da_sfc_pre.inc

1 subroutine da_sfc_pre (psfcm, psm, tsm, qsm, hsm, ho, to, qvo)
2 
3    !-----------------------------------------------------------------------
4    ! Purpose: Correct pressure between two levels. 
5    !
6    ! Reference: make use of the hydrosatic equation:
7    !
8    !  P2 = P1 * exp [-G/R * (z2-z1) / (tv1 + tv2)/2)
9    !
10    ! Where:
11    !  z1  = height at level 1
12    !  z1  = height at level 2
13    !  tv1 = temperature at level 1
14    !  tv2 = temperature at level 2
15    !  P1  = Pressure at level 1
16    !  P2  = Pressure at level 2
17    !---------------------------------------------------------------------------
18 
19    implicit none
20 
21    real, intent (out)   :: psfcm   ! model pressure at ho
22    real, intent (in)    :: psm, tsm, qsm
23 
24    real, intent (in)           :: hsm, ho
25    real, intent (in), optional :: to, qvo
26 
27    real                 :: tvo, tvsm, tv, dz, arg0, arg
28 
29    real, parameter      :: GASR =  gas_constant
30    real, parameter      :: G = gravity
31 
32    ! 1.  MODEL AND OBSERVATION VIRTUAL TEMPERATURE
33    ! ---------------------------------------------
34 
35    tvsm = tsm  * (1. + 0.608 * qsm)
36    if (present(to) .and. present(qvo)) then
37       tvo = to  * (1. + 0.608 * qvo)
38    else if (present(to) .and. .not.present(qvo)) then
39       tvo = to
40    else
41       tvo = tvsm
42    end if
43 
44    tv  = 0.5 * (tvsm + tvo)
45 
46    ! 2. HEIGHT DifFERENCE BEWTEEN MODEL SURFACE AND OBSERVATIONS
47    ! ------------------------------------------------------------
48 
49    dz = hsm - ho
50    arg0 = dz * g / gasr
51 
52    ! 3.  EXTRAPOLATE PRESSURE OBS TO MODEL SURFACE
53    ! ---------------------------------------------
54 
55    arg = arg0    / tv 
56 
57    psfcm = psm * exp (arg)
58 
59 end subroutine da_sfc_pre
60 
61