ocean_hvisc_config_t Derived Type

   type :: ocean_hvisc_config_t
      character(len=16) :: lateral_closure = "none"
         !! Lateral-mixing closure tag.  "none" (default) keeps the scalar
         !! `nu_h`.  "leith" = Leith vorticity-gradient scaling;
         !! "smagorinsky"/"smag" = Smagorinsky strain-rate scaling.  Either
         !! gives a per-face viscosity floored at `ah_bg`, capped at `ah_max`.
      real(wp) :: c_smag = 0.15_wp
         !! Dimensionless Smagorinsky coefficient.  Default 0.15 (range 0.1–0.2).
      real(wp) :: c_leith = 1.0_wp
         !! Dimensionless Leith coefficient.  Typical range 1.0–2.0.
      real(wp) :: kh_vel_scale = 0.0_wp
         !! Velocity scale (m/s) for the grid-resolution viscosity floor
         !! `nu_min = kh_vel_scale · dx`.  Typical 3e-3 m/s.
      real(wp) :: ah_bg = -1.0_wp
         !! Background harmonic viscosity (m²/s) for the lateral closure.
         !! Negative (default) = derive from nu_h and kh_vel_scale.
      real(wp) :: ah_max = 1.0e4_wp
         !! Hard cap on the per-face harmonic viscosity (m²/s).
      real(wp) :: kh_vel_scale_live = 0.0_wp
         !! Live velocity-scale viscosity coefficient (m/s).  When positive,
         !! a state-dependent `A_vel = kh_vel_scale_live·dx·|u|` is
         !! max-combined into the per-face harmonic viscosity every step.
         !! Default 0 ⇒ off, bit-identical.
      real(wp) :: kh_aniso = 0.0_wp
         !! Anisotropic Laplacian viscosity magnitude (m²/s; Smith &
         !! McWilliams 2003).  When positive AND `stress_tensor = .true.`,
         !! adds direction-tensor terms aligned with `aniso_dir`.  Default 0
         !! ⇒ isotropic, bit-identical.  Stress-tensor path only.
      integer :: aniso_mode = 0
         !! Anisotropy-direction mode.  Only mode 0 (grid-relative
         !! `(n1,n2) = aniso_dir`) is implemented; other values rejected at
         !! configure (no silent fall-back).
      real(wp) :: aniso_dir(2) = [1.0_wp, 0.0_wp]
         !! Anisotropy direction vector `(n1,n2)` in grid i,j components.
         !! Default `(1,0)` = grid-i.  Normalised internally.
      logical :: smag_ah = .false.
         !! Smagorinsky biharmonic.  When `.true.` the biharmonic viscosity
         !! becomes flow-aware `nu4_face = C_b · L⁴ · |D|`, clamped to
         !! `[nu_4_bg, nu_4_max]`; the scalar `nu_4` is bypassed.
      real(wp) :: smag_bi_const = 0.06_wp
         !! Nondimensional biharmonic Smagorinsky constant.  Typical 0.015–0.06.
      real(wp) :: c_leith_bi = 0.0_wp
         !! Nondimensional biharmonic Leith constant.  Drives
         !! `lateral_closure="leith_biharm"` (per-face
         !! `nu4_face = C_lb · grid_sp⁶ · inv_PI6 · |∇²ζ|`, clamped to
         !! `[nu_4_bg, nu_4_max]`).  Default 0.0 = no-op (startup warning if
         !! `leith_biharm` selected at 0.0).
      real(wp) :: nu_4_bg = 0.0_wp
         !! Background biharmonic viscosity floor (m⁴/s) for the flow-aware path.
      real(wp) :: nu_4_max = 1.0e12_wp
         !! Static hard cap on per-face biharmonic viscosity (m⁴/s).  The
         !! forward-Euler biharmonic-CFL bound is enforced automatically per
         !! cell (scaled by `bound_coef`), so this is just an extra ceiling.
      real(wp) :: nu_h = 0.0_wp
         !! Constant horizontal eddy viscosity for momentum (m²/s).  Default
         !! 0 = bit-identical; wind-driven runs need this set.
      real(wp) :: nu_4 = 0.0_wp
         !! Constant biharmonic eddy viscosity for momentum (m⁴/s).  Default 0.
      logical :: no_slip = .false.
         !! Lateral coast BC (static land mask).  `.false.` (default) =
         !! FREE-SLIP (corner vorticity/shear strain multiplied by `wet_q`);
         !! `.true.` = NO-SLIP (factor `2 - wet_q`).  All-wet ⇒ bit-identical.
      logical :: stress_tensor = .false.
         !! Horizontal-viscosity operator.  `.false.` (default) =
         !! velocity-Laplacian `A·∇²u` with global caps, bit-identical.
         !! `.true.` = thickness-weighted stress-divergence form
         !! (momentum-conserving) with per-cell CFL limiter and coast
         !! masking; reduces to the Laplacian on uniform-grid/h/all-wet.
      real(wp) :: bound_coef = 0.8_wp
         !! CFL safety coefficient for the per-cell viscosity limiters
         !! (harmonic when `stress_tensor`, biharmonic always when `nu_4 > 0`
         !! or Smag_AH).  Default 0.8; must be ≤ 1.
      logical :: bound_kh = .false.
         !! MOM6 `BOUND_KH` analogue for the velocity-Laplacian paths:
         !! per-face clamp of the harmonic viscosity to
         !! `bound_coef·0.125/(dt·(1/dx²+1/dy²))` (~¼ of the forward-
         !! Euler stability limit).  Prevents an over-large `nu_h` (or
         !! flow-aware coefficient) from putting the frozen depth-mean
         !! viscous forcing on the barotropic mode into the phase-
         !! reversed ANTI-damping regime at grid scale, which pumps an
         !! exponential rim-trapped barotropic mode via the split-RK2
         !! Δu corrector (the 600² Lagrangian double-gyre blow-up).
         !! Recommended `.true.` for any config where
         !! `nu·dt·(1/dx²+1/dy²) ≳ 0.1`.  Default `.false.` ⇒
         !! bit-identical.
      logical :: resoln_scaled_visc = .false.
         !! Resolution-scaled Laplacian viscosity (Hallberg 2013).  When
         !! `.true.` the dynamic Laplacian viscosity is multiplied by the
         !! VarMix resolution function `Res_fn ∈ [0,1]` before the `ah_max`
         !! clamp (biharmonic `nu_4` never scaled).  Requires
         !! `&ocean_varmix_nml enable=.true.`.  Default `.false.` ⇒ bit-identical.
   end type ocean_hvisc_config_t