rho_phy and unit of tracers into WRF atmosphere

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friedemann
Posts: 9
Joined: Mon Feb 10, 2020 8:48 am

rho_phy and unit of tracers into WRF atmosphere

Post by friedemann » Mon Jul 27, 2020 6:43 pm

Hi,
I'm using WRF to compute transport of passive greenhouse gas tracers (chem_opt=17, emiss_opt=17). Those are usually expressed as dry air mole fractions, and I assumed WRF does the same. A student's question prompted me to look into this and now I'm not sure whether emissions are actually emitted as wet air mole fractions.
The lines in question are in module_ghg_fluxes.F, lines 77f (in WRF 4.1.1):

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         conv_rho=8.0461e-6/rho_phy(i,k,j)*dtstep/dz8w(i,k,j)  ! 8.0461e-6=molar_mass(air)/3600, [g/mol/s]                                                                                
         chem(i,k,j,p_co2_ant)= chem(i,k,j,p_co2_ant) + conv_rho* emis_ant(i,k,j,p_e_co2)                                                                                                 
The question boils down to whether rho_phy is the density of dry air or the density of wet air. I traced the definition of rho_phy via emissions_driver.F to subroutine chem_prep in module_chem_utilities.F:

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rho(i,k,j) = 1./alt(i,k,j)*(1.+moist(i,k,j,P_QV))
I didn't track down 'alt' (the input to chem_prep is grid%alt), but this looks like density of wet air to me. Comments in e.g. module_mosaic_cloudchem.F and module_sorgam_vbs_aqchem.F suggest that, too. On the other hand, a comment in module_chem_plumerise_scalar.F says it's dry air density ('dn0' in subroutine plumerise). And if it's wet air, wouldn't the equation be inconsistent because 8.0461e-6 is computed with the molar mass of dry air...?
In emissions_driver, conversion factors for some species (called 'conv'; analogous to 'conv_rho' above) contain the factor 1/rho_phy just as conv_rho, while others contain the factor alt.

Can someone explain which density rho_phy actually is?

Best regards,
Friedemann

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