[MITgcm-support] Inconsistency in cubed sphere grid generator

[Dimitris Menemenlis]

Hi Nils, I don’t know (remember) answer to your question,
but the black cross does look familiar.

The two lines that you mention were introduced with following comment:
"% Fix truncation for x=0 or y=0 (aja)”
(aja is Alistair Adcroft)

I wonder if Alistair introduced this correction in order to keep the grid
fully spherical?  That is your “new” grid might eliminate the “cross”
anomaly in the area but at the expense of introducing a small bulge
near the center of the face?

Dimitris Menemenlis

> On Jul 4, 2018, at 9:19 AM, Nils Hutter <nils.hutter at awi.de> wrote:
> 
> Hi all,
> 
> 
> I am using the northern face of the cubed sphere grid for Arctic simulations. While refining the grid, we found that there is a jump in the cell area for all cells that lie on a line in grid direction, which crosses the center of the each face. This is nicely seen as a darker cross in the attached figure. We checked the matlab grid generator for cubed sphere (http://wwwcvs.mitgcm.org/viewvc/MITgcm/MITgcm_contrib/high_res_cube/matlab-grid-generator/) and found that the following changes in map_xy2xyz.m solves the problem (see attached figure of grid cell area for new mesh):
> 
> 
> RCS file: /u/gcmpack/MITgcm_contrib/high_res_cube/matlab-grid-generator/map_xy2xyz.m,v
> retrieving revision 1.1.1.1
> diff -r1.1.1.1 map_xy2xyz.m
> 20,21c20,21
> < kx=find(x<0);
> < ky=find(y<0);
> ---
> > kx=find(x<=0);
> > ky=find(y<=0);
> 23a24
> >
> 57,58c58,59
> < X(find(x==0))=0;
> < Y(find(y==0))=0;
> ---
> > %X(find(x==0))=0;
> > %Y(find(y==0))=0;
> 
> 
> For cs-tan-c-510-1020, the grid points generated with these modification are on average 20cm displaced compared to original grid. Maximum displacement is 79cm.
> 
> Does anyone know, why especially the last two lines that set some values to zero were necessary initially? These lead to the slightly lower dx at the boundary of the quadrant (-> the center of each face) and thereby smaller grid cell size.
> 
> 
> Thanks,
> 
> Nils
> 
> 
> 
> <cs-510-ra-new.png><cs-510-ra-old.png>_______________________________________________
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