<HTML> <BODY BGCOLOR=#ccccdd LINK=#0000aa VLINK=#0000ff ALINK=#ff0000 ><BASE TARGET="bottom_target"><PRE>
<A NAME='DA_SFC_PRE'><A href='../../html_code/physics/da_sfc_pre.inc.html#DA_SFC_PRE' TARGET='top_target'><IMG SRC="../../gif/bar_red.gif" border=0></A>
subroutine da_sfc_pre (psfcm, psm, tsm, qsm, hsm, ho, to, qvo) 18,2
!-----------------------------------------------------------------------
! 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) :: psfcm ! model pressure at ho
real, intent (in) :: psm, tsm, qsm
real, intent (in) :: hsm, ho
real, intent (in), optional :: to, qvo
real :: tvo, tvsm, tv, dz, arg0, arg
real, parameter :: GASR = gas_constant
real, parameter :: G = gravity
if (trace_use) call da_trace_entry
("da_sfc_pre")
! 1. MODEL AND OBSERVATION VIRTUAL TEMPERATURE
! ---------------------------------------------
tvsm = tsm * (1.0 + 0.608 * qsm)
if (present(to) .and. present(qvo)) then
tvo = to * (1.0 + 0.608 * qvo)
else if (present(to) .and. .not.present(qvo)) then
tvo = to
else
tvo = tvsm
end if
tv = 0.5 * (tvsm + tvo)
! 2. HEIGHT DifFERENCE BEWTEEN MODEL SURFACE AND OBSERVATIONS
! ------------------------------------------------------------
dz = hsm - ho
arg0 = dz * g / gasr
! 3. EXTRAPOLATE PRESSURE OBS TO MODEL SURFACE
! ---------------------------------------------
arg = arg0 / tv
psfcm = psm * exp (arg)
if (trace_use) call da_trace_exit("da_sfc_pre")
end subroutine da_sfc_pre