Improve fieldset error message on missing axis metadata (#2345)

Author: VeckoTheGecko

src/parcels/_core/fieldset.py

@@ -207,7 +207,10 @@ def from_copernicusmarine(ds: xr.Dataset):
         expected_axes = set("XYZT")  # TODO: Update after we have support for 2D spatial fields
         if missing_axes := (expected_axes - set(ds.cf.axes)):
             raise ValueError(
-                f"Dataset missing axes {missing_axes} to have coordinates for all {expected_axes} axes according to CF conventions."
+                f"Dataset missing CF compliant metadata for axes "
+                f"{missing_axes}. Expected 'axis' attribute to be set "
+                f"on all dimension axes {expected_axes}. "
+                "HINT: Add xarray metadata attribute 'axis' to dimension - e.g., ds['lat'].attrs['axis'] = 'Y'"
             )
 
         ds = _rename_coords_copernicusmarine(ds)

tests/test_fieldset.py

@@ -243,6 +243,20 @@ def test_fieldset_from_copernicusmarine(ds, caplog):
     assert "renamed it to 'V'" in caplog.text
 
 
+@pytest.mark.parametrize("ds", [datasets_circulation_models["ds_copernicusmarine"].copy()])
+def test_fieldset_from_copernicusmarine_missing_axis(ds, caplog):
+    del ds["latitude"].attrs["axis"]
+
+    with pytest.raises(
+        ValueError,
+        match="Dataset missing CF compliant metadata for axes "
+        ".*. Expected 'axis' attribute to be set "
+        "on all dimension axes .*. "
+        "HINT: Add xarray metadata attribute 'axis' to dimension .*",
+    ):
+        FieldSet.from_copernicusmarine(ds)
+
+
 def test_fieldset_from_copernicusmarine_no_currents(caplog):
     ds = datasets_circulation_models["ds_copernicusmarine"].cf.drop_vars(
         ["eastward_sea_water_velocity", "northward_sea_water_velocity"]