ocean_vcoord_init Subroutine

   subroutine ocean_vcoord_init(this, grid, nz_ml)
      !! Allocate every per-column array the slot owns: `dsig`,
      !! `z_ref_global`, `target_h`, `z_ref`.  All sized once at init —
      !! grids don't resize.  Host allocations only; `enter_data` ships
      !! them to the device.
      class(ocean_vcoord_t), intent(inout) :: this
      type(hgrid_t), intent(in) :: grid
      integer, intent(in), optional :: nz_ml
      integer :: nz_local, k
      nz_local = 1
      if (present(nz_ml)) nz_local = nz_ml
      if (nz_local < 1) nz_local = 1
      this%nx_total = grid%nx_total
      this%ny_total = grid%ny_total
      this%nz_ml = nz_local
      allocate (this%dsig(nz_local))
      do k = 1, nz_local
         this%dsig(k) = 1.0_wp/real(nz_local, wp)
      end do
      ! Default reference z-interfaces: uniform 0..1 normalised.  The
      ! ZSIGMA / ZSTAR_SIGMA branches that consume this expect absolute
      ! metre values from the namelist parser; the normalised default
      ! is only useful for VCOORD_SIGMA (which ignores it)
      ! and for unit tests that pre-populate before running.
      allocate (this%z_ref_global(0:nz_local))
      do k = 0, nz_local
         this%z_ref_global(k) = real(k, wp)/real(nz_local, wp)
      end do
      allocate (this%target_h(grid%nx_total, grid%ny_total, nz_local), source=0.0_wp)
      ! Per-column z* reference table — sized but not populated.  Callers
      ! invoke `build_zref_full(h_bed)` once at setup to fill it.  Until
      ! then, ZSTAR_FULL's `compute_target_h` walks a column of zeros,
      ! which the formula safely degrades to an all-h_min vanishing-
      ! layer column (the dry / shallow degenerate case).
      allocate (this%z_ref(grid%nx_total, grid%ny_total, 0:nz_local), source=0.0_wp)

      ! Geopotential depth of the column top.  Zero = the `z = 0` datum;
      ! `configure_ocean_cavity` overwrites it with the static ice draft
      ! when a cavity is configured.  Allocated unconditionally so the
      ! Z_FIXED kernel never sees a placeholder-sized array.
      allocate (this%z_top(grid%nx_total, grid%ny_total), source=0.0_wp)

      ! `VCOORD_Z_FIXED` stretched nominal profile — allocated
      ! unconditionally (zeros) so the target kernel's explicit-shape
      ! dummies never see a placeholder; filled by
      ! `ocean_vcoord_set_z_fixed_profile` only when a profile is set.
      allocate (this%z_fixed_zi(0:nz_local), source=0.0_wp)
      allocate (this%z_fixed_dz(nz_local), source=0.0_wp)

      ! Isopycnal target densities — sized `0:nz_ml`, populated by the
      ! setup wiring only when `coord_type == VCOORD_RHO`.  Default is a
      ! benign monotone ramp (1020..1030 kg/m³) so the slot is always
      ! valid; the setup path overwrites it from the namelist.
      allocate (this%rho_target(0:nz_local))
      do k = 0, nz_local
         this%rho_target(k) = 1020.0_wp + 10.0_wp*real(k, wp)/real(nz_local, wp)
      end do

      ! ALE remap scratch — persistent so the remap step doesn't
      ! allocate fresh host buffers per call (the DCs there were
      ! reading device-resident `ms%h_layer` and writing into freshly-
      ! allocated host arrays, forcing per-iteration H↔D transfers
      ! that dominated wallclock in production runs).
      allocate (this%remap_total_h(grid%nx_total, grid%ny_total), source=0.0_wp)
      allocate (this%remap_h_ref(grid%nx_total, grid%ny_total), source=0.0_wp)
      allocate (this%remap_h_old(grid%nx_total, grid%ny_total, nz_local), source=0.0_wp)
      allocate (this%remap_conc_t(grid%nx_total, grid%ny_total, nz_local), source=0.0_wp)
      allocate (this%remap_conc_s(grid%nx_total, grid%ny_total, nz_local), source=0.0_wp)
      this%is_init = .true.
   end subroutine ocean_vcoord_init