Problem with new multi-model statistics and variables on hybrid levels

[axel-lauer]

Variables on hybrid vertical levels (e.g. clw) have to be converted to pressure or height levels to be able to be processed by some diagnostics and to be able to calculate meaningful multi-model statistics across different models. For this conversion, some models provide an auxiliary coordinate 'p0'. Calculating the multi-model mean over such datasets converted to pressure or height levels fails because 'p0' is kept after converting the vertical levels but (as expected) not identical in all models:

iris.exceptions.MergeError: failed to merge into a single cube.
  Coordinates in cube.aux_coords (scalar) differ: p0.

p0 contains no useful information any more after vertical coordinate transformation has been done. Yet, this results in a failure to calculate the multi-model mean over such 3-dim variables (e.g. 3-dim cloud variables).

This issue has been introduced with the new multi-model statistics (#1150) and can be reproduced with the following example recipe: recipe_testp0.yml.txt

[zklaus]

Thanks for reporting this issue, @axel-lauer. Is there a recipe in ESMValTool that is impacted by this?

[axel-lauer]

As far as I know, this issue affects recipes under development only.

[zklaus]

Ok, in that case I will target 2.4.0 here.

[valeriupredoi]

do we have any recipes that have made it to main @axel-lauer - if not, we can bump this to 2.5.0 😁

[axel-lauer]

I don't think we have anything in main yet, so postponing this until v2.5 is fine but we shouldn't forget about it.

[axel-lauer]

Further investigation of this problem showed that in addition to auxiliary coordinate p0, also presence of ptop can cause the same problems and needs to be removed in order to be able to calculate multi-model statistics (again). A fix in _combine (esmvalcore/preprocessor/_multimodel.py) could look like this:

        for auxcoord in cube.aux_coords:
            if (auxcoord.var_name == 'p0' or auxcoord.var_name == 'ptop'):
                cube.remove_coord(auxcoord)

In addition to this, add_plev_from_altitude and add_altitude_from_plev in esmvalcore/cmor/_fixes/shared.py need to be modified in order to work properly by removing definition of var_name and of long_name, e.g. 38c1c01

[schlunma]

Moving this to v2.6 since there is not open PR yet.