Use of FZJ-CMIP-nitrogen-2-0 versus FZJ-CMIP-nitrogen-1-2 forcing

[jmulcahy11]

UKESM has already run the piControl plus a number of historical simulations with the now older FZJ-CMIP-nitrogen-1-2 forcing version of the Nitrogen deposition forcing. We are wondering for any new historical ensembles should we update to v2-0 or would this introduce a consistency as the piControl uses v1-2 and will not be rerun. We would appreciate some guidance on this. thanks a lot

[znichollscr]

Good question, @hegglinm: does it make any sense to run historical using FZJ-CMIP-nitrogen-2-0 if your pre-industrial is run with FZJ-CMIP-nitrogen-1-2?

[hegglinm]

Hello @jmulcahy11 Yes, I recommend you use the new v2-0 for new historical ensemble members if possible. The discontinuity going from PI v1.2 to historical v2.0 is expected to introduce smaller issues than using v1.2 over the historical period (which shows a rather strong latitudinal shift compared to the PI control. I would be keen to know any resulting issues if you find any with this approach, so that we can quantify biases and provide further guidance to other users in the future. Thanks!

[znichollscr]

Docs updated here: #437

[jmulcahy11]

Thanks @hegglinm for the clarification

[buffysummer]

Thanks for the clarification.
I have a follow-up question regarding emission-driven piControl experiments.
KMA-NIMS has been using FZJ-CMIP-nitrogen-1-2 for our concentration-driven piControl and historical simulations.
For CMIP7, we are now considering using version 2-0 for new emission-driven piControl experiments.
Would it be acceptable to use version 2.0 for the emission-driven piControl, while the existing concentration-driven piControl was performed with version 1-2?
In other words, would this version difference between the concentration-driven and emission-driven piControl experiments be considered acceptable?

[znichollscr]

My (inexpert two cents): yes, because concentration-driven and emissions-driven simulations will be different anyway. Others may have a different view so pinging @durack1 @vnaik60 @hegglinm (please pull in any others who you think need to be consulted on this, particularly anyone who may want to do analysis across concentration-driven and emissions-driven experiments would be helpful as this choice will matter most to them)