remove remaining attrs in atmophere data (#468)

Author: rhaegar325

src/access_moppy/atmosphere.py

@@ -335,6 +335,16 @@ def update_attributes(self):
         if not cmor_attrs.get("positive"):
             self.ds[self.cmor_name].attrs.pop("positive", None)
 
+        # Drop model-native attributes inherited from the source variable via the
+        # rename above that have no place in CMIP6 output:
+        #  - grid_mapping: a regular lat-lon CMIP6 grid carries none, and its
+        #    container variable is not carried into the output, so the attribute
+        #    is a dangling reference that fails the CF grid-mapping check.
+        #  - um_stash_source: a UM STASH provenance code, absent from the
+        #    published reference.
+        self.ds[self.cmor_name].attrs.pop("grid_mapping", None)
+        self.ds[self.cmor_name].attrs.pop("um_stash_source", None)
+
         var_type = cmor_attrs.get("type", "double")
         self.ds[self.cmor_name] = self.ds[self.cmor_name].astype(
             self.type_mapping.get(var_type, np.float64)

tests/unit/test_atmosphere.py

@@ -820,6 +820,22 @@ def test_datetime64_time_not_cast_to_float(self):
 
         assert np.issubdtype(cmoriser.ds["time"].dtype, np.datetime64)
 
+    @pytest.mark.unit
+    def test_model_native_attributes_dropped(self):
+        """Model-native attributes inherited via rename (grid_mapping with no
+        container variable, um_stash_source) must be dropped from the output."""
+        cf_time = xr.cftime_range(
+            "2020-01-31", periods=1, freq="ME", calendar="gregorian"
+        )
+        cmoriser = _make_cmoriser_for_update_attributes(cf_time)
+        cmoriser.ds["tasmax"].attrs["grid_mapping"] = "latitude_longitude"
+        cmoriser.ds["tasmax"].attrs["um_stash_source"] = "m01s03i236"
+
+        cmoriser.update_attributes()
+
+        assert "grid_mapping" not in cmoriser.ds["tasmax"].attrs
+        assert "um_stash_source" not in cmoriser.ds["tasmax"].attrs
+
     @pytest.mark.unit
     def test_numeric_time_is_cast_to_float(self):
         """Numeric (float64) time IS cast according to the type mapping."""