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how to set NSSL CCN mp scheme properly?

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Dear Colleges,

I am interested in using the NSSL CCN microphysics scheme (option 18) with predicted CCN. In the manual it is suggested to set a nssl_cccn and ccn_conc parameters to 0.7e9.
My question is if we do not set those parameters to 0.7e9 value what global CCN is used by default?
My next questions are:
Does the 0.7e9 value correspond to polluted or clean atmospheric conditions?
Can we set other values (higher or lower) for the nssl_cccn and ccn_conc parameters?
Where we can find more information on setting global CCN in the NSSL scheme (option 18)?

Kind regards,
According to the Registry.EM_COMMON file, it looks like the default value of nssl_cccn is 0.5e9, so that's what the value would be if you didn't set to 0.7e9. I don't know much about the details of this scheme, but I would assume that the developers of it think that for a global value, 0.7e9 is the appropriate value to use. As for other questions, I would suggest taking a look at the publication for this scheme to see if it's helpful is providing more detail:
Thanks a lot for the information! Can you also find information on the CCN used in the NSSL mp scheme option 17 (steady background CCN). What CCN is set in this scheme?

Kind regards,
If you look near the top of the code for this scheme (phys/module_mp_nssl_2mom.F), you will see:
!  ccn : base cloud condensation nuclei concentration (use namelist.input value "nssl_cccn")
So it looks like it's using the same value as you have for nssl_cccn.

Thanks for your reply!

When I run the WRF v4.1.4 with mp=18 and mp=17 the results are significantly different for precipitations. The mp=18 (with default ccn) produces substantially higher precipitation and rain mixing ratios, while the results from mp=17 and mp=22 (NSSL without hail) are very similar. The simulated case is for orograhpical wintertime precipitation associated with the passage of cold front.

Kind regards,
Unfortunately different schemes can produce significantly different results. I can't comment on the exact differences here, as we received these schemes from outside developers who would know more about them.