configure_ocean_top_drag Subroutine

   subroutine configure_ocean_top_drag(cfg, ocean_state, grid, compute_rank)
      !! Ice-shelf TOP drag (`&ocean_tdrag_nml`, Phase 4a): copy the
      !! variant + coefficients onto the slot, project the static
      !! cell-centred `metrics%cover_frac` onto the velocity FACES, and
      !! enforce the ONE-`C_d` rule against the melt slot.
      !!
      !! Runs AFTER `configure_ocean_cavity_melt` (which is where the melt
      !! slot's `cdrag_top` is seeded, and which this routine then
      !! overwrites from `&ocean_tdrag_nml cd` — `validate_config` has
      !! already refused a disagreement, so the assignment is structural,
      !! not a silent override) and AFTER the `cover_frac` halo exchange,
      !! and BEFORE `ocean_state_enter_data` so the host-filled face masks
      !! reach the device with the `copyin` map.
      !!
      !! Disabled ⇒ nothing is written, the slot keeps its placeholder
      !! arrays, and the run is bit-identical.
      type(config_t), intent(in) :: cfg
      type(ocean_state_t), intent(inout) :: ocean_state
      type(hgrid_t), intent(in) :: grid
      integer, intent(in) :: compute_rank

      integer :: nx, ny

      if (.not. cfg%ocean%tdrag%enable) return

      nx = grid%nx_total
      ny = grid%ny_total

      ocean_state%tdrag%variant = parse_tdrag_variant(cfg%ocean%tdrag%form)
      ocean_state%tdrag%c_drag = cfg%ocean%tdrag%cd
      ocean_state%tdrag%r_linear = cfg%ocean%tdrag%r
      ocean_state%tdrag%htbl = cfg%ocean%tdrag%htbl
      ocean_state%tdrag%drag_bg_vel = cfg%ocean%tdrag%bg_vel
      ocean_state%tdrag%tbl_thick_min = cfg%ocean%tdrag%tbl_thick_min
      ocean_state%tdrag%implicit = cfg%ocean%tdrag%implicit
      ! `&ocean_vdiff_nml implicit_top_drag` is the OTHER implicit form:
      ! the fold into the vdiff `k = nz` diagonal.  Latched on both slots
      ! — the top-drag kernel needs it to fill `lambda_top_u/v`, and the
      ! driver reads `td%implicit_fold` to skip the explicit apply.
      ! `validate_config` has already refused the double-count
      ! (`implicit_top_drag` × `&ocean_tdrag_nml implicit`) and the
      ! distributed case (`htbl > 0`).
      ocean_state%tdrag%implicit_fold = cfg%ocean%vdiff%implicit_top_drag
      ocean_state%vdiff%implicit_top_drag = cfg%ocean%vdiff%implicit_top_drag
      ! `rho0` is only ever used to scale the `stress_top` diagnostic into
      ! N/m^2; no dynamics reads it.  The one rho0 of record is
      ! `&ocean_ic_nml rho_0` -> `eos%rho0`, already resolved by
      ! `configure_ocean_reference_density` well before this point.
      ocean_state%tdrag%rho0 = ocean_state%eos%rho0

      ! Static face cover, OR of the two abutting cells (see
      ! `rdb_ocean_top_drag`'s module docstring for why OR at a calving
      ! front), plus the cell-centred copy the `stress_top` diagnostic
      ! reads.  `cover_frac` is `&ocean_cavity_dyn_nml` geometry: filled
      ! in the IC seed, halo-exchanged by the engine, never touched again.
      call top_drag_fill_face_cover_impl(ocean_state%tdrag%cover_u, &
                                         ocean_state%tdrag%cover_v, &
                                         ocean_state%metrics%cover_frac, nx, ny)
      ocean_state%tdrag%cover_t(:, :) = ocean_state%metrics%cover_frac(:, :)

      ! ONE drag coefficient for momentum and melt.  MOM6 carries two
      ! independent top-drag coefficients; we deliberately do not — see
      ! the agreement rule in `validate_config`, which has already failed
      ! loud if the user set two different values.
      if (cfg%ocean%cavity_melt%enable .and. &
          ocean_state%tdrag%variant == TDRAG_QUADRATIC) then
         ocean_state%cavity_flux%cdrag_top = cfg%ocean%tdrag%cd
      end if

      if (compute_rank == 0) then
         call logger%info("Ice-shelf top drag: "//trim(cfg%ocean%tdrag%form)// &
                          "  C_d="//to_string(cfg%ocean%tdrag%cd)// &
                          "  r="//to_string(cfg%ocean%tdrag%r)//" 1/s")
         if (cfg%ocean%tdrag%htbl > 0.0_wp) then
            call logger%info("  top BL:         HTBL="//to_string(cfg%ocean%tdrag%htbl)// &
                             " m  bg_vel="//to_string(cfg%ocean%tdrag%bg_vel)// &
                             " m/s  thick_min="// &
                             to_string(cfg%ocean%tdrag%tbl_thick_min)//" m")
         else
            call logger%info("  top BL:         layer-nz only (HTBL=0)")
         end if
         call logger%info("  covered faces:  u "// &
                          to_string(int(sum(ocean_state%tdrag%cover_u)))//", v "// &
                          to_string(int(sum(ocean_state%tdrag%cover_v)))// &
                          " (OR of the two abutting cells)")
         if (cfg%ocean%cavity_melt%enable) then
            call logger%info("  melt u* C_d:    taken from &ocean_tdrag_nml cd "// &
                             "(one coefficient for momentum and melt)")
         end if
      end if
   end subroutine configure_ocean_top_drag