! $Header$

SUBROUTINE clift(p, t, rr, rs, plcl, dplcldt, dplcldq)
  USE lmdz_yomcst

  IMPLICIT NONE
  ! ***************************************************************
  ! *                                                             *
  ! * CLIFT : COMPUTE LIFTING CONDENSATION LEVEL AND ITS          *
  ! *         DERIVATIVES RELATIVE TO T AND R                     *
  ! *   (WITHIN 0.2% OF FORMULA OF BOLTON, MON. WEA. REV.,1980)   *
  ! *                                                             *
  ! * written by   : GRANDPEIX Jean-Yves, 17/11/98, 12.39.01      *
  ! * modified by :                                               *
  ! ***************************************************************
  ! *
  ! *Arguments :
  ! *
  ! * Input :  P = pressure of level from wich lifting is performed
  ! *          T = temperature of level P
  ! *          RR = vapour mixing ratio at level P
  ! *          RS = vapour saturation mixing ratio at level P
  ! *
  ! * Output : PLCL = lifting condensation level
  ! *          DPLCLDT = derivative of PLCL relative to T
  ! *          DPLCLDQ = derivative of PLCL relative to R
  ! *
  ! cccccccccccccccccccccc
  ! constantes coherentes avec le modele du Centre Europeen
  ! RD = 1000.0 * 1.380658E-23 * 6.0221367E+23 / 28.9644
  ! RV = 1000.0 * 1.380658E-23 * 6.0221367E+23 / 18.0153
  ! CPD = 3.5 * RD
  ! CPV = 4.0 * RV
  ! CL = 4218.0
  ! CI=2090.0
  ! CPVMCL=CL-CPV
  ! CLMCI=CL-CI
  ! EPS=RD/RV
  ! ALV0=2.5008E+06
  ! ALF0=3.34E+05

  ! on utilise les constantes thermo du Centre Europeen: (sb)

  REAL :: p, t, rr, rs, plcl, dplcldt, dplcldq, cpd, cpv, cl, cpvmcl, eps, alv0, a, b
  REAL :: rh, chi, alv

  cpd = rcpd
  cpv = rcpv
  cl = rcw
  cpvmcl = cl - cpv
  eps = rd / rv
  alv0 = rlvtt


  ! Bolton formula coefficients :
  a = 1669.0
  b = 122.0

  rh = rr / rs
  chi = t / (a - b * rh - t)
  plcl = p * (rh**chi)

  alv = alv0 - cpvmcl * (t - 273.15)

  ! -- sb: correction:
  ! DPLCLDQ = PLCL*CHI*( 1./RR - B*CHI/T/RS*ALOG(RH) )
  dplcldq = plcl * chi * (1. / rr + b * chi / t / rs * alog(rh))
  ! sb --

  dplcldt = plcl * chi * ((a - b * rh * (1. + alv / rv / t)) / t**2 * chi * alog(rh) - alv / rv / t**2)

END SUBROUTINE clift