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  • !=== COPYRIGHT AND LICENSE STATEMENT ===
    !
    !  This file is part of the TensorProductMultigrid code.
    !  
    !  (c) The copyright relating to this work is owned jointly by the
    !  Crown, Met Office and NERC [2014]. However, it has been created
    !  with the help of the GungHo Consortium, whose members are identified
    !  at https://puma.nerc.ac.uk/trac/GungHo/wiki .
    !  
    !  Main Developer: Eike Mueller
    !  
    !  TensorProductMultigrid is free software: you can redistribute it and/or
    !  modify it under the terms of the GNU Lesser General Public License as
    !  published by the Free Software Foundation, either version 3 of the
    !  License, or (at your option) any later version.
    !  
    !  TensorProductMultigrid is distributed in the hope that it will be useful,
    !  but WITHOUT ANY WARRANTY; without even the implied warranty of
    !  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    !  GNU Lesser General Public License for more details.
    !  
    !  You should have received a copy of the GNU Lesser General Public License
    !  along with TensorProductMultigrid (see files COPYING and COPYING.LESSER).
    !  If not, see <http://www.gnu.org/licenses/>.
    !
    !=== COPYRIGHT AND LICENSE STATEMENT ===
    
    
    !==================================================================
    !
    !  Discretisation module of the model problem
    !
    !
    !  -omega2 * (d^2/dx^2 + d^2/dy^2 + lambda2 * d^2/dz^2 ) u
    !                                                + delta u = RHS
    !  [Cartesian]
    !
    !  or
    !
    !  -omega2 * (laplace_{2d} + lambda2/r^2 d/dr (r^2 d/dr)) u
    !                                                + delta u = RHS
    !  [Spherical]
    !
    !  We use a cell centered finite volume discretisation with
    !  The equation is discretised either in a unit cube or on 1/6th
    !  of a cubed sphere grid.
    !
    !  The vertical grid spacing is not necessarily uniform and can
    !  be chosen by specifying the vertical grid in a vector.
    !
    !  The following boundary conditions are used:
    !
    !  * Dirichlet in the horizontal
    !  * Neumann in the vertical
    !
    !  For delta = 0 the operator reduces to the Poisson operator.
    !
    !    Eike Mueller, University of Bath, Feb 2012
    !
    !==================================================================
    
    module discretisation
    
      use parameters
      use messages
      use datatypes
      use communication
      use mpi
    
      implicit none
    
    private
    
      type model_parameters
        real(kind=rl) :: omega2   ! omega^2
        real(kind=rl) :: lambda2  ! lambda^2
        real(kind=rl) :: delta    ! delta
      end type model_parameters
    
    ! --- Stencil ---
    !
    
    ! Grid traversal direction in SOR
      integer, parameter :: DIRECTION_FORWARD = 1
      integer, parameter :: DIRECTION_BACKWARD = 2
    
    ! Ordering in SOR
      ! Lexicographic ordering
      integer, parameter :: ORDERING_LEX = 1
      ! Red-black ordering
      integer, parameter :: ORDERING_RB = 2
    
      type smoother_parameters
        ! smoother
        integer :: smoother
        ! relaxation parameter
        real(kind=rl) :: rho
        ! ordering of degrees of freedom
        integer :: ordering
      end type smoother_parameters
    
      ! Allowed smoothers
      integer, parameter :: SMOOTHER_LINE_SOR = 3
      integer, parameter :: SMOOTHER_LINE_SSOR = 4
      integer, parameter :: SMOOTHER_LINE_JAC = 6
    
      ! Number of levels
      integer :: nlev
    
      ! Grid parameters
      type(grid_parameters) :: grid_param
    
      ! Model parameters
      type(model_parameters) :: model_param
    
      ! Smoother parameters
      type(smoother_parameters) :: smoother_param
    
      ! Arrays for measuring the residual reduction
      real(kind=rl), allocatable :: log_resreduction(:)
      integer, allocatable :: nsmooth_total(:)
    
      ! Data structure for storing the vertical discretisation
      type vertical_coefficients
        real(kind=rl), allocatable :: a(:)
        real(kind=rl), allocatable :: b(:)
        real(kind=rl), allocatable :: c(:)
        real(kind=rl), allocatable :: d(:)
      end type vertical_coefficients
    
      ! Stoarge for vertical coefficients
      type(vertical_coefficients) :: vert_coeff
    
    
    public::discretisation_initialise
    public::discretisation_finalise
    public::smooth
    public::line_SOR
    public::line_SSOR
    public::line_jacobi
    
    public::apply
    public::model_parameters
    public::smoother_parameters
    public::volume_of_element
    public::SMOOTHER_LINE_SOR
    public::SMOOTHER_LINE_SSOR
    public::SMOOTHER_LINE_JAC
    public::DIRECTION_FORWARD
    public::DIRECTION_BACKWARD
    public::ORDERING_LEX
    public::ORDERING_RB
    
    contains
    
    !==================================================================
    ! Initialise module
    !==================================================================
    
     subroutine discretisation_initialise_mnh(grid_param_in, &
                                           model_param_in, &
                                           smoother_param_in, &
                                           nlev_in)
        implicit none
        type(grid_parameters), intent(in)   :: grid_param_in
        type(model_parameters), intent(in)   :: model_param_in
        type(smoother_parameters), intent(in)   :: smoother_param_in
        integer, intent(in) :: nlev_in
        integer :: k
        grid_param = grid_param_in
        model_param = model_param_in
        smoother_param = smoother_param_in
        nlev = nlev_in
        allocate(log_resreduction(nlev))
        allocate(nsmooth_total(nlev))
        log_resreduction(:) = 0.0_rl
        nsmooth_total(:) = 0
        allocate(r_grid(grid_param%nz+1))
        if (grid_param%graded) then
          do k=1,grid_param%nz+1
            r_grid(k) = grid_param%H*(1.0_rl*(k-1.0_rl)/grid_param%nz)**2
          end do
        else
          do k=1,grid_param%nz+1
            r_grid(k) = grid_param%H*(1.0_rl*(k-1.0_rl)/grid_param%nz)
          end do
        end if
    #ifdef CARTESIANGEOMETRY
    #else
        r_grid(:) = 1.0_rl + r_grid(:)
    #endif
        ! Allocate arrays for vertical discretisation matrices
        ! and calculate matrix entries
        allocate(vert_coeff%a(grid_param%nz))
        allocate(vert_coeff%b(grid_param%nz))
        allocate(vert_coeff%c(grid_param%nz))
        allocate(vert_coeff%d(grid_param%nz))
        call construct_vertical_coeff_mnh()
      end subroutine discretisation_initialise_mnh
    
    
      subroutine discretisation_initialise(grid_param_in, &
                                           model_param_in, &
                                           smoother_param_in, &
                                           nlev_in)
        implicit none
        type(grid_parameters), intent(in)   :: grid_param_in
        type(model_parameters), intent(in)   :: model_param_in
        type(smoother_parameters), intent(in)   :: smoother_param_in
        integer, intent(in) :: nlev_in
        integer :: k
        grid_param = grid_param_in
        model_param = model_param_in
        smoother_param = smoother_param_in
        nlev = nlev_in
        allocate(log_resreduction(nlev))
        allocate(nsmooth_total(nlev))
        log_resreduction(:) = 0.0_rl
        nsmooth_total(:) = 0
        allocate(r_grid(grid_param%nz+1))
        if (grid_param%graded) then
          do k=1,grid_param%nz+1
            r_grid(k) = grid_param%H*(1.0_rl*(k-1.0_rl)/grid_param%nz)**2
          end do
        else
          do k=1,grid_param%nz+1
            r_grid(k) = grid_param%H*(1.0_rl*(k-1.0_rl)/grid_param%nz)
          end do
        end if
    #ifdef CARTESIANGEOMETRY
    #else
        r_grid(:) = 1.0_rl + r_grid(:)
    #endif
        ! Allocate arrays for vertical discretisation matrices
        ! and calculate matrix entries
        allocate(vert_coeff%a(grid_param%nz))
        allocate(vert_coeff%b(grid_param%nz))
        allocate(vert_coeff%c(grid_param%nz))
        allocate(vert_coeff%d(grid_param%nz))
        call construct_vertical_coeff()
      end subroutine discretisation_initialise
    
    !==================================================================
    ! Finalise module
    !==================================================================
      subroutine discretisation_finalise()
        implicit none
        integer :: level
        real(kind=rl) :: rho_avg
    #ifdef MEASURESMOOTHINGRATE
        if (i_am_master_mpi) then
          write(STDOUT,'("Average smoothing rates:")')
          do level=nlev,1,-1
            if (nsmooth_total(level) > 0) then
              rho_avg = exp(log_resreduction(level)/nsmooth_total(level))
            else
              rho_avg = 1.0_rl
            end if
            write(STDOUT,'("rho_{avg}(",I3,") = ",E10.4," ( ",I5," x )")') &
              level, rho_avg, nsmooth_total(level)
          end do
        end if
    #endif
        deallocate(log_resreduction)
        deallocate(nsmooth_total)
        deallocate(r_grid)
        ! Deallocate storage for vertical discretisation matrices
        deallocate(vert_coeff%a)
        deallocate(vert_coeff%b)
        deallocate(vert_coeff%c)
        deallocate(vert_coeff%d)
      end subroutine discretisation_finalise
    
    !==================================================================
    ! Construct alpha_{i',j'} and |T_{ij}| needed for the
    ! horizontal stencil
    ! ( alpha_{i+1,j},
    !   alpha_{i-1,j},
    !   alpha_{i,j+1},
    !   alpha_{i,j-1},
    !   alpha_{ij})
    ! (ix,iy) are LOCAL indices of the grid boxes, which are
    ! converted to global indices
    
    !==================================================================
      subroutine construct_alpha_T_mnh(grid_param,ix,iy,alpha_T,Tij)
        implicit none
        type(grid_parameters), intent(in) :: grid_param
        integer, intent(in) :: ix
        integer, intent(in) :: iy
        real(kind=rl), intent(inout), dimension(5) :: alpha_T
        real(kind=rl), intent(out) :: Tij
        real(kind=rl)                  :: h, rho_i, sigma_j
    #ifdef CARTESIANGEOMETRY
        h = grid_param%L/grid_param%n
        ! Cartesian coefficients
        Tij = h**2
        if (ix == grid_param%n) then
          alpha_T(1) = 2.0_rl
        else
          alpha_T(1) = 1.0_rl
        end if
        if (ix == 1) then
          alpha_T(2) = 2.0_rl
        else
          alpha_T(2) = 1.0_rl
        end if
        if (iy == grid_param%n) then
          alpha_T(3) = 2.0_rl
        else
          alpha_T(3) = 1.0_rl
        end if
        if (iy == 1) then
          alpha_T(4) = 2.0_rl
        else
          alpha_T(4) = 1.0_rl
        end if
    #else
        ! Coefficients in cubed sphere geometry
        ! (rho_i,sigma_j) \in [-1,1] x [-1,1] are the coordinates of the
        ! cubed sphere segment
        h = 2.0_rl/grid_param%n
        Tij = volume_of_element(ix,iy,grid_param)
        rho_i = 2.0_rl*(1.0_rl*ix-0.5_rl)/grid_param%n-1.0_rl
        sigma_j = 2.0_rl*(1.0_rl*iy-0.5_rl)/grid_param%n-1.0_rl
        ! alpha_{i+1,j}
        if (ix == grid_param%n) then
          alpha_T(1) = 2.0_rl*DSQRT((1.0_rl+(rho_i+0.25_rl*h)**2)/(1.0_rl+sigma_j**2))
        else
          alpha_T(1) = DSQRT((1.0_rl+(rho_i+0.5_rl*h)**2)/(1.0_rl+sigma_j**2))
        end if
        ! alpha_{i-1,j}
        if (ix == 1) then
          alpha_T(2) = 2.0_rl*DSQRT((1.0_rl+(rho_i-0.25_rl*h)**2)/(1.0_rl+sigma_j**2))
        else
          alpha_T(2) = DSQRT((1.0_rl+(rho_i-0.5_rl*h)**2)/(1.0_rl+sigma_j**2))
        end if
        ! alpha_{i,j+1}
        if (iy == grid_param%n) then
          alpha_T(3) = 2.0_rl*DSQRT((1.0_rl+(sigma_j+0.25_rl*h)**2)/(1.0_rl+rho_i**2))
        else
          alpha_T(3) = DSQRT((1.0_rl+(sigma_j+0.5_rl*h)**2)/(1.0_rl+rho_i**2))
        end if
        ! alpha_{i,j-1}
        if (iy == 1) then
          alpha_T(4) = 2.0_rl*DSQRT((1.0_rl+(sigma_j-0.25_rl*h)**2)/(1.0_rl+rho_i**2))
        else
          alpha_T(4) = DSQRT((1.0_rl+(sigma_j-0.5_rl*h)**2)/(1.0_rl+rho_i**2))
        end if
    #endif
        alpha_T(5) = alpha_T(1) + alpha_T(2) + alpha_T(3) + alpha_T(4)
      end subroutine construct_alpha_T_mnh
    
    !==================================================================
      subroutine construct_alpha_T(grid_param,ix,iy,alpha_T,Tij)
        implicit none
        type(grid_parameters), intent(in) :: grid_param
        integer, intent(in) :: ix
        integer, intent(in) :: iy
        real(kind=rl), intent(inout), dimension(5) :: alpha_T
        real(kind=rl), intent(out) :: Tij
        real(kind=rl)                  :: h, rho_i, sigma_j
    #ifdef CARTESIANGEOMETRY
        h = grid_param%L/grid_param%n
        ! Cartesian coefficients
        Tij = h**2
        if (ix == grid_param%n) then
          alpha_T(1) = 2.0_rl
        else
          alpha_T(1) = 1.0_rl
        end if
        if (ix == 1) then
          alpha_T(2) = 2.0_rl
        else
          alpha_T(2) = 1.0_rl
        end if
        if (iy == grid_param%n) then
          alpha_T(3) = 2.0_rl
        else
          alpha_T(3) = 1.0_rl
        end if
        if (iy == 1) then
          alpha_T(4) = 2.0_rl
        else
          alpha_T(4) = 1.0_rl
        end if
    #else
        ! Coefficients in cubed sphere geometry
        ! (rho_i,sigma_j) \in [-1,1] x [-1,1] are the coordinates of the
        ! cubed sphere segment
        h = 2.0_rl/grid_param%n
        Tij = volume_of_element(ix,iy,grid_param)
        rho_i = 2.0_rl*(1.0_rl*ix-0.5_rl)/grid_param%n-1.0_rl
        sigma_j = 2.0_rl*(1.0_rl*iy-0.5_rl)/grid_param%n-1.0_rl
        ! alpha_{i+1,j}
        if (ix == grid_param%n) then
          alpha_T(1) = 2.0_rl*DSQRT((1.0_rl+(rho_i+0.25_rl*h)**2)/(1.0_rl+sigma_j**2))
        else
          alpha_T(1) = DSQRT((1.0_rl+(rho_i+0.5_rl*h)**2)/(1.0_rl+sigma_j**2))
        end if
        ! alpha_{i-1,j}
        if (ix == 1) then
          alpha_T(2) = 2.0_rl*DSQRT((1.0_rl+(rho_i-0.25_rl*h)**2)/(1.0_rl+sigma_j**2))
        else
          alpha_T(2) = DSQRT((1.0_rl+(rho_i-0.5_rl*h)**2)/(1.0_rl+sigma_j**2))
        end if
        ! alpha_{i,j+1}
        if (iy == grid_param%n) then
          alpha_T(3) = 2.0_rl*DSQRT((1.0_rl+(sigma_j+0.25_rl*h)**2)/(1.0_rl+rho_i**2))
        else
          alpha_T(3) = DSQRT((1.0_rl+(sigma_j+0.5_rl*h)**2)/(1.0_rl+rho_i**2))
        end if
        ! alpha_{i,j-1}
        if (iy == 1) then
          alpha_T(4) = 2.0_rl*DSQRT((1.0_rl+(sigma_j-0.25_rl*h)**2)/(1.0_rl+rho_i**2))
        else
          alpha_T(4) = DSQRT((1.0_rl+(sigma_j-0.5_rl*h)**2)/(1.0_rl+rho_i**2))
        end if
    #endif
        alpha_T(5) = alpha_T(1) + alpha_T(2) + alpha_T(3) + alpha_T(4)
      end subroutine construct_alpha_T
    !==================================================================
    ! Construct coefficients of tridiagonal matrix A_T
    ! describing the coupling in the vertical direction and the
    ! diagonal matrix diag(d)
    !==================================================================
    
    subroutine construct_vertical_coeff_mnh()
      implicit none
      real(kind=rl) :: a_k_tmp, b_k_tmp, c_k_tmp, d_k_tmp
      real(kind=rl) :: omega2, lambda2, delta, vol_r, surface_k, surface_kp1
      integer :: k
      omega2 = model_param%omega2
      lambda2 = model_param%lambda2
      delta = model_param%delta
      do k = 1, grid_param%nz
    #ifdef  CARTESIANGEOMETRY
        vol_r = r_grid(k+1)-r_grid(k)
        surface_k = 1.0_rl
        surface_kp1 = 1.0_rl
    #else
        vol_r = (r_grid(k+1)**3 - r_grid(k)**3)/3.0_rl
        surface_k = r_grid(k)**2
        surface_kp1 = r_grid(k+1)**2
    #endif
        ! Diagonal element
        a_k_tmp = delta*vol_r
        ! off diagonal elements
        ! Boundary conditions
        ! Top
        if (k == grid_param%nz) then
          if (grid_param%vertbc == VERTBC_DIRICHLET) then
            b_k_tmp = - 2.0_rl * omega2*lambda2 &
                               * surface_kp1/(r_grid(k+1)-r_grid(k))
          else
            b_k_tmp = 0.0_rl
          end if
        else
          b_k_tmp = - 2.0_rl*omega2*lambda2 &
                  * surface_kp1/(r_grid(k+2)-r_grid(k))
        end if
        ! Bottom
        if (k == 1) then
          if (grid_param%vertbc == VERTBC_DIRICHLET) then
            c_k_tmp = - 2.0_rl * omega2*lambda2 &
                               * surface_k/(r_grid(k+1)-r_grid(k))
          else
            c_k_tmp = 0.0_rl
          end if
        else
          c_k_tmp = - 2.0_rl * omega2 * lambda2 &
                  * surface_k/(r_grid(k+1)-r_grid(k-1))
        end if
        ! Diagonal matrix d_k
        d_k_tmp = - omega2 * (r_grid(k+1)-r_grid(k))
        vert_coeff%a(k) = a_k_tmp/d_k_tmp
        vert_coeff%b(k) = b_k_tmp/d_k_tmp
        vert_coeff%c(k) = c_k_tmp/d_k_tmp
        vert_coeff%d(k) = d_k_tmp
      end do
    end subroutine construct_vertical_coeff_mnh
    
    
    subroutine construct_vertical_coeff()
      implicit none
      real(kind=rl) :: a_k_tmp, b_k_tmp, c_k_tmp, d_k_tmp
      real(kind=rl) :: omega2, lambda2, delta, vol_r, surface_k, surface_kp1
      integer :: k
      omega2 = model_param%omega2
      lambda2 = model_param%lambda2
      delta = model_param%delta
      do k = 1, grid_param%nz
    #ifdef  CARTESIANGEOMETRY
        vol_r = r_grid(k+1)-r_grid(k)
        surface_k = 1.0_rl
        surface_kp1 = 1.0_rl
    #else
        vol_r = (r_grid(k+1)**3 - r_grid(k)**3)/3.0_rl
        surface_k = r_grid(k)**2
        surface_kp1 = r_grid(k+1)**2
    #endif
        ! Diagonal element
        a_k_tmp = delta*vol_r
        ! off diagonal elements
        ! Boundary conditions
        ! Top
        if (k == grid_param%nz) then
          if (grid_param%vertbc == VERTBC_DIRICHLET) then
            b_k_tmp = - 2.0_rl * omega2*lambda2 &
                               * surface_kp1/(r_grid(k+1)-r_grid(k))
          else
            b_k_tmp = 0.0_rl
          end if
        else
          b_k_tmp = - 2.0_rl*omega2*lambda2 &
                  * surface_kp1/(r_grid(k+2)-r_grid(k))
        end if
        ! Bottom
        if (k == 1) then
          if (grid_param%vertbc == VERTBC_DIRICHLET) then
            c_k_tmp = - 2.0_rl * omega2*lambda2 &
                               * surface_k/(r_grid(k+1)-r_grid(k))
          else
            c_k_tmp = 0.0_rl
          end if
        else
          c_k_tmp = - 2.0_rl * omega2 * lambda2 &
                  * surface_k/(r_grid(k+1)-r_grid(k-1))
        end if
        ! Diagonal matrix d_k
        d_k_tmp = - omega2 * (r_grid(k+1)-r_grid(k))
        vert_coeff%a(k) = a_k_tmp/d_k_tmp
        vert_coeff%b(k) = b_k_tmp/d_k_tmp
        vert_coeff%c(k) = c_k_tmp/d_k_tmp
        vert_coeff%d(k) = d_k_tmp
      end do
    end subroutine construct_vertical_coeff
    
    !==================================================================
    ! Calculate local residual r = b - A.u
    !==================================================================
    
      subroutine calculate_residual_mnh(level,m,b,u,r)
        implicit none
        integer, intent(in)                :: level
        integer, intent(in)                :: m
        type(scalar3d), intent(in)         :: b
        type(scalar3d), intent(inout)      :: u
        type(scalar3d), intent(inout)      :: r
        integer :: ix,iy,iz
    
        ! r <- A.u
        call apply_mnh(u,r)
        ! r <- b - r = b - A.u
        do ix=u%icompx_min,u%icompx_max
          do iy=u%icompy_min,u%icompy_max
            do iz=1,u%grid_param%nz
              r%s(iz,iy,ix) = b%s(iz,iy,ix) - r%s(iz,iy,ix)
            end do
          end do
        end do
      end subroutine calculate_residual_mnh
    
    
      subroutine calculate_residual(level,m,b,u,r)
        implicit none
        integer, intent(in)                :: level
        integer, intent(in)                :: m
        type(scalar3d), intent(in)         :: b
        type(scalar3d), intent(inout)      :: u
        type(scalar3d), intent(inout)      :: r
        integer :: ix,iy,iz
    
        ! r <- A.u
        call apply(u,r)
        ! r <- b - r = b - A.u
        do ix=u%icompx_min,u%icompx_max
          do iy=u%icompy_min,u%icompy_max
            do iz=1,u%grid_param%nz
              r%s(iz,iy,ix) = b%s(iz,iy,ix) - r%s(iz,iy,ix)
            end do
          end do
        end do
      end subroutine calculate_residual
    
    !==================================================================
    ! Apply operator v = A.u
    !==================================================================
    
      subroutine apply_mnh(u,v)
        implicit none
        type(scalar3d), intent(in)         :: u
        type(scalar3d), intent(inout)      :: v
        real(kind=rl), dimension(5) :: alpha_T
        real(kind=rl) :: Tij
        real(kind=rl) :: a_k, b_k, c_k, d_k
        integer :: ix,iy,iz
        real(kind=rl) :: tmp
    
        do ix=u%icompx_min,u%icompx_max
          do iy=u%icompy_min,u%icompy_max
            ! Construct horizontal part of stencil
            call construct_alpha_T_mnh(u%grid_param,  &
                                   ix+u%ix_min-1, &
                                   iy+u%iy_min-1, &
                                   alpha_T,Tij)
            do iz=1,u%grid_param%nz
              a_k = vert_coeff%a(iz)
              b_k = vert_coeff%b(iz)
              c_k = vert_coeff%c(iz)
              d_k = vert_coeff%d(iz)
              tmp = ((a_k-b_k-c_k)*Tij - alpha_T(5)) * u%s(iz,iy,ix)
              if (iz < grid_param%nz) then
                tmp = tmp + b_k*Tij * u%s(iz+1,iy,ix)
              end if
              if (iz > 1) then
                tmp = tmp + c_k*Tij * u%s(iz-1,iy,ix)
              end if
              tmp = tmp + alpha_T(1) * u%s(iz,  iy  ,ix+1) &
                        + alpha_T(2) * u%s(iz,  iy  ,ix-1) &
                        + alpha_T(3) * u%s(iz,  iy+1,ix  ) &
                        + alpha_T(4) * u%s(iz,  iy-1,ix  )
              v%s(iz,iy,ix) = d_k*tmp
            end do
          end do
        end do
      end subroutine apply_mnh
    
    
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      subroutine apply(u,v)
        implicit none
        type(scalar3d), intent(in)         :: u
        type(scalar3d), intent(inout)      :: v
        real(kind=rl), dimension(5) :: alpha_T
        real(kind=rl) :: Tij
        real(kind=rl) :: a_k, b_k, c_k, d_k
        integer :: ix,iy,iz
        real(kind=rl) :: tmp
    
        do ix=u%icompx_min,u%icompx_max
          do iy=u%icompy_min,u%icompy_max
            ! Construct horizontal part of stencil
            call construct_alpha_T(u%grid_param,  &
                                   ix+u%ix_min-1, &
                                   iy+u%iy_min-1, &
                                   alpha_T,Tij)
            do iz=1,u%grid_param%nz
              a_k = vert_coeff%a(iz)
              b_k = vert_coeff%b(iz)
              c_k = vert_coeff%c(iz)
              d_k = vert_coeff%d(iz)
              tmp = ((a_k-b_k-c_k)*Tij - alpha_T(5)) * u%s(iz,iy,ix)
              if (iz < grid_param%nz) then
                tmp = tmp + b_k*Tij * u%s(iz+1,iy,ix)
              end if
              if (iz > 1) then
                tmp = tmp + c_k*Tij * u%s(iz-1,iy,ix)
              end if
              tmp = tmp + alpha_T(1) * u%s(iz,  iy  ,ix+1) &
                        + alpha_T(2) * u%s(iz,  iy  ,ix-1) &
                        + alpha_T(3) * u%s(iz,  iy+1,ix  ) &
                        + alpha_T(4) * u%s(iz,  iy-1,ix  )
              v%s(iz,iy,ix) = d_k*tmp
            end do
          end do
        end do
      end subroutine apply
    !==================================================================
    !==================================================================
    !
    !     S M O O T H E R S
    !
    !==================================================================
    !==================================================================
    
    !==================================================================
    ! Perform nsmooth smoother iterations
    !==================================================================
      subroutine smooth(level,m,nsmooth,direction,b,u)
        implicit none
        integer, intent(in) :: level
        integer, intent(in) :: m
        integer, intent(in) :: nsmooth       ! Number of smoothing steps
        integer, intent(in) :: direction     ! Direction
        type(scalar3d), intent(inout) :: b   ! RHS
        type(scalar3d), intent(inout) :: u   ! solution vector
        integer :: i
        real(kind=rl) :: log_res_initial, log_res_final
        type(scalar3d) :: r
        integer :: halo_size
        integer :: nlocal, nz
    
    #ifdef MEASURESMOOTHINGRATE
        r%ix_min = u%ix_min
        r%ix_max = u%ix_max
        r%iy_min = u%iy_min
        r%iy_max = u%iy_max
        r%icompx_min = u%icompx_min
        r%icompx_max = u%icompx_max
        r%icompy_min = u%icompy_min
        r%icompy_max = u%icompy_max
        r%halo_size = u%halo_size
        r%isactive = u%isactive
        r%grid_param = u%grid_param
        nlocal = r%ix_max-r%ix_min+1
        halo_size = r%halo_size
        nz = r%grid_param%nz
        allocate(r%s(0:nz+1,                       &
                     1-halo_size:nlocal+halo_size, &
                     1-halo_size:nlocal+halo_size))
        call calculate_residual(level,m,b,u,r)
        log_res_initial = log(l2norm(r))
    #endif
        ! Carry out nsmooth iterations of the smoother
        if (smoother_param%smoother == SMOOTHER_LINE_SOR) then
          do i=1,nsmooth
            call line_SOR(level,m,direction,b,u)
          end do
        else if (smoother_param%smoother == SMOOTHER_LINE_SSOR) then
          do i=1,nsmooth
            call line_SSOR(level,m,direction,b,u)
          end do
        else if (smoother_param%smoother == SMOOTHER_LINE_JAC) then
          do i=1,nsmooth
            call line_jacobi(level,m,b,u)
          end do
        end if
    #ifdef MEASURESMOOTHINGRATE
        call calculate_residual(level,m,b,u,r)
        log_res_final = log(l2norm(r))
        log_resreduction(level) = log_resreduction(level) &
                                + (log_res_final - log_res_initial)
        nsmooth_total(level) = nsmooth_total(level) + nsmooth
        deallocate(r%s)
    #endif
      end subroutine smooth
    
    !==================================================================
    ! SOR line smoother
    !==================================================================
      subroutine line_SOR(level,m,direction,b,u)
    
        implicit none
    
        integer, intent(in)                :: level
        integer, intent(in)                :: m
        integer, intent(in)                :: direction
        type(scalar3d), intent(in)         :: b
        type(scalar3d), intent(inout)      :: u
        real(kind=rl), allocatable :: r(:)
        integer :: nz, nlocal
        real(kind=rl), allocatable :: c(:), utmp(:)
        integer :: ixmin(5), ixmax(5), dix
        integer :: iymin(5), iymax(5), diy
        integer :: color
        integer :: nsweeps, isweep
        integer :: ordering
        real(kind=rl) :: rho
        integer, dimension(4) :: send_requests, recv_requests
        integer :: tmp, ierr
        integer :: iblock
        logical :: overlap_comms
    
        ordering = smoother_param%ordering
        rho = smoother_param%rho
    
        nz = u%grid_param%nz
    
        ! Create residual vector
        allocate(r(nz))
        ! Allocate memory for auxiliary vectors for Thomas algorithm
        allocate(c(nz))
        allocate(utmp(nz))
        nlocal = u%ix_max-u%ix_min+1
    #ifdef OVERLAPCOMMS
        overlap_comms = (nlocal > 2)
    #else
        overlap_comms = .false.
    #endif
        ! Block 1 (N)
        ixmin(1) = 1
        ixmax(1) = nlocal
        iymin(1) = 1
        iymax(1) = 1
        ! Block 2 (S)
        ixmin(2) = 1
        ixmax(2) = nlocal
        iymin(2) = nlocal
        iymax(2) = nlocal
        ! Block 3 (W)
        ixmin(3) = 1
        ixmax(3) = 1
        iymin(3) = 2
        iymax(3) = nlocal-1
        ! Block 4 (E)
        ixmin(4) = nlocal
        ixmax(4) = nlocal
        iymin(4) = 2
        iymax(4) = nlocal-1
        ! Block 5 (INTERIOR)
        if (overlap_comms) then
          ixmin(5) = 2
          ixmax(5) = nlocal-1
          iymin(5) = 2
          iymax(5) = nlocal-1
        else
          ! If there are no interior cells, do not overlap
          ! communications and calculations, just loop over interior cells
          ixmin(5) = 1
          ixmax(5) = nlocal
          iymin(5) = 1
          iymax(5) = nlocal
        end if
        dix = +1
        diy = +1
        color = 1
        ! When iteration backwards over the grid, reverse the direction
        if (direction == DIRECTION_BACKWARD) then
          do iblock = 1, 5
            tmp = ixmax(iblock)
            ixmax(iblock) = ixmin(iblock)
            ixmin(iblock) = tmp
            tmp = iymax(iblock)
            iymax(iblock) = iymin(iblock)
            iymin(iblock) = tmp
          end do
          dix = -1
          diy = -1
          color = 0
        end if
        nsweeps = 1
        if (ordering == ORDERING_LEX) then
          nsweeps = 1
        else if (ordering == ORDERING_RB) then
          nsweeps = 2
        end if
        do isweep = 1, nsweeps
          if (overlap_comms) then
            ! Loop over cells next to boundary (iblock = 1,...,4)
            do iblock = 1, 4
              call loop_over_grid(iblock)
            end do
            ! Initiate halo exchange
            call ihaloswap(level,m,u,send_requests,recv_requests)
          end if
          ! Loop over INTERIOR cells
          iblock = 5
          call loop_over_grid(iblock)
          if (overlap_comms) then
            if (m > 0) then
              call mpi_waitall(4,recv_requests, MPI_STATUSES_IGNORE, ierr)
            end if
          else
            call haloswap(level,m,u)
          end if
          color = 1-color
        end do
    
        ! Free memory again
        deallocate(r)
        deallocate(c)
        deallocate(utmp)
    
        contains
    
        !------------------------------------------------------------------
        ! Loop over grid, for a given block
        !------------------------------------------------------------------
        subroutine loop_over_grid(iblock)
          implicit none
          integer, intent(in) :: iblock
          integer :: ix,iy,iz
          do ix=ixmin(iblock),ixmax(iblock),dix
            do iy=iymin(iblock),iymax(iblock),diy
              if (ordering == ORDERING_RB) then
                if (mod((ix+u%ix_min)+(iy+u%iy_min),2) .ne. color) cycle
              end if
              call apply_tridiag_solve(ix,iy,r,c,b,         &
                                       u%s(1:nz,iy  ,ix+1), &
                                       u%s(1:nz,iy  ,ix-1), &
                                       u%s(1:nz,iy+1,ix  ), &
                                       u%s(1:nz,iy-1,ix  ), &
                                       utmp)
               ! Add to field with overrelaxation-factor
              do iz=1,nz
                u%s(iz,iy,ix) = (1.0_rl-rho)*u%s(iz,iy,ix) + rho*utmp(iz)
              end do
            end do
          end do
        end subroutine loop_over_grid
    
      end subroutine line_SOR
    
    !==================================================================
    ! SSOR line smoother
    !==================================================================
      subroutine line_SSOR(level,m,direction,b,u)
        implicit none
        integer, intent(in)                :: level
        integer, intent(in)                :: m
        integer, intent(in)                :: direction
        type(scalar3d), intent(in)         :: b
        type(scalar3d), intent(inout)      :: u
        if (direction == DIRECTION_FORWARD) then
          call line_SOR(level,m,DIRECTION_FORWARD,b,u)
          call line_SOR(level,m,DIRECTION_BACKWARD,b,u)
        else
          call line_SOR(level,m,DIRECTION_BACKWARD,b,u)
          call line_SOR(level,m,DIRECTION_FORWARD,b,u)
        end if
      end subroutine line_SSOR
    
    !==================================================================
    ! Jacobi line smoother
    !==================================================================
      subroutine line_Jacobi(level,m,b,u)
        implicit none
        integer, intent(in)                :: level
        integer, intent(in)                :: m
        type(scalar3d), intent(in)         :: b
        type(scalar3d), intent(inout)      :: u
        real(kind=rl), allocatable :: r(:)
        integer :: ix,iy,iz, nz
        real(kind=rl), dimension(5) :: alpha_T
        real(kind=rl), allocatable :: c(:), utmp(:)
        real(kind=rl), allocatable :: u0(:,:,:)
        integer :: nlocal, halo_size
        real(kind=rl) :: rho
        logical :: overlap_comms
        integer, dimension(4) :: send_requests, recv_requests
        integer :: ixmin(5), ixmax(5)
        integer :: iymin(5), iymax(5)
        integer :: iblock, ierr
    
        ! Set optimal smoothing parameter on each level
        rho = 2.0_rl/(2.0_rl+4.0_rl*model_param%omega2*u%grid_param%n**2/(1.0_rl+4.0_rl*model_param%omega2*u%grid_param%n**2))
    
        nz = u%grid_param%nz
        nlocal = u%ix_max -u%ix_min + 1
        halo_size = u%halo_size
    
    #ifdef OVERLAPCOMMS
        overlap_comms = (nlocal > 2)
    #else
        overlap_comms = .false.
    #endif
    
        ! Block 1 (N)
        ixmin(1) = 1
        ixmax(1) = nlocal
        iymin(1) = 1
        iymax(1) = 1
        ! Block 2 (S)
        ixmin(2) = 1
        ixmax(2) = nlocal
        iymin(2) = nlocal
        iymax(2) = nlocal
        ! Block 3 (W)
        ixmin(3) = 1
        ixmax(3) = 1
        iymin(3) = 2
        iymax(3) = nlocal-1
        ! Block 4 (E)
        ixmin(4) = nlocal
        ixmax(4) = nlocal
        iymin(4) = 2
        iymax(4) = nlocal-1
        ! Block 5 (INTERIOR)
        if (overlap_comms) then
          ixmin(5) = 2
          ixmax(5) = nlocal-1
          iymin(5) = 2
          iymax(5) = nlocal-1
        else
          ! If there are no interior cells, do not overlap
          ! communications and calculations, just loop over interior cells
          ixmin(5) = 1
          ixmax(5) = nlocal
          iymin(5) = 1
          iymax(5) = nlocal
        end if
    
        ! Temporary vector
        allocate(u0(0:u%grid_param%nz+1,            &
                    1-halo_size:nlocal+halo_size,   &
                    1-halo_size:nlocal+halo_size) )
        u0(:,:,:) = u%s(:,:,:)
        ! Create residual vector
        allocate(r(nz))
        ! Allocate memory for auxiliary vectors for Thomas algorithm
        allocate(c(nz))
        allocate(utmp(nz))
    
        ! Loop over grid
        if (overlap_comms) then
        ! Loop over cells next to boundary (iblock = 1,...,4)
          do iblock = 1, 4
            call loop_over_grid(iblock)
          end do
          ! Initiate halo exchange
          call ihaloswap(level,m,u,send_requests,recv_requests)
        end if
        ! Loop over INTERIOR cells
        iblock = 5
        call loop_over_grid(iblock)
        if (overlap_comms) then
          if (m > 0) then
            call mpi_waitall(4,recv_requests, MPI_STATUSES_IGNORE, ierr)