Choice of namtra_ldf namelist parameters

!----------------------------------------------------------------------------------
&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)
/

If the namelist variable ln_traldf_grif is set true (and key_ ldfslp is set), NEMO updates both active and passive tracers using the Griffies triad representation of iso-neutral diffusion and the eddy-induced advective skew (GM) fluxes. Otherwise (by default) the filtered version of Cox's original scheme is employed (§9.2). In the present implementation of the Griffies scheme, the advective skew fluxes are implemented even if key_ traldf_eiv is not set.

Values of iso-neutral diffusivity and GM coefficient are set as described in §9.1. If none of the keys key_ traldf_cNd, N=1,2,3 is set (the default), spatially constant iso-neutral $A_l$ and GM diffusivity $A_e$ are directly set by rn_aeih_0 and rn_aeiv_0. If 2D-varying coefficients are set with key_ traldf_c2d then $A_l$ is reduced in proportion with horizontal scale factor according to (9.1) ^D.1. In idealised setups with key_ traldf_c2d, $A_e$ is reduced similarly, but if key_ traldf_eiv is set in the global configurations with key_ traldf_c2d, a horizontally varying $A_e$ is instead set from the Held-Larichev parameterisation^D.2 (ldfeiv.F90) and rn_aeiv_0 is ignored unless it is zero.

The options specific to the Griffies scheme include:

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ln_traldf_gdia

Default value is false. See §D.3.6. If this is set true, time-mean eddy-advective (GM) velocities are output for diagnostic purposes, even though the eddy advection is accomplished by means of the skew fluxes.

ln_traldf_iso

See §D.2.10. If this is set false (the default), then `iso-neutral' mixing is accomplished within the surface mixed-layer along slopes linearly decreasing with depth from the value immediately below the mixed-layer to zero (flat) at the surface (§D.2.10). This is the same treatment as used in the default implementation §9.2.2; Fig. 9.2. Where ln_traldf_iso is set true, the vertical skew flux is further reduced to ensure no vertical buoyancy flux, giving an almost pure horizontal diffusive tracer flux within the mixed layer. This is similar to the tapering suggested by Gerdes et al. [1991]. See §D.2.10

ln_traldf_botmix

See §D.2.8. If this is set false (the default) then the lateral diffusive fluxes associated with triads partly masked by topography are neglected. If it is set true, however, then these lateral diffusive fluxes are applied, giving smoother bottom tracer fields at the cost of introducing diapycnal mixing.

Footnotes

...Eq_title ^D.1

Except in global ORCA $0.5^{\circ}$ runs with key_ traldf_eiv, where $A_l$ is set like $A_e$ but with a minimum vale of $100\;\mathrm{m}^2\;\mathrm{s}^{-1}$

... parameterisation^D.2

In this case, $A_e$ at low latitudes $\vert\theta\vert<20^{\circ}$ is further reduced by a factor $\vert f/f_{20}\vert$, where $f_{20}$ is the value of $f$ at $20^{\circ}$ N