sentinel-1 image from Polar View (available for 30 days): https://www.polarview.aq/images/105_S1jpgfull/S1B_EW_GRDM_1SSH_20191016T041914_9DC8_S_1.final.jpg

If you wish to use another image, make sure that the full rectangle, 100 to 110 degrees West and 74 to 75 degrees South is visible.  Your scaling numbers may vary slightly from the ones below.

Download Figure 2 from Millan 2017 (link above) using the "Open in Figure Viewer" link then "Save Image as..."

I opened the satellite image in GIMP first and then "Open as Layers..." Figure 2.  (It only works in that order because the satellite image is larger.)

I measured the 100 to 110 degrees West and 74 to 75 degrees South rectangle in both images and estimated that I needed to scale the satellite image down by a factor of 4.46.  (I confess that this calculation was non-trivial, but the results were surprisingly good.)  I scaled the image from 14060x14406 to 3152x3230.

Then I just reduced the opacity of the top layer to 50% and lined up the corners of the rectangle.  It was not off by more than a pixel so I was happy.  Finally I cropped to image to the edges of the black box (b) and saved it.

Let me know if you have any problems duplicating this.

baking

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Re: Thwaites Glacier Discussion
« Reply #152 on: October 19, 2019, 08:09:34 AM »
A quick update.  I was able to fairly easily (details below) add the grounding lines through late November 2017 (2017.91 in decimal years) as a very faint (30% opacity) overlay.  It's a very busy diagram, but I hope at some point to be able to mask out the extraneous details and increase the opacity.

In general, I was quite surprised at the size of the ground line retreat.  Seeing it laid on top of the satellite images I have grown quite familiar with was a bit of a shock.  Also, at first glance, it would appear that there has been some additional grounding line retreat in the last two years.  In particular, the West side of the "Butterfly" looks like it is no longer grounded, although it should probably be compared to older satellite images to see if it is a new feature or not.

Edit: Added an annotated version.

This is Figure 1(B) from Milillo 2019 "Heterogeneous retreat and ice melt of Thwaites Glacier, West Antarctica"
https://advances.sciencemag.org/content/5/1/eaau3433

The only way I found to obtain a high resolution image of the figure was to click on the "View this article with LENS" button, then click on "Figures" and click on Figure 1.  I cropped and masked the figure to just get the bathymetry, then rotated it clockwise 70 degrees and scaled it down 4% (0.96 scaling.)  The 1996 grounding lines then lined up quite nicely, yellow in Milillo and Red in Millan.
« Last Edit: October 19, 2019, 08:32:29 AM by baking »

baking

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Re: Thwaites Glacier Discussion
« Reply #153 on: October 29, 2019, 09:21:04 PM »
This is a follow up to Reply #132 about the underwater peaks under Thwaites Tongue and the effects they have had over the past two weeks.  The concern is that the peaks will cause shearing of the Western side of the Tongue.  Already we can see increased rifting from what had occurred before.

The first image is a radar image from Sentinel-1 taken on October 26, 2019.  It shows the approximate location of the highest two peaks identified previously, and cracks and rifts that have widened significantly since October 14.

The second is a 3 image GIF from October 14, 20, and 26.  The motion is relative to the rest of the Tongue, not absolute, to better see the effect of the rifting.

baking

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Re: Thwaites Glacier Discussion
« Reply #154 on: November 02, 2019, 04:52:49 AM »
Here is an updated GIF through November 1, showing the rifting in the Tongue described in the post above which is showing no sign of slowing down.

Tor Bejnar

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Re: Thwaites Glacier Discussion
« Reply #155 on: November 02, 2019, 02:23:06 PM »
Baking's 2nd image (the GIF) shows the latitude or longitude line moving about.  It appears the one iceberg that moves the most in the GIF moves in sync with with red line, suggesting everything else is 'actually moving' and that one berg is (relatively) standing still.  An alternate interpretation would be the red line is approximate (and therefore is not stationary): if so why include it in the first place?  (I know this is not Baking's doing.)

Other that that, I really appreciate Baking's close look at the relative movements of the mostly glued-together icebergs.
Arctic ice is healthy for children and other living things.

baking

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Re: Thwaites Glacier Discussion
« Reply #156 on: November 02, 2019, 04:03:29 PM »
Baking's 2nd image (the GIF) shows the latitude or longitude line moving about.  It appears the one iceberg that moves the most in the GIF moves in sync with with red line, suggesting everything else is 'actually moving' and that one berg is (relatively) standing still.  An alternate interpretation would be the red line is approximate (and therefore is not stationary): if so why include it in the first place?  (I know this is not Baking's doing.)

Other that that, I really appreciate Baking's close look at the relative movements of the mostly glued-together icebergs.

Great questions, Tor.  I tried to explain in the post before that these GIFs are relative to the regular movement of the Tongue.  The red line is the 75 South line of latitude, so its movement in the GIF reflects the absolute motion of the Tongue.  The lone iceberg that is moving with the line is in fact grounded and has been for over a year.  It rests on a underwater peak and there is another peak directly above it in the GIF, just below the midline where you can see cracks widening.  This second peak is also responsible for the expansion of the rifts to the right (South or upstream.)

Most of my GIFs show absolute motion, but this is one case where the relative motion is more subtle and easier to see with movement of the main flow removed.  Make sure you follow the discussion at Reply #132 and above and don't hesitate to ask any questions.  I'd be happy to clarify.

The significance of this is best highlighted in this GIF https://twitter.com/StefLhermitte/status/1101870380623511552 which shows how the underwater peaks shear lines of icebergs off the Western side of the Tongue making it ever thinner.

All of my pictures are from PolarView.aq who are responsible for the lines of latitude and longitude for which I am forever grateful.

Tor Bejnar

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Re: Thwaites Glacier Discussion
« Reply #157 on: November 02, 2019, 05:13:16 PM »
Thanks!
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baking

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Re: Thwaites Glacier Discussion
« Reply #158 on: November 09, 2019, 12:46:55 AM »
Here is a quick update on the Western side of Thwaite's Tongue.  I'm only showing two images in this GIF, November 1 and 7, to emphasize that the rifting is still continuing to expand at a rapid rate.  All motion here is relative.

If the underwater peak continues to push against the Tongue, you can see where a row of icebergs, 3-4 across, will be stripped off reducing the Tongue's width by about a third.

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Re: Thwaites Glacier Discussion
« Reply #159 on: November 13, 2019, 09:49:40 PM »
Another 6-day Sentinal-1 radar update on the rift in the Western side of Thwaites Tongue.  This GIF shows the last 3 months.  Motion is relative to the Tongue.  This rifting is being caused by an underwater peak just to the left of the red latitude line