C $Header$
C $Name$

#include "MOM_VECINV_OPTIONS.h"

      SUBROUTINE MOM_VI_V_VERTSHEAR(
     I        bi,bj,K,
     I        vFld,wFld,
     U        vShearTerm,
     I        myThid)
      IMPLICIT NONE
C     *==========================================================*
C     | S/R MOM_V_VERTSHEAR
C     *==========================================================*
C     *==========================================================*

C     == Global variables ==
#include "SIZE.h"
#include "EEPARAMS.h"
#include "GRID.h"
#include "PARAMS.h"

C     == Routine arguments ==
      INTEGER bi,bj,K
      _RL vFld(1-OLx:sNx+OLx,1-OLy:sNy+OLy,Nr,nSx,nSy)
      _RL wFld(1-OLx:sNx+OLx,1-OLy:sNy+OLy,Nr,nSx,nSy)
      _RL vShearTerm(1-OLx:sNx+OLx,1-OLy:sNy+OLy)
      INTEGER myThid

C     == Local variables ==
      INTEGER I,J,Kp1,Km1
      _RL mask_Kp1,mask_Km1,wBarYm,wBarYp
      _RL vZm,vZp
      LOGICAL  rAdvAreaWeight
c     _RL vmask_Kp1,vmask_K,vmask_Km1
c     LOGICAL freeslipK,noslipK
c     PARAMETER(freeslipK=.TRUE.)
c     PARAMETER(noslipK=.NOT.freeslipK)
c     LOGICAL freeslip1,noslip1
c     PARAMETER(freeslip1=.TRUE.)
c     PARAMETER(noslip1=.NOT.freeslip1)
c1    _RL wBarYZ,vZbarZ

      rAdvAreaWeight =.TRUE.
C-    Area-weighted average either in KE or in vert. advection:
      IF ( selectKEscheme.EQ.1 .OR. selectKEscheme.EQ.3 )
     &  rAdvAreaWeight =.FALSE.

      Kp1=min(K+1,Nr)
      mask_Kp1=1.
      IF (K.EQ.Nr) mask_Kp1=0.
      Km1=max(K-1,1)
      mask_Km1=1.
      IF (K.EQ.1) mask_Km1=0.

      DO J=2-OLy,sNy+OLy
       DO I=1-OLx,sNx+OLx

c       vmask_K=_maskS(i,j,k,bi,bj)

C barZ( barY( W ) )
c       wBarYm=0.5*(wFld(I,J,K,bi,bj)+wFld(I,J-1,K,bi,bj))
c       wBarYp=0.5*(wFld(I,J,Kp1,bi,bj)+wFld(I,J-1,Kp1,bi,bj))
c    &              *mask_Kp1

       IF ( rAdvAreaWeight ) THEN
C       Transport at interface k : Area weighted average
        wBarYm=0.5*(
     &    wFld(I,J,K,bi,bj)*rA(i,j,bi,bj)*maskC(i,j,Km1,bi,bj)
     &   +wFld(I,J-1,K,bi,bj)*rA(i,j-1,bi,bj)*maskC(i,j-1,Km1,bi,bj)
     &             )*mask_Km1*deepFac2F(K)*rhoFacF(K)
     &              *recip_rAs(i,j,bi,bj)

C       Transport at interface k+1 (here wFld is already masked)
        wBarYp=0.5*(
     &    wFld(I,J,Kp1,bi,bj)*rA(i,j,bi,bj)
     &   +wFld(I,J-1,Kp1,bi,bj)*rA(i,j-1,bi,bj)
     &             )*mask_Kp1*deepFac2F(Kp1)*rhoFacF(Kp1)
     &              *recip_rAs(i,j,bi,bj)
       ELSE
C       Transport at interface k : simple average
        wBarYm=0.5*(
     &    wFld(I,J,K,bi,bj)*maskC(i,j,Km1,bi,bj)
     &   +wFld(I,J-1,K,bi,bj)*maskC(i,j-1,Km1,bi,bj)
     &             )*mask_Km1*deepFac2F(K)*rhoFacF(K)

C       Transport at interface k+1 (here wFld is already masked)
        wBarYp=0.5*(
     &    wFld(I,J,Kp1,bi,bj)
     &   +wFld(I,J-1,Kp1,bi,bj)
     &             )*mask_Kp1*deepFac2F(Kp1)*rhoFacF(Kp1)
       ENDIF

C delta_Z( V )  @ interface k
c       vmask_Km1=mask_Km1*maskS(i,j,Km1,bi,bj)
        vZm=(vFld(I,J,K,bi,bj)-mask_Km1*vFld(I,J,Km1,bi,bj))*rkSign
c2   &      *recip_dRC(K)
c       IF (freeslip1) vZm=vZm*vmask_Km1
c       IF (noslip1.AND.vmask_Km1.EQ.0.) vZm=vZm*2.

C delta_Z( V )  @ interface k+1
c       vmask_Kp1=mask_Kp1*maskS(i,j,Kp1,bi,bj)
        vZp=(mask_Kp1*vFld(I,J,Kp1,bi,bj)-vFld(I,J,K,bi,bj))*rkSign
c2   &      *recip_dRC(Kp1)
c       IF (freeslipK) vZp=vZp*vmask_Kp1
c       IF (noslipK.AND.vmask_Kp1.EQ.0.) vZp=vZp*2.

c1      IF (upwindShear) THEN
c1       wBarYZ=0.5*( wBarXm + wBarXp )
c1       IF (wBarYZ.GT.0.) THEN
c1        vZbarZ=vZp
c1       ELSE
c1        vZbarZ=vZm
c1       ENDIF
c1      ELSE
c1       vZbarZ=0.5*(vZm+vZp)
c1      ENDIF
c1      vShearTerm(I,J)=-wBarYZ*vZbarZ*_maskS(I,J,K,bi,bj)

c2      vShearTerm(I,J)=-0.5*(wBarYp*vZp+wBarYm*vZm)
c2   &                  *_maskS(I,J,K,bi,bj)
        IF (upwindShear) THEN
          vShearTerm(I,J)=-0.5*
     &                   (     (wBarYp*vZp+wBarYm*vZm)
     &                        +(ABS(wBarYp)*vZp-ABS(wBarYm)*vZm)
     &                   )*_recip_hFacS(i,j,k,bi,bj)
     &                    * recip_drF(K)
     &                    * recip_deepFac2C(K)*recip_rhoFacC(K)
        ELSE
          vShearTerm(I,J)=-0.5*(wBarYp*vZp+wBarYm*vZm)
     &                    *_recip_hFacS(i,j,k,bi,bj)
     &                    * recip_drF(K)
     &                    * recip_deepFac2C(K)*recip_rhoFacC(K)
        ENDIF
       ENDDO
      ENDDO

      RETURN
      END
