A problem about the compute the conputation of the upward longwave flux in module_sf_bep_bem.F

[Panzhan]

I am working with WRF v4.7.1 and examining the implementation of PV panels in phys/module_sf_bep_bem.F, specifically within the upward_rad() subroutine. I have a question regarding the calculation of gfl when PV panels are present on the roof.

In the current code, gfl is calculated as:
gfl=(1.-albr_u)*rs*(1-pv_frac_roof)+emr_u*rld*(1-pv_frac_roof)+pv_frac_roof*emr_u*sigma*tpvlev(id,iz)**4 &
-emr_u*sigma*(tr_av(id,iz)**4.)+(1-gr_frac_roof)*sfr(id,iz)+(sfrv(id,iz)+lfrv(id,iz))*gr_frac_roof+(1.-gr_frac_roof)*lfr(id,iz)
where emr_u is the emissivity of the roof.

However, I am wondering why the longwave emission term from the PV panels uses emr_u instead of the emissivity of the PV panels themselves. Would it not be more consistent to write:
gfl=(1.-albr_u)*rs*(1-pv_frac_roof)+emr_u*rld*(1-pv_frac_roof)+pv_frac_roof*em_pv*sigma*tpvlev(id,iz)**4 &
-emr_u*sigma*(tr_av(id,iz)**4.)+(1-gr_frac_roof)*sfr(id,iz)+(sfrv(id,iz)+lfrv(id,iz))*gr_frac_roof+(1.-gr_frac_roof)*lfr(id,iz)
where em_pv is the emissivity of the PV panels (e.g., 0.79)?

 

[cenlinhe]

This term "pv_frac_roof*emr_u*sigma*tpvlev(id,iz)**4" you mentioned here is the net longwave radiation from the PV panel to the roof; therefore, theoretically speaking, it should use the emissivity of the PV panel.
One thing to note is that em_pv (0.79) you mentioned is the emissivity of the glass face (upwards face of PV). In the above term, you should use the emissivity of the PV array downwards, which is 0.95 (monocrystalline silicon downward face of the PVP) (Zonato et al., 2021), not 0.79. This 0.95 is close to 0.90 (the default roof emissivity in the urban parameter table), so the difference will be small.