remove Zhao-Carr cloud schemes and combine Xu-Randall cloud fraction calculation

[Qingfu-Liu]

Description of Changes:

1. Remove Zhao-Carr cloud schemes which are no longer used in the CCPP physics suites. The new code changes remove cloud calculation for "progcld_zhao_carr" and "progcld_zhao_carr_pdf".
2. Create a new function cld_frac_XuRandall in the module_radiation_clouds, and replaced the subroutines "cloud_fraction_mass_flx_2", "cloud_fraction_mass_flx_1", and "cloud_fraction_XuRandall" with the new function.
3. remove the function cld_frac_XuRandall from program GFS_rrtmgp_cloud_mp.F90, and replaced it with the newly created function cld_frac_XuRandall in the module_radiation_clouds.

Tests Conducted:

Regression tests are conducted on Ursa, and the results are binary identical

Dependencies:

No dependence

Documentation:

No new document (not needed)

Issue (optional):

No issues. The changes remove Zhao-Carr cloud schemes which are no longer used, and combine some similar subroutines

[dustinswales]

@Qingfu-Liu Thanks for making these changes.

1. I love to see radiation_clouds.f getting some long overdue housekeeping. Good work!
2. It's great that the RRTMG-CLDMP coupling now has only one (configurable) flavor of Xu-Randall, but it appears that we now have two identical Xu-Randall cloud fraction routines; The one being introduced here in radiation_clouds.f for the RRTMG cloud coupling, and one used for the RRTGMP cloud coupling, GFS_rrtmgp_cloud_mp.F90.

Could we create a new module, say physics/Radiation/radiation_cloud_fraction.F90, which contains a single Xu-Randall cloud fraction routine that both RRTMG (physics/Radiation/radiation_clouds.f) and RRTMGP (physics/Interstitials/UFS_SCM_NEPTUNE/GFS_rrtmgp_cloud_mp.F90) uses?

[Qingfu-Liu]

@dustinswales OK. I will make the changes. Thanks.

[Qingfu-Liu]

@dustinswales I cross-checked the code between cld_frac_XuRandall (RRTMGP) and cloud_fraction_XuRandall (RRTMG), there is small differences between the two:
Line 806 (/scratch3/NCEPDEV/global/Qingfu.Liu/git/ufs-weather-model_20251229/UFSATM/ccpp/physics/physics/Interstitials/UFS_SCM_NEPTUNE/GFS_rrtmgp_cloud_mp.F90)
tem2 = max(min(tem1*(cld_mr - clwt), 50.0 ), 0.0 ) (where clwt = 1.0e-6 * (plyr(i,k)0.001))
Line 3160 (/scratch3/NCEPDEV/global/Qingfu.Liu/git/ufs-weather-model_20260105/UFSATM/ccpp/physics/physics/Radiation/radiation_clouds.f_save)
value = max( min( tem1(clwf(i,k)-clwm), 50.0 ), 0.0 ) (where clwm=0. here)
Do you have any comments/suggestions (I prefer to use the RRTMG version here since no impact on the GFSv17 regression tests)

[dustinswales]

@Qingfu-Liu I don't understand the need for this line? (clwmin =0 above)

[Qingfu-Liu]

Yes. we can remove this line here. Should we change the line 806 in GFS_rrtmgp_cloud_mp.F90 as:
tem2 = max(min(tem1*(cld_mr-clwm), 50.0 ), 0.0 ), where clwm=0. in RRTMG
My guess is that clwm=clwt when you change the code for RRTMGP

[dustinswales]

@Qingfu-Liu If clwm = 0, then why subtract it from here.

[dustinswales]

@Qingfu-Liu I don't want to make the call here, but I "think" the GP formulation is correct, and the G formulation is not. At least when looking back at https://journals.ametsoc.org/view/journals/atsc/53/21/1520-0469_1996_053_3084_ascpfu_2_0_co_2.xml?tab_body=pdf.
We should select one, and whichever way we choose, it will impact the other radiation scheme. Thoughts?

[Qingfu-Liu]

@dustinswales I will do some more research. From the code, the two cloud fraction calculations for RRTMG and RRTMGP are different.

[dustinswales]

@Qingfu-Liu As you pointed out, the algorithms are nearly the same, with small differences in the exponent.
For RRTMG, exponent = cloud mixing ratio - clwm (set = 0)
For RRTMGP, exponent = cloud mixing ratio - minimum allowable condensate (~1e-8)

So whatever way is "correct", it will impact the other and cause answer changes. But they should be the same.

[Qingfu-Liu]

@dustinswales I have modified the RRTMG code to make the Xu-Randall subroutine elemental and can be used for RRTMGP ( slightly modify the input variables). I will upload the code after the regression tests

[Qingfu-Liu]

@dustinswales I uploaded the new code and change the RRTMG Xu-Randall subroutine to be a function and elemental, and can be called in the RRTMGP scheme with extra input control variable "factor".

[dustinswales]

@Qingfu-Liu Thanks for doing this.
For GP (GFS_rrtmgp_cloud_mp.F90) we will need to:

• Import cld_frac_XuRandall from module_radiation_clouds here
• Remove the cld_frac_XuRandall subroutine from here
• Modify calls to cld_frac_XuRandall in GFS_rrtmgp_cloud_mp to provide "factor=1".

Ultimately, I think FactorG=FactorGP, but this will not give us B4B reproducibility for the RRTMGP test if we adopt this here.
Even though the change is probably insignificant, I'm reluctant to support this w/o some testing in a GP enabled application.

[Qingfu-Liu]

@dustinswales I have removed the cld_frac_XuRandall function from RRTMGP scheme, and replaced the function from module_radiation_clouds. The regression tests from RRTMGP (two suites in the default rt.conf) produce binary identical results.
I am going to run the regression test for all the suites in the default rt.conf.

[dustinswales]

@Qingfu-Liu Thanks for addressing all my concerns.
This is a great contribution! and it gets us closer to unifying our interfaces to RRTMG/RRTMGP.

[grantfirl]

@Qingfu-Liu Should we remove the entire Zhao-Carr scheme from the repo? @ligiabernardet I can't remember if we have an official process for retiring schemes, do you?

[Qingfu-Liu]

@grantfirl Yes, we should remove the entire Zhao-Carr scheme from the repository. I do not see any suites will use it in the future

[dustinswales]

@grantfirl I think we may still have a RT in the SCM that uses ZC? If so, we should remove it.

[grantfirl]

@Qingfu-Liu Ligia is going to ask representatives of the ccpp-physics stakeholders at the CCPP Physics Management meeting whether they have any reason to keep the ZC scheme in the repo. If not, we'll need to more completely remove all of the ZC "hooks" within other schemes and the files themselves.

[Qingfu-Liu]

@grantfirl I have removed the Zhao-Carr schemes in this PR, only left a message if someone calls the Zhao-Carr schemes.

[grantfirl]

@grantfirl I have removed the Zhao-Carr schemes in this PR, only left a message if someone calls the Zhao-Carr schemes.

I don't understand. The ZC microphysics scheme files are still in the repository (ccpp/physics/physics/MP/Zhao_Carr) and there are plenty of instances where identifier_for_zhao_carr_microphysics_scheme and identifier_for_zhao_carr_pdf_microphysics_scheme variables are being used in other schemes.

[Qingfu-Liu]

@grantfirl Good comment. I did not remove the zhao-carr identifier, but stopped the model and give the error message once it is used. The Zhao-Carr cloud schemes are removed from the code radiation_clouds.f. You mean I should remove all the Zhao-Carr identifier. I will go through the code and upload a new version

[grantfirl]

@grantfirl Good comment. I did not remove the zhao-carr identifier, but stopped the model and give the error message once it is used. The Zhao-Carr cloud schemes are removed from the code radiation_clouds.f. You mean I should remove all the Zhao-Carr identifier. I will go through the code and upload a new version

Ya, otherwise we're leaving "abandoned" pieces of code in the repository that will never be executed.

[Qingfu-Liu]

@grantfirl I only modified the code under the directory "UFSATM/ccpp". There are more changes in the ccpp-physics directories "physics/Interstitials/UFS_SCM_NEPTUNE" and "physics/MP/Zhao_Carr". I did not change the code in the directory ""upp/sorc/ncep_post.fd" which has modules related to Zhao-Carr MP schemes (not sure I should change the post). I will upload more changes in the "UFSATM" repository.

[grantfirl]

@grantfirl I only modified the code under the directory "UFSATM/ccpp". There are more changes in the ccpp-physics directories "physics/Interstitials/UFS_SCM_NEPTUNE" and "physics/MP/Zhao_Carr". I did not change the code in the directory ""upp/sorc/ncep_post.fd" which has modules related to Zhao-Carr MP schemes (not sure I should change the post). I will upload more changes in the "UFSATM" repository.

@Qingfu-Liu These changes look good/complete to me. Thanks!

[RuiyuSun]

Thanks @Qingfu-Liu