14.4.6 Mixing

Namelist /mixing/

These variables are for setting mixing and related values.

•  am = lateral viscosity coefficient in cm²/sec for option consthmix. Refer to Bryan, Manabe, Pacanowski (1975) for estimating a value.

•  ah = lateral diffusion coefficient in cm²/sec for option consthmix.
•  ambi = absolute value of lateral viscosity coefficient in cm⁴/sec for option velocity_horz_biharmonic. Refer to Section 33.4 for estimating a value.
•  ahbi = absolute value of lateral diffusion coefficient in cm⁴/sec for option tracer_horz_biharmonic. Refer to Section 33.4 for estimating a value.

•  kappa_m = vertical viscosity coefficient in cm²/sec for option constvmix.

•  kappa_h = vertical diffusion coefficient in cm²/sec for option constvmix.

•  cdbot = bottom drag coefficient which is unitless. A typical value would be around $2.5 \mbox{x} 10^{-3}$.

•  aidif = implicit vertical diffusion factor. In cases where the vertical mixing coefficients severely limit the time step (because of fine vertical resolution or large vertical eddy coefficients), this constraint on the time step can be relaxed by solving the vertical diffusion term implicitly. The vertical diffusion term is solved implicitly when option implicitvmix or isoneutralmix is enabled. Otherwise, aidif is not used. When solving implicitly, $0 \le aidif \le 1$ with aidif=1 giving fully implicit and aidif=0giving fully explicit treatment. Why should semi-implicit vertical diffusion be used? The recommendation from Haltiner and Williams (1980) is for aidif=0.5 when solving semi-implicitly. This gives the Crank-Nicholson implicit scheme which is always stable. This setting is supposed to be the most accurate one. However, this is not the case when solving a time dependent problem as discussed in Section 37.5.

•  ncon = number of passes on the convective adjustment routine. Only meaningful when option fullconvect is not enabled and implicitvmix is not enabled.

•  nmix = number of time steps between mixing time steps. A mixing time step is either a Forward or Euler Backward time step as opposed to the normal leapfrog time step.

•  eb = Boolean for using Euler backward mixing scheme used on mixing time steps.

•  acor = implicit Coriolis factor for treating the Coriolis term semi-implicitly. For semi-implicit treatment, 0.5 < acor < 1and option damp_inertial_oscillation must be enabled. Refer to Section 27.2.1 for a discussion of when this is appropriate.

•  dampts = Newtonian damping time scale in days used with option restorst. Note that damping time scale may by set differently for each tracer.

•  annlev = Boolean for replacing seasonal sponge data by annual means when enabling option sponges.

•  filter_reflat_s = southern latitude (degrees) below which polar filtering operates if option fourfil is enabled. The filter will remove zonal scales from the solution which are less than the zonal grid spacing at latitude filter_reflat_s.

•  filter_reflat_n = northern latitude (degrees) above which polar filtering operates if option fourfil is enabled. The filter will remove zonal scales from the solution which are less than the zonal grid spacing at latitude filter_reflat_n.

•  rjfrst = southern latitude (degrees) below which polar filtering is turned off when option fourfil is enabled. The purpose of this is to save computations over the land mass of Antarctica.