C $Header: /home/ubuntu/mnt/e9_copy/MITgcm/pkg/generic_advdiff/gad_c4_adv_r.F,v 1.1 2001/05/30 19:34:48 adcroft Exp $
C $Name:  $

#include "GAD_OPTIONS.h"

      SUBROUTINE GAD_C4_ADV_R( 
     I           bi,bj,k,
     I           rTrans,
     I           tracer,
     O           wT,
     I           myThid )
      IMPLICIT NONE

C     == GLobal variables ==
#include "SIZE.h"
#include "GRID.h"
#include "GAD.h"

C     == Routine arguments ==
      INTEGER bi,bj,k
      _RL rTrans(1-OLx:sNx+OLx,1-OLy:sNy+OLy)
      _RL tracer(1-OLx:sNx+OLx,1-OLy:sNy+OLy,Nr,nSx,nSy)
      _RL wT    (1-OLx:sNx+OLx,1-OLy:sNy+OLy)
      INTEGER myThid

C     == Local variables ==
      INTEGER i,j,kp1,km1,km2
      _RL Cr,Rjm,Rj,Rjp,Rjjm,Rjjp
#include "GAD_FLUX_LIMITER.h"

      km2=MAX(1,k-2)
      km1=MAX(1,k-1)
      kp1=MIN(Nr,k+1)

      IF ( k.GT.Nr) THEN
       DO j=1-Oly,sNy+Oly
        DO i=1-Olx,sNx+Olx
         wT(i,j) = 0.
        ENDDO
       ENDDO
      ELSE
       DO j=1-Oly,sNy+Oly
        DO i=1-Olx,sNx+Olx
         Rjp=(tracer(i,j,kp1,bi,bj)-tracer(i,j,k,bi,bj))
     &        *maskC(i,j,kp1,bi,bj)
         Rj=(tracer(i,j,k,bi,bj)-tracer(i,j,kM1,bi,bj))
     &        *maskC(i,j,k,bi,bj)
         Rjm=(tracer(i,j,km1,bi,bj)-tracer(i,j,kM2,bi,bj))
     &        *maskC(i,j,km1,bi,bj)
         Rjjp=Rjp-Rj
         Rjjm=Rj-Rjm
         wT(i,j) =
     &     rTrans(i,j)*(
     &       (maskC(i,j,kM1,bi,bj)*
     &        (Tracer(i,j,k,bi,bj)+Tracer(i,j,kM1,bi,bj))*0.5 _d 0
     &       +(maskC(i,j,k,bi,bj)-maskC(i,j,kM1,bi,bj))*
     &        Tracer(i,j,k,bi,bj)
     &       )
     &     -maskC(i,j,km1,bi,bj)*oneSixth*(Rjjm+Rjjp)*0.5 _d 0 )
        ENDDO
       ENDDO
      ENDIF

      RETURN
      END