Add mesh fix for CMIP6 ICON-ESM-LR conservative regridding

[alioacar]

Description

Closes #3055

This PR fixes conservative regridding for CMIP6 ICON-ESM-LR data on its native unstructured grid. Some ICON-ESM-LR variables provide 1D latitude and longitude auxiliary coordinates with polygon bounds. When these variables are used with an ESMF conservative regridding scheme, esmf_regrid treats the source grid as a rectilinear grid and fails.

This follows the same principle as the native ICON fix: avoid Iris MeshXY.from_coords because it would create duplicate mesh nodes from shared polygon vertices. CMIP6 ICON-ESM-LR data do not provide the ICON vertex_of_cell connectivity, so the connectivity is reconstructed from the latitude and longitude polygon bounds instead.

I tested this with a reduced model only version of ref/recipe_ref_cre.yml

Model only test recipe

documentation:
  title: ICON-ESM-LR cloud radiative effect regrid test
  description: >
    Reduced model-only version of ref/recipe_ref_cre.yml for testing
    conservative regridding of derived ICON-ESM-LR cloud radiative effects.
  authors:
    - bock_lisa
    - lauer_axel
  maintainer:
    - bock_lisa

datasets:
  - project: CMIP6
    activity: CMIP
    dataset: ICON-ESM-LR
    ensemble: r1i1p1f1
    institute: MPI-M
    exp: historical
    grid: gn
    mip: Amon

preprocessors:
  full_climatology:
    climate_statistics:
      period: full
    regrid:
      target_grid: 1x1
      scheme:
        reference: esmf_regrid.schemes:ESMFAreaWeighted
  zonal_mean:
    climate_statistics:
      period: full
    regrid:
      scheme:
        reference: esmf_regrid.schemes:ESMFAreaWeighted
      target_grid: 1x1
    zonal_statistics:
      operator: mean

diagnostics:
  plot_maps:
    description: Plot ICON-ESM-LR lwcre and swcre climatology maps.
    variables:
      lwcre:
        timerange: 1996/1996
        mip: Amon
        preprocessor: full_climatology
        derive: true
      swcre:
        timerange: 1996/1996
        mip: Amon
        preprocessor: full_climatology
        derive: true
    scripts:
      plot:
        script: monitor/multi_datasets.py
        plot_folder: "{plot_dir}"
        plot_filename: "{plot_type}_{real_name}_{dataset}_{mip}"
        plots:
          map:
            common_cbar: true
            x_pos_stats_avg: -0.1
            fontsize: 10

  plot_profiles:
    description: Plot ICON-ESM-LR lwcre and swcre zonal means.
    variables:
      lwcre:
        timerange: 1996/1996
        mip: Amon
        preprocessor: zonal_mean
        derive: true
      swcre:
        timerange: 1996/1996
        mip: Amon
        preprocessor: zonal_mean
        derive: true
    scripts:
      plot:
        script: monitor/multi_datasets.py
        plot_folder: "{plot_dir}"
        plot_filename: "{plot_type}_{real_name}_{dataset}_{mip}"
        plots:
          variable_vs_lat:
            legend_kwargs:
              loc: upper right

Link to documentation:

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[schlunma]

Thanks @alioacar for your contribution, this looks really nice! I will take a look at this after my vacation by the end of June. Hope that works for you!

[alioacar]

Sure @schlunma, enjoy your vacation! 😊