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Implement equation of time in RRTMG shortwave radiation #201

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Implement equation of time in RRTMG shortwave radiation #201

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1 change: 1 addition & 0 deletions src/core_atmosphere/physics/mpas_atmphys_driver_radiation_sw.F
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Original file line number Diff line number Diff line change
Expand Up @@ -1023,6 +1023,7 @@ subroutine driver_radiation_sw(itimestep,configs,mesh,state,time_lev,diag_physic
swdnt = swdnt_p , swdntc = swdntc_p , swupb = swupb_p , &
swupbc = swupbc_p , swdnb = swdnb_p , swdnbc = swdnbc_p , &
swddir = swddir_p , swddni = swddni_p , swddif = swddif_p , &
julian = curr_julday , &
ids = ids , ide = ide , jds = jds , jde = jde , kds = kds , kde = kde , &
ims = ims , ime = ime , jms = jms , jme = jme , kms = kms , kme = kme , &
its = its , ite = ite , jts = jts , jte = jte , kts = kts , kte = kte &
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13 changes: 10 additions & 3 deletions src/core_atmosphere/physics/physics_wrf/module_ra_rrtmg_sw.F
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Expand Up @@ -10036,7 +10036,7 @@ subroutine rrtmg_swrad( &
swupt,swuptc,swdnt,swdntc, &
swupb,swupbc,swdnb,swdnbc, &
swupflx, swupflxc, swdnflx, swdnflxc, &
swddir,swddni,swddif, &
swddir,swddni,swddif,julian, &
ids,ide, jds,jde, kds,kde, &
ims,ime, jms,jme, kms,kme, &
its,ite, jts,jte, kts,kte &
Expand All @@ -10054,6 +10054,7 @@ subroutine rrtmg_swrad( &
integer,intent(in):: icloud,cldovrlp,idcor
integer,intent(in):: has_reqc,has_reqi,has_reqs
integer,intent(in),optional:: o3input
real,intent(in):: julian

real,intent(in):: radt,degrad,xtime,declin,solcon,gmt
real,intent(in),dimension(ims:ime,jms:jme):: xlat,xlong
Expand Down Expand Up @@ -10101,6 +10102,7 @@ subroutine rrtmg_swrad( &
integer:: i,j,k,n

real:: coszrs,xt24,tloctm,hrang,xxlat,adjes,scon
real:: da,eot
real:: ro,dz
real:: corr
real:: gliqwp,gicewp,gsnowp,gravmks
Expand Down Expand Up @@ -10197,9 +10199,14 @@ subroutine rrtmg_swrad( &

!--- calculate the cosine of the solar zenith angle at the current time step to determine if the sun is
! above or below the horizon (xt24 is the fractional part of simulation days plus half of radt in
! units of minutes, julian is in days, and radt is in minutes). do not call rrtmg_sw is night-time:
! units of minutes, julian is in days, and radt is in minutes). eot (equation of time) is a correction
! of mean solar time due to the difference between that time and the apparent solar time.
! do not call rrtmg_sw is night-time:
dorrsw = .true.
xt24 = mod(xtime+radt*0.5,1440.)
da=6.2831853071795862*(julian-1.)/365.
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@dustinswales dustinswales Jan 14, 2026

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Can any of these magic number be parameterized?

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@clark-evans clark-evans Jan 14, 2026

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Probably - the 6.283... value in line 10206 is 2*pi. I'm not sure on the others, though. I don't know the source for the form of this equation that @weiwangncar used -- Wei, perhaps you know?

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@weiwangncar weiwangncar Jan 14, 2026

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@clark-evans The number comes from the contributor to WRF, Jose Arias. Will check with Jimy.

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@weiwangncar weiwangncar Jan 14, 2026

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@clark-evans It looks like the PR in MPAS has some discussion on this.

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@clark-evans clark-evans Jan 14, 2026

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Thanks for reminding me of that, @weiwangncar! @dustinswales -- what do you think? Maybe we implement 2*pi and add a comment above the equation pointing to the GML reference?

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@dudhia dudhia Jan 15, 2026

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Yes that is 2*pi. This addition to the MPAS repo got delayed but will occur. WRF radiation has had it for a while already where we did not implement it directly in the radiation routine but in a separate cosine zenith angle routine.

eot=(0.000075+0.001868*cos(da)-0.032077*sin(da) &
-0.014615*cos(2*da)-0.04089*sin(2*da))*(229.18)
xt24 = mod(xtime+radt*0.5,1440.)+eot
tloctm = gmt + xt24/60. + xlong(i,j)/15.
hrang = 15. * (tloctm-12.) * degrad
xxlat = xlat(i,j) * degrad
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