Dear all,

I am currently trying to set up a grid for a nested model using the NEMOGCM/TOOLS/NESTING routines. The grid of the parent model has curvilinear coordinates and the i,j axis are rotated by an angle from geographical north.

Running create_coordinates.exe with following definitions in the namelist.input

&coarse_grid_files
    parent_coordinate_file = '/path/to/parent/model/coordinates.nc'
    parent_meshmask_file   = '/path/to/parent/model/mesh_mask.nc'
/
&nesting
    imin = 259
    imax = 310
    jmin = 183
    jmax = 202
    rho  = 4
    rhot = 2
/
&nemo_coarse_grid
    jpizoom = 1
    jpjzoom = 1
/

creates a new coordinates.nc file with the desired smaller grid spacing, however, I have noticed some issues - in particular with regards to the grid staggering. The problem is illustrated in attached figure.

• The first thing I was wondering about is why the T-points of the parent grid (red dots) do not coincide with every fourth T-point of the nested grid (blue x)?

• The more striking feature is that the staggering of the nested grid does not take into account the grid rotation which means that T-points (blue) and U-points (yellow) share the same latitude and T-points (blue) and V-points (green) share the same longitudes.

My question is if I have overlooked something of if rotated grid are currently not supported in the nesting tools?

Any input on that is much appreciated.

Thanks, Christoph