When do i have to use/drop the area cell method?

[larsbuntemeyer]

In #74, we have areosol variables defined on pressure levels, e.g., plev19. However, i see from the CMIP6 cmor tables, that they often drop the area cell method for variables on pressure levels, while single variables on pressure levels preserve the area mean cell method, for example here:

https://github.com/PCMDI/cmip6-cmor-tables/blob/087fe45d21c082e28723e0f930e4266abe91b853/Tables/CMIP6_Eday.json#L1745-L1780

To my understanding, temperature is an intensive variable independent of the vertical coordinate. Honestly, I'm never sure about this, when to use area: mean cell method. I see that for CMIP7 cmor tables, this area cell method is indeed introduced in the pressure level air temperature:

https://github.com/WCRP-CMIP/cmip7-cmor-tables/blob/e98f002bf876982bd15f048db89278bc81c97fb7/tables/CMIP7_atmos.json#L4830-L4846

I'm, in general, not sure what it means if a variable has no area cell method. I can understand, that, e.g., time independent fx variables have no time cell method since they are static, but shouldn't all variables (except for spatial aggregations) have an area cell method defined? In general, it seems that in CMIP7 cmor tables that is the case...

I would appreciate feedback by @taylor13 on this topic!

[taylor13]

@larsbuntemeyer As you probably read in the conventions document:

Because the default interpretation for an intensive quantity differs from that of an extensive quantity and because this distinction may not be understood by some users of the data, it is recommended that every data variable include for each of its dimensions and each of its scalar coordinate variables the cell_methods information of interest (unless this information would not be meaningful). It is especially recommended that cell_methods be explicitly specified for each spatio-temporal dimension and each spatio-temporal scalar coordinate variable.

In CMIP7, I think this advice is followed more often than in CMIP6. For model output, the values reported as a function of a horizontal spatial dimension almost always represent spatial means. In some model formulations, I think the velocities might represent something closer to a point value. For any quantity that you would like to preserve a global total or global mean (i.e., a conservative quantity), we would like values reported that represent area means.

Happy to answer any follow-up questions on this.

[larsbuntemeyer]

Thanks Karl for the quick reply!