larger than expected diffs in cice tx0.661 mesh versus internal grid

[mvertens]

I have turned on actual error trapping rather than warning to compare the input meshe lat/lon values  to the internally generated values in cice5. I am finding some differences that are much bigger than I would expect for the tx0.66v1 mesh. These errors were present before - but only warnings were printed out. Now the model actually stops.

The mesh file 
/glade/p/cesmdata/cseg/inputdata/share/meshes/tx0.66v1_190314_ESMFmesh.nc
was generated from the following SCRIP input file:   /glade/p/cesmdata/cseg/mapping/grids/tx0.66v1_SCRIP_190314.nc 
which @alperaltuntas created in March of 2019.

The internal ice grid is
/glade/p/cesmdata/cseg/inputdata/ocn/mom/tx0.66v1/grid/horiz_grid_20190315.ieeer8

Errors of the following type are seen

ERROR: CICE n, lonmesh, lon, diff_lon =    352    140.2982497816467       140.2993809030784             0.11311D-02
ERROR: CICE n, lonmesh, lon, diff_lon =    353    143.3065378697909       143.3075936600567             0.10558D-02
ERROR: CICE n, lonmesh, lon, diff_lon =    390    110.5901027413569       110.5911242235564             0.10215D-02

I am not seeing any differences in latitude that are trapped - just in longitude.
This same type of problem I expect would also be in cice6 - but needs to be verified.
For now - I can turn off the aborts but we really need to resolve this as an issue.

[dabail10]

Is CICE not reading in the same mesh file? CICE should be reading in the corner lat/lon (B-grid velocity points) and then does an internal interpolation using the SCRIP methods to go to the gridcell centers (T points). The lat/lon values are compared between the sea ice and ocean at the gridcell centers in the mediator. Does MOM use a slightly different method for finding the grid cell centers? Should we just be reading the center lat/lon values from the mesh file?

[mvertens]

@dabail10 @alperaltuntas - only the center lats/lons are read in from the mesh for both cice and mom. There is no comparison in the mediator (this was the case in cpl7 - but in cmeps its done in the component caps). So maybe its the interpolation to cell centers that is giving the diffs - but there are no diffs that are triggered for the gx1v7 mesh. Its just that these diffs for tx0.661v1 are larger than I would have expected.

[mvertens]

Also - the cice lats and lons from the mesh are not really used I believe. Its only a sanity check. The internal lats and lons are used to obtain the solar zenith angle from the calendar day that is sent by the mediatory (from the atm input into the mediator).
The areas from the mesh are used to carry out conservative first order interpolation of cice states and fluxes from cice to the atm.

[mvertens]

@dabail10 @alperaltuntas - What is the status of this issue?

[dabail10]

Still in progress. I haven't had time to look at it.

[mvertens]

@dabail10 - thanks for the update.