bt_wide_init Subroutine

   subroutine bt_wide_init(this, grid, dx, dy, lon_west, lat_south, &
                           rad_earth, grid_config, &
                           f_0, beta, y_ref, coriolis_scheme, omega)
      !! Allocate the wide shadow state.  Builds `grid_w` (same nx_phys/ny_phys
      !! as `grid`, nghost = grid%nghost + bt_halo), fills wide metrics via the
      !! same formula generator, fills wide f_corner.
      !! `grid_config` must be GRID_CONFIG_CARTESIAN or GRID_CONFIG_SPHERICAL;
      !! supergrid/tripolar are excluded at configure time.
      class(bt_wide_t), intent(inout) :: this
      type(hgrid_t), intent(in) :: grid
         !! Normal-width grid descriptor for this subdomain.
      real(wp), intent(in) :: dx, dy
         !! Cell spacing (m for Cartesian; deg for spherical).
      real(wp), intent(in) :: lon_west, lat_south
         !! South-west corner (used only for spherical; ignored for Cartesian).
      real(wp), intent(in) :: rad_earth
         !! Earth radius (m; used only for spherical; ignored for Cartesian).
      integer, intent(in) :: grid_config
         !! GRID_CONFIG_CARTESIAN or GRID_CONFIG_SPHERICAL.
      real(wp), intent(in) :: f_0, beta, y_ref
         !! Beta-plane Coriolis parameters.
      integer, intent(in) :: coriolis_scheme
         !! CORIOLIS_SCHEME_BETA_PLANE or CORIOLIS_SCHEME_PLANETARY.
      real(wp), intent(in), optional :: omega
         !! Planetary rotation rate for the planetary scheme.  Absent => 0.
      real(wp) :: omega_l

      integer :: nx_w, ny_w, bt_h

      omega_l = 0.0_wp
      if (present(omega)) omega_l = omega
      bt_h = this%bt_halo
      this%ng_wide = grid%nghost + bt_h
      this%num_cycles = bt_h/2

      ! Build the wide grid descriptor.  Physical size identical to the normal
      ! grid; nghost increases by bt_halo.  Global offsets must match so the
      ! formula-metric and f_corner fills land on the correct physical rows.
      call this%grid_w%init(grid%nx_phys, grid%ny_phys, this%ng_wide, dx, dy)
      this%grid_w%i_offset_global = grid%i_offset_global
      this%grid_w%j_offset_global = grid%j_offset_global
      this%grid_w%nx_global = grid%nx_global
      this%grid_w%ny_global = grid%ny_global

      nx_w = this%grid_w%nx_total
      ny_w = this%grid_w%ny_total

      ! Build wide metrics via the same formula generator.
      call this%metrics_w%init(this%grid_w)
      select case (grid_config)
      case (GRID_CONFIG_SPHERICAL)
         call metrics_fill_spherical(this%metrics_w, this%grid_w, &
                                     lon_west, lat_south, dx, dy, rad_earth)
      case default   ! GRID_CONFIG_CARTESIAN
         call metrics_fill_cartesian(this%metrics_w, this%grid_w, dx, dy)
      end select
      call metrics_finalize(this%metrics_w)

      ! Fill wide f_corner via metrics_fill_coriolis.
      allocate (this%f_corner_w(nx_w + 1, ny_w + 1), source=0.0_wp)
      block
         real(wp), allocatable :: f_centre_scratch(:, :)
         allocate (f_centre_scratch(nx_w, ny_w), source=0.0_wp)
         call metrics_fill_coriolis(this%metrics_w, coriolis_scheme, &
                                    f_0, beta, y_ref, omega_l, &
                                    this%grid_w, this%f_corner_w, f_centre_scratch)
         deallocate (f_centre_scratch)
      end block

      ! Allocate wide shadow arrays, initialised to zero.
      allocate (this%w_eta(nx_w, ny_w), source=0.0_wp)
      allocate (this%w_H_ref(nx_w, ny_w), source=0.0_wp)
      allocate (this%w_eta_new(nx_w, ny_w), source=0.0_wp)
      allocate (this%w_ke(nx_w, ny_w), source=0.0_wp)
      allocate (this%w_eta_sum(nx_w, ny_w), source=0.0_wp)
      allocate (this%w_eta_end(nx_w, ny_w), source=0.0_wp)
      allocate (this%w_ubt(nx_w + 1, ny_w), source=0.0_wp)
      allocate (this%w_ubt_prev(nx_w + 1, ny_w), source=0.0_wp)
      allocate (this%w_rem_u(nx_w + 1, ny_w), source=1.0_wp)
      allocate (this%w_ubt_sum(nx_w + 1, ny_w), source=0.0_wp)
      allocate (this%w_uhbt_sum(nx_w + 1, ny_w), source=0.0_wp)
      allocate (this%w_uhbt(nx_w + 1, ny_w), source=0.0_wp)
      allocate (this%w_ubt_end(nx_w + 1, ny_w), source=0.0_wp)
      allocate (this%w_vbt(nx_w, ny_w + 1), source=0.0_wp)
      allocate (this%w_vbt_prev(nx_w, ny_w + 1), source=0.0_wp)
      allocate (this%w_rem_v(nx_w, ny_w + 1), source=1.0_wp)
      allocate (this%w_vbt_sum(nx_w, ny_w + 1), source=0.0_wp)
      allocate (this%w_vhbt_sum(nx_w, ny_w + 1), source=0.0_wp)
      allocate (this%w_vhbt(nx_w, ny_w + 1), source=0.0_wp)
      allocate (this%w_vbt_end(nx_w, ny_w + 1), source=0.0_wp)
      allocate (this%w_zeta(nx_w + 1, ny_w + 1), source=0.0_wp)
      allocate (this%w_force_u(nx_w + 1, ny_w), source=0.0_wp)
      allocate (this%w_force_v(nx_w, ny_w + 1), source=0.0_wp)

      this%is_init = .true.
   end subroutine bt_wide_init