Fix ERA5 native6 fix to handle single monthly-averaged NetCDF files.

esmvalcore/cmor/_fixes/native6/era5.py

@@ -22,15 +22,18 @@
         return 'fx'
 
     time.convert_units('days since 1850-1-1 00:00:00.0')
-    if len(time.points) == 1:
-        if cube.long_name != 'Geopotential':
-            raise ValueError('Unable to infer frequency of cube '
-                             f'with length 1 time dimension: {cube}')
+
+    if (len(time.points) == 1 and cube.long_name == 'Geopotential'):
         return 'fx'
 
-    interval = time.points[1] - time.points[0]
-    if interval - 1 / 24 < 1e-4:
-        return 'hourly'
+    if len(time.points) > 1:
+        interval = time.points[1] - time.points[0]
+        if interval - 1 / 24 < 1e-4:
+            return 'hourly'
+    else:
+        logger.warning("Unable to infer frequency of cube "
+                       f"with length 1 time dimension: {cube},"
+                       "assuming 'monthly' frequency")
 
     return 'monthly'
 

tests/integration/cmor/_fixes/native6/test_era5.py

@@ -86,8 +86,7 @@ def test_get_frequency_fx():
     )
     assert get_frequency(cube) == 'fx'
     cube.long_name = 'Not geopotential'
-    with pytest.raises(ValueError):
-        get_frequency(cube)
+    assert get_frequency(cube) == 'monthly'
 
 
 def _era5_latitude():