[MITgcm-support] question about cube-sphere topology log output

[Jean-Michel Campin]

Hi Dan,

I took a quick look at what the docs say about pkg/exch2 and I have the impression that this
is seriously outdated. Also I remember that I changed (a long time ago, when moving the
matlab pkg/exch2 code generator inside pkg/exch2 fortran code) the meaning of the 
index mapping function (the "exch2_oi" described in the docs), so this part of the docs
is no longer relevant.

Regarding this:
> what is pij?
> what is oi,oj?

The best place for a minimal description is W2_EXCH2_PARAMS.h, lines 58-68 :

C--   COMMON /W2_EXCH2_BUILD_I/ Integer variables used to build topology
C     facet_owns  :: Range of tiles this facet "owns"
C     facet_pij   \  ::
C     facet_oi     } :: indices correspondence matrix (facet_pij) & offsets:
C     facet_oj    /  ::
C-with:  suffix "so" for indices of source facet j ;
C        suffix "tg" for indices of target facet jj= INT(facet_link(i,j))
C      pij(:,i,j) : matrix which gives so indices when applied to tg indices
C        iso = pij(1)*itg + pij(2)*jtg + oi
C        jso = pij(3)*itg + pij(4)*jtg + oj
C-----

these are used to fill-up the halo-region of the target facet "tg" from the interior of
the source facet "so"; (convention: 1,2,3,4 <==> N,S,E,W)

Note that these are not really used for I/O and global-mapping of the domain,
but just to fill the halo regions.

Does this answer your question ? and if you need more info about I/O,
let me know.

Cheers,
Jean-Michel

On Tue, Oct 15, 2024 at 02:26:14PM +0000, Daniel Goldberg wrote:
> hello all
> 
> Im interested in cube sphere topology. the reason is i would like to code up a solver off-line on a cube-sphere grid ? the intention is for it to eventually enable STREAMICE to run in a cube-sphere grid in order to be coupled with AIM.
> 
> i read the page https://mitgcm.readthedocs.io/en/latest/phys_pkgs/exch2.html and didn't find that it answered my questions that i could see, at least not directly enough! however, i ran the solid-body.cs-32x32x1 verification and looked in the file w2_tile_topology.0000.log that are produced, and it might be closer to what i was looking for, but i would like to make sure i understand:
> 
>   N.Edge Facet  1 <-- W.Edge Facet  3 : pij=  0 -1  1  0 ; oi,oj=    33    32
>   S.Edge Facet  1 <-- N.Edge Facet  6 : pij=  1  0  0  1 ; oi,oj=     0   -32
>   E.Edge Facet  1 <-- W.Edge Facet  2 : pij=  1  0  0  1 ; oi,oj=    32     0
>   W.Edge Facet  1 <-- N.Edge Facet  5 : pij=  0  1 -1  0 ; oi,oj=   -32    33
> 
> the above is from the log, so my understanding is that
> 
>   *
> facet (face) 1 shares its northern edge with the western edge of face 3.
>   *
> "North", "West" etc refer to the top row and the left column in an array sense, not geographic directions.
>   *
> if i were to use the matlab utility read_cs_face to read binary output, the "Facet" number above would match the "face" argument.
> 
> Is all/any of this correct? If so i have two further questions:
> 
>   *
> what is pij?
>   *
> what is oi,oj?
> 
> Many thanks!
> Dan
> 
> 
> --
> Daniel Goldberg, PhD (he/him)
> Reader in Glaciology
> School of Geosciences, University of Edinburgh
> Geography Building, Drummond Street, Edinburgh EH8 9XP
> https://dngoldberg.github.io/
> The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336. Is e buidheann carthannais a th? ann an Oilthigh Dhùn Èideann, clàraichte an Alba, àireamh clàraidh SC005336.

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