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Convert the BareGroundFluxesMod dewpoint calculation into a function #3706

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Convert the BareGroundFluxesMod dewpoint calculation into a function #3706

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53 changes: 36 additions & 17 deletions src/biogeophys/BareGroundFluxesMod.F90
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Original file line number Diff line number Diff line change
Expand Up @@ -148,15 +148,6 @@ subroutine BareGroundFluxes(bounds, num_noexposedvegp, filter_noexposedvegp, &
real(r8) :: forc_e ! vapor pressure at forcing height [Pa]
real(r8) :: forc_qs ! saturated specific humidity at forcing height [kg/kg]
real(r8) :: forc_esat ! saturated vapor pressure at forcing height [Pa]
! Coefficients from Alduchov and Eskridge (1996) as used by Lawrence (2005) for Magnus dewpoint
! equation over liquid
real(r8), parameter :: A1_liq = 17.625_r8 ! [-]
real(r8), parameter :: B1_liq = 243.04_r8 ! [degC]
real(r8), parameter :: C1_liq = 610.94_r8 ! [Pa]
! Coefficients from Alduchov and Eskridge (1996) for Magnus dewpoint equation over ice
real(r8), parameter :: A1_ice = 22.587_r8 ! [-]
real(r8), parameter :: B1_ice = 273.86_r8 ! [degC]
real(r8), parameter :: C1_ice = 611.21_r8 ! [Pa]
!------------------------------------------------------------------------------

associate( &
Expand Down Expand Up @@ -425,14 +416,8 @@ subroutine BareGroundFluxes(bounds, num_noexposedvegp, filter_noexposedvegp, &
! corresponding to 1% relative humidity
forc_e = max( (forc_q(c)*forc_pbot(c)) / (forc_q(c)+0.622_r8), 0.01_r8*forc_esat)

! Magnus expression for dew point
! Equation (7) from Lawrence (2005)
if (t_grnd(c) < tfrz) then ! ice coefficients
forc_dewpoint = B1_ice*log(forc_e/C1_ice) / (A1_ice - log(forc_e/C1_ice))
else ! liquid water coefficients
forc_dewpoint = B1_liq*log(forc_e/C1_liq) / (A1_liq - log(forc_e/C1_liq))
end if
forc_dewpoint = forc_dewpoint + tfrz
! Calculate dewpoint at forcing height
forc_dewpoint = dewpoint(forc_e,t_grnd(c))

!changed by K.Sakaguchi. Soilbeta is used for evaporation
if (dqh(p) > 0._r8) then !dew (beta is not applied, just like rsoil used to be)
Expand Down Expand Up @@ -559,4 +544,38 @@ subroutine BareGroundFluxes(bounds, num_noexposedvegp, filter_noexposedvegp, &

end subroutine BareGroundFluxes
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@wwieder wwieder Jan 28, 2026

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Way to go @olyson, this seems to be a b4b refactor that cleans up the code by creating a function for dew and frost over bare ground. Assuming this is really b4b can we just merge the PR to b4b_dev?


!------------------------------------------------------------------------------
real(r8) function dewpoint(e,t)
!
! DESCRIPTION:
! Calculates dewpoint (degK) from input vapor pressure
! Uses Equation 7 from Lawrence 2005, https://doi.org/10.1175/BAMS-86-2-225

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@wwieder wwieder Jan 28, 2026

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Do we also need to update our documentation to reflect this?

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@olyson olyson Jan 28, 2026

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I will open an issue for the documentation. I'll also open an issue to document #3643

use clm_varcon, only : tfrz

! ARGUMENTS
real(r8), intent(in) :: e ! vapor pressure [Pa]
real(r8), intent(in) :: t ! temperature (K)
! Coefficients from Alduchov and Eskridge (1996) as used by Lawrence (2005) for Magnus dewpoint
! equation over liquid
real(r8), parameter :: A1_liq = 17.625_r8 ! [-]
real(r8), parameter :: B1_liq = 243.04_r8 ! [degC]
real(r8), parameter :: C1_liq = 610.94_r8 ! [Pa]
! Coefficients from Alduchov and Eskridge (1996) for Magnus dewpoint equation over ice
real(r8), parameter :: A1_ice = 22.587_r8 ! [-]
real(r8), parameter :: B1_ice = 273.86_r8 ! [degC]
real(r8), parameter :: C1_ice = 611.21_r8 ! [Pa]

! Magnus expression for dew point
! Equation (7) from Lawrence (2005)
if (t < tfrz) then ! ice coefficients
dewpoint = B1_ice*log(e/C1_ice) / (A1_ice - log(e/C1_ice))
else ! liquid water coefficients
dewpoint = B1_liq*log(e/C1_liq) / (A1_liq - log(e/C1_liq))
end if
dewpoint = dewpoint + tfrz

end function dewpoint
!------------------------------------------------------------------------------

end module BareGroundFluxesMod