Add an iris-esmf-regrid based regridding scheme

[bouweandela]

Description

Add an iris-esmf-regrid-based regridding scheme and use it as the default scheme for irregular grids and meshes

Example usage (as generic scheme) in preprocessor in recipe:

regrid:
  target_grid: 1x1
  scheme:
    reference: esmvalcore.preprocessor.regrid_schemes:IrisESMFRegrid
    method: bilinear
    mdtol: 0.7

Example plots for regridding 2D variable tos to a 1x1 degree grid are available here.
Example plots for regridding 3D variable thetao to a 1x1 degree grid and subsequently extracting the 500m depth level are available here.

Closes #2405.

Links to documentation:

• Regridding (interpolation, extrapolation) schemes
• esmvalcore.preprocessor.regrid_schemes.IrisESMFRegrid

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.72%. Comparing base (343368e) to head (a848468).
Report is 1 commits behind head on main.

Additional details and impacted files

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[schlunma]

Looks very good already Bouwe, it's great to see our custom ESMF code being replaced. Some additional comments:

1. In the PR description, please replace "unstructured" with "irregular" (you are replacing the schemes for irregular)
2. Would it make sense to deprecate our own ESMFPy... schemes? I don't think they are really useful anymore given that we have a much better solution for this now.
3. Would it maybe even make sense and go one step further by also replacing the schemes for regular regridding? This would make our code even easier. I think esmf-regrid also supports that kind of regridding with the same schemes. In principle, it would even work for unstructured grids, but in this case the data would need to be UGRID-compliant, and I am not sure we can enforce that (yet).

[bouweandela]

Thanks for reviewing @schlunma!

In the PR description, please replace "unstructured" with "irregular" (you are replacing the schemes for irregular)

I mixed those two up, it should be better now.

Would it make sense to deprecate our own ESMFPy... schemes? I don't think they are really useful anymore given that we have a much better solution for this now.

Yes, but maybe with the next release of iris-esmf-regrid where the nearest scheme is also supported. @stephenworsley mentioned that he expects to sort out SciTools-incubator/iris-esmf-regrid#276 and SciTools-incubator/iris-esmf-regrid#369 and make a new release soon.

Would it maybe even make sense and go one step further by also replacing the schemes for regular regridding? This would make our code even easier. I think esmf-regrid also supports that kind of regridding with the same schemes. In principle, it would even work for unstructured grids, but in this case the data would need to be UGRID-compliant, and I am not sure we can enforce that (yet).

I like this idea, but there is an issue with ESMF that prevents parallelisation using Dask (see #2316). I have reported it to the ESMF developers and they said they saw opportunities to solve it, but I'm not sure when this will be solved. I would be keen to improve our parallisation scheme for those cases where ESMF is not needed and then it is relevant to only use it when is really needed.

[schlunma]

Thanks for addressing my comments @bouweandela! Could you please add some documentation about the new schemes for meshes? It would also be great to use a common naming convention for this, e.g., at the moment, the documentation only talks about UGRID instead of iris meshes (in the ICON doc). Maybe you could move that part to regridding docs and only reference that in the ICON-specific section?

[bouweandela]

Would it make sense to deprecate our own ESMFPy... schemes? I don't think they are really useful anymore given that we have a much better solution for this now.

Done in 9aaf1b2

[bouweandela]

@schlunma I've improved the documentation and testing and also added support for lazy nearest neighbour regridding now because this is at least partly supported with the new iris-esmf-regrid v0.11. Could you do another review, please?

[schlunma]

Great! I just tested this extensively with all different grid types, and everything works as excepted 🚀

Just have a couple of minor comments on the doc. Cheers!

[bouweandela]

Thanks for reviewing @schlunma. I think I have addressed all comments now, could you have another look please?

[schlunma]

Awesome, thanks @bouweandela! This will facilitate regridding enormously 🚀