Read derived variables from other MIP tables (#2256)

doc/quickstart/configure.rst

@@ -683,7 +683,8 @@ related to CMOR table settings available:
   extended with variables from the :ref:`custom_cmor_tables` (by default loaded
   from the ``esmvalcore/cmor/tables/custom`` directory) and it is possible to
   use variables with a ``mip`` which is different from the MIP table in which
-  they are defined.
+  they are defined. Note that this option is always enabled for
+  :ref:`derived <Variable derivation>` variables.
 * ``cmor_path``: path to the CMOR table.
   Relative paths are with respect to `esmvalcore/cmor/tables`_.
   Defaults to the value provided in ``cmor_type`` written in lower case.

doc/recipe/preprocessor.rst

@@ -201,13 +201,15 @@ the ozone 3D field. In this case, a derivation function is provided to
 vertically integrate the ozone and obtain total column ozone for direct
 comparison with the observations.
 
-To contribute a new derived variable, it is also necessary to define a name for
-it and to provide the corresponding CMOR table. This is to guarantee the proper
-metadata definition is attached to the derived data. Such custom CMOR tables
-are collected as part of the `ESMValCore package
-<https://github.com/ESMValGroup/ESMValCore>`_. By default, the variable
-derivation will be applied only if the variable is not already available in the
-input data, but the derivation can be forced by setting the appropriate flag.
+The tool will also look in other ``mip`` tables for the same ``project`` to find
+the definition of derived variables. To contribute a completely new derived
+variable, it is necessary to define a name for it and to provide the
+corresponding CMOR table. This is to guarantee the proper metadata definition
+is attached to the derived data. Such custom CMOR tables are collected as part
+of the `ESMValCore package <https://github.com/ESMValGroup/ESMValCore/tree/main/esmvalcore/cmor/tables/custom>`_.
+By default, the variable derivation will be applied only if the variable is not
+already available in the input data, but the derivation can be forced by
+setting the ``force_derivation`` flag.
 
 .. code-block:: yaml
 

esmvalcore/cmor/table.py

@@ -296,8 +296,8 @@ def get_variable(
                     pass
 
         # If that didn't work, look in all tables (i.e., other MIPs) if
-        # cmor_strict=False
-        var_info = self._look_in_all_tables(alt_names_list)
+        # cmor_strict=False or derived=True
+        var_info = self._look_in_all_tables(derived, alt_names_list)
 
         # If that didn' work either, look in default table if cmor_strict=False
         # or derived=True
@@ -324,10 +324,10 @@ def _look_in_default(self, derived, alt_names_list, table_name):
                     break
         return var_info
 
-    def _look_in_all_tables(self, alt_names_list):
+    def _look_in_all_tables(self, derived, alt_names_list):
         """Look for variable in all tables."""
         var_info = None
-        if not self.strict:
+        if (not self.strict or derived):
             for alt_names in alt_names_list:
                 var_info = self._look_all_tables(alt_names)
                 if var_info:

tests/integration/cmor/test_table.py

@@ -105,6 +105,11 @@ def test_get_variable_from_alt_names(self):
         var = self.variables_info.get_variable('SImon', 'sic')
         self.assertEqual(var.short_name, 'siconc')
 
+    def test_get_variable_derived(self):
+        """Test that derived variable are looked up from other MIP tables."""
+        var = self.variables_info.get_variable('3hr', 'sfcWind', derived=True)
+        self.assertEqual(var.short_name, 'sfcWind')
+
     def test_get_variable_from_custom(self):
         """Get a variable from default."""
         self.variables_info.strict = False