Follow CF convention for temperature difference (#429)

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### Pull Request Checklist:
- [x] This PR addresses an already opened issue (for bug fixes /
features)
    - This PR fixes #428
- [ ] (If applicable) Documentation has been added / updated (for bug
fixes / features).
- [x] (If applicable) Tests have been added.
- [x] This PR does not seem to break the templates.
- [x] CHANGELOG.rst has been updated (with summary of main changes).
- [x] Link to issue (:issue:`number`) and pull request (:pull:`number`)
has been added.

### What kind of change does this PR introduce?

* Add unit_metadata for the dtr conversion to be clear that this is a
difference and not convert the units in a non-sensical way.
* Added unit update for delta. If it's a temperature xclim will add the
unit_metadata. Do we want this for the climatological delta ?

### Does this PR introduce a breaking change?
yes, changes the attrs

### Other information:
* this needs to wait for the next xclim release for the new functions. 
* Should we add `temperature: on_scale` for `tasmax_from_dtr` and
`tasmin_from_dtr` or is it unecessary?
* Is it necessary somewhere else ?

CHANGELOG.rst

@@ -26,6 +26,7 @@ Bug fixes
 
 Internal changes
 ^^^^^^^^^^^^^^^^
+* Include CF convention for temperature differences and on scale (:pull:`428`, :issue:`428`).
 * Bumped the version of `xclim` to 0.53.2. (:pull:`482`).
 
 v0.10.0 (2024-09-30)

src/xscen/aggregate.py

@@ -23,6 +23,7 @@
 except ImportError:
     xe = None
 from xclim.core.indicator import Indicator
+from xclim.core.units import pint2cfattrs, units2pint
 
 from .config import parse_config
 from .extract import subset_warming_level
@@ -565,6 +566,10 @@ def compute_deltas(  # noqa: C901
             if (isinstance(kind, dict) and kind[vv] == "+") or kind == "+":
                 _kind = "abs."
                 deltas[v_name] = other_hz[vv] - ref[vv]
+                unit = pint2cfattrs(
+                    units2pint(other_hz[vv].attrs["units"]), is_difference=True
+                )
+                deltas[v_name].attrs.update(unit)
             elif (isinstance(kind, dict) and kind[vv] == "/") or kind == "/":
                 _kind = "rel."
                 deltas[v_name] = other_hz[vv] / ref[vv]

src/xscen/xclim_modules/conversions.py

@@ -99,12 +99,11 @@ def dtr_from_minmax(tasmin: xr.DataArray, tasmax: xr.DataArray) -> xr.DataArray:
 
     Returns
     -------
-    xr.DataArray, K
+    xr.DataArray
       Daily temperature range
     """
-    # We force K to overcome the "delta degree" ambiguity
-    tasmin = convert_units_to(tasmin, "K")
-    tasmax = convert_units_to(tasmax, "K")
+    tasmin = convert_units_to(tasmin, tasmax)
     dtr = tasmax - tasmin
-    dtr.attrs["units"] = "K"
+    dtr.attrs["units"] = tasmin.attrs["units"]
+    dtr.attrs["units_metadata"] = "temperature: difference"
     return dtr

tests/test_aggregate.py

@@ -75,6 +75,8 @@ def test_options(self, kind, rename_variables, to_level):
         assert deltas[variable].attrs["delta_kind"] == delta_kind
         assert deltas[variable].attrs["delta_reference"] == "1981-2010"
         assert deltas[variable].attrs["units"] == units
+        if kind == "+":
+            assert deltas[variable].attrs["units_metadata"] == "temperature: difference"
         np.testing.assert_array_equal(deltas[variable], results)
 
     @pytest.mark.parametrize("cal", ["proleptic_gregorian", "noleap", "360_day"])

tests/test_xclimmod_conversions.py

@@ -57,5 +57,6 @@ def test_dtr():
     )
 
     dtr = conv.dtr_from_minmax(tasmin, tasmax)
-    assert dtr.attrs["units"] == "K"
+    assert dtr.attrs["units"] == "°C"
+    assert dtr.attrs["units_metadata"] == "temperature: difference"
     np.testing.assert_array_equal(dtr, (tasmax + 273.15) - tasmin)