How to prescribe monthly-varying forcing with fixed background state in CAM6 (perpetual winter setup)

[Dajeong]

Hi everyone,

I am trying to design a CAM6 experiment in which the background state is fixed, while the boundary forcings varies monthly.
Conceptually, this is similar to a perpetual winter experiment, but with month-dependent forcing fields.

More specifically, my goal is:

• To keep the model background state fixed (e.g., winter climatology)
• While prescribing forcing fields that vary from month to month (e.g., monthly mean fields)

Now I am using CESM2.2.2.
The output of ./desribe_version is:

------------------------------------------------------------------------
git describe:
release-cesm2.2.2-0-g779b0a3
------------------------------------------------------------------------

------------------------------------------------------------------------
git status:
HEAD detached at release-cesm2.2.2
nothing to commit, working tree clean
------------------------------------------------------------------------

Thank you very much for your help!

Best regards,
Dajeong

 

[peverley]

Hi Dajeong,

I asked around and this is the advice I heard from the scientists:

We usually use the nudging functionality to 'fix' the background state, either nudging towards reanalysis or another model run.

Does that give you a starting point?

Courtney

 

[Dajeong]

Hi Courtney,

Thank you for your suggestion about using nudging.

I would like to clarify my experimental design more concretely. My goal is to isolate the atmospheric response to a prescribed SST forcing while keeping the background state close to a fixed seasonal climatology.

In practice, I am considering using nudging toward a winter climatology for U, V, and T. However, I am concerned that strong nudging of the winds and temperature may suppress the dynamically generated response to the SST forcing, especially the development of pressure anomalies.

From a modeling perspective, I would really appreciate your advice on the following points:

1. Is it reasonable to nudge all of U, V, and T in this type of experiment, or is it more common to leave winds free and only constrain thermodynamic fields such as temperature?
2. How sensitive are large-scale circulation and pressure responses to the nudging timescale in practice? Is there a typical relaxation timescale that balances maintaining the background state while still allowing a meaningful forced response?
3. Would an alternative approach, such as using a perpetual seasonal radiation setup instead of nudging, be more appropriate for isolating SST-forced responses? If so, are there any example namelist settings or configurations that you would recommend for this type of setup?

Please feel free to respond only to the parts that are most relevant or convenient. Any guidance would be greatly appreciated.

Best regards,
Dajeong