I’m using WRF-CMAQ coupled model now. I notice that for sea salt aerosols, coupled model uses the real and imaginary parts of the complex refractive index at 14 different wavelengths at MODULE module_ra_rrtmg_sw_cmaq in module_ra_rrtmg_aero_optical_util_cmaq.F. And these complex refractive index from OPAC (Hess et al., 1998) are based on RH=0%.
So how does the model apply RH=0% data to any relative humidity SSA? Is there a function between RH and complex refractive index used in coupled model?
In addition, I have seen some reports on the asymmetry factor of inhomogeneous SSA, and the RRTMG used in the coupling model seems to regard SSA as completely homogeneous. Is it possible to add the judgment of sea salt RH when calculating the asymmetry factor?
While hydrated SSA does not have it’s own species-specific refractive indices, total aerosol complex refractive indices are calculated which would include SSA+Aerosol liquid water (aggregating volume-weighted elemental carbon, SSA, aerosol liquid water, water soluble, and water insoluble aerosols). Aerosol liquid water is calculated based off of aerosol water activity and RH (for the inorganic via ISOROPPIA II; for organic aerosols starting with CMAQv5.3.2 with encoded kappa values - see AERO_DATA.F).
With regard to the asymmetry factor of inhomogeneous SSA - the current assumption for SSA aerosol is that they are homogeneously mixed. The only inhomogenously mixed representation encoded currently is that of a core-shell black carbon/elemental carbon (core) surrounded by all other aerosol types (shell). I would recommend following ‘BHCOAT’ and ‘CS’ as keywords within module_ra_rrtmg_aero_optical_util_cmaq.F to see what exactly is going on. I’m not sure how exactly you would go about implementing your idea but this might be a good place to familiarize yourself. Good luck and let me know if you have any follow-up questions!