Running WRF-Chem model with aerosol direct effects off and indirect effects on

[wrf-chem]

Hi,

I am performing several simulations with aerosol direct effects both turned on and with the emission source of interest turned on/off, in different simulation experiments.
I want to separate the direct effects from the indirect effects by running additional simulations with the direct effects turned off.
How can I do it?
I’ve been following the WRF-Chem user’s guide from the version 3.9.1 (the version of the model that I am running with), and there seems to be some contradictory information in it.
The chemical mechanism that I am using is the CBMZ-MOSAIC with 8 bins (aq). To run the model with direct and indirect effects, I am using the Morrison double-moment microphysics option and the following namelist.input options, considering three one-way nested domains (36 km, 12 km, 4 km horizontal grid resolution):

aer_ra_feedback = 1, 1, 1,
aer_op_opt = 1, 1, 1,
cu_rad_feedback = .true., .true., .false.,
cu_diag = 1, 1, 0,
progn = 1, 1, 1,
wetscav_onoff = 1, 1, 1,
cldchem_onoff = 1, 1, 1,

Turning off the aerosol direct effects should be achieved by making aer_ra_feedback = 0, 0, 0. However, in section 4.3.3 of the WRF-Chem User’s guide v3.9.1.1 says the following:
“To run with indirect effect on, one should turn on the aerosol direct effect (aer_rad_feeedback = 1 and aer_op_opt > 0).”
Thus, believing in what the user’s guide says, it seems that having the direct effect turned on is necessary for running the model with aerosol indirect effects turned on.
Any advice on how to run the model with aerosol indirect effects on and aerosol direct effects off?

Thank you.

Kind regards,
Rui Silva

PS: I know that it is not possible to run only with aerosol direct effects with a chem_opt that includes aqueous chemistry, and that for running with indirect effects the chem_opt should include aqueous reactions. Hence, the only option should be turning off the direct effects. The question is how?

[Mary Barth]

Hi Rui,

I can see how that text in the User's Guide is misleading.  I recommend doing these simulations to understand the indirect effect of aerosols:

1. CBMZ-MOSAIC with 8 bins (aq). and aer_ra_feedback = 1 with "control" emission levels.

2. CBMZ-MOSAIC with 8 bins (aq). and aer_ra_feedback = 1 with "perturbed" emission levels, where perturbed is reducing or increasing emissions in the simulation from what was done in the first simulation. This is closer to reality but does not isolate the effect of indirect feedback because it does not remove the aer_ra_feedback effect. So the differences between simulation 1 and 2 are due to both indirect and direct feedback because aerosol concentrations are different between the two simulations. 

It is okay to run those same two simulations with aer_ra_feedback = 0 to isolate the indirect effect. Eidhammer et al. (2014) did that, among other studies. 

I hope this helps, and if anyone else wants to comment, please do!

Mary

[wrf-chem]

Hi Dr. Mary,

Thank you for helping me with these questions and in the setup of the simulation scenarios. 

From a quick test with the model, it seems that it is possible to turn off aer_ra_feedback while running with indirect effects. 

At least I don't have any errors while running the model. However, I need to check the results to be sure that it is working.

If I have any problem, I will let you know. 

Kind regards,

Rui Silva

[Lin Lu]

Hello all, 

Thanks for this thread. It is very useful to know.

Can I also ask;  when we say in the model  aer_ra_feedback = 0 turned off the feedbacks, exactly what does that mean?

Which variables in the model are directly affected by this option?

Where can I find out more about this?

Thanks,

 Lin

[Suqian]

Hello,

Thanks for the useful information! I am wondering if turning off aerosol direct effects means turning off the feedback of chemistry to meteorology in WRF-Chem. To disable chem impacts on met in WRF-Chem, is there anything elase needed to be done?

Thanks,

Suqian