# Lateral Ocean Physics (LDF)

The lateral physics terms in the momentum and tracer equations have been described in §2.5.1 and their discrete formulation in §5.2 and §6.6). In this section we further discuss each lateral physics option. Choosing one lateral physics scheme means for the user defining, (1) the space and time variations of the eddy coefficients ; (2) the direction along which the lateral diffusive fluxes are evaluated (model level, geopotential or isopycnal surfaces); and (3) the type of operator used (harmonic, or biharmonic operators, and for tracers only, eddy induced advection on tracers). These three aspects of the lateral diffusion are set through namelist parameters and CPP keys (see the nam_traldf and nam_dynldf below). Note that this chapter describes the default implementation of iso-neutral tracer mixing, and Griffies's implementation, which is used if traldf_grif=true, is described in AppdxD

```!----------------------------------------------------------------------------------
&namtra_ldf    !   lateral diffusion scheme for tracers
!----------------------------------------------------------------------------------
!                       !  Operator type:
ln_traldf_lap    =  .true.   !  laplacian operator
ln_traldf_bilap  =  .false.  !  bilaplacian operator
!                       !  Direction of action:
ln_traldf_level  =  .false.  !  iso-level
ln_traldf_hor    =  .false.  !  horizontal (geopotential)   (needs "key_ldfslp" when ln_sco=T)
ln_traldf_iso    =  .true.   !  iso-neutral                 (needs "key_ldfslp")
!		       	   !  Griffies parameters              (all need "key_ldfslp")
ln_traldf_grif   =  .false.  !  use griffies triads
ln_traldf_gdia   =  .false.  !  output griffies eddy velocities
ln_triad_iso     =  .false.  !  pure lateral mixing in ML
ln_botmix_grif   =  .false.  !  lateral mixing on bottom
!                       !  Coefficients
! Eddy-induced (GM) advection always used with Griffies; otherwise needs "key_traldf_eiv"
! Value rn_aeiv_0 is ignored unless = 0 with Held-Larichev spatially varying aeiv
!                                  (key_traldf_c2d & key_traldf_eiv & key_orca_r2, _r1 or _r05)
rn_aeiv_0        =  2000.    !  eddy induced velocity coefficient [m2/s]
rn_aht_0         =  2000.    !  horizontal eddy diffusivity for tracers [m2/s]
rn_ahtb_0        =     0.    !  background eddy diffusivity for ldf_iso [m2/s]
!                                           (normally=0; not used with Griffies)
rn_slpmax        =     0.01  !  slope limit
rn_chsmag        =     1.    !  multiplicative factor in Smagorinsky diffusivity
rn_smsh          =     1.    !  Smagorinsky diffusivity: = 0 - use only sheer
rn_aht_m         =  2000.    !  upper limit or stability criteria for lateral eddy diffusivity (m2/s)
/
```

```!-----------------------------------------------------------------------
&namdyn_ldf    !   lateral diffusion on momentum
!-----------------------------------------------------------------------
!                       !  Type of the operator :
ln_dynldf_lap    =  .true.   !  laplacian operator
ln_dynldf_bilap  =  .false.  !  bilaplacian operator
!                       !  Direction of action  :
ln_dynldf_level  =  .false.  !  iso-level
ln_dynldf_hor    =  .true.   !  horizontal (geopotential)            (require "key_ldfslp" in s-coord.)
ln_dynldf_iso    =  .false.  !  iso-neutral                          (require "key_ldfslp")
!                       !  Coefficient
rn_ahm_0_lap     = 40000.    !  horizontal laplacian eddy viscosity   [m2/s]
rn_ahmb_0        =     0.    !  background eddy viscosity for ldf_iso [m2/s]
rn_ahm_0_blp     =     0.    !  horizontal bilaplacian eddy viscosity [m4/s]
rn_cmsmag_1      =     3.    !  constant in laplacian Smagorinsky viscosity
rn_cmsmag_2      =     3     !  constant in bilaplacian Smagorinsky viscosity
rn_cmsh          =     1.    !  1 or 0 , if 0 -use only shear for Smagorinsky viscosity
rn_ahm_m_blp     =    -1.e12 !  upper limit for bilap  abs(ahm) < min( dx^4/128rdt, rn_ahm_m_blp)
rn_ahm_m_lap     = 40000.    !  upper limit for lap  ahm < min(dx^2/16rdt, rn_ahm_m_lap)
/
```

Subsections

Gurvan Madec and the NEMO Team
NEMO European Consortium2017-02-17