ESMF regrid fails when there is more than one variable on an axis

[nhsavage]

🐛 Bug Report

When using Iris ESMF-regid to regrid data such as rotated pole data that includes auxiliary coords of true longitude two coords are returned not a single one

How To Reproduce

Steps to reproduce the behaviour:

1. load CORDEX data on rotated pole grid wtih lat and lon 2D coords e.g cube = iris.load_cube('pr_AFR-22_NCC-NorESM1-M_historical_r1i1p1_CLMcom-KIT-CCLM5-0-15_v1_day_19500101-19501231.nc')
2. outcube2 = cube.regrid(newcube, ESMFAreaWeighted())

Expected behaviour

the aux coord should be ignored in favour of the dim coord

Screenshots

Environment

• MO VDI RHEL7
• esmf_regrid version:'0.4.0'

Additional context

Click to expand this section...

We believe this can be resolved by having iris-esmf-regrid search Coords using dim_coord=True in these searches:

https://github.com/SciTools-incubator/iris-esmf-regrid/blob/7d6f568e8659109b5c8226f53dd27f5a5dd31f51/esmf_regrid/schemes.py#L184-L187

It should also fall back on the current implementation by using a try-except block.

The only scenario producing an error would then be no DimCoords and multiple AuxCoords on the same axis, which is truly ambiguous and would need the user to help by massaging the input.

See here for further details.

[sloosvel]

Example of the error message:

  File "/home/b/b381943/mambaforge/envs/coretool26rc4/lib/python3.10/site-packages/iris/cube.py", line 4359, in regrid
    regridder = scheme.regridder(self, grid)
  File "/home/b/b381943/mambaforge/envs/coretool26rc4/lib/python3.10/site-packages/esmf_regrid/schemes.py", line 338, in regridder
    return ESMFAreaWeightedRegridder(src_grid, tgt_grid, mdtol=self.mdtol)
  File "/home/b/b381943/mambaforge/envs/coretool26rc4/lib/python3.10/site-packages/esmf_regrid/schemes.py", line 367, in __init__
    regrid_info = _regrid_rectilinear_to_rectilinear__prepare(src_grid, tgt_grid)
  File "/home/b/b381943/mambaforge/envs/coretool26rc4/lib/python3.10/site-packages/esmf_regrid/schemes.py", line 186, in _regrid_rectilinear_to_rectilinear__prepare
    src_x = src_grid_cube.coord(axis="x")
  File "/home/b/b381943/mambaforge/envs/coretool26rc4/lib/python3.10/site-packages/iris/cube.py", line 1908, in coord
    raise iris.exceptions.CoordinateNotFoundError(emsg)
iris.exceptions.CoordinateNotFoundError: 'Expected to find exactly 1 coordinate, but found 2. They were: projection_x_coordinate, longitude.'

[stephenworsley]

This should be closed by #217