#include "cppdefs.h"
      MODULE ad_zetabc_mod
#ifdef ADJOINT
!
!svn $Id$
!================================================== Hernan G. Arango ===
!  Copyright (c) 2002-2020 The ROMS/TOMS Group       Andrew M. Moore   !
!    Licensed under a MIT/X style license                              !
!    See License_ROMS.txt                                              !
!=======================================================================
!                                                                      !
!  This subroutine sets adjoint lateral boundary conditions for        !
!  free-surface. It updates the specified "kout" index.                !
!                                                                      !
!  BASIC STATE variables needed: zeta                                  !
!                                                                      !
!=======================================================================
!
      implicit none

      PRIVATE
      PUBLIC  :: ad_zetabc, ad_zetabc_tile

      CONTAINS
!
!***********************************************************************
      SUBROUTINE ad_zetabc (ng, tile, kout)
!***********************************************************************
!
      USE mod_param
      USE mod_ocean
      USE mod_stepping
!
!  Imported variable declarations.
!
      integer, intent(in) :: ng, tile, kout
!
!  Local variable declarations.
!
# include "tile.h"
!
      CALL ad_zetabc_tile (ng, tile,                                    &
     &                     LBi, UBi, LBj, UBj,                          &
     &                     IminS, ImaxS, JminS, JmaxS,                  &
     &                     krhs(ng), kstp(ng), kout,                    &
     &                     OCEAN(ng) % zeta,                            &
     &                     OCEAN(ng) % ad_zeta)
      RETURN
      END SUBROUTINE ad_zetabc
!
!***********************************************************************
      SUBROUTINE ad_zetabc_tile (ng, tile,                              &
     &                           LBi, UBi, LBj, UBj,                    &
     &                           IminS, ImaxS, JminS, JmaxS,            &
     &                           krhs, kstp, kout,                      &
     &                           zeta, ad_zeta)
!***********************************************************************
!
      USE mod_param
      USE mod_boundary
      USE mod_grid
      USE mod_ncparam
      USE mod_scalars
!
!  Imported variable declarations.
!
      integer, intent(in) :: ng, tile
      integer, intent(in) :: LBi, UBi, LBj, UBj
      integer, intent(in) :: IminS, ImaxS, JminS, JmaxS
      integer, intent(in) :: krhs, kstp, kout
!
# ifdef ASSUMED_SHAPE
      real(r8), intent(in) :: zeta(LBi:,LBj:,:)

      real(r8), intent(inout) :: ad_zeta(LBi:,LBj:,:)
# else
      real(r8), intent(in) :: zeta(LBi:UBi,LBj:UBj,3)

      real(r8), intent(inout) :: ad_zeta(LBi:UBi,LBj:UBj,3)
# endif
!
!  Local variable declarations.
!
      integer :: i, j, know

      real(r8) :: Ce, Cx
      real(r8) :: cff, cff1, cff2, dt2d, tau

      real(r8) :: ad_Ce, ad_Cx
      real(r8) :: ad_cff1, ad_cff2
      real(r8) :: adfac

      real(r8), dimension(IminS:ImaxS,JminS:JmaxS) :: ad_grad

# include "set_bounds.h"
!
!-----------------------------------------------------------------------
!  Initialize adjoint private variables.
!-----------------------------------------------------------------------
!
      ad_Ce=0.0_r8
      ad_Cx=0.0_r8
      ad_cff1=0.0_r8
      ad_cff2=0.0_r8

      ad_grad(LBi:UBi,LBj:UBj)=0.0_r8
!
!-----------------------------------------------------------------------
!  Set time-indices
!-----------------------------------------------------------------------
!
      IF (FIRST_2D_STEP) THEN
        know=krhs
        dt2d=dtfast(ng)
      ELSE IF (PREDICTOR_2D_STEP(ng)) THEN
        know=krhs
        dt2d=2.0_r8*dtfast(ng)
      ELSE
        know=kstp
        dt2d=dtfast(ng)
      END IF

# if defined WET_DRY
!
!-----------------------------------------------------------------------
! Ensure that water level on boundary cells is above bed elevation.
!-----------------------------------------------------------------------
!
      cff=Dcrit(ng)-eps

      IF (.not.(EWperiodic(ng).or.NSperiodic(ng))) THEN
        IF (DOMAIN(ng)%NorthEast_Corner(tile)) THEN
          IF (LBC_apply(ng)%north(Iend+1).and.                          &
     &        LBC_apply(ng)%east (Jend+1)) THEN
            IF (zeta(Iend+1,Jend+1,kout).le.                            &
     &          (Dcrit(ng)-GRID(ng)%h(Iend+1,Jend+1))) THEN
!>            tl_zeta(Iend+1,Jend+1,kout)=-GRID(ng)%tl_h(Iend+1,Jend+1)
!>
              GRID(ng)%ad_h(Iend+1,Jend+1)=GRID(ng)%ad_h(Iend+1,Jend+1)-&
     &                                     ad_zeta(Iend+1,Jend+1,kout)
              ad_zeta(Iend+1,Jend+1,kout)=0.0_r8
            END IF
          END IF
        END IF
        IF (DOMAIN(ng)%NorthWest_Corner(tile)) THEN
          IF (LBC_apply(ng)%north(Istr-1).and.                          &
     &        LBC_apply(ng)%west (Jend+1)) THEN
            IF (zeta(Istr-1,Jend+1,kout).le.                            &
     &          (Dcrit(ng)-GRID(ng)%h(Istr-1,Jend+1))) THEN
!>            tl_zeta(Istr-1,Jend+1,kout)=-GRID(ng)%tl_h(Istr-1,Jend+1)
!>
              GRID(ng)%ad_h(Istr-1,Jend+1)=GRID(ng)%ad_h(Istr-1,Jend+1)-&
     &                                     ad_zeta(Istr-1,Jend+1,kout)
              ad_zeta(Istr-1,Jend+1,kout)=0.0_r8
            END IF
          END IF
        END IF
        IF (DOMAIN(ng)%SouthEast_Corner(tile)) THEN
          IF (LBC_apply(ng)%south(Iend+1).and.                          &
     &        LBC_apply(ng)%east (Jstr-1)) THEN
            IF (zeta(Iend+1,Jstr-1,kout).le.                            &
     &          (Dcrit(ng)-GRID(ng)%h(Iend+1,Jstr-1))) THEN
!>            tl_zeta(Iend+1,Jstr-1,kout)=-GRID(ng)%tl_h(Iend+1,Jstr-1)
!>
              GRID(ng)%ad_h(Iend+1,Jstr-1)=GRID(ng)%ad_h(Iend+1,Jstr-1)-&
     &                                     ad_zeta(Iend+1,Jstr-1,kout)
              tl_zeta(Iend+1,Jstr-1,kout)=0.0_r8
            END IF
          END IF
        END IF
        IF (DOMAIN(ng)%SouthWest_Corner(tile)) THEN
          IF (LBC_apply(ng)%south(Istr-1).and.                          &
     &        LBC_apply(ng)%west (Jstr-1)) THEN
            IF (zeta(Istr-1,Jstr-1,kout).le.                            &
     &          (Dcrit(ng)-GRID(ng)%h(Istr-1,Jstr-1))) THEN
!>            tl_zeta(Istr-1,Jstr-1,kout)=-GRID(ng)%tl_h(Istr-1,Jstr-1)
!>
              GRID(ng)%ad_h(Istr-1,Jstr-1)=GRID(ng)%ad_h(Istr-1,Jstr-1)-&
     &                                     ad_zeta(Istr-1,Jstr-1,kout)
              ad_zeta(Istr-1,Jstr-1,kout)=0.0_r8
            END IF
          END IF
        END IF
      END IF

      IF (.not.NSperiodic(ng)) THEN
        IF (DOMAIN(ng)%Northern_Edge(tile)) THEN
          DO i=Istr,Iend
            IF (LBC_apply(ng)%north(i)) THEN
              IF (zeta(i,Jend+1,kout).le.                               &
     &            (Dcrit(ng)-GRID(ng)%h(i,Jend+1))) THEN
!>              tl_zeta(i,Jend+1,kout)=-GRID(ng)%tl_h(i,Jend+1)
!>
                GRID(ng)%ad_h(i,Jend+1)=GRID(ng)%ad_h(i,Jend+1)-        &
     &                                  ad_zeta(i,Jend+1,kout)
                ad_zeta(i,Jend+1,kout)=0.0_r8
              END IF
            END IF
          END DO
        END IF
        IF (DOMAIN(ng)%Southern_Edge(tile)) THEN
          DO i=Istr,Iend
            IF (LBC_apply(ng)%south(i)) THEN
              IF (zeta(i,Jstr-1,kout).le.                               &
     &            (Dcrit(ng)-GRID(ng)%h(i,Jstr-1))) THEN
!>              tl_zeta(i,Jstr-1,kout)=-GRID(ng)%tl_h(i,Jstr-1)
!>
                GRID(ng)%ad_h(i,Jstr-1)=GRID(ng)%ad_h(i,Jstr-1)-        &
     &                                  ad_zeta(i,Jstr-1,kout)
                ad_zeta(i,Jstr-1,kout)=0.0_r8
              END IF
            END IF
          END DO
        END IF
      END IF

      IF (.not.EWperiodic(ng)) THEN
        IF (DOMAIN(ng)%Eastern_Edge(tile)) THEN
          DO j=Jstr,Jend
            IF (LBC_apply(ng)%east(j)) THEN
              IF (zeta(Iend+1,j,kout).le.                               &
     &            (Dcrit(ng)-GRID(ng)%h(Iend+1,j))) THEN
!>              tl_zeta(Iend+1,j,kout)=-GRID(ng)%tl_h(Iend+1,j)
!>
                GRID(ng)%ad_h(Iend+1,j)=GRID(ng)%ad_h(Iend+1,j)-        &
     &                                  ad_zeta(Iend+1,j,kout)
                ad_zeta(Iend+1,j,kout)=0.0_r8
              END IF
            END IF
          END DO
        END IF
        IF (DOMAIN(ng)%Western_Edge(tile)) THEN
          DO j=Jstr,Jend
            IF (LBC_apply(ng)%west(j)) THEN
              IF (zeta(Istr-1,j,kout).le.                               &
     &            (Dcrit(ng)-GRID(ng)%h(Istr-1,j))) THEN
!>              tl_zeta(Istr-1,j,kout)=-GRID(ng)%tl_h(Istr-1,j)
!>
                GRID(ng)%ad_h(Istr-1,j)=GRID(ng)%ad_h(Istr-1,j)-        &
     &                                  ad_zeta(Istr-1,j,kout)
                ad_zeta(Istr-1,j,kout)=0.0_r8
              END IF
            END IF
          END DO
        END IF
      END IF
# endif
!
!-----------------------------------------------------------------------
!  Boundary corners.
!-----------------------------------------------------------------------
!
      IF (.not.(EWperiodic(ng).or.NSperiodic(ng))) THEN
        IF (DOMAIN(ng)%NorthEast_Corner(tile)) THEN
          IF (LBC_apply(ng)%north(Iend+1).and.                          &
     &        LBC_apply(ng)%east (Jend+1)) THEN
!>          tl_zeta(Iend+1,Jend+1,kout)=0.5_r8*                         &
!>   &                                  (tl_zeta(Iend+1,Jend  ,kout)+   &
!>   &                                   tl_zeta(Iend  ,Jend+1,kout))
!>
            adfac=0.5_r8*ad_zeta(Iend+1,Jend+1,kout)
            ad_zeta(Iend  ,Jend+1,kout)=ad_zeta(Iend  ,Jend+1,kout)+    &
     &                                  adfac
            ad_zeta(Iend+1,Jend  ,kout)=ad_zeta(Iend+1,Jend  ,kout)+    &
     &                                  adfac
            ad_zeta(Iend+1,Jend+1,kout)=0.0_r8
          END IF
        END IF
        IF (DOMAIN(ng)%NorthWest_Corner(tile)) THEN
          IF (LBC_apply(ng)%north(Istr-1).and.                          &
     &        LBC_apply(ng)%west (Jend+1)) THEN
!>          tl_zeta(Istr-1,Jend+1,kout)=0.5_r8*                         &
!>   &                                  (tl_zeta(Istr-1,Jend  ,kout)+   &
!>   &                                   tl_zeta(Istr  ,Jend+1,kout))
!>
            adfac=0.5_r8*ad_zeta(Istr-1,Jend+1,kout)
            ad_zeta(Istr-1,Jend  ,kout)=ad_zeta(Istr-1,Jend  ,kout)+    &
     &                                  adfac
            ad_zeta(Istr  ,Jend+1,kout)=ad_zeta(Istr  ,Jend+1,kout)+    &
     &                                  adfac
            ad_zeta(Istr-1,Jend+1,kout)=0.0_r8
          END IF
        END IF
        IF (DOMAIN(ng)%SouthEast_Corner(tile)) THEN
          IF (LBC_apply(ng)%south(Iend+1).and.                          &
     &        LBC_apply(ng)%east (Jstr-1)) THEN
!>          tl_zeta(Iend+1,Jstr-1,kout)=0.5_r8*                         &
!>   &                                  (tl_zeta(Iend  ,Jstr-1,kout)+   &
!>   &                                   tl_zeta(Iend+1,Jstr  ,kout))
!>
            adfac=0.5_r8*ad_zeta(Iend+1,Jstr-1,kout)
            ad_zeta(Iend  ,Jstr-1,kout)=ad_zeta(Iend  ,Jstr-1,kout)+    &
     &                                  adfac
            ad_zeta(Iend+1,Jstr  ,kout)=ad_zeta(Iend+1,Jstr  ,kout)+    &
     &                                  adfac
            ad_zeta(Iend+1,Jstr-1,kout)=0.0_r8
          END IF
        END IF
        IF (DOMAIN(ng)%SouthWest_Corner(tile)) THEN
          IF (LBC_apply(ng)%south(Istr-1).and.                          &
     &        LBC_apply(ng)%west (Jstr-1)) THEN
!>          tl_zeta(Istr-1,Jstr-1,kout)=0.5_r8*                         &
!>   &                                  (tl_zeta(Istr  ,Jstr-1,kout)+   &
!>   &                                   tl_zeta(Istr-1,Jstr  ,kout))
!>
            adfac=0.5_r8*ad_zeta(Istr-1,Jstr-1,kout)
            ad_zeta(Istr  ,Jstr-1,kout)=ad_zeta(Istr  ,Jstr-1,kout)+    &
     &                                  adfac
            ad_zeta(Istr-1,Jstr  ,kout)=ad_zeta(Istr-1,Jstr  ,kout)+    &
     &                                  adfac
            ad_zeta(Istr-1,Jstr-1,kout)=0.0_r8
          END IF
        END IF
      END IF
!
!-----------------------------------------------------------------------
!  Lateral boundary conditions at the northern edge.
!-----------------------------------------------------------------------
!
      IF (DOMAIN(ng)%Northern_Edge(tile)) THEN
!
!  Northern edge, implicit upstream radiation condition.
!
        IF (ad_LBC(inorth,isFsur,ng)%radiation) THEN
          IF (iic(ng).ne.0) THEN
            DO i=Istr,Iend
              IF (LBC_apply(ng)%north(i)) THEN
# if defined CELERITY_READ && defined FORWARD_READ
                IF (ad_LBC(inorth,isFsur,ng)%nudging) THEN
                  IF (BOUNDARY(ng)%zeta_north_Ce(i).eq.0.0_r8) THEN
                    tau=FSobc_in(ng,inorth)
                  ELSE
                    tau=FSobc_out(ng,inorth)
                  END IF
                  tau=tau*dt2d
                END IF
#  ifdef RADIATION_2D
                Cx=BOUNDARY(ng)%zeta_north_Cx(i)
#  else
                Cx=0.0_r8
#  endif
                Ce=BOUNDARY(ng)%zeta_north_Ce(i)
                cff=BOUNDARY(ng)%zeta_north_C2(i)
# endif
# ifdef MASKING
!>              tl_zeta(i,Jend+1,kout)=tl_zeta(i,Jend+1,kout)*          &
!>   &                                 GRID(ng)%rmask(i,Jend+1)
!>
                ad_zeta(i,Jend+1,kout)=ad_zeta(i,Jend+1,kout)*          &
     &                                 GRID(ng)%rmask(i,Jend+1)
# endif
                IF (ad_LBC(inorth,isFsur,ng)%nudging) THEN
!>                tl_zeta(i,Jend+1,kout)=tl_zeta(i,Jend+1,kout)-        &
!>   &                                   tau*tl_zeta(i,Jend+1,know)
!>
                  ad_zeta(i,Jend+1,know)=ad_zeta(i,Jend+1,know)-        &
     &                                   tau*ad_zeta(i,Jend+1,kout)
                END IF
!>              tl_zeta(i,Jend+1,kout)=(cff*tl_zeta(i,Jend+1,know)+     &
!>   &                                  Ce *tl_zeta(i,Jend  ,kout)-     &
!>   &                                  MAX(Cx,0.0_r8)*                 &
!>   &                                     tl_grad(i  ,Jend+1)-         &
!>   &                                  MIN(Cx,0.0_r8)*                 &
!>   &                                     tl_grad(i+1,Jend+1))/        &
!>   &                                 (cff+Ce)
!>
                adfac=ad_zeta(i,Jend+1,kout)/(cff+Ce)
                ad_grad(i  ,Jend+1)=ad_grad(i  ,Jend+1)-                &
     &                              MAX(Cx,0.0_r8)*adfac
                ad_grad(i+1,Jend+1)=ad_grad(i+1,Jend+1)-                &
     &                              MIN(Cx,0.0_r8)*adfac
                ad_zeta(i,Jend  ,kout)=ad_zeta(i,Jend  ,kout)+Ce *adfac
                ad_zeta(i,Jend+1,know)=ad_zeta(i,Jend+1,know)+cff*adfac
                ad_zeta(i,Jend+1,kout)=0.0_r8
              END IF
            END DO
          END IF
!
!  Northern edge, explicit Chapman boundary condition.
!
        ELSE IF (ad_LBC(inorth,isFsur,ng)%Chapman_explicit) THEN
          DO i=Istr,Iend
            IF (LBC_apply(ng)%north(i)) THEN
              cff=dt2d*GRID(ng)%pn(i,Jend)
              cff1=SQRT(g*(GRID(ng)%h(i,Jend)+                          &
     &                     zeta(i,Jend,know)))
              Ce=cff*cff1
# ifdef MASKING
!>            tl_zeta(i,Jend+1,kout)=tl_zeta(i,Jend+1,kout)*            &
!>   &                               GRID(ng)%rmask(i,Jend+1)
!>
              ad_zeta(i,Jend+1,kout)=ad_zeta(i,Jend+1,kout)*            &
     &                               GRID(ng)%rmask(i,Jend+1)
# endif
!>            tl_zeta(i,Jend+1,kout)=(1.0_r8-Ce)*tl_zeta(i,Jend+1,know)+&
!>   &                               tl_Ce*(zeta(i,Jend+1,know)+        &
!>   &                                      zeta(i,Jend  ,know))+       &
!>   &                               Ce*tl_zeta(i,Jend,know)
!>
              ad_zeta(i,Jend+1,know)=ad_zeta(i,Jend+1,know)+            &
     &                               (1.0_r8-Ce)*ad_zeta(i,Jend+1,kout)
              ad_Ce=ad_Ce+(zeta(i,Jend+1,know)+                         &
     &                     zeta(i,Jend  ,know))*ad_zeta(i,Jend+1,kout)
              ad_zeta(i,Jend,know)=ad_zeta(i,Jend,know)+                &
     &                             Ce*ad_zeta(i,Jend+1,kout)
              ad_zeta(i,Jend+1,kout)=0.0_r8
!>            tl_Ce=cff*tl_cff1
!>
              ad_cff1=ad_cff1+cff*ad_Ce
              ad_Ce=0.0_r8
!>            tl_cff1=0.5_r8*g*(GRID(ng)%tl_h(i,Jend)+                  &
!>   &                          tl_zeta(i,Jend,know))/cff1
!>
              adfac=0.5_r8*g*ad_cff1/cff1
              GRID(ng)%ad_h(i,Jend)=GRID(ng)%ad_h(i,Jend)+adfac
              ad_zeta(i,Jend,know)=ad_zeta(i,Jend,know)+adfac
              ad_cff1=0.0_r8
            END IF
          END DO
!
!  Northern edge, implicit Chapman boundary condition.
!
        ELSE IF (ad_LBC(inorth,isFsur,ng)%Chapman_implicit) THEN
          DO i=Istr,Iend
            IF (LBC_apply(ng)%north(i)) THEN
              cff=dt2d*GRID(ng)%pn(i,Jend)
              cff1=SQRT(g*(GRID(ng)%h(i,Jend)+                          &
     &                     zeta(i,Jend,know)))
              Ce=cff*cff1
              cff2=1.0_r8/(1.0_r8+Ce)
# ifdef MASKING
!>            tl_zeta(i,Jend+1,kout)=tl_zeta(i,Jend+1,kout)*            &
!>   &                               GRID(ng)%rmask(i,Jend+1)
!>
              ad_zeta(i,Jend+1,kout)=ad_zeta(i,Jend+1,kout)*            &
     &                               GRID(ng)%rmask(i,Jend+1)
# endif
!>            tl_zeta(i,Jend+1,kout)=tl_cff2*(zeta(i,Jend+1,know)+      &
!>   &                                        Ce*zeta(i,Jend,kout))+    &
!>   &                               cff2*(tl_zeta(i,Jend+1,know)+      &
!>   &                                     tl_Ce*zeta(i,Jend,kout)+     &
!>   &                                     Ce*tl_zeta(i,Jend,kout))
!>
              adfac=cff2*ad_zeta(i,Jend+1,kout)
              ad_zeta(i,Jend  ,kout)=ad_zeta(i,Jend  ,kout)+Ce*adfac
              ad_zeta(i,Jend+1,know)=ad_zeta(i,Jend+1,know)+adfac
              ad_Ce=ad_Ce+zeta(i,Jend,kout)*adfac
              ad_cff2=ad_cff2+                                          &
     &                (zeta(i,Jend+1,know)+                             &
     &                 Ce*zeta(i,Jend,kout))*ad_zeta(i,Jend+1,kout)
              ad_zeta(i,Jend+1,kout)=0.0_r8
!>            tl_cff2=-cff2*cff2*tl_Ce
!>
              ad_Ce=ad_Ce-cff2*cff2*ad_cff2
              ad_cff2=0.0_r8
!>            tl_Ce=cff*tl_cff1
!>
              ad_cff1=ad_cff1+cff*ad_Ce
              ad_Ce=0.0_r8
!>            tl_cff1=0.5_r8*g*(GRID(ng)%tl_h(i,Jend)+                  &
!>   &                          tl_zeta(i,Jend,know))/cff1
!>
              adfac=0.5_r8*g*ad_cff1/cff1
              GRID(ng)%ad_h(i,Jend)=GRID(ng)%ad_h(i,Jend)+adfac
              ad_zeta(i,Jend,know)=ad_zeta(i,Jend,know)+adfac
              ad_cff1=0.0_r8
            END IF
          END DO
!
!  Northern edge, clamped boundary condition.
!
        ELSE IF (ad_LBC(inorth,isFsur,ng)%clamped) THEN
          DO i=Istr,Iend
            IF (LBC_apply(ng)%north(i)) THEN
# ifdef MASKING
!>            tl_zeta(i,Jend+1,kout)=tl_zeta(i,Jend+1,kout)*            &
!>   &                               GRID(ng)%rmask(i,Jend+1)
!>
              ad_zeta(i,Jend+1,kout)=ad_zeta(i,Jend+1,kout)*            &
     &                               GRID(ng)%rmask(i,Jend+1)
# endif
# ifdef ADJUST_BOUNDARY
              IF (Lobc(inorth,isFsur,ng)) THEN
!>              tl_zeta(i,Jend+1,kout)=BOUNDARY(ng)%tl_zeta_north(i)
!>
                BOUNDARY(ng)%ad_zeta_north(i)=BOUNDARY(ng)%             &
     &                                                ad_zeta_north(i)+ &
     &                                        ad_zeta(i,Jend+1,kout)
                ad_zeta(i,Jend+1,kout)=0.0_r8
              ELSE
!>              tl_zeta(i,Jend+1,kout)=0.0_r8
!>
                ad_zeta(i,Jend+1,kout)=0.0_r8
              END IF
# else
!>            tl_zeta(i,Jend+1,kout)=0.0_r8
!>
              ad_zeta(i,Jend+1,kout)=0.0_r8
# endif
            END IF
          END DO
!
!  Northern edge, gradient boundary condition.
!
        ELSE IF (ad_LBC(inorth,isFsur,ng)%gradient) THEN
          DO i=Istr,Iend
            IF (LBC_apply(ng)%north(i)) THEN
# ifdef MASKING
!>            tl_zeta(i,Jend+1,kout)=tl_zeta(i,Jend+1,kout)*            &
!>   &                               GRID(ng)%rmask(i,Jend+1)
!>
              ad_zeta(i,Jend+1,kout)=ad_zeta(i,Jend+1,kout)*            &
     &                               GRID(ng)%rmask(i,Jend+1)
# endif
!>            tl_zeta(i,Jend+1,kout)=tl_zeta(i,Jend,kout)
!>
              ad_zeta(i,Jend  ,kout)=ad_zeta(i,Jend  ,kout)+            &
     &                               ad_zeta(i,Jend+1,kout)
              ad_zeta(i,Jend+1,kout)=0.0_r8
            END IF
          END DO
!
!  Northern edge, closed boundary condition.
!
        ELSE IF (ad_LBC(inorth,isFsur,ng)%closed) THEN
          DO i=Istr,Iend
            IF (LBC_apply(ng)%north(i)) THEN
# ifdef MASKING
!>            tl_zeta(i,Jend+1,kout)=tl_zeta(i,Jend+1,kout)*            &
!>   &                               GRID(ng)%rmask(i,Jend+1)
!>
              ad_zeta(i,Jend+1,kout)=ad_zeta(i,Jend+1,kout)*            &
     &                               GRID(ng)%rmask(i,Jend+1)
# endif
!>            tl_zeta(i,Jend+1,kout)=tl_zeta(i,Jend,kout)
!>
              ad_zeta(i,Jend  ,kout)=ad_zeta(i,Jend  ,kout)+            &
     &                               ad_zeta(i,Jend+1,kout)
              ad_zeta(i,Jend+1,kout)=0.0_r8
            END IF
          END DO
        END IF
      END IF
!
!-----------------------------------------------------------------------
!  Lateral boundary conditions at the southern edge.
!-----------------------------------------------------------------------
!
      IF (DOMAIN(ng)%Southern_Edge(tile)) THEN
!
!  Southern edge, implicit upstream radiation condition.
!
        IF (ad_LBC(isouth,isFsur,ng)%radiation) THEN
          IF (iic(ng).ne.0) THEN
            DO i=Istr,Iend
              IF (LBC_apply(ng)%south(i)) THEN
# if defined CELERITY_READ && defined FORWARD_READ
                IF (ad_LBC(isouth,isFsur,ng)%nudging) THEN
                  IF (BOUNDARY(ng)%zeta_south_Ce(i).eq.0.0_r8) THEN
                    tau=FSobc_in(ng,isouth)
                  ELSE
                    tau=FSobc_out(ng,isouth)
                  END IF
                  tau=tau*dt2d
                END IF
#  ifdef RADIATION_2D
                Cx=BOUNDARY(ng)%zeta_south_Cx(i)
#  else
                Cx=0.0_r8
#  endif
                Ce=BOUNDARY(ng)%zeta_south_Ce(i)
                cff=BOUNDARY(ng)%zeta_south_C2(i)
# endif
# ifdef MASKING
!>              tl_zeta(i,Jstr-1,kout)=tl_zeta(i,Jstr-1,kout)*          &
!>   &                                 GRID(ng)%rmask(i,Jstr-1)
!>
                ad_zeta(i,Jstr-1,kout)=ad_zeta(i,Jstr-1,kout)*          &
     &                                 GRID(ng)%rmask(i,Jstr-1)
# endif
                IF (ad_LBC(isouth,isFsur,ng)%nudging) THEN
!>                tl_zeta(i,Jstr-1,kout)=tl_zeta(i,Jstr-1,kout)-        &
!>   &                                   tau*tl_zeta(i,Jstr-1,know)
!>
                  ad_zeta(i,Jstr-1,know)=ad_zeta(i,Jstr-1,know)-        &
     &                                   tau*ad_zeta(i,Jstr-1,kout)
                END IF
!>              tl_zeta(i,Jstr-1,kout)=(cff*tl_zeta(i,Jstr-1,know)+     &
!>   &                                  Ce *tl_zeta(i,Jstr  ,kout)-     &
!>   &                                  MAX(Cx,0.0_r8)*                 &
!>   &                                     tl_grad(i  ,Jstr-1)-         &
!>   &                                  MIN(Cx,0.0_r8)*                 &
!>   &                                     tl_grad(i+1,Jstr-1))/        &
!>   &                                 (cff+Ce)
!>
                adfac=ad_zeta(i,Jstr-1,kout)/(cff+Ce)
                ad_grad(i  ,Jstr-1)=ad_grad(i  ,Jstr-1)-                &
     &                              MAX(Cx,0.0_r8)*adfac
                ad_grad(i+1,Jstr-1)=ad_grad(i+1,Jstr-1)-                &
     &                              MIN(Cx,0.0_r8)*adfac
                ad_zeta(i,Jstr-1,know)=ad_zeta(i,Jstr-1,know)+cff*adfac
                ad_zeta(i,Jstr  ,kout)=ad_zeta(i,Jstr  ,kout)+Ce *adfac
                ad_zeta(i,Jstr-1,kout)=0.0_r8
              END IF
            END DO
          END IF
!
!  Southern edge, explicit Chapman boundary condition.
!
        ELSE IF (ad_LBC(isouth,isFsur,ng)%Chapman_explicit) THEN
          DO i=Istr,Iend
            IF (LBC_apply(ng)%south(i)) THEN
              cff=dt2d*GRID(ng)%pn(i,Jstr)
              cff1=SQRT(g*(GRID(ng)%h(i,Jstr)+                          &
     &                     zeta(i,Jstr,know)))
              Ce=cff*cff1
# ifdef MASKING
!>            tl_zeta(i,Jstr-1,kout)=tl_zeta(i,Jstr-1,kout)*            &
!>   &                               GRID(ng)%rmask(i,Jstr-1)
!>
              ad_zeta(i,Jstr-1,kout)=ad_zeta(i,Jstr-1,kout)*            &
     &                               GRID(ng)%rmask(i,Jstr-1)
# endif
!>            tl_zeta(i,Jstr-1,kout)=(1.0_r8-Ce)*tl_zeta(i,Jstr-1,know)+&
!>   &                               tl_Ce*(zeta(i,Jstr-1,know)+        &
!>   &                                      zeta(i,Jstr  ,know))+       &
!>   &                               Ce*tl_zeta(i,Jstr,know)
!>
              ad_zeta(i,Jstr-1,know)=ad_zeta(i,Jstr-1,know)+            &
     &                               (1.0_r8-Ce)*ad_zeta(i,Jstr-1,kout)
              ad_Ce=ad_Ce+(zeta(i,Jstr-1,know)+                         &
     &                     zeta(i,Jstr  ,know))*ad_zeta(i,Jstr-1,kout)
              ad_zeta(i,Jstr,know)=ad_zeta(i,Jstr,know)+                &
     &                             Ce*ad_zeta(i,Jstr-1,kout)
              ad_zeta(i,Jstr-1,kout)=0.0_r8
!>            tl_Ce=cff*tl_cff1
!>
              ad_cff1=ad_cff1+cff*ad_Ce
              ad_Ce=0.0_r8
!>            tl_cff1=0.5_r8*g*(GRID(ng)%tl_h(i,Jstr)+                  &
!>   &                          tl_zeta(i,Jstr,know))/cff1
!>
              adfac=0.5_r8*g*ad_cff1/cff1
              GRID(ng)%ad_h(i,Jstr)=GRID(ng)%ad_h(i,Jstr)+adfac
              ad_zeta(i,Jstr,know)=ad_zeta(i,Jstr,know)+adfac
              ad_cff1=0.0_r8
            END IF
          END DO
!
!  Southern edge, implicit Chapman boundary condition.
!
        ELSE IF (ad_LBC(isouth,isFsur,ng)%Chapman_implicit) THEN
          DO i=Istr,Iend
            IF (LBC_apply(ng)%south(i)) THEN
              cff=dt2d*GRID(ng)%pn(i,Jstr)
              cff1=SQRT(g*(GRID(ng)%h(i,Jstr)+                          &
     &                     zeta(i,Jstr,know)))
              Ce=cff*cff1
              cff2=1.0_r8/(1.0_r8+Ce)
# ifdef MASKING
!>            tl_zeta(i,Jstr-1,kout)=tl_zeta(i,Jstr-1,kout)*            &
!>   &                               GRID(ng)%rmask(i,Jstr-1)
!>
              ad_zeta(i,Jstr-1,kout)=ad_zeta(i,Jstr-1,kout)*            &
     &                               GRID(ng)%rmask(i,Jstr-1)
# endif
!>            tl_zeta(i,Jstr-1,kout)=tl_cff2*(zeta(i,Jstr-1,know)+      &
!>   &                                        Ce*zeta(i,Jstr,kout))+    &
!>   &                               cff2*(tl_zeta(i,Jstr-1,know)+      &
!>   &                                     tl_Ce*zeta(i,Jstr,kout)+     &
!>   &                                     Ce*tl_zeta(i,Jstr,kout))
!>
              adfac=cff2*ad_zeta(i,Jstr-1,kout)
              ad_zeta(i,Jstr-1,know)=ad_zeta(i,Jstr-1,know)+adfac
              ad_zeta(i,Jstr  ,kout)=ad_zeta(i,Jstr  ,kout)+Ce*adfac
              ad_Ce=ad_Ce+zeta(i,Jstr,kout)*adfac
              ad_cff2=ad_cff2+                                          &
     &                (zeta(i,Jstr-1,know)+                             &
     &                 Ce*zeta(i,Jstr,kout))*ad_zeta(i,Jstr-1,kout)
              ad_zeta(i,Jstr-1,kout)=0.0_r8
!>            tl_cff2=-cff2*cff2*tl_Ce
!>
              ad_Ce=ad_Ce-cff2*cff2*ad_cff2
              ad_cff2=0.0_r8
!>            tl_Ce=cff*tl_cff1
!>
              ad_cff1=ad_cff1+cff*ad_Ce
              ad_Ce=0.0_r8
!>            tl_cff1=0.5_r8*g*(GRID(ng)%tl_h(i,Jstr)+                  &
!>   &                          tl_zeta(i,Jstr,know))/cff1
!>
              adfac=0.5_r8*g*ad_cff1/cff1
              GRID(ng)%ad_h(i,Jstr)=GRID(ng)%ad_h(i,Jstr)+adfac
              ad_zeta(i,Jstr,know)=ad_zeta(i,Jstr,know)+adfac
              ad_cff1=0.0_r8
            END IF
          END DO
!
!  Southern edge, clamped boundary condition.
!
        ELSE IF (ad_LBC(isouth,isFsur,ng)%clamped) THEN
          DO i=Istr,Iend
            IF (LBC_apply(ng)%south(i)) THEN
# ifdef MASKING
!>            tl_zeta(i,Jstr-1,kout)=tl_zeta(i,Jstr-1,kout)*              &
!>   &                               GRID(ng)%rmask(i,Jstr-1)
!>
              ad_zeta(i,Jstr-1,kout)=ad_zeta(i,Jstr-1,kout)*              &
     &                               GRID(ng)%rmask(i,Jstr-1)
# endif
# ifdef ADJUST_BOUNDARY
              IF (Lobc(isouth,isFsur,ng)) THEN
!>              tl_zeta(i,Jstr-1,kout)=BOUNDARY(ng)%tl_zeta_south(i)
!>
                BOUNDARY(ng)%ad_zeta_south(i)=BOUNDARY(ng)%             &
     &                                                ad_zeta_south(i)+ &
     &                                        ad_zeta(i,Jstr-1,kout)
                ad_zeta(i,Jstr-1,kout)=0.0_r8
              ELSE
!>              tl_zeta(i,Jstr-1,kout)=0.0_r8
!>
                ad_zeta(i,Jstr-1,kout)=0.0_r8
              END IF
# else
!>            tl_zeta(i,Jstr-1,kout)=0.0_r8
!>
              ad_zeta(i,Jstr-1,kout)=0.0_r8
# endif
            END IF
          END DO
!
!  Southern edge, gradient boundary condition.
!
        ELSE IF (ad_LBC(isouth,isFsur,ng)%gradient) THEN
          DO i=Istr,Iend
            IF (LBC_apply(ng)%south(i)) THEN
# ifdef MASKING
!>            tl_zeta(i,Jstr-1,kout)=tl_zeta(i,Jstr-1,kout)*            &
!>   &                               GRID(ng)%rmask(i,Jstr-1)
!>
              ad_zeta(i,Jstr-1,kout)=ad_zeta(i,Jstr-1,kout)*            &
     &                               GRID(ng)%rmask(i,Jstr-1)
# endif
!>            tl_zeta(i,Jstr-1,kout)=tl_zeta(i,Jstr,kout)
!>
              ad_zeta(i,Jstr  ,kout)=ad_zeta(i,Jstr  ,kout)+            &
     &                               ad_zeta(i,Jstr-1,kout)
              ad_zeta(i,Jstr-1,kout)=0.0_r8
            END IF
          END DO
!
!  Southern edge, closed boundary condition.
!
        ELSE IF (ad_LBC(isouth,isFsur,ng)%closed) THEN
          DO i=Istr,Iend
            IF (LBC_apply(ng)%south(i)) THEN
# ifdef MASKING
!>            tl_zeta(i,Jstr-1,kout)=tl_zeta(i,Jstr-1,kout)*            &
!>   &                               GRID(ng)%rmask(i,Jstr-1)
!>
              ad_zeta(i,Jstr-1,kout)=ad_zeta(i,Jstr-1,kout)*            &
     &                               GRID(ng)%rmask(i,Jstr-1)
# endif
!>            tl_zeta(i,Jstr-1,kout)=tl_zeta(i,Jstr,kout)
!>
              ad_zeta(i,Jstr  ,kout)=ad_zeta(i,Jstr  ,kout)+            &
     &                               ad_zeta(i,Jstr-1,kout)
              ad_zeta(i,Jstr-1,kout)=0.0_r8
            END IF
          END DO
        END IF
      END IF
!
!-----------------------------------------------------------------------
!  Lateral boundary conditions at the eastern edge.
!-----------------------------------------------------------------------
!
      IF (DOMAIN(ng)%Eastern_Edge(tile)) THEN
!
!  Eastern edge, implicit upstream radiation condition.
!
        IF (ad_LBC(ieast,isFsur,ng)%radiation) THEN
          IF (iic(ng).ne.0) THEN
            DO j=Jstr,Jend
              IF (LBC_apply(ng)%east(j)) THEN
# if defined CELERITY_READ && defined FORWARD_READ
                IF (ad_LBC(ieast,isFsur,ng)%nudging) THEN
                  IF (BOUNDARY(ng)%zeta_east_Cx(j).eq.0.0_r8) THEN
                    tau=FSobc_in(ieast)
                  ELSE
                    tau=FSobc_out(ieast)
                  END IF
                  tau=tau*dt2d
                END IF
                Cx=BOUNDARY(ng)%zeta_east_Cx(j)
#  ifdef RADIATION_2D
                Ce=BOUNDARY(ng)%zeta_east_Ce(j)
#  else
                Ce=0.0_r8
#  endif
                cff=BOUNDARY(ng)%zeta_east_C2(j)
# endif
# ifdef MASKING
!>              tl_zeta(Iend+1,j,kout)=tl_zeta(Iend+1,j,kout)*          &
!>   &                                 GRID(ng)%rmask(Iend+1,j)
!>
                ad_zeta(Iend+1,j,kout)=ad_zeta(Iend+1,j,kout)*          &
     &                                 GRID(ng)%rmask(Iend+1,j)
# endif
                IF (ad_LBC(ieast,isFsur,ng)%nudging) THEN
!>                tl_zeta(Iend+1,j,kout)=tl_zeta(Iend+1,j,kout)-        &
!>   &                                   tau*tl_zeta(Iend+1,j,know)
!>
                  ad_zeta(Iend+1,j,know)=ad_zeta(Iend+1,j,know)-        &
     &                                   tau*ad_zeta(Iend+1,j,kout)
                END IF
!>              tl_zeta(Iend+1,j,kout)=(cff*tl_zeta(Iend+1,j,know)+     &
!>   &                                  Cx *tl_zeta(Iend  ,j,kout)-     &
!>   &                                  MAX(Ce,0.0_r8)*                 &
!>   &                                     tl_grad(Iend+1,j  )-         &
!>   &                                  MIN(Ce,0.0_r8)*                 &
!>   &                                     tl_grad(Iend+1,j+1))/        &
!>   &                                 (cff+Cx)
!>
                adfac=ad_zeta(Iend+1,j,kout)/(cff+Cx)
                ad_grad(Iend+1,j  )=ad_grad(Iend+1,j  )-                &
     &                              MAX(Ce,0.0_r8)*adfac
                ad_grad(Iend+1,j+1)=ad_grad(Iend+1,j+1)-                &
     &                              MIN(Ce,0.0_r8)*adfac
                ad_zeta(Iend  ,j,kout)=ad_zeta(Iend  ,j,kout)+Cx *adfac
                ad_zeta(Iend+1,j,know)=ad_zeta(Iend+1,j,know)+cff*adfac
                ad_zeta(Iend+1,j,kout)=0.0_r8
              END IF
            END DO
          END IF
!
!  Eastern edge, explicit Chapman boundary condition.
!
        ELSE IF (ad_LBC(ieast,isFsur,ng)%Chapman_explicit) THEN
          DO j=Jstr,Jend
            IF (LBC_apply(ng)%east(j)) THEN
              cff=dt2d*GRID(ng)%pm(Iend,j)
              cff1=SQRT(g*(GRID(ng)%h(Iend,j)+                          &
     &                     zeta(Iend,j,know)))
              Cx=cff*cff1
# ifdef MASKING
!>            tl_zeta(Iend+1,j,kout)=tl_zeta(Iend+1,j,kout)*            &
!>   &                               GRID(ng)%rmask(Iend+1,j)
!>
              ad_zeta(Iend+1,j,kout)=ad_zeta(Iend+1,j,kout)*            &
     &                               GRID(ng)%rmask(Iend+1,j)
# endif
!>            tl_zeta(Iend+1,j,kout)=(1.0_r8-Cx)*tl_zeta(Iend+1,j,know)+&
!>   &                               tl_Cx*(zeta(Iend+1,j,know)+        &
!>   &                                      zeta(Iend  ,j,know))+       &
!>   &                               Cx*tl_zeta(Iend,j,know)
!>
              ad_zeta(Iend+1,j,know)=ad_zeta(Iend+1,j,know)+            &
     &                               (1.0_r8-Cx)*ad_zeta(Iend+1,j,kout)
              ad_Cx=ad_Cx+(zeta(Iend+1,j,know)+                         &
     &                     zeta(Iend  ,j,know))*ad_zeta(Iend+1,j,kout)
              ad_zeta(Iend,j,know)=ad_zeta(Iend,j,know)+                &
     &                             Cx*ad_zeta(Iend+1,j,kout)
              ad_zeta(Iend+1,j,kout)=0.0_r8
!>            tl_Cx=cff*tl_cff1
!>
              ad_cff1=ad_cff1+cff*ad_Cx
              ad_Cx=0.0_r8
!>            tl_cff1=0.5_r8*g*(GRID(ng)%tl_h(Iend,j)+                  &
!>   &                          tl_zeta(Iend,j,know))/cff1
!>
              adfac=0.5_r8*g*ad_cff1/cff1
              GRID(ng)%ad_h(Iend,j)=GRID(ng)%ad_h(Iend,j)+adfac
              ad_zeta(Iend,j,know)=ad_zeta(Iend,j,know)+adfac
              ad_cff1=0.0_r8
            END IF
          END DO
!
!  Eastern edge, implicit Chapman boundary condition.
!
        ELSE IF (ad_LBC(ieast,isFsur,ng)%Chapman_implicit) THEN
          DO j=Jstr,Jend
            IF (LBC_apply(ng)%east(j)) THEN
              cff=dt2d*GRID(ng)%pm(Iend,j)
              cff1=SQRT(g*(GRID(ng)%h(Iend,j)+                          &
     &                     zeta(Iend,j,know)))
              Cx=cff*cff1
              cff2=1.0_r8/(1.0_r8+Cx)
# ifdef MASKING
!>            tl_zeta(Iend+1,j,kout)=tl_zeta(Iend+1,j,kout)*            &
!>   &                               GRID(ng)%rmask(Iend+1,j)
!>
              ad_zeta(Iend+1,j,kout)=ad_zeta(Iend+1,j,kout)*            &
     &                               GRID(ng)%rmask(Iend+1,j)
# endif
!>            tl_zeta(Iend+1,j,kout)=tl_cff2*(zeta(Iend+1,j,know)+      &
!>   &                                        Cx*zeta(Iend,j,kout))+    &
!>   &                               cff2*(tl_zeta(Iend+1,j,know)+      &
!>   &                                     tl_Cx*zeta(Iend,j,kout)+     &
!>   &                                     Cx*tl_zeta(Iend,j,kout))
!>
              adfac=cff2*ad_zeta(Iend+1,j,kout)
              ad_zeta(Iend  ,j,kout)=ad_zeta(Iend  ,j,kout)+Cx*adfac
              ad_zeta(Iend+1,j,know)=ad_zeta(Iend+1,j,know)+adfac
              ad_Cx=ad_Cx+zeta(Iend,j,kout)*adfac
              ad_cff2=ad_cff2+                                          &
     &                (zeta(Iend+1,j,know)+                             &
     &                 Cx*zeta(Iend,j,kout))*ad_zeta(Iend+1,j,kout)
              ad_zeta(Iend+1,j,kout)=0.0_r8
!>            tl_cff2=-cff2*cff2*tl_Cx
!>
              ad_Cx=ad_Cx-cff2*cff2*ad_cff2
              ad_cff2=0.0_r8
!>            tl_Cx=cff*tl_cff1
!>
              ad_cff1=ad_cff1+cff*ad_Cx
              ad_Cx=0.0_r8
!>            tl_cff1=0.5_r8*g*(GRID(ng)%tl_h(Iend,j)+                  &
!>   &                          tl_zeta(Iend,j,know))/cff1
!>
              adfac=0.5_r8*g*ad_cff1/cff1
              GRID(ng)%ad_h(Iend,j)=GRID(ng)%ad_h(Iend,j)+adfac
              ad_zeta(Iend,j,know)=ad_zeta(Iend,j,know)+adfac
              ad_cff1=0.0_r8
            END IF
          END DO
!
!  Eastern edge, clamped boundary condition.
!
        ELSE IF (ad_LBC(ieast,isFsur,ng)%clamped) THEN
          DO j=Jstr,Jend
            IF (LBC_apply(ng)%east(j)) THEN
# ifdef MASKING
!>            tl_zeta(Iend+1,j,kout)=tl_zeta(Iend+1,j,kout)*            &
!>   &                               GRID(ng)%rmask(Iend+1,j)
!>
              ad_zeta(Iend+1,j,kout)=ad_zeta(Iend+1,j,kout)*            &
     &                               GRID(ng)%rmask(Iend+1,j)
# endif
# ifdef ADJUST_BOUNDARY
              IF (Lobc(ieast,isFsur,ng)) THEN
!>              tl_zeta(Iend+1,j,kout)=BOUNDARY(ng)%tl_zeta_east(j)
!>
                BOUNDARY(ng)%ad_zeta_east(j)=BOUNDARY(ng)%              &
     &                                               ad_zeta_east(j)+   &
     &                                       ad_zeta(Iend+1,j,kout)
                ad_zeta(Iend+1,j,kout)=0.0_r8
              ELSE
!>              tl_zeta(Iend+1,j,kout)=0.0_r8
!>
                ad_zeta(Iend+1,j,kout)=0.0_r8
              END IF
# else
!>            tl_zeta(Iend+1,j,kout)=0.0_r8
!>
              ad_zeta(Iend+1,j,kout)=0.0_r8
# endif
            END IF
          END DO
!
!  Eastern edge, gradient boundary condition.
!
        ELSE IF (ad_LBC(ieast,isFsur,ng)%gradient) THEN
          DO j=Jstr,Jend
            IF (LBC_apply(ng)%east(j)) THEN
# ifdef MASKING
!>            tl_zeta(Iend+1,j,kout)=tl_zeta(Iend+1,j,kout)*            &
!>   &                               GRID(ng)%rmask(Iend+1,j)
!>
              ad_zeta(Iend+1,j,kout)=ad_zeta(Iend+1,j,kout)*            &
     &                               GRID(ng)%rmask(Iend+1,j)
# endif
!>            tl_zeta(Iend+1,j,kout)=tl_zeta(Iend,j,kout)
!>
              ad_zeta(Iend  ,j,kout)=ad_zeta(Iend  ,j,kout)+            &
     &                               ad_zeta(Iend+1,j,kout)
              ad_zeta(Iend+1,j,kout)=0.0_r8
            END IF
          END DO
!
!  Eastern edge, closed boundary condition.
!
        ELSE IF (ad_LBC(ieast,isFsur,ng)%closed) THEN
          DO j=Jstr,Jend
            IF (LBC_apply(ng)%east(j)) THEN
# ifdef MASKING
!>            tl_zeta(Iend+1,j,kout)=tl_zeta(Iend+1,j,kout)*              &
!>   &                               GRID(ng)%rmask(Iend+1,j)
!>
              ad_zeta(Iend+1,j,kout)=ad_zeta(Iend+1,j,kout)*              &
     &                               GRID(ng)%rmask(Iend+1,j)
# endif
!>            tl_zeta(Iend+1,j,kout)=tl_zeta(Iend,j,kout)
!>
              ad_zeta(Iend  ,j,kout)=ad_zeta(Iend  ,j,kout)+              &
     &                               ad_zeta(Iend+1,j,kout)
              ad_zeta(Iend+1,j,kout)=0.0_r8
            END IF
          END DO
        END IF
      END IF
!
!-----------------------------------------------------------------------
!  Lateral boundary conditions at the western edge.
!-----------------------------------------------------------------------
!
      IF (DOMAIN(ng)%Western_Edge(tile)) THEN
!
!  Western edge, implicit upstream radiation condition.
!
        IF (ad_LBC(iwest,isFsur,ng)%radiation) THEN
          IF (iic(ng).ne.0) THEN
            DO j=Jstr,Jend
              IF (LBC_apply(ng)%west(j)) THEN
# if defined CELERITY_READ && defined FORWARD_READ
                IF (ad_LBC(iwest,isFsur,ng)%nudging) THEN
                  IF (BOUNDARY(ng)%zeta_west_Cx(j).eq.0.0_r8) THEN
                    tau=FSobc_in(ng,iwest)
                  ELSE
                    tau=FSobc_out(ng,iwest)
                  END IF
                  tau=tau*dt2d
                END IF
                Cx=BOUNDARY(ng)%zeta_west_Cx(j)
#  ifdef RADIATION_2D
                Ce=BOUNDARY(ng)%zeta_west_Ce(j)
#  else
                Ce=0.0_r8
#  endif
                cff=BOUNDARY(ng)%zeta_west_C2(j)
# endif
# ifdef MASKING
!>              tl_zeta(Istr-1,j,kout)=tl_zeta(Istr-1,j,kout)*          &
!>   &                                 GRID(ng)%rmask(Istr-1,j)
!>
                ad_zeta(Istr-1,j,kout)=ad_zeta(Istr-1,j,kout)*          &
     &                                 GRID(ng)%rmask(Istr-1,j)
# endif
                IF (ad_LBC(iwest,isFsur,ng)%nudging) THEN
!>                tl_zeta(Istr-1,j,kout)=tl_zeta(Istr-1,j,kout)-        &
!>   &                                   tau*tl_zeta(Istr-1,j,know)
!>
                  ad_zeta(Istr-1,j,know)=ad_zeta(Istr-1,j,know)-        &
     &                                   tau*ad_zeta(Istr-1,j,kout)

                END IF
!>              tl_zeta(Istr-1,j,kout)=(cff*tl_zeta(Istr-1,j,know)+     &
!>   &                                  Cx *tl_zeta(Istr  ,j,kout)-     &
!>   &                                  MAX(Ce,0.0_r8)*                 &
!>   &                                     tl_grad(Istr-1,j  )-         &
!>   &                                  MIN(Ce,0.0_r8)*                 &
!>   &                                     tl_grad(Istr-1,j+1))/        &
!>   &                                 (cff+Cx)
!>
                adfac=ad_zeta(Istr-1,j,kout)/(cff+Cx)
                ad_grad(Istr-1,j  )=ad_grad(Istr-1,j  )-                &
     &                              MAX(Ce,0.0_r8)*adfac
                ad_grad(Istr-1,j+1)=ad_grad(Istr-1,j+1)-                &
     &                              MIN(Ce,0.0_r8)*adfac
                ad_zeta(Istr-1,j,know)=ad_zeta(Istr-1,j,know)+cff*adfac
                ad_zeta(Istr  ,j,kout)=ad_zeta(Istr  ,j,kout)+Cx *adfac
                ad_zeta(Istr-1,j,kout)=0.0_r8
              END IF
            END DO
          END IF
!
!  Western edge, explicit Chapman boundary condition.
!
        ELSE IF (ad_LBC(iwest,isFsur,ng)%Chapman_explicit) THEN
          DO j=Jstr,Jend
            IF (LBC_apply(ng)%west(j)) THEN
              cff=dt2d*GRID(ng)%pm(Istr,j)
              cff1=SQRT(g*(GRID(ng)%h(Istr,j)+                          &
     &                     zeta(Istr,j,know)))
              Cx=cff*cff1
# ifdef MASKING
!>            tl_zeta(Istr-1,j,kout)=tl_zeta(Istr-1,j,kout)*            &
!>   &                               GRID(ng)%rmask(Istr-1,j)
!>
              ad_zeta(Istr-1,j,kout)=ad_zeta(Istr-1,j,kout)*            &
     &                               GRID(ng)%rmask(Istr-1,j)
# endif
!>            tl_zeta(Istr-1,j,kout)=(1.0_r8-Cx)*tl_zeta(Istr-1,j,know)+&
!>   &                               tl_Cx*(zeta(Istr-1,j,know)+        &
!>   &                                      zeta(Istr  ,j,know))+       &
!>   &                               Cx*tl_zeta(Istr,j,know)
!>
              ad_zeta(Istr-1,j,know)=ad_zeta(Istr-1,j,know)+            &
     &                               (1.0_r8-Cx)*ad_zeta(Istr-1,j,kout)
              ad_Cx=ad_Cx+(zeta(Istr-1,j,know)+                         &
     &                     zeta(Istr  ,j,know))*ad_zeta(Istr-1,j,kout)
              ad_zeta(Istr,j,know)=ad_zeta(Istr,j,know)+                &
     &                             Cx*ad_zeta(Istr-1,j,kout)
              ad_zeta(Istr-1,j,kout)=0.0_r8
!>            tl_Cx=cff*tl_cff1
!>
              ad_cff1=ad_cff1+cff*ad_Cx
              ad_Cx=0.0_r8
!>            tl_cff1=0.5_r8*g*(GRID(ng)%tl_h(Istr,j)+                  &
!>   &                          tl_zeta(Istr,j,know))/cff1
!>
              adfac=0.5_r8*g*ad_cff1/cff1
              GRID(ng)%ad_h(Istr,j)=GRID(ng)%ad_h(Istr,j)+adfac
              ad_zeta(Istr,j,know)=ad_zeta(Istr,j,know)+adfac
              ad_cff1=0.0_r8
            END IF
          END DO
!
!  Western edge, implicit Chapman boundary condition.
!
        ELSE IF (ad_LBC(iwest,isFsur,ng)%Chapman_implicit) THEN
          DO j=Jstr,Jend
            IF (LBC_apply(ng)%west(j)) THEN
              cff=dt2d*GRID(ng)%pm(Istr,j)
              cff1=SQRT(g*(GRID(ng)%h(Istr,j)+                          &
     &                     zeta(Istr,j,know)))
              Cx=cff*cff1
              cff2=1.0_r8/(1.0_r8+Cx)
# ifdef MASKING
!>            tl_zeta(Istr-1,j,kout)=tl_zeta(Istr-1,j,kout)*            &
!>   &                               GRID(ng)%rmask(Istr-1,j)
!>
              ad_zeta(Istr-1,j,kout)=ad_zeta(Istr-1,j,kout)*            &
     &                               GRID(ng)%rmask(Istr-1,j)
#  endif
!>            tl_zeta(Istr-1,j,kout)=tl_cff2*(zeta(Istr-1,j,know)+      &
!>   &                                        Cx*zeta(Istr,j,kout))+    &
!>   &                               cff2*(tl_zeta(Istr-1,j,know)+      &
!>   &                                     tl_Cx*zeta(Istr,j,kout)+     &
!>   &                                     Cx*tl_zeta(Istr,j,kout))
!>
              adfac=cff2*ad_zeta(Istr-1,j,kout)
              ad_zeta(Istr-1,j,know)=ad_zeta(Istr-1,j,know)+adfac
              ad_zeta(Istr  ,j,kout)=ad_zeta(Istr  ,j,kout)+Cx*adfac
              ad_Cx=ad_Cx+zeta(Istr,j,kout)*adfac
              ad_cff2=ad_cff2+                                          &
     &                (zeta(Istr-1,j,know)+                             &
     &                 Cx*zeta(Istr,j,kout))*ad_zeta(Istr-1,j,kout)
              ad_zeta(Istr-1,j,kout)=0.0_r8
!>            tl_cff2=-cff2*cff2*tl_Cx
!>
              ad_Cx=ad_Cx-cff2*cff2*ad_cff2
              ad_cff2=0.0_r8
!>            tl_Cx=cff*tl_cff1
!>
              ad_cff1=ad_cff1+cff*ad_Cx
              ad_Cx=0.0_r8
!>            tl_cff1=0.5_r8*g*(GRID(ng)%tl_h(Istr,j)+                  &
!>   &                          tl_zeta(Istr,j,know))/cff1
!>
              adfac=0.5_r8*g*ad_cff1/cff1
              GRID(ng)%ad_h(Istr,j)=GRID(ng)%ad_h(Istr,j)+adfac
              ad_zeta(Istr,j,know)=ad_zeta(Istr,j,know)+adfac
              ad_cff1=0.0_r8
            END IF
          END DO
!
!  Western edge, clamped boundary condition.
!
        ELSE IF (ad_LBC(iwest,isFsur,ng)%gradient) THEN
          DO j=Jstr,Jend
            IF (LBC_apply(ng)%west(j)) THEN
# ifdef MASKING
!>            tl_zeta(Istr-1,j,kout)=tl_zeta(Istr-1,j,kout)*            &
!>   &                               GRID(ng)%rmask(Istr-1,j)
!>
              ad_zeta(Istr-1,j,kout)=ad_zeta(Istr-1,j,kout)*            &
     &                               GRID(ng)%rmask(Istr-1,j)
# endif
# ifdef ADJUST_BOUNDARY
              IF (Lobc(iwest,isFsur,ng)) THEN
!>              tl_zeta(Istr-1,j,kout)=BOUNDARY(ng)%tl_zeta_west(j)
!>
                BOUNDARY(ng)%ad_zeta_west(j)=BOUNDARY(ng)%              &
     &                                               ad_zeta_west(j)+   &
     &                                       ad_zeta(Istr-1,j,kout)
                ad_zeta(Istr-1,j,kout)=0.0_r8
              ELSE
!>              tl_zeta(Istr-1,j,kout)=0.0_r8
!>
                ad_zeta(Istr-1,j,kout)=0.0_r8
              END IF
# else
!>            tl_zeta(Istr-1,j,kout)=0.0_r8
!>
              ad_zeta(Istr-1,j,kout)=0.0_r8
# endif
            END IF
          END DO
!
!  Western edge, gradient boundary condition.
!
        ELSE IF (ad_LBC(iwest,isFsur,ng)%gradient) THEN
          DO j=Jstr,Jend
            IF (LBC_apply(ng)%west(j)) THEN
# ifdef MASKING
!>            tl_zeta(Istr-1,j,kout)=tl_zeta(Istr-1,j,kout)*            &
!>   &                               GRID(ng)%rmask(Istr-1,j)
!>
              ad_zeta(Istr-1,j,kout)=ad_zeta(Istr-1,j,kout)*            &
     &                               GRID(ng)%rmask(Istr-1,j)
# endif
!>            tl_zeta(Istr-1,j,kout)=tl_zeta(Istr,j,kout)
!>
              ad_zeta(Istr  ,j,kout)=ad_zeta(Istr  ,j,kout)+            &
     &                               ad_zeta(Istr-1,j,kout)
              ad_zeta(Istr-1,j,kout)=0.0_r8
            END IF
          END DO
!
!  Western edge, closed boundary condition.
!
        ELSE IF (ad_LBC(iwest,isFsur,ng)%closed) THEN
          DO j=Jstr,Jend
            IF (LBC_apply(ng)%west(j)) THEN
# ifdef MASKING
!>            tl_zeta(Istr-1,j,kout)=tl_zeta(Istr-1,j,kout)*            &
!>   &                               GRID(ng)%rmask(Istr-1,j)
!>
              ad_zeta(Istr-1,j,kout)=ad_zeta(Istr-1,j,kout)*            &
     &                               GRID(ng)%rmask(Istr-1,j)
# endif
!>            tl_zeta(Istr-1,j,kout)=tl_zeta(Istr,j,kout)
!>
              ad_zeta(Istr  ,j,kout)=ad_zeta(Istr  ,j,kout)+            &
     &                               ad_zeta(Istr-1,j,kout)
              ad_zeta(Istr-1,j,kout)=0.0_r8
            END IF
          END DO
        END IF
      END IF

      RETURN
      END SUBROUTINE ad_zetabc_tile
#endif
      END MODULE ad_zetabc_mod
