#include "cppdefs.h"
#if defined NONLINEAR && !defined SOLVE3D
      SUBROUTINE main2d (RunInterval)
!
!svn $Id$
!=======================================================================
!  Copyright (c) 2002-2020 The ROMS/TOMS Group                         !
!    Licensed under a MIT/X style license                              !
!    See License_ROMS.txt                           Hernan G. Arango   !
!========================================== Alexander F. Shchepetkin ===
!                                                                      !
!  This subroutine is the main driver for nonlinear ROMS/TOMS when     !
!  configurated as shallow water (barotropic) ocean model only. It     !
!  advances forward  the vertically integrated primitive equations     !
!  for all nested grids,  if any,  by the specified  time interval     !
!  (seconds), RunInterval.                                             !
!                                                                      !
!=======================================================================
!
      USE mod_param
      USE mod_parallel
# if defined MODEL_COUPLING && defined MCT_LIB
      USE mod_coupler
# endif
      USE mod_iounits
# ifdef NESTING
      USE mod_nesting
# endif
      USE mod_scalars
      USE mod_stepping
!
      USE dateclock_mod,        ONLY : time_string
      USE diag_mod,             ONLY : diag
# ifdef TLM_CHECK
      USE dotproduct_mod,       ONLY : nl_dotproduct
# endif
# if defined NLM_OUTER           || defined W4DPSAS                 ||\
     defined W4DPSAS_SENSITIVITY || defined W4DPSAS_FCT_SENSITIVITY
      USE forcing_mod,          ONLY : forcing
# endif
# ifdef ADJUST_WSTRESS
      USE frc_adjust_mod,       ONLY : frc_adjust
# endif
      USE ini_fields_mod,       ONLY : ini_fields, ini_zeta
# ifdef NESTING
      USE nesting_mod,          ONLY : nesting
#  ifndef ONE_WAY
      USE nesting_mod,          ONLY : do_twoway
#  endif
# endif
# if defined ADJUST_BOUNDARY
      USE obc_adjust_mod,       ONLY : obc_adjust, load_obc
# endif
# if defined ATM_COUPLING && defined MCT_LIB
      USE ocean_coupler_mod,    ONLY : ocn2atm_coupling
# endif
# if defined WAV_COUPLING && defined MCT_LIB
      USE ocean_coupler_mod,    ONLY : ocn2wav_coupling
# endif
# ifdef NEARSHORE_MELLOR
      USE radiation_stress_mod, ONLY : radiation_stress
# endif
# ifdef BULK_FLUXES2D
#  ifdef CCSM_FLUXES2D
      USE ccsm_flux_mod, ONLY : ccsm_flux
#  else
      USE bulk_flux_mod, ONLY : bulk_flux
#  endif
# endif
# ifdef AVERAGES
      USE set_avg_mod,          ONLY : set_avg
# endif
# if defined SSH_TIDES || defined UV_TIDES
      USE set_tides_mod,        ONLY : set_tides
# endif
      USE set_vbc_mod,          ONLY : set_vbc
      USE step2d_mod,           ONLY : step2d
# ifdef FLOATS
      USE step_floats_mod,      ONLY : step_floats
# endif
      USE strings_mod,          ONLY : FoundError
!
      implicit none
!
!  Imported variable declarations.
!
      real(dp), intent(in) :: RunInterval
!
!  Local variable declarations.
!
      logical :: DoNestLayer, Time_Step

      integer :: Nsteps, Rsteps
      integer :: ig, il, istep, ng, nl, tile
      integer :: next_indx1
# ifdef FLOATS
      integer :: Lend, Lstr, chunk_size
# endif
!
!=======================================================================
!  Time-step vertically integrated equations.
!=======================================================================
!
!  Time-step the 3D kernel for the specified time interval (seconds),
!  RunInterval.
!
      Time_Step=.TRUE.
      DoNestLayer=.TRUE.
!
      KERNEL_LOOP : DO WHILE (Time_Step)
!
!  In nesting applications, the number of nesting layers (NestLayers) is
!  used to facilitate refinement grids and composite/refinament grids
!  combinations. Otherwise, the solution it is looped once for a single
!  grid application (NestLayers = 1).
!
        nl=0
#ifdef NESTING
        TwoWayInterval(1:Ngrids)=0.0_r8
#endif
!
        NEST_LAYER : DO WHILE (DoNestLayer)
!
!  Determine number of time steps to compute in each nested grid layer
!  based on the specified time interval (seconds), RunInterval. Non
!  nesting applications have NestLayers=1. Notice that RunInterval is
!  set in the calling driver. Its value may span the full period of the
!  simulation, a multi-model coupling interval (RunInterval > ifac*dt),
!  or just a single step (RunInterval=0).
!
          CALL ntimesteps (iNLM, RunInterval, nl, Nsteps, Rsteps)
          IF (FoundError(exit_flag, NoError, __LINE__,                  &
     &                   __FILE__)) RETURN
          IF ((nl.le.0).or.(nl.gt.NestLayers)) EXIT
!
!  Time-step governing equations for Nsteps.
!
          STEP_LOOP : DO istep=1,Nsteps
!
!  Set time indices and time clock.
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
              iic(ng)=iic(ng)+1
              time(ng)=time(ng)+dt(ng)
              tdays(ng)=time(ng)*sec2day
              CALL time_string (time(ng), time_code(ng))
              IF (step_counter(ng).eq.Rsteps) Time_Step=.FALSE.
            END DO

# if defined NLM_OUTER           || defined W4DPSAS                 ||\
     defined W4DPSAS_SENSITIVITY || defined W4DPSAS_FCT_SENSITIVITY
!
!-----------------------------------------------------------------------
!  If appropriate, add convolved adjoint solution impulse forcing to
!  the nonlinear model solution. Notice that the forcing is only needed
!  after finishing all the inner loops. The forcing is continuous.
!  That is, it is time interpolated at every time-step from available
!  snapshots (FrequentImpulse=TRUE).
!-----------------------------------------------------------------------
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
              IF (FrequentImpulse(ng)) THEN
                DO tile=first_tile(ng),last_tile(ng),+1
                  CALL forcing (ng, tile, kstp(ng), nstp(ng))
                END DO
!$OMP BARRIER
              END IF
            END DO
# endif
!
!-----------------------------------------------------------------------
!  Read in required data, if any, from input NetCDF files.
!-----------------------------------------------------------------------
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
!$OMP MASTER
              CALL get_data (ng)
!$OMP END MASTER
!$OMP BARRIER
              IF (FoundError(exit_flag, NoError, __LINE__,              &
     &                       __FILE__)) RETURN
            END DO
!
!-----------------------------------------------------------------------
!  If applicable, process input data: time interpolate between data
!  snapshots.
!-----------------------------------------------------------------------
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
              DO tile=first_tile(ng),last_tile(ng),+1
                CALL set_data (ng, tile)
              END DO
!$OMP BARRIER
            END DO
            IF (FoundError(exit_flag, NoError, __LINE__,                &
     &                     __FILE__)) RETURN
!
!-----------------------------------------------------------------------
!  Initialize all time levels and compute other initial fields.
!-----------------------------------------------------------------------
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
              IF (iic(ng).eq.ntstart(ng)) THEN
!
!  Initialize free-surface.
!
                DO tile=first_tile(ng),last_tile(ng),+1
                  CALL ini_zeta (ng, tile, iNLM)
                END DO
!$OMP BARRIER
!
!  Initialize other state variables.
!
                DO tile=last_tile(ng),first_tile(ng),-1
                  CALL ini_fields (ng, tile, iNLM)
                END DO
!$OMP BARRIER

# ifdef NESTING
!
!  Extract donor grid initial data at contact points and store it in
!  REFINED structure so it can be used for the space-time interpolation.
!
                IF (RefinedGrid(ng)) THEN
                  CALL nesting (ng, iNLM, ngetD)
                END IF
# endif
              END IF
            END DO
!
!-----------------------------------------------------------------------
!  Compute and report diagnostics. If appropriate, accumulate time-
!  averaged output data which needs a irreversible loop in shared-memory
!  jobs.
!-----------------------------------------------------------------------
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
              DO tile=first_tile(ng),last_tile(ng),+1     ! irreversible
# ifdef AVERAGES
#  ifdef FILTERED
                CALL set_filter (ng, tile)
#  else
                CALL set_avg (ng, tile)
#  endif
# endif
# ifdef DIAGNOSTICS
                CALL set_diags (ng, tile)
# endif
                CALL diag (ng, tile)
# ifdef TLM_CHECK
                CALL nl_dotproduct (ng, tile, Lnew(ng))
# endif
              END DO
!$OMP BARRIER
            END DO
            IF (FoundError(exit_flag, NoError, __LINE__,                &
     &                     __FILE__)) RETURN

# if defined ATM_COUPLING && defined MCT_LIB
!
!-----------------------------------------------------------------------
!  Couple ocean to atmosphere model every "CoupleSteps(Iatmos)"
!  timesteps: get air/sea fluxes.
!-----------------------------------------------------------------------
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
              IF ((iic(ng).ne.ntstart(ng)).and.                         &
     &            MOD(iic(ng),CoupleSteps(Iatmos,ng)).eq.0) THEN
                DO tile=last_tile(ng),first_tile(ng),-1
                  CALL ocn2atm_coupling (ng, tile)
                END DO
!$OMP BARRIER
              END IF
            END DO
# endif

# ifdef ADJUST_BOUNDARY
!
!-----------------------------------------------------------------------
!  Interpolate open boundary increments and adjust open boundary.
!  Load open boundary into storage arrays. Skip the last output
!  timestep.
!-----------------------------------------------------------------------
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
              IF (iic(ng).lt.(ntend(ng)+1)) THEN
                DO tile=first_tile(ng),last_tile(ng),+1
                  CALL obc_adjust (ng, tile, Lbinp(ng))
                  CALL load_obc (ng, tile, Lbout(ng))
                END DO
!$OMP BARRIER
              END IF
            END DO
# endif

# ifdef ADJUST_WSTRESS
!
!-----------------------------------------------------------------------
!  Interpolate surface forcing increments and adjust surface forcing.
!  Load surface forcing to storage arrays.
!-----------------------------------------------------------------------
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
              IF (iic(ng).lt.(ntend(ng)+1)) THEN
                DO tile=first_tile(ng),last_tile(ng),+1
                  CALL frc_adjust (ng, tile, Lfinp(ng))
                END DO
!$OMP BARRIER
              END IF
            END DO
# endif

# if defined WAV_COUPLING && defined MCT_LIB
!
!-----------------------------------------------------------------------
!  Couple ocean to waves model every "CoupleSteps(Iwaves)"
!  timesteps: get waves/sea fluxes.
!-----------------------------------------------------------------------
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
              IF ((iic(ng).ne.ntstart(ng)).and.                         &
     &            MOD(iic(ng)-1,CoupleSteps(Iwaves,ng)).eq.0) THEN
                DO tile=first_tile(ng),last_tile(ng),+1
                  CALL ocn2wav_coupling (ng, tile)
                END DO
!$OMP BARRIER
              END IF
            END DO
# endif

# ifdef NEARSHORE_MELLOR
!
!-----------------------------------------------------------------------
!  Compute radiation stress terms.
!-----------------------------------------------------------------------
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
              DO tile=last_tile(ng),first_tile(ng),-1
                CALL radiation_stress (ng, tile)
              END DO
!$OMP BARRIER
            END DO
# endif
!
!-----------------------------------------------------------------------
!  Set vertical boundary conditions. Process tidal forcing.
!-----------------------------------------------------------------------
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
              DO tile=first_tile(ng),last_tile(ng),+1
# ifdef BULK_FLUXES2D
#  ifdef CCSM_FLUXES2D
                CALL ccsm_flux (ng, tile)
#  else
                CALL bulk_flux (ng, tile)
#  endif
# endif
                CALL set_vbc (ng, tile)
# if defined SSH_TIDES || defined UV_TIDES
                CALL set_tides (ng, tile)
# endif
              END DO
!$OMP BARRIER
            END DO

# ifdef NESTING
!
!  If composite or mosaic grids, process additional points in the
!  contact zone between connected grids for bottom stress variables.
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
              IF (ANY(CompositeGrid(:,ng))) THEN
                CALL nesting (ng, iNLM, nbstr)
              END IF
            END DO
# endif
!
!-----------------------------------------------------------------------
!  If appropriate, write out fields into output NetCDF files.  Notice
!  that IO data is written in delayed and serial mode.  Exit if last
!  time step.
!-----------------------------------------------------------------------
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
!$OMP MASTER
              CALL output (ng)
!$OMP END MASTER
!$OMP BARRIER
              IF ((FoundError(exit_flag, NoError, __LINE__,             &
     &                        __FILE__)).or.                            &
     &            ((iic(ng).eq.(ntend(ng)+1)).and.(ng.eq.Ngrids))) THEN
                RETURN
              END IF
            END DO

# ifdef NESTING
!
!-----------------------------------------------------------------------
!  If refinement grid, interpolate (space, time) state variables
!  contact points from donor grid extracted data.
#  ifdef NESTING_DEBUG
!
!  Also, fill BRY_CONTACT(:,:)%Mflux to check for mass conservation
!  between coarse and fine grids.  This is only done for diagnostic
!  purposes. Also, debugging is possible with very verbose output
!  to fort.300 is allowed by activating uppercase(nesting_debug).
#  endif
!-----------------------------------------------------------------------
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
              IF (RefinedGrid(ng).and.(RefineScale(ng).gt.0)) THEN
                CALL nesting (ng, iNLM, nputD)
#  ifdef NESTING_DEBUG
                CALL nesting (ng, iNLM, nmflx)
#  endif
              END IF
            END DO
# endif
!
!-----------------------------------------------------------------------
!  Solve the vertically integrated primitive equations for the
!  free-surface and momentum components.
!-----------------------------------------------------------------------
!
# ifndef OFFLINE_FLOATS
!  Set time indices for predictor step. The PREDICTOR_2D_STEP switch
!  it is assumed to be false before the first time-step.
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
              iif(ng)=1
              nfast(ng)=1
              next_indx1=3-indx1(ng)
              IF (.not.PREDICTOR_2D_STEP(ng)) THEN
                PREDICTOR_2D_STEP(ng)=.TRUE.
                IF (FIRST_2D_STEP) THEN
                  kstp(ng)=indx1(ng)
                ELSE
                  kstp(ng)=3-indx1(ng)
                END IF
                knew(ng)=3
                krhs(ng)=indx1(ng)
              END IF
!
!  Predictor step - Advance barotropic equations using 2D time-step
!  ==============   predictor scheme.
!
              DO tile=last_tile(ng),first_tile(ng),-1
                CALL step2d (ng, tile)
              END DO
!$OMP BARRIER
            END DO

# ifdef NESTING
!
!  If composite or mosaic grids, process additional points in the
!  contact zone between connected grids for the state variables
!  associated with the 2D engine Predictor Step section.
!  If refinement, check mass flux conservation between coarse and
!  fine grids during debugging.
#  ifdef NESTING_DEBUG
!  Warning: very verbose output to fort.300 ascii file to check
!           mass flux conservation.
#  endif
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
              IF (ANY(CompositeGrid(:,ng))) THEN
                CALL nesting (ng, iNLM, n2dPS)
              END IF
              IF (RefinedGrid(ng).and.(RefineScale(ng).gt.0)) THEN
                CALL nesting (ng, iNLM, nmflx)
              END IF
            END DO
# endif
!
!  Set time indices for corrector step.
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
              IF (PREDICTOR_2D_STEP(ng)) THEN
                PREDICTOR_2D_STEP(ng)=.FALSE.
                knew(ng)=next_indx1
                kstp(ng)=3-knew(ng)
                krhs(ng)=3
                IF (iif(ng).lt.(nfast(ng)+1)) indx1(ng)=next_indx1
              END IF
!
!  Corrector step - Apply 2D time-step corrector scheme.  Notice that
!  ==============   there is not need for a corrector step during the
!  auxiliary (nfast+1) time-step.
!
              IF (iif(ng).lt.(nfast(ng)+1)) THEN
                DO tile=first_tile(ng),last_tile(ng),+1
                  CALL step2d (ng, tile)
                END DO
!$OMP BARRIER
              END IF
            END DO

# ifdef NESTING
#  if defined MASKING && defined WET_DRY
!
!  If nesting and wetting and drying, scale horizontal interpolation
!  weights to account for land/sea masking in contact areas. This needs
!  to be done at very time-step since the Land/Sea masking is time
!  dependent.
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
              CALL nesting (ng, iNLM, nmask)
            END DO
#  endif
!
!  If composite or mosaic grids, process additional points in the
!  contact zone between connected grids for the state variables
!  associated with the 2D engine Corrector Step section.
!  If refinement, check mass flux conservation between coarse and
!  fine grids during debugging.
#  ifdef NESTING_DEBUG
!  Warning: very verbose output to fort.300 ascii file to check
!           mass flux conservation.
#  endif
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
              IF (ANY(CompositeGrid(:,ng))) THEN
                CALL nesting (ng, iNLM, n2dCS)
              END IF
              IF (RefinedGrid(ng).and.(RefineScale(ng).gt.0)) THEN
                CALL nesting (ng, iNLM, nmflx)
              END IF
            END DO
# endif

# ifdef NESTING
#  ifndef ONE_WAY
!
!-----------------------------------------------------------------------
!  If refinement grids, perform two-way coupling between fine and
!  coarse grids. Correct coarse grid tracers values at the refinement
!  grid with refined accumulated fluxes.  Then, replace coarse grid
!  state variable with averaged refined grid values (two-way nesting).
!  Update coarse grid depth variables.
!
!  The two-way exchange of infomation between nested grids needs to be
!  done at the correct time-step and in the right sequence.
!-----------------------------------------------------------------------
!
            DO il=NestLayers,1,-1
              DO ig=1,GridsInLayer(il)
                ng=GridNumber(ig,il)
                IF (do_twoway(iNLM, nl, il, ng, istep)) THEN
                  CALL nesting (ng, iNLM, n2way)
                END IF
              END DO
            END DO
#  endif
!
!-----------------------------------------------------------------------
!  If donor to a finer grid, extract data for the external contact
!  points. This is the latest solution for the coarser grid.
!
!  It is stored in the REFINED structure so it can be used for the
!  space-time interpolation.
!-----------------------------------------------------------------------
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
              IF (DonorToFiner(ng)) THEN
                CALL nesting (ng, iNLM, ngetD)
              END IF
            END DO
# endif
# endif

# ifdef FLOATS
!
!-----------------------------------------------------------------------
!  Compute Lagrangian drifters trajectories: Split all the drifters
!  between all the computational threads, except in distributed-memory
!  and serial configurations. In distributed-memory, the parallel node
!  containing the drifter is selected internally since the state
!  variables do not have a global scope.
!-----------------------------------------------------------------------
!
            DO ig=1,GridsInLayer(nl)
              ng=GridNumber(ig,nl)
              IF (Lfloats(ng)) THEN
#  ifdef _OPENMP
                chunk_size=(Nfloats(ng)+numthreads-1)/numthreads
                Lstr=1+MyThread*chunk_size
                Lend=MIN(Nfloats(ng),Lstr+chunk_size-1)
#  else
                Lstr=1
                Lend=Nfloats(ng)
#  endif
                CALL step_floats (ng, Lstr, Lend)
!$OMP BARRIER
!
!  Shift floats time indices.
!
                nfp1(ng)=MOD(nfp1(ng)+1,NFT+1)
                nf  (ng)=MOD(nf  (ng)+1,NFT+1)
                nfm1(ng)=MOD(nfm1(ng)+1,NFT+1)
                nfm2(ng)=MOD(nfm2(ng)+1,NFT+1)
                nfm3(ng)=MOD(nfm3(ng)+1,NFT+1)
              END IF
            END DO
# endif

          END DO STEP_LOOP

        END DO NEST_LAYER

      END DO KERNEL_LOOP

      RETURN
      END SUBROUTINE main2d
#else
      SUBROUTINE main2d
      RETURN
      END SUBROUTINE main2d
#endif
