Neven: things have been real slow since that late July cyclone
Right. That memorable event was worth a separate forum of its own but didn’t get one. Some favorite resources are transient so have to be captured on the spot: WorldView can over-write a great swath with a cloudy one the same day; Rammb is only served for 20 days; PolarView offers Sentinel-1AB for just a month.
Instead there was a lot of nonsense posted about ‘compaction’ (in-house definition) even though OsiSaf showed decisively no net compressive inward ice motion ever occurred. OsiSaf arrows show observed gridded ice motion but exaggerated 6x in daily displacement: 3x for visibility x 48 hr interval. Ice motion tracked wind motion closely.
Centrifugal rotational forces and pack inertial forces evidently offset a weak coriolis (real slow ice velocity x sine of high latitude). The wind can only push against rough surfaces, pressure ridges and exposed freeboard. Mosaic measured ice pressure, not compaction, with sensors welded along the Polarstern's hull.
The last ten days of slow change can be visualized in a couple of ways. It takes high resolution to do this accurately because big square pixels don’t fit well over two curved ice edges being differenced — new open water (and lowering concentration) appear almost exclusively on the ice pack periphery. One dodgy extent pixel at 35x35 km holds 125 smaller AMSR2 pixels of 3.1 x 3.1 km.
Oren suggested a while back it would be helpful to have an operational description of how Venn diagrams from image pairs are made. Fortunately all manner of boolean logic operations are baked right into Gimp’s color picker. The first image below shows new open water (orange) in the August 10th AMSR2 uhh LARGE relative to areas filled with ice on August 1st. (Very little open water on the 1st was not open on the 10th, not shown).
Although the polar stereographic projection used in the archive is not equal area, it is very close to that about the 75º parallel, meaning a simple scaled pixel count suffices.
However collapsing a complex geometric object to a plain number loses too much information — surprising facts such as melt nesting, uniform perimeter shrinkage, supplemented in special regions such as ESS and northern Chukchi, lack of significant internal polynya development, opening fjords of the northern CAA, and rare deterioration above NE Greenland. Given just a number for area opened, none of this can be recovered from inversion.
Steps are simple; the trick is keeping color picker at the right setting operating on the right active layer:
open five dates before Aug 10th as a short stack in Gimp, 10th on top
set mode on each to ‘darken only’ to deprecate cloud cover artifacts
capture the view with with ‘new from visible’
delete the other dates and add Aug 01 from below.
set the color picker to desired concentration range on the embedded palette
crop stack to a non-excessive region of interest that works for both
select open water blue in the Bering 10th, change active layer to 1st, change color picker to ’subtract’, make new layer, name to avoid confusion, fill.
select open water blue on the 1st, change active layer to 10th, change color picker to ’subtract’, make new layer, name and fill with 2nd color.
set the color picker to ‘intersect’ to find open water common to both, make new layer, name and fill with 2nd color.