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subroutine da_intpsfc_prs (val, ho, po, hm, tm, qm, to, qo) 3,2
!----------------------------------------------------------------------------
! Purpose: Correct pressure between two levels.
!
! Reference: make use of the hydrosatic equation:
!
! P2 = P1 * exp [-G/R * (z2-z1) / (tv1 + tv2)/2)
!
! Where:
! z1 = height at level 1
! z1 = height at level 2
! tv1 = temperature at level 1
! tv2 = temperature at level 2
! P1 = Pressure at level 1
! P2 = Pressure at level 2
!----------------------------------------------------------------------------
implicit none
real, intent (out) :: val
real, intent (in) :: ho, po
real, intent (in) :: hm, tm, qm
real, intent (in), optional :: to, qo
real :: tvo, tvm, tv, dz, arg
if (trace_use) call da_trace_entry
("da_intpsfc_prs")
! 1. model and observation virtual temperature
! ---------------------------------------------
tvm = tm * (1.0 + 0.608 * qm)
if (present(to) .and. present(qo)) then
tvo = to * (1.0 + 0.608 * qo)
else if (present(to) .and. .not.present(qo)) then
tvo = to
else
tvo = tvm
end if
tv = 0.5 * (tvm + tvo)
! 2. height difference bewteen model surface and observations
! ------------------------------------------------------------
dz = hm - ho
! 3. extrapolate pressure obs to model surface
! ---------------------------------------------
arg = dz * gravity / gas_constant
arg = arg / tv
val = po * exp (-arg)
if (trace_use) call da_trace_exit("da_intpsfc_prs")
end subroutine da_intpsfc_prs