regridding NEMO outputs with UGRID

[jupalm]

Hi,

I am trying to regrid NEMO outputs (irregular grid) on a regular grid to make some nice zonal average plots with iris.
i manage to use the normal cube.regrid(reg_cube, iris.analysis.PointInCell())
Which works well almost everywhere but at high latitude, where there is a lot of missing data.

So i ended up trying to use iris-esmf-regrid which sounds like exactly what i need, all functions to regrid from unstructured to regular grid are there.
But the first answer i got when trying is that my loaded cube has no mesh
so i tried loading the cube as said in the doc :
using the

from iris.experimental.ugrid import PARSE_UGRID_ON_LOAD

with PARSE_UGRID_ON_LOAD.context():
            cube = iris.load_cube(file_name) 

but here again, iris struggles to get the mesh from the netcdf file.
i tried to use the NEMO mesh file from the same run without much success.

In the end, i am not sure if this UGRID functions can be used with NEMO nc files,
Could you explain if possible, how to use these functions to read a NEMO nc files and get a cube with a mesh that the regrid functions would be happy with.
It looks like the step i miss to get the whole thing running.

Huge thanks in advance for your help!

Best regards,

Julien.

[trexfeathers]

Hi @jupalm, thanks for getting in touch!

Here are the regridders that can take curvilinear grids (like NEMO/ORCA) as inputs:

• iris.analysis.PointInCell - handles anything 'unstructured', including curvilinear grids
• iris.analysis.UnstructuredNearest - handles anything 'unstructured', including curvilinear grids
• esmf_regrid.schemes.ESMFAreaWeighted - handles rectilinear or curvilinear grids

A mesh, usually under the UGRID specification, is a different concept to a curvilinear grid - you do not need to use PARSE_UGRID_ON_LOAD for any of the regridders above; they will all operate on your Cube when loaded normally.

Mesh handling

iris-esmf-regrid also has a mesh-specific regridder - esmf_regrid.experimental.unstructured_scheme.MeshToGridESMFRegridder - which may be worth trying to see if the results are better for you. To try this out, you need to first convert your ORCA grid into a UGRID mesh format - see here. Once you have a 'mesh-Cube', see here for instructions on regridding (I think you might have seen this already).

[jupalm]

Thanks!!
That's super useful!
I only saw the PointInCell method as the documentation on the old experimental tools only point to this one here
The explanations on how to convert the ORCA file are brilliant exactly what i needed.
Might be worth doing a conversion toolbox (maybe in iris-esmf-regrid) that could be useful.

Thanks again for your help!