FUNCTION f_qv_from_rh (RH, T_K, RH0, T_K0, P_PA0) RESULT (QV_KG)
!!!--------------------------------------------------------------------------
!!!
!!!                       FUNCTION F_QV_FROM_RH
!!!                     **************************
!!!
!!!  PURPOSE: 
!!!  -------
!!!     TANGENT LINEAR CODE FOR
!!!     COMPUTING MIXING RATIO FROM RELATIVE HUMIDITY, TEMPERATURE AND PRESSURE
!!!
!!!     THE ERROR DERIVATION SHOULD USED THE TANGENT LINEAR CODE, NOT THE
!!!     ORIGINAL NON-LINEAR CODE.
!!!
!!   METHOD:
!!   ------
!!      LINEAR OR LOGARITHMIC VERTICAL INTERPOLATION
!!      OUT OF BOUND LOCATIONS ARE EXTRAPOLATED
!!
!!   INPUT:
!!   -----
!!      RH:    RELAITVE HUMIDITY in %
!!      P_PA:  PRESSURE          in Pa
!!      T_K:   TEMPERATURE       in K
!!
!!   OUTPUT:
!!   ------
!!      QV_KG: MIXING RATIO IN kg/kg
!!
!!   COMMON:           NO
!!   -------
!!   EXTERNAL:         NO                   
!!   --------
!!
!!   REFERENCES:
!!   -----------
!!    R. R. ROGERS AND M. K. YAU, 1989: A SHORT COURSE IN CLOUD PHYSICS,
!!                                      3ND EDITION, PERGAMON PRESS, PAGE 14-19.
!!
!!   VERIFICATION SET:
!!   -----------------
!!    T_K  = 268.15 K,  
!!    TD_K = 262.55 K
!!    RH   = 65 %, 
!!    P_PA = 80000  Pa, 
!!    QV   = 2.11E-03 kg/kg,
!!
!!  MODIFICATIONS:
!!   ------------
!!       Developed by Yong-Run Guo (11/07/00)
!!----------------------------------------------------------------------------CC

      IMPLICIT NONE

      REAL T_K , RH , QV_KG
      REAL P_PA0, T_K0, RH0
      ! REAL P_MB, W_KG
      REAL ES , QS
      REAL P_MB0, ES0, QS0, QV_KG0
!------------------------------------------------------------------------------C

!...P in mb

      P_MB0 = P_PA0 / 100.

!...VAPOR PRESSURE in mb

      ES  = 6.112 * 17.67 * 243.5 * T_K /                       &
                    ((T_K0-273.15+243.5)*(T_K0-273.15+243.5)) * &
                    EXP (17.67*(T_K0-273.15)/(T_K0-273.15+243.5))
      ES0 = 6.112 * EXP (17.67*(T_K0-273.15)/(T_K0-273.15+243.5)) 

!...SATURATION MIXING RATIO in kg/kg

      QS  = 0.622 * (P_MB0 * ES ) /  &
                   ((P_MB0 - ES0) * (P_MB0 - ES0))
      QS0 = 0.622 * ES0 /(P_MB0-ES0)            

!...MIXING RATIO in kg/kg

      QV_KG  = 0.01 * (RH0 * QS + RH * QS0)
      QV_KG0 = 0.01 * RH0 * QS0

!...Mixing ratio must be positive

      IF (QV_KG0 < 0.) QV_KG = 0.

      RETURN

      END FUNCTION f_qv_from_rh