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 and
GM diffusivity are directly set by rn_aeih_0 and
rn_aeiv_0. If 2D-varying coefficients are set with
key_ traldf_c2d then is reduced in proportion with horizontal
scale factor according to (9.1) ^{D.1}. In idealised setups with
key_ traldf_c2d, is reduced similarly, but if key_ traldf_eiv
is set in the global configurations with key_ traldf_c2d, a horizontally varying 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:

- [font=]
**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.

- ...Eq_title
^{D.1} - Except in global ORCA runs with key_ traldf_eiv, where is set like but with a minimum vale of
- ... parameterisation
^{D.2} - In this case, at low latitudes is further reduced by a factor , where is the value of at N

Gurvan Madec and the NEMO Team

NEMO European Consortium2017-02-17