Add plev/altitude conversion to extract_levels

[axel-lauer]

This pull requests extends the existing preprocessor function extract_levels. The possibility to convert pressure levels (air_pressure) into height (altitude) and vice versa is added. For the conversion, existing code in cmor/_fixes/shared.py is used that uses the US standard atmosphere to convert from height to pressure. A similar function has been implemented to also allow conversion from pressure to height.

Tasks

• This pull request has a descriptive title that can be used in a changelog
• Public functions should have a numpy-style docstring so they appear properly in the API documentation. For all other functions a one line docstring is sufficient.
• If writing a new/modified preprocessor function, please update the documentation
• Circle/CI tests pass. Status can be seen below your pull request. If the tests are failing, click the link to find out why.
• Codacy code quality checks pass. Status can be seen below your pull request. If there is an error, click the link to find out why. If you suspect Codacy may be wrong, please ask by commenting.

[schlunma]

Is there a specific reason why you don't calculate the altitude bounds?

[axel-lauer]

Calculating altitude bounds did not work for the MPI-ESM1-2-LR model (variable clw)...

[schlunma]

I guess that model also needs a fix similar to MPI-ESM1-2-HR to get correct bounds. Can you maybe add that to #635? I will add an if statement and only add the bounds if possible.

[axel-lauer]

@schlunma Sounds good!

[schlunma]

Scientific review: Successfully tested the new function on real data (about 20 CMIP6 models) and on synthetic data (see tests) 👍

Technical review: Fine from my side, but since I wrote parts of the code myself maybe @valeriupredoi can have a brief look on the changes?

@axel-lauer I think it would be nice to have one sentence about this functionality in the documentation? And maybe also the docstring of extract_levels?

[axel-lauer]

@schlunma good point! I just added some documentation.

[bouweandela]

Looks good to me, apart from the minor technical improvement mentioned in the comment that could be made.

[bouweandela]

It would also be nice to mention the functionality here: https://docs.esmvaltool.org/projects/esmvalcore/en/latest/recipe/preprocessor.html#vertical-interpolation

[axel-lauer]

I thought the information in https://docs.esmvaltool.org/projects/esmvalcore/en/latest/recipe/preprocessor.html#vertical-interpolation is taken from the docstrings. These have been updated accordingly...

[bouweandela]

No, that's the info here: https://docs.esmvaltool.org/projects/esmvalcore/en/latest/api/esmvalcore.preprocessor.html#esmvalcore.preprocessor.extract_levels

The preprocessor as it can be used in the recipe has it's own documentation page:
https://github.com/ESMValGroup/ESMValCore/blob/master/doc/recipe/preprocessor.rst#vertical-interpolation

[axel-lauer]

@bouweandela Thanks for pointing me to the documentation page. I added a brief description of the new functionality.