rdb_constants Module

Specific heat capacity of seawater (J/(kg·K)). Converts surface heat flux Q_net [W/m²] to kinematic heat flux Q_net/(rho_0·cp) [m·K/s].

Degrees → radians conversion factor.

Minimum depth to consider a cell wet (m)

&ocean_bt_nml frhat_scheme = "arithmetic" (DEFAULT pre-port behaviour). Every barotropic depth mean (rdb_barotropic_coupling::derive_bt_from_layers, face_depth_mean_u/v, face_depth_mean_rem_u/v, apply_bt_correction’s open/visc_rem folds, compute_bt_rem_from_visc_rem’s av_rem) weights each layer’s face thickness by the plain two-abutting-cell arithmetic mean, h_face(k) = 0.5*(h_L(k) + h_R(k)). Bit-identical to the pre-frhat-port tree.

&ocean_bt_nml frhat_scheme = "hybrid" — MOM6’s HVEL_SCHEME = HYBRID face-thickness closure (btcalc’s default scheme there). Above the shallower of the two abutting columns’ bed depths the face thickness is the arithmetic mean (as FRHAT_ARITHMETIC); below it, the harmonic mean h_L*h_R/(h_L+h_R) — which vanishes with the thinner side instead of being dragged up by the thicker one — with a linear blend across the transition layer. Suppresses a thin partial-bed layer’s weight next to a thick abutting layer, the mechanism docs/ visc_rem_bt_rem_plan.md Section 7 identifies as still missing from every roundabout barotropic depth mean. See rdb_barotropic_coupling::frhat_h_face_step (src/shared_module_utilities/rdb_frhat_face.inc) for the bottom-up port of MOM6’s e_u/D_shallow_u recursion.

Maximum Froude number allowed at wet/dry fronts. If |u|/sqrt(g*h) exceeds this, momentum is rescaled. Physical flows rarely exceed Fr=3; Fr=10 is very permissive.

Gravitational acceleration (m/s^2)

Pure division-safety epsilon (m), far below any physical thickness: adding it (1/(h+H_DIV_EPS)) only prevents a 1/0, never changes a well-posed result. Use where a divisor is ALREADY guaranteed positive (MOM6 H_subroundoff role).

Dynamic-vanish threshold (m). A layer thinner than this is dynamically VANISHED — skip / merge into a neighbour, do NOT clamp. When dividing by h_layer for a PHYSICAL result, gate on this. Tests of it are STRICT >: a layer sitting exactly ON the marker reads as vanished, which is what the geometric vcoord families rely on (rdb_vcoord :: vcoord_h_min_role).

Do not hand-roll the test. Tracers are stored as CONTENT (hTr = h·c) and the tree holds one invariant about vanished layers —

I1′:  `h <= H_VANISHED  ⇒  hTr = h·c_live`, every registered
      tracer, `c_live` the concentration of the filler's donor
      (nearest live layer above; topmost live layer for the
      fillers above it; `hTr = 0` with no live layer)

— with ONE definition of the predicate, of “the concentration of layer k”, and of the column merge that restores it: src/shared_module_utilities/rdb_vanished_layer.inc (rdb_vl_is_live, rdb_vl_conc, rdb_vl_column_conc, rdb_vl_holds_live_conc, rdb_vl_merge_content), ONE enforcement point (multilayer_state_t%enforce_vanished_content, at the tail of the outer step) and a fail-loud tripwire (&vcoord_nml check_vanished_content). The contract, and the table of consumers that deliberately substitute something ELSE (the EOS’s reference T/S, the sponge’s nearest massive layer, the diagnostics’ NaN, the melt sampler’s skip), is written down once in src/core/ocean/README.md, “The vanished-layer content rule”. A new raw hTr/h divide or a new comparison against this constant outside the sanctioned modules is flagged by the vanished-layer pre-commit hook.

Canuto et al. (2001) Model A second-moment-closure stability functions (rational alpha_M / alpha_N form per Umlauf & Burchard 2003).

Constant c_mu (=0.09) / Pr_t (=1) – standard high-Re k-epsilon (default; bit-identical to the original closure).

Galperin et al. (1988) quasi-equilibrium stratification-aware stability functions (Mellor-Yamada level-2.5 algebraic form).

Cell bathymetry depth (m, positive-down) below which an ocean cell is treated as LAND for surface-forcing masking. Consumed by ocean_state_seed_from_cfg to populate multilayer%wet_mask.

Latent heat of vaporization of water (J/kg). A representative near-surface value (2.501e6 at 0 °C, weakly T-dependent). Used by the evaporative latent-heat surface cooling: a net evaporation of E [m/s] withdraws a heat flux ρ_water·E·L_v [W/m²] from the top of the water column as the evaporated mass changes phase.

Minimum depth for non-hydrostatic pressure operations (m). Cells shallower than this are treated as hydrostatic to avoid 1/h singularities in the NH Poisson operator.

Maximum vertical layers for per-column stack workspace inside do concurrent. Stack arrays in the BPG, remap, kappa-shear, Redi and diag-remap kernels are dimensioned by this constant so the compiler can emit fixed-size thread-local storage on GPU.

The requirement is NZ_STACK_MAX >= nz + 1 — see nz_stack_required below for the derivation and the survey of every consumer. Enforced fail-loud by validate_config.

History: 100 -> 256 (2026-06-19), on the belief that the Redi neutral-diffusion sweep needed 2*nz+2 of this constant. It does not: Redi’s nsurf locals are declared 2*NZ_STACK_MAX+2 (rdb_ocean_redi.F90 PoLc/PoRc/KoLc/KoRc/hEc), so they scale with the constant and only need NZ_STACK_MAX >= nz. Lowered 256 -> 128 (2026-08-05) after that measurement: 256 was ~2x larger than any consumer needed and cost ~4 GB of CUDA per-thread local memory, which the driver reserves at each kernel’s first launch and never returns. 128 covers the largest shipped case (nz=90, acc_channel_kitchensink_xl.nml) and the largest test (nz=100) with headroom.

Heap-allocated workspace arrays (PPM snapshots, remap snapshots) have no such limit — they are allocated at runtime.

Earth angular velocity (rad/s)

Pi

Radians → degrees conversion factor.

Piecewise constant (0th order, donor cell)

Piecewise linear with minmod limiter (1st order)

Piecewise parabolic (2nd order, Colella & Woodward)

Piecewise parabolic with non-uniform 4th-order (H4) edge values — thickness-weighted edge estimate (White & Adcroft 2008) feeding the same Colella & Woodward parabola + monotonicity limiter as REMAP_PPM

Piecewise quartic (4th-order, PQM_IH4IH3 — White & Adcroft 2008): implicit-h4 edge values + implicit-h3 edge slopes + a quartic per-cell reconstruction with the White & Adcroft monotonicity limiter and a conservative quartic overlap integral. Requires nz >= 5; falls back to REMAP_PPM for thinner columns. Boundary cells are PCM.

Seawater density (kg/m^3)

Below this depth, momentum is zeroed to prevent huge velocities at wet/dry fronts (m)

2*Pi

Ignore SSH when forming target_h; layer thicknesses follow the static H · dsig(k) decomposition. Used by the ocean path as the “do nothing, leave the IC layers alone” default before the user picks a remap-aware coord_type. Was originally declared in rdb_ocean_vcoord as an orphan; moved here so the VCOORD enum lives in one place.

Hybrid z/isopycnal coordinate (P3, Bleck 2002 / MOM6 coord_hycom). Runs the exact VCOORD_RHO density-space inversion (same ocean_vcoord_compute_target_h_rho kernel, hybrid=.true.) but adds two HYCOM deltas around it: a bottom-up density monotonize before the inversion, and a z nominal-floor sweep after it that pushes too-shallow interfaces down to a fixed-resolution surface band (dsig·(H+η)). Net effect: a fixed-res near-surface z* band (no surface collapse) with an isopycnal interior — the production GVC coordinate. Reuses rho_target / rho_ref_pressure; no new knobs.

Pure Lagrangian / isopycnal — h_layer evolves freely and is never remapped. compute_target_h returns the current h unchanged and the ALE remap step is a no-op. Distinct from VCOORD_EULERIAN_Z (which targets the static H · dsig decomposition): LAGRANGIAN tracks material surfaces, so for adiabatic flow it reproduces MOM6’s COORD_CONFIG="gprime" NK=2 isopycnal layering. Parsed from “lagrangian” / “isopycnal”.

Isopycnal coordinate (P2). Layer interfaces are placed on prescribed potential-density surfaces rho_target(0:nz) (monotone-increasing, referenced to rho_ref_pressure). The ALE regrid inverts a PPM reconstruction of the column density profile for the depth where ρ equals each interior target, then remaps T/S/tracers/velocities onto the new grid. Bottom-up convention: rho_target(0) (lightest) anchors the surface interface (k=nz); rho_target(nz) (densest) the bed (k=1). Validation-grade alone (weakly-stratified columns collapse); HYCOM hybrid is the production follow-on (P3). See rdb_ocean_vcoord :: ocean_vcoord_compute_target_h_rho_impl.

Pure terrain-following sigma coordinate (default)

z-sigma hybrid: z-levels in deep/steep, sigma in shallow

z-star “lite” (MOM6-like quasi-horizontal): single global z_ref pattern stretched proportionally per column by H/z_ref(nz)

z-star “full” MOM6: per-column z_ref anchored to local bathymetry with vanishing layers below the bed. Surface layers stay at a fixed physical thickness independent of H

z/sigma hybrid: smoothstep blend from pure sigma in shallow water (H ≤ depth_transition) to z-lite in deep water (H ≥ depth_transition + blend_width). Coastal wet/dry friendliness in shallow zones, SSH-tracking in deep zones

Fixed-z interfaces with vanishing layers in shallow water. ALE remap pulls h_layer(k) → h_target(k) where the targets come from absolute-depth interfaces z_target = (0, h_nominal, 2·h_nominal, ..., h_ref) with h_nominal = h_ref/nz_ml — or, under &vcoord_nml z_fixed_profile = "list" | "tanh", from a stretched per-layer table (ocean_vcoord_t%z_fixed_zi). In deep cells the interfaces stay locked at those depths; in shallow cells the bed-side layers vanish to a minimum thickness h_min while the surface layer absorbs the residual. Mirrors MOM6’s COORD_CONFIG = "gprime" layer structure and the existing seed_h_layer_z_fixed_impl IC algorithm — used as a per-step ALE target rather than an init-only seed so the interface stays anchored as the simulation evolves.

Working precision kind parameter (single)