ERA5 on-the-fly CMORizer produces scientifically wrong results for some radiation variables #2475
Description
Describe the bug
Following ESMValGroup/ESMValTool#3694 (comment), we (@axel-lauer, @LisaBock and myself) had a more detailed look at the radiation variables produced by the ERA5 on-the-fly CMORizer. Here is a summary of what we found:
rlds (surface_downwelling_longwave_flux_in_air; W m-2)
- ERA5 variable we currently use:
surface_thermal_radiation_downwards(J m-2) - ERA5 variable we should use:
mean_surface_downward_long_wave_radiation_flux(W m-2)
These two ERA5 variables are basically identical after accounting for the time accumulation. However, we can skip the time accumulation in our code if we use second variable here.
-> Not a big problem, but this would simplify the code
rlns or rls (surface_net_downward_longwave_flux; W m-2)
Note: these variables are identical. The first is our custom version, the second the CMIP6 version.
- ERA5 variable we currently use for rlns:
surface_net_thermal_radiation(J m-2) - ERA5 variable we currently use for rls:
mean_surface_net_long_wave_radiation_flux(W m-2) - ERA5 variable we should use for both:
mean_surface_net_long_wave_radiation_flux(W m-2)
These two ERA5 variables are basically identical after accounting for the time accumulation. However, we can skip the time accumulation in our code if we use second variable here.
-> Not a big problem, but this would simplify the code
rlus (surface_upwelling_longwave_flux_in_air; W m-2)
- ERA5 variable we currently use:
mean_surface_net_long_wave_radiation_flux(J m-2) - ERA5 variable we should use: custom derivation from the net and downwards flux, but this cannot be done in the fix yet (Variable derivation for ERA5 on-the-fly CMORizer #1806)
On the surface, outgoing longwave flux is not the same as the net flux (downwards minus upwards)!
-> This is scientifically wrong!
rsds (surface_downwelling_shortwave_flux_in_air; W m-2)
- ERA5 variable we currently use:
surface_solar_radiation_downwards(J m-2) - ERA5 variable we should use:
mean_surface_downward_short_wave_radiation_flux(W m-2)
These two ERA5 variables are basically identical after accounting for the time accumulation. However, we can skip the time accumulation in our code if we use second variable here.
-> Not a big problem, but this would simplify the code
rsns or rss (surface_net_downward_shortwave_flux; W m-2)
Note: these variables are identical. The first is our custom version, the second the CMIP6 version.
- ERA5 variable we currently use for both:
surface_net_solar_radiation(J m-2) - ERA5 variable we should use for both:
mean_surface_net_short_wave_radiation_flux(W m-2)
These two ERA5 variables are basically identical after accounting for the time accumulation. However, we can skip the time accumulation in our code if we use second variable here.
-> Not a big problem, but this would simplify the code
rsus (surface_upwelling_shortwave_flux_in_air; W m-2)
- ERA5 variable we currently use:
mean_surface_net_short_wave_radiation_flux(J m-2) - ERA5 variable we should use: custom derivation from the net and downwards flux, but this cannot be done in the fix yet (Variable derivation for ERA5 on-the-fly CMORizer #1806)
On the surface, outgoing shortwave flux is not the same as the net flux (downwards minus upwards)!
-> This is scientifically wrong!
Thus, at least for rlus and rsus, we need to adapt the CMORizer.