Copy additional coordinates when regridding

[bouweandela]

Description

The regrid preprocessor function regrids one set of coordinates to another. However, CORDEX data typically has two sets of coordinates: rotated_latitude/rotated_longitude or projection_y_coordinate/projection_x_coordinate as a iris.coords.DimCoord and latitude/longitude as an additional iris.coords.AuxCoord. The changes in this pull request ensure that if the target grid has multiple coordinates, these will be available on the regridded cube, even if only one set is used for the regridding.

Closes #2839

Link to documentation:

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✅ All modified and coverable lines are covered by tests.
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[bouweandela]

For testing purposes, here are four types of grids we commonly encounter:

In [1]: from esmvalcore.dataset import Dataset
   ...: from esmvalcore.preprocessor import regrid

In [2]: cordex_lambert_conformal = Dataset(
   ...:     project="CORDEX",
   ...:     mip="mon",
   ...:     short_name="tas",
   ...:     dataset="RegCM4-6",
   ...:     ensemble="r1i1p1",
   ...:     driver="CNRM-CERFACS-CNRM-CM5",
   ...:     institute="ICTP",
   ...:     domain="EUR-11",
   ...:     exp="historical",
   ...:     rcm_version="v2",
   ...:     timerange="2001/2005",
   ...:     use_standard_grid=True,
   ...: ).load()
   ...: 
   ...: cordex_rotated_pole = Dataset(
   ...:     project="CORDEX",
   ...:     mip="day",
   ...:     short_name="ts",
   ...:     dataset="HIRHAM5",
   ...:     ensemble="r1i1p1",
   ...:     driver="IPSL-IPSL-CM5A-MR",
   ...:     institute="DMI",
   ...:     domain="EUR-11",
   ...:     exp="historical",
   ...:     rcm_version="v1",
   ...:     timerange="2001/2005",
   ...: ).load()
   ...: 
   ...: tas = Dataset(
   ...:     project="CMIP6",
   ...:     mip="Amon",
   ...:     short_name="tas",
   ...:     dataset="CanESM5",
   ...:     ensemble="r1i1p1f1",
   ...:     exp="historical",
   ...:     grid="gn",
   ...:     timerange="2001/2005",
   ...: ).load()
   ...: 
   ...: tos = Dataset(
   ...:     project="CMIP6",
   ...:     mip="Omon",
   ...:     short_name="tos",
   ...:     dataset="CanESM5",
   ...:     ensemble="r1i1p1f1",
   ...:     exp="historical",
   ...:     grid="gn",
   ...:     timerange="2001/2005",
   ...: ).load()
   ...: 
WARNING:esmvalcore.cmor._fixes.cordex.cordex_fixes:Support for CORDEX datasets in a Lambert Conformal coordinate system is ongoing. Certain preprocessor functions may fail.
WARNING:esmvalcore.cmor._fixes.cordex.cordex_fixes:Maximum difference between original projection_x_coordinate points and standard EUR-11 domain points for variable tas from dataset RegCM4-6 is 131500.0 m. Consider visualizing the data on a map and comparing with recognizable features such as coastlines to check that the grid is correct.
WARNING:esmvalcore.cmor._fixes.cordex.cordex_fixes:Maximum difference between original projection_y_coordinate points and standard EUR-11 domain points for variable tas from dataset RegCM4-6 is 131500.0 m. Consider visualizing the data on a map and comparing with recognizable features such as coastlines to check that the grid is correct.
WARNING:esmvalcore.cmor._fixes.cordex.cordex_fixes:Maximum difference between original latitude points and standard EUR-11 domain points for variable tas from dataset RegCM4-6 is 1.331701984850012 degrees_north. Consider visualizing the data on a map and comparing with recognizable features such as coastlines to check that the grid is correct.
WARNING:esmvalcore.cmor._fixes.cordex.cordex_fixes:Maximum difference between original longitude points and standard EUR-11 domain points for variable tas from dataset RegCM4-6 is 359.7994925609495 degrees_east. Consider visualizing the data on a map and comparing with recognizable features such as coastlines to check that the grid is correct.
/home/bandela/src/esmvalgroup/esmvalcore/.pixi/envs/default/lib/python3.14/site-packages/intake_esgf/catalog.py:73: UserWarning: You have enabled at least one index which uses the old Solr technology. ESGF is moving away from this technology and you may find that some indicesfail to return a response.
  warnings.warn(
/home/bandela/src/esmvalgroup/esmvalcore/.pixi/envs/default/lib/python3.14/site-packages/intake_esgf/catalog.py:73: UserWarning: You have enabled at least one index which uses the old Solr technology. ESGF is moving away from this technology and you may find that some indicesfail to return a response.
  warnings.warn(

Note that with this branch, the regridded data has all horizontal coordinates of the target grid, instead of a subset:

In [3]: regrid(tos, target_grid=cordex_rotated_pole, scheme="linear").coords()
Out[3]: 
[<DimCoord: time / (days since 1850-1-1 00:00:00)  [...]+bounds  shape(60,)>,
 <DimCoord: grid_latitude / (degrees)  [-23.375, ...]+bounds  shape(412,)>,
 <DimCoord: grid_longitude / (degrees)  [-28.375, ...]+bounds  shape(424,)>,
 <AuxCoord: latitude / (degrees_north)  [...]+bounds  shape(412, 424)>,
 <AuxCoord: longitude / (degrees_east)  [...]+bounds  shape(412, 424)>]

In [4]: regrid(cordex_rotated_pole, target_grid=tos, scheme="linear").coords()
Out[4]: 
[<DimCoord: time / (days since 1850-1-1 00:00:00)  [...]+bounds  shape(1825,)>,
 <DimCoord: cell index along second dimension / (1)  [ 0, 1, ..., 289, 290]  shape(291,)>,
 <DimCoord: cell index along first dimension / (1)  [ 0, 1, ..., 358, 359]  shape(360,)>,
 <AuxCoord: latitude / (degrees_north)  [...]+bounds  shape(291, 360)>,
 <AuxCoord: longitude / (degrees_east)  [[73.5, 74.5, ..., 71.5, 72.5], ...]+bounds  shape(291, 360)>]

while with the current main branch the result is:

In [3]: regrid(tos, target_grid=cordex_rotated_pole, scheme="linear").coords()
Out[3]: 
[<DimCoord: time / (days since 1850-1-1 00:00:00)  [...]+bounds  shape(60,)>,
 <DimCoord: grid_latitude / (degrees)  [-23.375, ...]+bounds  shape(412,)>,
 <DimCoord: grid_longitude / (degrees)  [-28.375, ...]+bounds  shape(424,)>]

In [4]: regrid(cordex_rotated_pole, target_grid=tos, scheme="linear").coords()
Out[4]: 
[<DimCoord: time / (days since 1850-1-1 00:00:00)  [...]+bounds  shape(1825,)>,
 <AuxCoord: latitude / (degrees_north)  [...]+bounds  shape(291, 360)>,
 <AuxCoord: longitude / (degrees_east)  [[73.5, 74.5, ..., 71.5, 72.5], ...]+bounds  shape(291, 360)>]

[flicj191]

Thanks, I've tested this in a recipe regridding to a tos dataset and its working, makes sense to copy the dim coords.

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[bouweandela]

Thank you for testing and reviewing @flicj191 and @rbeucher ✨