Always write target coordinates to source cube after regridding (#2673)

Author: schlunma

esmvalcore/preprocessor/_regrid.py

@@ -876,12 +876,14 @@ def regrid(
     if cube.coords("latitude") and cube.coords("longitude"):
         if _horizontal_grid_is_close(cube, target_grid_cube):
             for coord in ["latitude", "longitude"]:
-                cube.coord(coord).points = target_grid_cube.coord(
-                    coord
-                ).core_points()
-                cube.coord(coord).bounds = target_grid_cube.coord(
-                    coord
-                ).core_bounds()
+                is_dim_coord = cube.coords(coord, dim_coords=True)
+                coord_dims = cube.coord_dims(coord)
+                cube.remove_coord(coord)
+                target_coord = target_grid_cube.coord(coord).copy()
+                if is_dim_coord:
+                    cube.add_dim_coord(target_coord, coord_dims)
+                else:
+                    cube.add_aux_coord(target_coord, coord_dims)
             return cube
 
     # Load scheme and reuse existing regridder if possible

tests/unit/preprocessor/_regrid/test_regrid.py

@@ -226,17 +226,48 @@ def test_horizontal_grid_is_close(cube2_spec: dict, expected: bool):
     assert _horizontal_grid_is_close(cube1, cube2) == expected
 
 
-def test_regrid_is_skipped_if_grids_are_the_same():
+def test_regrid_is_skipped_if_grids_are_the_same_dim_coord(mocker):
     """Test that regridding is skipped if the grids are the same."""
+    mock_get_regridder = mocker.patch(
+        "esmvalcore.preprocessor._regrid._get_regridder", autospec=True
+    )
+    cube = _make_cube(lat=LAT_SPEC1, lon=LON_SPEC1)
+
+    expected_same_cube = regrid(cube, target_grid="10x10", scheme="linear")
+
+    mock_get_regridder.assert_not_called()
+    np.testing.assert_equal(expected_same_cube.shape, cube.shape)
+    assert cube.coords("latitude", dim_coords=True)
+    assert cube.coords("longitude", dim_coords=True)
+
+
+def test_regrid_is_skipped_if_grids_are_the_same_aux_coord(mocker):
+    """Test that regridding is skipped if the grids are the same."""
+    mock_get_regridder = mocker.patch(
+        "esmvalcore.preprocessor._regrid._get_regridder", autospec=True
+    )
+    cube = _make_cube(lat=LAT_SPEC1, lon=LON_SPEC1)
+    lat = cube.coord("latitude")
+    lon = cube.coord("longitude")
+    cube.remove_coord(lat)
+    cube.remove_coord(lon)
+    cube.add_aux_coord(lat, 0)
+    cube.add_aux_coord(lon, 1)
+
+    expected_same_cube = regrid(cube, target_grid="10x10", scheme="linear")
+
+    mock_get_regridder.assert_not_called()
+    np.testing.assert_equal(expected_same_cube.shape, cube.shape)
+    assert cube.coords("latitude", dim_coords=False)
+    assert cube.coords("longitude", dim_coords=False)
+
+
+def test_regrid_is_not_skipped_if_grids_are_different():
+    """Test that regridding is not skipped if the grids are different."""
     cube = _make_cube(lat=LAT_SPEC1, lon=LON_SPEC1)
-    scheme = "linear"
 
-    # regridding to the same spec returns the same cube
-    expected_same_cube = regrid(cube, target_grid="10x10", scheme=scheme)
-    assert expected_same_cube == cube
+    expected_different_cube = regrid(cube, target_grid="5x5", scheme="linear")
 
-    # regridding to a different spec returns a different cube
-    expected_different_cube = regrid(cube, target_grid="5x5", scheme=scheme)
     assert expected_different_cube is not cube