KPP Boundary Layer Mixing

This page explains how to enable, configure, and use Omega K-Profile Parameterization (KPP) boundary layer mixing in runs. The implementation follows directly from the MPAS-Ocean implementation.

Related pages:

• KPP design/theory: Design KPP document

• KPP implementation details: Developer KPP document

• Broader vertical mixing options: Vertical Mixing Coefficients

What KPP Provides

KPP computes:

• Ocean surface boundary layer depth (OSBLDepth)

• Vertical viscosity (VertVisc)

• Vertical diffusivity (VertDiff)

• Non-local tracer flux profile (VertNonLocalFlux)

It uses a bulk Richardson depth search followed by profile-based coefficient construction. The non-local flux is a standard part of the KPP formulation and is included by default.

How KPP Is Used in Time Stepping

KPP is connected to all three Omega time steppers: Forward-Backward, RungeKutta2, and RungeKutta4. For whichever stepper is active, KPP is computed once per time step, at the start of the step (in contrast to MPAS-Ocean), before any tendency is evaluated. Boundary-layer depth, viscosity, diffusivity, and the non-local flux profile are then held fixed for the rest of the step and are used by:

• the non-local tracer tendency at every internal stage of the stepper, and

• the implicit vertical mixing solve applied after the time levels are advanced.

Because both use the same KPP fields, the non-local flux and the diffusivity it is paired with are always consistent with one another.

The KPP diagnostics written to output for a step therefore describe the ocean state at the beginning of that step, not the updated state at the end of it. This is the same convention used by the MPAS-Ocean split-explicit stepper.

Configuration

KPP settings are under VertMix: KPP in omega.yml.

Example

VertMix:
  KPP:
    Enable: true
    CriticalBulkRichardsonNumber: 0.25
    SurfaceLayerExtent: 0.1
    MatchTechnique: SimpleShapes
    InterpType2: LMD94
    UseEnhancedDiffusion: true
    UseOSBLSmoothing: true
    UseLangmuirTurbulence: true
    IceFractionThresholdForLangmuir: 0.05
    IceFractionThresholdForMinimumOSBL: 0.15
    MinimumOSBLUnderSeaIce: 5.0
    DebugDiagnostics: false

Key Options

Key	Meaning	Typical default
Enable	Enable KPP mixing	true
CriticalBulkRichardsonNumber	OBL depth criterion threshold	0.25
SurfaceLayerExtent	Surface layer thickness as a fraction of the OBL depth (\(\epsilon\) in Large et al. 1994)	0.1
MatchTechnique	How the K profile meets interior mixing at the OBL base: SimpleShapes or MatchBoth	SimpleShapes
InterpType2	Interpolation type used near OBL matching/base logic	LMD94
UseEnhancedDiffusion	Enable enhanced diffusion treatment near OBL base	true
UseOSBLSmoothing	Apply horizontal smoothing to the OSBL depth	true
UseLangmuirTurbulence	Apply Langmuir enhancement to the turbulent velocity scale	true
IceFractionThresholdForLangmuir	Above this ice fraction, disable Langmuir enhancement	0.05
IceFractionThresholdForMinimumOSBL	Above this ice fraction, enforce minimum OSBL depth	0.15
MinimumOSBLUnderSeaIce	Minimum OSBL depth under sea ice (m)	5.0
DebugDiagnostics	Enable additional KPP diagnostics/logging in debug workflows, and extend the Ri diagnostic profiles below the boundary layer base	false

Output and Diagnostics

To diagnose KPP, include KPP fields in output stream contents. Common fields:

• OSBLDepth

• VertNonLocalFlux

• BulkRichardsonNumber

• BulkRichardsonShear

• UnresolvedShear

• BuoyancyJump

• TurbulentVelocityScale

• PotentialDensity

• SurfaceFrictionVelocity

• SurfaceBuoyancyFlux

The boundary layer search stops at the first level where the bulk Richardson number reaches its critical value, so BulkRichardsonNumber, BulkRichardsonShear, UnresolvedShear and BuoyancyJump are zero below the boundary layer base. Set DebugDiagnostics: true to compute and output the full water column profile of these four fields; it does not change OSBLDepth or any mixing coefficient.

Typical Workflow

1. Enable KPP and set baseline options in omega.yml.

2. Run a short case.

3. Inspect OSBLDepth and coefficient fields.

4. If needed, tune CriticalBulkRichardsonNumber, MatchTechnique, and InterpType2.

5. Re-run and compare diagnostics.

Practical Notes

• Non-local tracer transport is required for physically correct KPP boundary layer tracer fluxes. It is applied through the Tendencies: TracerNonLocalFluxTendencyEnable flag, which is on by default and should be left enabled; see Tendency Terms.

• Use DebugDiagnostics sparingly for troubleshooting targeted cases.

• When studying sea-ice regions, review minimum-OBL and ice-threshold options.