ERA5 on-the-fly CMORizer: changed sign of evspsbl and evspsblpot

[katjaweigel]

Change sign of evspsbl and evspsblpot to have the same direction as CMIP models. Added both variables for ERA5_land.

Description

For ERA5 evapotranspiration and potential evapotranspiration are negative, if they go from the surface into the atmosphere for CMIP models they are positive:

According to the description of Evapotranspiration on the Copernicus page (https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-single-levels-monthly-means?tab=overview):

The ECMWF Integrated Forecasting System (IFS) convention is that downward fluxes are positive. Therefore, negative values indicate evaporation and positive values indicate condensation.

In the CMOR table, these fluxes are defined as positive, if they go from the surface into the atmosphere:

Evaporation at surface (also known as evapotranspiration): flux of water into the atmosphere due to conversion of both liquid and solid phases to vapor (from underlying surface and vegetation)

Therefore the fix to cmorize the ERA5 (and ERA5-Land) data needs to switch their sign.

Note that the sign of these variables would change, everywhere where they are used. So far, I did not find any plot or calculation based on these variables from ERA5, but they are included in
ESMValTool/esmvaltool/recipes/cmorizers/recipe_daily_era5.yml
and
ESMValTool/esmvaltool/recipes/examples/recipe_check_obs.yml
and could have been used to create data to use outside ESMValTool diagnostics.

Closes #2116

Link to documentation: https://github.com/ESMValGroup/ESMValCore/blob/fix_era5_evspsbl/doc/quickstart/find_data.rst

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[katjaweigel]

This is not ready, but I accidentally pressed the wrong button, when I wanted to make a draft PR.

[katjaweigel]

It seems to fail on hourly data - I don't understand if there is really an issue with them (could be, because there is a time dependent unit conversion involved, but that has actually not changed) or if it does not have the hourly data (we don't have them on Levante but they exist on the Copernicus page)?

[rswamina]

@valeriupredoi - Calling your attention to this as requested in the morning session of the workshop.

[valeriupredoi]

@katjaweigel it was just a test needing fixing for the two evspbl hourly variables - if the expected data before fixing is negative then the test data has to be negative as well 😁

[codecov]

Codecov Report

Merging #2115 (b9158c8) into main (f5aef4a) will increase coverage by 0.00%.
Report is 25 commits behind head on main.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2115   +/-   ##
=======================================
  Coverage   93.09%   93.09%           
=======================================
  Files         237      237           
  Lines       12806    12810    +4     
=======================================
+ Hits        11922    11926    +4     
  Misses        884      884           

Files Changed	Coverage Δ
esmvalcore/cmor/_fixes/native6/era5.py	91.20% <100.00%> (+0.07%)	⬆️
esmvalcore/cmor/_fixes/native6/era5_land.py	100.00% <100.00%> (ø)

[katjaweigel]

@valeriupredoi thanks a lot, I'd have never found that test.

[valeriupredoi]

@valeriupredoi thanks a lot, I'd have never found that test.

my pleasure, am the chief mechanic after all 😁

[valeriupredoi]

many thanks @katjaweigel 🍺

[remi-kazeroni]

Thanks, @katjaweigel! Looks good to me. I just have a couple of minor suggestions for the documentation. I have edited the title of the PR to mention ERA5, feel free to revert back of you prefer.

I have also removed the "backward incompatible" label because this PR corrects a bug in a CMORization for variables not following CMOR standards. I think that label is not really needed here. Question: do we have recipes in main that would be affected by this change? If so, could you please list them in the description of this PR?

[katjaweigel]

Dear @remi-kazeroni thanks for checking! I included your comments. the only recipes in main, which contain these variables from ERA5 I found are:
ESMValTool/esmvaltool/recipes/examples/recipe_check_obs.yml and
ESMValTool/esmvaltool/recipes/cmorizers/recipe_daily_era5.yml
I included that in the description above.

[axel-lauer]

Looks good to me. Thanks fot spotting and fixing this! Maybe consider changing the wording in line 132 of find_data.rst e.g. as suggested.

[katjaweigel]

@axel-lauer thanks a lot and sorry that I forgot that change when I included the other ones.