How to change CO2 concentration in CESM2 FHIST experiment

[changmao]

Hi all,

I am learning how to use CESM FHIST for CO2 concentration increase experiments. Although I have read some posts and tutorials, it is still confused to me about how to modify CO2 concentration.

CCSM-CO2-PPMV seems to be suitable when CO2 is a constant value, but some also said that this parameter can be mainly used to set the initial concentration of atmospheric CO2,which seemed to be applicable to my experiment.

Flbc_file is also a file containing CO2 concentration, but I'm not sure if I should modify this file. If so, how should I proceed?

Background of my experiment: It included control experiments (all climate variables were set to pre-industrial level and remained unchanged throughout the simulation process) and CO2 concentration increase experiments (CO2 concentration increased to four times the pre-industrial level at the beginning of the simulation 4XCO2, while other variables remained unchanged).

Any help will be deeply appreciated!

 

[islas]

I think you should modify the flbc_file. I think that's where the CO2 forcing is coming from in a transient run.

Isla

 

[changmao]

I think you should modify the flbc_file. I think that's where the CO2 forcing is coming from in a transient run.

Isla

Thanks very much for your help. But I really don't know how to modify Flbc_file. Should I directly change the CO2 value in the file to 4 times? Thank you again for any guidance.

 

[islas]

I didn't really understand your experiment. You want to run the whole historical period with transient CO2 but starting out at a baseline of 4xCO2 at the beginning of the historical period? So you want to run with CO2 forcing that's equal to 4 times the piControl + the historical transient anomalies? it sounds like a bit of a strange experiment so I may not have understood properly. If that's the case the you probably want to compute the anomalies for the historical period compared to the piControl and add them on to a 4 times piControl baseline. If you simply want to quadruple the CO2 and see how the run evolves as opposed to running with the transient historical CO2 then you can do that by changing co2vmr in user_nl_cam and multiplying the preindustrial value by 4.

 

[changmao]

I didn't really understand your experiment. You want to run the whole historical period with transient CO2 but starting out at a baseline of 4xCO2 at the beginning of the historical period? So you want to run with CO2 forcing that's equal to 4 times the piControl + the historical transient anomalies? it sounds like a bit of a strange experiment so I may not have understood properly. If that's the case the you probably want to compute the anomalies for the historical period compared to the piControl and add them on to a 4 times piControl baseline. If you simply want to quadruple the CO2 and see how the run evolves as opposed to running with the transient historical CO2 then you can do that by changing co2vmr in user_nl_cam and multiplying the preindustrial value by 4.

I think it should be the second scenario, which is to see the result of 4xCO2. My understanding of what you meant is, set co2vmr value to 4 times in user_cl_cam firstly, and then multiply the pre industrial value by 4? If so, what does "multiplying the preindustrial value by 4" mean and where should I specifically modify it? Is it flbc_file?

 

[islas]

OK, if you simply want to run with 4xCO2 then you don't need to change it in the flbc file. You just need to change co2vmr. If you set up a B1850 case and from your case directory run ./preview_namelists, you should see in ./CaseDocs/atm_in that co2vmr is set as follows...

co2vmr = 284.7e-6

You can change this by adding the following line to user_nl_cam in your case directory

co2vmr = 1138.8e-6

where the value here is 4 times the default. If you run ./preview_namelists again you should see that change propagated to ./CaseDocs/atm_in and that is the CO2 concentration the simulation will use. There's also a compset BCO2x4cmip6 which has CO2 concentrations quadrupled. You can also find more information on changing the CO2 concentration when you just want to use a constant value (not transient) in the examples of changing namelist parameters here:

Namelists — CESM Tutorial