Fix unstructured mesh handling

Author: alioacar

esmvalcore/cmor/_fixes/cmip6/icon_esm_lr.py

@@ -1,11 +1,76 @@
 """Fixes for ICON-ESM-LR model."""
 
+from iris.mesh import MeshXY
+
 from esmvalcore.cmor._fixes.fix import Fix
+from esmvalcore.iris_helpers import has_unstructured_grid
 
 
 class AllVars(Fix):
     """Fixes for all variables."""
 
+    @staticmethod
+    def _add_mesh_from_vertices(cube, lat_vertices, lon_vertices):
+        """Add Iris mesh from cell vertices if this is an unstructured grid."""
+        # skip if the mesh topology is already provided
+        if cube.mesh is not None:
+            return False
+        # only create meshes for 1D unstructured grids.
+        if not has_unstructured_grid(cube):
+            return False
+
+        lat = cube.coord("latitude")
+        lon = cube.coord("longitude")
+        lat_dims = cube.coord_dims(lat)
+        lon_dims = cube.coord_dims(lon)
+
+        # check the dims
+        if lat_dims != lon_dims or len(lat_dims) != 1:
+            return False
+
+        # at least triangular cell bounds
+        if (
+            lat_vertices.shape != lon_vertices.shape
+            or lat_vertices.ndim != 2
+            or lat_vertices.shape[0] != lat.shape[0]
+            or lat_vertices.shape[1] < 3
+        ):
+            return False
+
+        lat.bounds = lat_vertices
+        lon.bounds = lon_vertices
+
+        mesh = MeshXY.from_coords(lon, lat)
+        # replace aux coords with mesh aware ones
+        cube.remove_coord("latitude")
+        cube.remove_coord("longitude")
+        for mesh_coord in mesh.to_MeshCoords("face"):
+            cube.add_aux_coord(mesh_coord, lat_dims)
+
+        return True
+
+    @classmethod
+    def _add_mesh_from_coord_bounds(cls, cube):
+        """Add Iris mesh from existing coordinate bounds if possible."""
+        if not cube.coords("latitude") or not cube.coords("longitude"):
+            return False
+
+        lat = cube.coord("latitude")
+        lon = cube.coord("longitude")
+        if not lat.has_bounds() or not lon.has_bounds():
+            return False
+
+        return cls._add_mesh_from_vertices(
+            cube,
+            lat.core_bounds(),
+            lon.core_bounds(),
+        )
+
+    def fix_data(self, cube):
+        """Add Iris mesh for unstructured data if possible."""
+        self._add_mesh_from_coord_bounds(cube)
+        return cube
+
     def fix_metadata(self, cubes):
         """Rename ``var_name`` of latitude and longitude.
 

tests/integration/cmor/_fixes/cmip6/test_icon_esm_lr.py

@@ -95,3 +95,43 @@ def test_allvars_fix_metadata_no_lat_lon(cubes):
     fix = AllVars(None)
     out_cubes = fix.fix_metadata(cubes)
     assert cubes is out_cubes
+
+
+def test_allvars_fix_data_adds_mesh_from_bounds():
+    """Test ``fix_metadata`` adds a mesh for unstructured ICON data."""
+    lat_coord = AuxCoord(
+        [0.0, 1.0],
+        bounds=[[0.0, 0.0, 1.0], [0.0, 1.0, 1.0]],
+        var_name="latitude",
+        standard_name="latitude",
+        units="degrees_north",
+    )
+    lon_coord = AuxCoord(
+        [0.0, 1.0],
+        bounds=[[0.0, 1.0, 0.0], [1.0, 1.0, 2.0]],
+        var_name="longitude",
+        standard_name="longitude",
+        units="degrees_east",
+    )
+    cube = Cube(
+        [10.0, 20.0],
+        var_name="tas",
+        aux_coords_and_dims=[(lat_coord, 0), (lon_coord, 0)],
+        attributes={"grid_type": "unstructured"},
+    )
+    cubes = CubeList([cube])
+    fix = AllVars(None)
+
+    out_cubes = fix.fix_metadata(cubes)
+
+    assert cubes is out_cubes
+    assert cube.mesh is None
+    assert cube.coord("latitude").var_name == "lat"
+    assert cube.coord("longitude").var_name == "lon"
+
+    out_cube = fix.fix_data(cube)
+
+    assert out_cube is cube
+    assert cube.mesh is not None
+    assert cube.coord("latitude").mesh is cube.mesh
+    assert cube.coord("longitude").mesh is cube.mesh