porous_eta_face Function

   pure function porous_eta_face(z_a, z_b, interp) result(eta)
      !! Interface height at a velocity point from the two adjacent
      !! cell-centre interface heights.  MOM6's `PORBAR_ETA_INTERP`
      !! options.  `POROUS_ETA_MAX` (the higher, i.e. shallower,
      !! interface) is the default and the LEAST blocking: the open width
      !! `w` is monotone increasing in the interface height, so the rule
      !! that returns the larger height leaves the most of the face open.
      !! `POROUS_ETA_MIN` is the most blocking.
      real(wp), intent(in) :: z_a
         !! Interface height in the first adjacent cell (m, positive up).
      real(wp), intent(in) :: z_b
         !! Interface height in the second adjacent cell (m, positive up).
      integer, intent(in) :: interp
         !! One of the `POROUS_ETA_*` enum values.
      real(wp) :: eta

      select case (interp)
      case (POROUS_ETA_MAX)
         eta = max(z_a, z_b)
      case (POROUS_ETA_MIN)
         eta = min(z_a, z_b)
      case (POROUS_ETA_ARITH)
         eta = 0.5_wp*(z_a + z_b)
      case (POROUS_ETA_HARM)
         ! Only meaningful when both interfaces are on the SAME side of
         ! the datum (the ocean case, both negative).  A straddling pair
         ! can drive the denominator through zero, where the H_DIV_EPS
         ! armour prevents the division but not a large result.
         eta = 2.0_wp*(z_a*z_b)/(z_a + z_b + H_DIV_EPS)
      case default
         ! UNREACHABLE.  MOM6 issues a FATAL here; this is a `pure`
         ! device-side function and cannot, so the fail-loud lives at the
         ! host boundary instead: `parse_porous_eta_interp` returns -1 for
         ! any unrecognised string and `configure_ocean_porous`
         ! `error stop`s on it, so no other value can reach this kernel.
         ! Falling back to MAX keeps the function total.
         eta = max(z_a, z_b)
      end select
   end function porous_eta_face