MOM_sponge.F90

!> Implements sponge regions in isopycnal mode
module mom_sponge

! This file is part of MOM6. See LICENSE.md for the license.

use mom_coms, only : sum_across_pes
use mom_diag_mediator, only : post_data, query_averaging_enabled, register_diag_field
use mom_diag_mediator, only : diag_ctrl
use mom_error_handler, only : mom_error, fatal, note, warning, is_root_pe
use mom_file_parser, only : get_param, log_param, log_version, param_file_type
use mom_grid, only : ocean_grid_type
use mom_spatial_means, only : global_i_mean
use mom_time_manager, only : time_type
use mom_unit_scaling, only : unit_scale_type
use mom_verticalgrid, only : verticalgrid_type

! Planned extension:  Support for time varying sponge targets.

implicit none ; private

#include <MOM_memory.h>

public set_up_sponge_field, set_up_sponge_ml_density
public initialize_sponge, apply_sponge, sponge_end, init_sponge_diags

! A note on unit descriptions in comments: MOM6 uses units that can be rescaled for dimensional
! consistency testing. These are noted in comments with units like Z, H, L, and T, along with
! their mks counterparts with notation like "a velocity [Z T-1 ~> m s-1]".  If the units
! vary with the Boussinesq approximation, the Boussinesq variant is given first.

!> A structure for creating arrays of pointers to 3D arrays
type, public :: p3d
  real, dimension(:,:,:), pointer :: p => null() !< A pointer to a 3D array
end type p3d
!> A structure for creating arrays of pointers to 2D arrays
type, public :: p2d
  real, dimension(:,:), pointer :: p => null() !< A pointer to a 2D array
end type p2d

!> This control structure holds memory and parameters for the MOM_sponge module
type, public :: sponge_cs ; private
  logical :: bulkmixedlayer  !< If true, a refined bulk mixed layer is used with
                       !! nkml sublayers and nkbl buffer layer.
  integer :: nz        !< The total number of layers.
  integer :: isc       !< The starting i-index of the computational domain at h.
  integer :: iec       !< The ending i-index of the computational domain at h.
  integer :: jsc       !< The starting j-index of the computational domain at h.
  integer :: jec       !< The ending j-index of the computational domain at h.
  integer :: isd       !< The starting i-index of the data domain at h.
  integer :: ied       !< The ending i-index of the data domain at h.
  integer :: jsd       !< The starting j-index of the data domain at h.
  integer :: jed       !< The ending j-index of the data domain at h.
  integer :: num_col   !< The number of sponge points within the computational domain.
  integer :: fldno = 0 !< The number of fields which have already been
                       !! registered by calls to set_up_sponge_field
  integer, pointer :: col_i(:) => null() !< Array of the i-indicies of each of the columns being damped.
  integer, pointer :: col_j(:) => null() !< Array of the j-indicies of each of the columns being damped.
  real, pointer :: iresttime_col(:) => null() !< The inverse restoring time of each column [T-1 ~> s-1].
  real, pointer :: rcv_ml_ref(:) => null() !< The value toward which the mixed layer
                             !! coordinate-density is being damped [R ~> kg m-3].
  real, pointer :: ref_eta(:,:) => null() !< The value toward which the interface
                             !! heights are being damped [Z ~> m].
  type(p3d) :: var(max_fields_) !< Pointers to the fields that are being damped.
  type(p2d) :: ref_val(max_fields_) !< The values to which the fields are damped.

  logical :: do_i_mean_sponge !< If true, apply sponges to the i-mean fields.
  real, pointer :: iresttime_im(:) => null() !< The inverse restoring time of
                             !! each row for i-mean sponges [T-1 ~> s-1].
  real, pointer :: rcv_ml_ref_im(:) => null() !! The value toward which the i-mean
                             !< mixed layer coordinate-density is being damped [R ~> kg m-3].
  real, pointer :: ref_eta_im(:,:) => null() !< The value toward which the i-mean
                             !! interface heights are being damped [Z ~> m].
  type(p2d) :: ref_val_im(max_fields_) !< The values toward which the i-means of
                             !! fields are damped.

  type(diag_ctrl), pointer :: diag => null() !< A structure that is used to
                             !! regulate the timing of diagnostic output.
  integer :: id_w_sponge = -1 !< A diagnostic ID
end type sponge_cs

contains

!> This subroutine determines the number of points which are within
!! sponges in this computational domain.  Only points that have
!! positive values of Iresttime and which mask2dT indicates are ocean
!! points are included in the sponges.  It also stores the target interface
!! heights.
subroutine initialize_sponge(Iresttime, int_height, G, param_file, CS, GV, &
                             Iresttime_i_mean, int_height_i_mean)
  type(ocean_grid_type),   intent(in) :: g          !< The ocean's grid structure
  real, dimension(SZI_(G),SZJ_(G)), &
                           intent(in) :: iresttime  !< The inverse of the restoring time [T-1 ~> s-1].
  real, dimension(SZI_(G),SZJ_(G),SZK_(G)+1), &
                           intent(in) :: int_height !< The interface heights to damp back toward [Z ~> m].
  type(param_file_type),   intent(in) :: param_file !< A structure to parse for run-time parameters
  type(sponge_cs),         pointer    :: cs         !< A pointer that is set to point to the control
                                                    !! structure for this module
  type(verticalgrid_type), intent(in) :: gv         !< The ocean's vertical grid structure
  real, dimension(SZJ_(G)), &
                 optional, intent(in) :: iresttime_i_mean !< The inverse of the restoring time for
                                                          !! the zonal mean properties [T-1 ~> s-1].
  real, dimension(SZJ_(G),SZK_(G)+1), &
                 optional, intent(in) :: int_height_i_mean !< The interface heights toward which to
                                                           !! damp the zonal mean heights [Z ~> m].


! This include declares and sets the variable "version".
#include "version_variable.h"
  character(len=40)  :: mdl = "MOM_sponge"  ! This module's name.
  logical :: use_sponge
  integer :: i, j, k, col, total_sponge_cols

  if (associated(cs)) then
    call mom_error(warning, "initialize_sponge called with an associated "// &
                            "control structure.")
    return
  endif

! Set default, read and log parameters
  call log_version(param_file, mdl, version)
  call get_param(param_file, mdl, "SPONGE", use_sponge, &
                 "If true, sponges may be applied anywhere in the domain. "//&
                 "The exact location and properties of those sponges are "//&
                 "specified from MOM_initialization.F90.", default=.false.)

  if (.not.use_sponge) return
  allocate(cs)

  if (present(iresttime_i_mean) .neqv. present(int_height_i_mean)) &
    call mom_error(fatal, "initialize_sponge:  The optional arguments \n"//&
           "Iresttime_i_mean and int_height_i_mean must both be present \n"//&
           "if either one is.")

  cs%do_i_mean_sponge = present(iresttime_i_mean)

  cs%nz = g%ke
!  CS%isc = G%isc ; CS%iec = G%iec ; CS%jsc = G%jsc ; CS%jec = G%jec
!  CS%isd = G%isd ; CS%ied = G%ied ; CS%jsd = G%jsd ; CS%jed = G%jed
  ! CS%bulkmixedlayer may be set later via a call to set_up_sponge_ML_density.
  cs%bulkmixedlayer = .false.

  cs%num_col = 0 ; cs%fldno = 0
  do j=g%jsc,g%jec ; do i=g%isc,g%iec
    if ((iresttime(i,j)>0.0) .and. (g%mask2dT(i,j)>0)) &
      cs%num_col = cs%num_col + 1
  enddo ; enddo

  if (cs%num_col > 0) then

    allocate(cs%Iresttime_col(cs%num_col)) ; cs%Iresttime_col = 0.0
    allocate(cs%col_i(cs%num_col))         ; cs%col_i = 0
    allocate(cs%col_j(cs%num_col))         ; cs%col_j = 0

    col = 1
    do j=g%jsc,g%jec ; do i=g%isc,g%iec
      if ((iresttime(i,j)>0.0) .and. (g%mask2dT(i,j)>0)) then
        cs%col_i(col) = i ; cs%col_j(col) = j
        cs%Iresttime_col(col) = iresttime(i,j)
        col = col +1
      endif
    enddo ; enddo

    allocate(cs%Ref_eta(cs%nz+1,cs%num_col))
    do col=1,cs%num_col ; do k=1,cs%nz+1
      cs%Ref_eta(k,col) = int_height(cs%col_i(col),cs%col_j(col),k)
    enddo ; enddo

  endif

  if (cs%do_i_mean_sponge) then
    allocate(cs%Iresttime_im(g%jsd:g%jed)) ; cs%Iresttime_im(:) = 0.0
    allocate(cs%Ref_eta_im(g%jsd:g%jed,g%ke+1)) ; cs%Ref_eta_im(:,:) = 0.0

    do j=g%jsc,g%jec
      cs%Iresttime_im(j) = iresttime_i_mean(j)
    enddo
    do k=1,cs%nz+1 ; do j=g%jsc,g%jec
      cs%Ref_eta_im(j,k) = int_height_i_mean(j,k)
    enddo ; enddo
  endif

  total_sponge_cols = cs%num_col
  call sum_across_pes(total_sponge_cols)

  call log_param(param_file, mdl, "!Total sponge columns", total_sponge_cols, &
                 "The total number of columns where sponges are applied.")

end subroutine initialize_sponge

!> This subroutine sets up diagnostics for the sponges.  It is separate
!! from initialize_sponge because it requires fields that are not readily
!! available where initialize_sponge is called.
subroutine init_sponge_diags(Time, G, GV, US, diag, CS)
  type(time_type),       target, intent(in)    :: time !< The current model time
  type(ocean_grid_type),         intent(in)    :: g    !< The ocean's grid structure
  type(verticalgrid_type),       intent(in)    :: gv   !< The ocean's vertical grid structure
  type(unit_scale_type),         intent(in)    :: us   !< A dimensional unit scaling type
  type(diag_ctrl),       target, intent(inout) :: diag !< A structure that is used to regulate diagnostic output
  type(sponge_cs),               pointer       :: cs   !< A pointer to the control structure for this module that
                                                       !! is set by a previous call to initialize_sponge.

  if (.not.associated(cs)) return

  cs%diag => diag
  cs%id_w_sponge = register_diag_field('ocean_model', 'w_sponge', diag%axesTi, &
      time, 'The diapycnal motion due to the sponges', 'm s-1', conversion=us%s_to_T)

end subroutine init_sponge_diags

!> This subroutine stores the reference profile for the variable
!! whose address is given by f_ptr. nlay is the number of layers in
!! this variable.
subroutine set_up_sponge_field(sp_val, f_ptr, G, nlay, CS, sp_val_i_mean)
  type(ocean_grid_type), intent(in) :: g      !< The ocean's grid structure
  real, dimension(SZI_(G),SZJ_(G),SZK_(G)), &
                         intent(in) :: sp_val !< The reference profiles of the quantity being registered.
  real, dimension(SZI_(G),SZJ_(G),SZK_(G)), &
                 target, intent(in) :: f_ptr  !< a pointer to the field which will be damped
  integer,               intent(in) :: nlay   !< the number of layers in this quantity
  type(sponge_cs),       pointer    :: cs     !< A pointer to the control structure for this module that
                                              !! is set by a previous call to initialize_sponge.
  real, dimension(SZJ_(G),SZK_(G)),&
               optional, intent(in) :: sp_val_i_mean !< The i-mean reference value for
                                              !! this field with i-mean sponges.

  integer :: j, k, col
  character(len=256) :: mesg ! String for error messages

  if (.not.associated(cs)) return

  cs%fldno = cs%fldno + 1

  if (cs%fldno > max_fields_) then
    write(mesg,'("Increase MAX_FIELDS_ to at least ",I3," in MOM_memory.h or decrease &
           &the number of fields to be damped in the call to &
           &initialize_sponge." )') cs%fldno
    call mom_error(fatal,"set_up_sponge_field: "//mesg)
  endif

  allocate(cs%Ref_val(cs%fldno)%p(cs%nz,cs%num_col))
  cs%Ref_val(cs%fldno)%p(:,:) = 0.0
  do col=1,cs%num_col
    do k=1,nlay
      cs%Ref_val(cs%fldno)%p(k,col) = sp_val(cs%col_i(col),cs%col_j(col),k)
    enddo
    do k=nlay+1,cs%nz
      cs%Ref_val(cs%fldno)%p(k,col) = 0.0
    enddo
  enddo

  cs%var(cs%fldno)%p => f_ptr

  if (nlay/=cs%nz) then
    write(mesg,'("Danger: Sponge reference fields require nz (",I3,") layers.&
        & A field with ",I3," layers was passed to set_up_sponge_field.")') &
          cs%nz, nlay
    if (is_root_pe()) call mom_error(warning, "set_up_sponge_field: "//mesg)
  endif

  if (cs%do_i_mean_sponge) then
    if (.not.present(sp_val_i_mean)) call mom_error(fatal, &
      "set_up_sponge_field: sp_val_i_mean must be present with i-mean sponges.")

    allocate(cs%Ref_val_im(cs%fldno)%p(cs%jsd:cs%jed,cs%nz))
    cs%Ref_val(cs%fldno)%p(:,:) = 0.0
    do k=1,cs%nz ; do j=cs%jsc,cs%jec
      cs%Ref_val_im(cs%fldno)%p(j,k) = sp_val_i_mean(j,k)
    enddo ; enddo
  endif

end subroutine set_up_sponge_field


!> This subroutine stores the reference value for mixed layer density.  It is handled differently
!! from other values because it is only used in determining which layers can be inflated.
subroutine set_up_sponge_ml_density(sp_val, G, CS, sp_val_i_mean)
  type(ocean_grid_type), intent(in) :: g    !< The ocean's grid structure
  real, dimension(SZI_(G),SZJ_(G)), &
                         intent(in) :: sp_val !< The reference values of the mixed layer density [R ~> kg m-3]
  type(sponge_cs),       pointer    :: cs   !< A pointer to the control structure for this module that is
                                            !! set by a previous call to initialize_sponge.
  real, dimension(SZJ_(G)), &
               optional, intent(in) :: sp_val_i_mean !< the reference values of the zonal mean mixed
                                            !! layer density [R ~> kg m-3], for use if Iresttime_i_mean > 0.
!   This subroutine stores the reference value for mixed layer density.  It is
! handled differently from other values because it is only used in determining
! which layers can be inflated.

! Arguments: sp_val - The reference values of the mixed layer density.
!  (in/out)  CS - A pointer to the control structure for this module that is
!                 set by a previous call to initialize_sponge.

  integer :: j, col
  character(len=256) :: mesg ! String for error messages

  if (.not.associated(cs)) return

  if (associated(cs%Rcv_ml_ref)) then
    call mom_error(fatal, "set_up_sponge_ML_density appears to have been "//&
                           "called twice.")
  endif

  cs%bulkmixedlayer = .true.
  allocate(cs%Rcv_ml_ref(cs%num_col)) ; cs%Rcv_ml_ref(:) = 0.0
  do col=1,cs%num_col
    cs%Rcv_ml_ref(col) = sp_val(cs%col_i(col),cs%col_j(col))
  enddo

  if (cs%do_i_mean_sponge) then
    if (.not.present(sp_val_i_mean)) call mom_error(fatal, &
      "set_up_sponge_field: sp_val_i_mean must be present with i-mean sponges.")

    allocate(cs%Rcv_ml_ref_im(cs%jsd:cs%jed)) ; cs%Rcv_ml_ref_im(:) = 0.0
    do j=cs%jsc,cs%jec
      cs%Rcv_ml_ref_im(j) = sp_val_i_mean(j)
    enddo
  endif

end subroutine set_up_sponge_ml_density

!> This subroutine applies damping to the layers thicknesses, mixed layer buoyancy, and a variety of
!! tracers for every column where there is damping.
subroutine apply_sponge(h, dt, G, GV, US, ea, eb, CS, Rcv_ml)
  type(ocean_grid_type),   intent(inout) :: g   !< The ocean's grid structure
  type(verticalgrid_type), intent(in)    :: gv  !< The ocean's vertical grid structure
  type(unit_scale_type),   intent(in)    :: us  !< A dimensional unit scaling type
  real, dimension(SZI_(G),SZJ_(G),SZK_(G)), &
                           intent(inout) :: h   !< Layer thicknesses [H ~> m or kg m-2]
  real,                    intent(in)    :: dt  !< The amount of time covered by this call [T ~> s].
  real, dimension(SZI_(G),SZJ_(G),SZK_(G)), &
                           intent(inout) :: ea  !< An array to which the amount of fluid entrained
                                                !! from the layer above during this call will be
                                                !! added [H ~> m or kg m-2].
  real, dimension(SZI_(G),SZJ_(G),SZK_(G)), &
                           intent(inout) :: eb  !< An array to which the amount of fluid entrained
                                                !! from the layer below during this call will be
                                                !! added [H ~> m or kg m-2].
  type(sponge_cs),         pointer       :: cs  !< A pointer to the control structure for this module
                                                !! that is set by a previous call to initialize_sponge.
  real, dimension(SZI_(G),SZJ_(G)), &
                 optional, intent(inout) :: rcv_ml !<  The coordinate density of the mixed layer [R ~> kg m-3].

! This subroutine applies damping to the layers thicknesses, mixed
! layer buoyancy, and a variety of tracers for every column where
! there is damping.

  ! Local variables
  real, dimension(SZI_(G), SZJ_(G), SZK_(G)+1) :: &
    w_int, &       ! Water moved upward across an interface within a timestep,
                   ! [H ~> m or kg m-2].
    e_d            ! Interface heights that are dilated to have a value of 0
                   ! at the surface [Z ~> m].
  real, dimension(SZI_(G), SZJ_(G)) :: &
    eta_anom, &    ! Anomalies in the interface height, relative to the i-mean
                   ! target value [Z ~> m].
    fld_anom       ! Anomalies in a tracer concentration, relative to the
                   ! i-mean target value.
  real, dimension(SZJ_(G), SZK_(G)+1) :: &
    eta_mean_anom  ! The i-mean interface height anomalies [Z ~> m].
  real, allocatable, dimension(:,:,:) :: &
    fld_mean_anom  ! THe i-mean tracer concentration anomalies.
  real, dimension(SZI_(G), SZK_(G)+1) :: &
    h_above, &     ! The total thickness above an interface [H ~> m or kg m-2].
    h_below        ! The total thickness below an interface [H ~> m or kg m-2].
  real, dimension(SZI_(G)) :: &
    dilate         ! A nondimensional factor by which to dilate layers to
                   ! give 0 at the surface [nondim].

  real :: e(szk_(g)+1)  ! The interface heights [Z ~> m], usually negative.
  real :: e0       ! The height of the free surface [Z ~> m].
  real :: e_str    ! A nondimensional amount by which the reference
                   ! profile must be stretched for the free surfaces
                   ! heights in the two profiles to agree.
  real :: w        ! The thickness of water moving upward through an
                   ! interface within 1 timestep [H ~> m or kg m-2].
  real :: wm       ! wm is w if w is negative and 0 otherwise [H ~> m or kg m-2].
  real :: wb       ! w at the interface below a layer [H ~> m or kg m-2].
  real :: wpb      ! wpb is wb if wb is positive and 0 otherwise [H ~> m or kg m-2].
  real :: ea_k, eb_k ! [H ~> m or kg m-2]
  real :: damp     ! The timestep times the local damping  coefficient [nondim].
  real :: i1pdamp  ! I1pdamp is 1/(1 + damp). [nondim]
  real :: damp_1pdamp ! damp_1pdamp is damp/(1 + damp). [nondim]
  real :: idt      ! 1.0/dt times a height unit conversion factor [m H-1 T-1 ~> s-1 or m3 kg-1 s-1].
  integer :: c, m, nkmb, i, j, k, is, ie, js, je, nz
  is = g%isc ; ie = g%iec ; js = g%jsc ; je = g%jec ; nz = g%ke

  if (.not.associated(cs)) return
  if (cs%bulkmixedlayer) nkmb = gv%nk_rho_varies
  if (cs%bulkmixedlayer .and. (.not.present(rcv_ml))) &
    call mom_error(fatal, "Rml must be provided to apply_sponge when using "//&
                           "a bulk mixed layer.")

  if ((cs%id_w_sponge > 0) .or. cs%do_i_mean_sponge) then
    do k=1,nz+1 ; do j=js,je ; do i=is,ie
      w_int(i,j,k) = 0.0
    enddo ; enddo ; enddo
  endif

  if (cs%do_i_mean_sponge) then
    ! Apply forcing to restore the zonal-mean properties to prescribed values.

    if (cs%bulkmixedlayer) call mom_error(fatal, "apply_sponge is not yet set up to "//&
                  "work properly with i-mean sponges and a bulk mixed layer.")

    do j=js,je ; do i=is,ie ; e_d(i,j,nz+1) = -g%bathyT(i,j) ; enddo ; enddo
    do k=nz,1,-1 ; do j=js,je ; do i=is,ie
      e_d(i,j,k) = e_d(i,j,k+1) + h(i,j,k)*gv%H_to_Z
    enddo ; enddo ; enddo
    do j=js,je
      do i=is,ie
        dilate(i) = g%bathyT(i,j) / (e_d(i,j,1) + g%bathyT(i,j))
      enddo
      do k=1,nz+1 ; do i=is,ie
        e_d(i,j,k) = dilate(i) * (e_d(i,j,k) + g%bathyT(i,j)) - g%bathyT(i,j)
      enddo ; enddo
    enddo

    do k=2,nz
      do j=js,je ; do i=is,ie
        eta_anom(i,j) = e_d(i,j,k) - cs%Ref_eta_im(j,k)
        if (cs%Ref_eta_im(j,k) < -g%bathyT(i,j)) eta_anom(i,j) = 0.0
      enddo ; enddo
      call global_i_mean(eta_anom(:,:), eta_mean_anom(:,k), g, tmp_scale=us%Z_to_m)
    enddo

    if (cs%fldno > 0) allocate(fld_mean_anom(g%isd:g%ied,nz,cs%fldno))
    do m=1,cs%fldno
      do j=js,je ; do i=is,ie
        fld_anom(i,j) = cs%var(m)%p(i,j,k) - cs%Ref_val_im(m)%p(j,k)
      enddo ; enddo
      call global_i_mean(fld_anom(:,:), fld_mean_anom(:,k,m), g, h(:,:,k))
    enddo

    do j=js,je ; if (cs%Iresttime_im(j) > 0.0) then
      damp = dt * cs%Iresttime_im(j) ; damp_1pdamp = damp / (1.0 + damp)

      do i=is,ie
        h_above(i,1) = 0.0 ; h_below(i,nz+1) = 0.0
      enddo
      do k=nz,1,-1 ; do i=is,ie
        h_below(i,k) = h_below(i,k+1) + max(h(i,j,k)-gv%Angstrom_H, 0.0)
      enddo ; enddo
      do k=2,nz+1 ; do i=is,ie
        h_above(i,k) = h_above(i,k-1) + max(h(i,j,k-1)-gv%Angstrom_H, 0.0)
      enddo ; enddo
      do k=2,nz
        ! w is positive for an upward (lightward) flux of mass, resulting
        ! in the downward movement of an interface.
        w = damp_1pdamp * eta_mean_anom(j,k) * gv%Z_to_H
        do i=is,ie
          if (w > 0.0) then
            w_int(i,j,k) = min(w, h_below(i,k))
            eb(i,j,k-1) = eb(i,j,k-1) + w_int(i,j,k)
          else
            w_int(i,j,k) = max(w, -h_above(i,k))
            ea(i,j,k) = ea(i,j,k) - w_int(i,j,k)
          endif
        enddo
      enddo
      do k=1,nz ; do i=is,ie
        ea_k = max(0.0, -w_int(i,j,k))
        eb_k = max(0.0, w_int(i,j,k+1))
        do m=1,cs%fldno
          cs%var(m)%p(i,j,k) = (h(i,j,k)*cs%var(m)%p(i,j,k) + &
              cs%Ref_val_im(m)%p(j,k) * (ea_k + eb_k)) / &
                     (h(i,j,k) + (ea_k + eb_k)) - &
              damp_1pdamp * fld_mean_anom(j,k,m)
        enddo

        h(i,j,k) = max(h(i,j,k) + (w_int(i,j,k+1) - w_int(i,j,k)), &
                       min(h(i,j,k), gv%Angstrom_H))
      enddo ; enddo
    endif ; enddo

    if (cs%fldno > 0) deallocate(fld_mean_anom)

  endif

  do c=1,cs%num_col
! c is an index for the next 3 lines but a multiplier for the rest of the loop
! Therefore we use c as per C code and increment the index where necessary.
    i = cs%col_i(c) ; j = cs%col_j(c)
    damp = dt * cs%Iresttime_col(c)

    e(1) = 0.0 ; e0 = 0.0
    do k=1,nz
      e(k+1) = e(k) - h(i,j,k)*gv%H_to_Z
    enddo
    e_str = e(nz+1) / cs%Ref_eta(nz+1,c)

    if ( cs%bulkmixedlayer ) then
      i1pdamp = 1.0 / (1.0 + damp)
      if (associated(cs%Rcv_ml_ref)) &
        rcv_ml(i,j) = i1pdamp * (rcv_ml(i,j) + cs%Rcv_ml_ref(c)*damp)
      do k=1,nkmb
        do m=1,cs%fldno
          cs%var(m)%p(i,j,k) = i1pdamp * &
              (cs%var(m)%p(i,j,k) + cs%Ref_val(m)%p(k,c)*damp)
        enddo
      enddo

      wpb = 0.0; wb = 0.0
      do k=nz,nkmb+1,-1
        if (gv%Rlay(k) > rcv_ml(i,j)) then
          w = min((((e(k)-e0) - e_str*cs%Ref_eta(k,c)) * damp)*gv%Z_to_H, &
                    ((wb + h(i,j,k)) - gv%Angstrom_H))
          wm = 0.5*(w-abs(w))
          do m=1,cs%fldno
            cs%var(m)%p(i,j,k) = (h(i,j,k)*cs%var(m)%p(i,j,k) + &
                     cs%Ref_val(m)%p(k,c)*(damp*h(i,j,k) + (wpb - wm))) / &
                     (h(i,j,k)*(1.0 + damp) + (wpb - wm))
          enddo
        else
          do m=1,cs%fldno
            cs%var(m)%p(i,j,k) = i1pdamp * &
              (cs%var(m)%p(i,j,k) + cs%Ref_val(m)%p(k,c)*damp)
          enddo
          w = wb + (h(i,j,k) - gv%Angstrom_H)
          wm = 0.5*(w-abs(w))
        endif
        eb(i,j,k) = eb(i,j,k) + wpb
        ea(i,j,k) = ea(i,j,k) - wm
        h(i,j,k)  = h(i,j,k)  + (wb - w)
        wb = w
        wpb = w - wm
      enddo

      if (wb < 0) then
        do k=nkmb,1,-1
          w = min((wb + (h(i,j,k) - gv%Angstrom_H)),0.0)
          h(i,j,k)  = h(i,j,k)  + (wb - w)
          ea(i,j,k) = ea(i,j,k) - w
          wb = w
        enddo
      else
        w = wb
        do k=gv%nkml,nkmb
          eb(i,j,k) = eb(i,j,k) + w
        enddo

        k = gv%nkml
        h(i,j,k) = h(i,j,k) + w
        do m=1,cs%fldno
          cs%var(m)%p(i,j,k) = (cs%var(m)%p(i,j,k)*h(i,j,k) + &
                                cs%Ref_val(m)%p(k,c)*w) / (h(i,j,k) + w)
        enddo
      endif

      do k=1,nkmb
        do m=1,cs%fldno
          cs%var(m)%p(i,j,k) = i1pdamp * &
              (cs%var(m)%p(i,j,k) + cs%Ref_val(m)%p(gv%nkml,c)*damp)
        enddo
      enddo

    else                                          ! not BULKMIXEDLAYER

      wpb = 0.0
      wb = 0.0
      do k=nz,1,-1
        w = min((((e(k)-e0) - e_str*cs%Ref_eta(k,c)) * damp)*gv%Z_to_H, &
                  ((wb + h(i,j,k)) - gv%Angstrom_H))
        wm = 0.5*(w - abs(w))
        do m=1,cs%fldno
          cs%var(m)%p(i,j,k) = (h(i,j,k)*cs%var(m)%p(i,j,k) + &
              cs%Ref_val(m)%p(k,c) * (damp*h(i,j,k) + (wpb - wm))) / &
                     (h(i,j,k)*(1.0 + damp) + (wpb - wm))
        enddo
        eb(i,j,k) = eb(i,j,k) + wpb
        ea(i,j,k) = ea(i,j,k) - wm
        h(i,j,k)  = h(i,j,k)  + (wb - w)
        wb = w
        wpb = w - wm
      enddo

    endif                                         ! end BULKMIXEDLAYER
  enddo ! end of c loop

  if (associated(cs%diag)) then ; if (query_averaging_enabled(cs%diag)) then
    if (cs%id_w_sponge > 0) then
      idt = gv%H_to_m / dt  ! Do any height unit conversion here for efficiency.
      do k=1,nz+1 ; do j=js,je ; do i=is,ie
        w_int(i,j,k) = w_int(i,j,k) * idt ! Scale values by clobbering array since it is local
      enddo ; enddo ; enddo
      call post_data(cs%id_w_sponge, w_int(:,:,:), cs%diag)
    endif
  endif ; endif

end subroutine apply_sponge

!> This call deallocates any memory in the sponge control structure.
subroutine sponge_end(CS)
  type(sponge_cs),         pointer    :: cs   !< A pointer to the control structure for this module
                                              !! that is set by a previous call to initialize_sponge.
  integer :: m

  if (.not.associated(cs)) return

  if (associated(cs%col_i)) deallocate(cs%col_i)
  if (associated(cs%col_j)) deallocate(cs%col_j)

  if (associated(cs%Iresttime_col)) deallocate(cs%Iresttime_col)
  if (associated(cs%Rcv_ml_ref)) deallocate(cs%Rcv_ml_ref)
  if (associated(cs%Ref_eta)) deallocate(cs%Ref_eta)

  if (associated(cs%Iresttime_im)) deallocate(cs%Iresttime_im)
  if (associated(cs%Rcv_ml_ref_im)) deallocate(cs%Rcv_ml_ref_im)
  if (associated(cs%Ref_eta_im)) deallocate(cs%Ref_eta_im)

  do m=1,cs%fldno
    if (associated(cs%Ref_val(cs%fldno)%p)) deallocate(cs%Ref_val(cs%fldno)%p)
    if (associated(cs%Ref_val_im(cs%fldno)%p)) &
      deallocate(cs%Ref_val_im(cs%fldno)%p)
  enddo

  deallocate(cs)

end subroutine sponge_end

!> \namespace mom_sponge
!!
!! By Robert Hallberg, March 1999-June 2000
!!
!!   This program contains the subroutines that implement sponge
!! regions, in which the stratification and water mass properties
!! are damped toward some profiles.  There are three externally
!! callable subroutines in this file.
!!
!!   initialize_sponge determines the mapping from the model
!! variables into the arrays of damped columns.  This remapping is
!! done for efficiency and to conserve memory.  Only columns which
!! have positive inverse damping times and which are deeper than a
!! supplied depth are placed in sponges.  The inverse damping
!! time is also stored in this subroutine, and memory is allocated
!! for all of the reference profiles which will subsequently be
!! provided through calls to set_up_sponge_field.  The first two
!! arguments are a two-dimensional array containing the damping
!! rates, and the interface heights to damp towards.
!!
!!   set_up_sponge_field is called to provide a reference profile
!! and the location of the field that will be damped back toward
!! that reference profile.  A third argument, the number of layers
!! in the field is also provided, but this should always be nz.
!!
!!   Apply_sponge damps all of the fields that have been registered
!! with set_up_sponge_field toward their reference profiles.  The
!! four arguments are the thickness to be damped, the amount of time
!! over which the damping occurs, and arrays to which the movement
!! of fluid into a layer from above and below will be added. The
!! effect on momentum of the sponge may be accounted for later using
!! the movement of water recorded in these later arrays.

end module mom_sponge