Add a user namelist for reduced output settings

[JorgSchwinger]

This PR add a file user_nl_blom_outvol_reduced to the directory cime_config. The purpose of this file is to provide a namelist with reduced output settings for the NorESM3 spin-up simulations or test simulations. The following general rules are applied:

• no daily output
• no output on isopycnic layers
• no (HAMOCC) or minimal (BLOM) monthly 3d-output on levels
• reduced annual 3d output

For convenience, and since it increases the output volume only little, I have repeated the output of 2d fields also in annual files.

@matsbn, @TomasTorsvik, @jmaerz , @milicak , @AleksiNummelin please review these output settings and let me know if you see the need for changes (please put a comment at the corresponding line).

@matsbn, one could further reduce the BLOM output by using 2-byte compressed output. After my experience this results in in ~20-30% smaller files after compression to netCDF4 and the accuracy is still good enough for most applications.

[jmaerz]

Hi @JorgSchwinger , is there any good reason to provide this as a user_nl_blom and not similar to/in line with #478?

[JorgSchwinger]

This was discussed here NorESMhub/noresm3_dev_simulations#88 and the conclusion was that it is easier to do it as user-mods_directory

[jmaerz]

Hi @JorgSchwinger , apologize my ignorance, but are user-mods equivalent to the source-mods directory? - if so, I really feel that this is a step backward rather than forward and I would advocate for introducing ONE xml-switch name at the NorESM level for let say CMIP7-spinup (and one for CMIP7-production runs or alike plus other switches needed) that switches on all components output via namelists. Admittedly, that's what I read when looking into issue NorESMhub/noresm3_dev_simulations#88 - the latter has never really conclusively decided to my understanding (and use-mods escaped my understanding...).

[JorgSchwinger]

user-mods are activated with an option when you create the case. The corresponding namelist files are maintained in cime (we could have a copy in BLOM's cime_config, as suggested by this PR, but that's not strictly necessary).

It is much easier, particularly for the other components that are not exclusively used for NorESM. If you don't agree with this solution, you should bring this up for the other components here NorESMhub/noresm3_dev_simulations#88

[jmaerz]

Hi @JorgSchwinger , in any case, many thanks for providing this - let's see, where NorESMhub/noresm3_dev_simulations#88 leads to. In the case that user-mods will be used, I would suggest i) to push this PR ONLY to NorESM (not to BLOM) and ii) potentially include your settings rather with a switch into the currently used namelist_definition_blom.xml-file.