[MITgcm-support] exch2 and autogenerated grids (llc, cs, etc.)

[Ryan Abernathey]

Hi,

After avoiding this issue for a long time, I am finally trying to
understand how the new auto-generated grids work (in particular the llc
grids).

Is this documentation up to date?
http://mitgcm.org/public/r2_manual/latest/online_documents/node233.html

For the global_oce_llc90 setup, the input files seem very simple, i.e. data
file just has
  usingCurvilinearGrid=.TRUE.,

and data.exch2 has
  W2_printMsg= 0,
  W2_mapIO   = 1,
  preDefTopol=0,
  dimsFacets(1:10) = 90, 270, 90, 270, 90, 90, 270, 90, 270, 90,
  facetEdgeLink(1:4,1)= 3.4, 0. , 2.4, 5.1,
  facetEdgeLink(1:4,2)= 3.2, 0. , 4.2, 1.3,
  facetEdgeLink(1:4,3)= 5.4, 2.1, 4.4, 1.1,
  facetEdgeLink(1:4,4)= 5.2, 2.3, 0. , 3.3,
  facetEdgeLink(1:4,5)= 1.4, 4.1, 0. , 3.1,

This is evidently enough information to completely specify the grid
geometry. But I can't figure out where in the code the geometry is actually
calculated. In ini_curvilinear_grid.F, it looks like it is just reading the
geometry from horizGridFile (i.e. tile.0001.mitgrid). So when do these
horizGridFiles get written?

Just looking for some hints to get started on understanding this important
but intimidating subject.

Thanks,
Ryan
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