[0.25deg config] Lateral Friction

[minghangli-uni]

As discussed in the namelist discussion,

Lateral viscosity, smag etc - try using same values as ACCESS-OM2

The following implementation is for OM3.

SMAGORINSKY_AH = True           !   [Boolean] default = False
                                ! If true, use a biharmonic Smagorinsky nonlinear eddy viscosity.
SMAG_BI_CONST = 0.05066         !   [nondim] default = 0.0
                                ! The nondimensional biharmonic Smagorinsky constant, typically 0.015 - 0.06.
LAPLACIAN = True                !   [Boolean] default = False
                                ! If true, use a Laplacian horizontal viscosity.
AH_VEL_SCALE = 0.01             !   [m s-1] default = 0.0
                                ! The velocity scale which is multiplied by the cube of the grid spacing to
                                ! calculate the biharmonic viscosity. The final viscosity is the largest of this
                                ! scaled viscosity, the Smagorinsky and Leith viscosities, and AH.
NOSLIP = False                  !   [Boolean] default = False
                                ! If true, no slip boundary conditions are used; otherwise free slip boundary
                                ! conditions are assumed. The implementation of the free slip BCs on a C-grid is
                                ! much cleaner than the no slip BCs. The use of free slip BCs is strongly
                                ! encouraged, and no slip BCs are not used with the biharmonic viscosity.

Comments

1. To be consistent with OM2, the lateral friction is implemented using a biharmonic operator. The Smagorinsky biharmonic viscosity is defined as $B_{smag} = A_{smag}\Delta^2/8$, where $A_{smag} = (C\Delta/\pi)^2|D|$ represents the Smagorinsky Laplacian viscosity. Here $C$ is a dimensionless scaling parameter, $\Delta$ is the grid size, $|D|$ is the deformation rate. In OM2, the isotropic Smagorinsky scaling parameter $C$ is set to 2.0, hence for OM3, the biharmonic Smagorinsky constant (SMAG_BI_CONST) $C^2/8\pi^2$ is equivalent to 0.05066.
2. A static biharmonic viscosity AH_VEL_SCALE is applied as referenced from GFDL OM5 and OM4.
3. In OM2, the lateral boundary condition (BC) for velocity is no slip, which is the only option for MOM5. However in OM3, the NOSLIP cannot be used together with biharmonic viscosity. The free slip BC is strongly encouraged for MOM6.
4. Another discussion can be found here MOM6 1deg parameterization 2 - lateral friction #105

[minghangli-uni]

The relevant action is being tracked in #253, so this issue is now closed.