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Vertical mixing and convection parameterizations

Several vertical mixing parameterizations are available within the POP model and are described in much more detail in the Reference Manual. The value of vmix_choice determines whether a simple constant mixing, a Richardson-number dependent mixing or the KPP mixing parameterization is used. Additional mixing parameters for each of these schemes are set in individual namelists shown below; only the namelist associated with the mixing choice is actually read.

The treatment of convection is also specified in the vertical mixing namelist through the convection_type variable. Convection can be treated using either convective adjustment or by specifying large diffusion coefficients in convectively unstable regions. The KPP vertical mixing parameterization must use the diffusion option. If convective adjustment is chosen, the number of passes through the vertical column to adjust is determined by the parameter nconvad. The treatment of convection by diffusion is governed by the input diffusion coefficients convect_diff and convect_visc. Note that for constant vertical mixing, you can apply diffusion to tracers only by setting convect_visc to zero; this is not true for Richardson number mixing or KPP.

Table: Vertical mixing namelist
&vertical_mix_nml LANL
Vertical mixing parameterizations
vmix_choice 'const' ''kpp' 'const', 'kpp', 'rich' method of computing vertical diffusion
implicit_vertical_mix .true. .true. .true., .false. if .true., vertical mixing is solved implicitly in time
aidif 1.0 1.0 0.5-1.0 time-centering parameter for implicit vertical mixing; use of the default value [1.0] is recommended
bottom_drag 1.0E-03 1.0E-03   (dimensionless) coefficient used in quadratic bottom drag formula
convection_type 'diffusion' 'diffusion' 'adjustment', 'diffusion' Convection treated by adjustment or by large mixing coefficients
nconvad 2 2 integer number of passes through the convective adjustment algorithm
convect_diff 1000.0 10000.0   tracer mixing coefficient to use with diffusion option
convect_visc 1000.0 10000.0   momentum mixing coefficient to use with diffusion option
bottom_heat_flx 0.0 0.0   constant (geothermal) heat flux (W/m2) to apply to bottom layers
bottom_heat_flx_depth 100000.0 100000.0   depth (cm) below which to apply bottom heat flux

Some vertical and horizontal mixing parameterizations (e.g. KPP and Gent-McWilliams to be discussed later) create large vertical mixing coefficients. In addition, when diffusion is used as the method for treating convection, the diffusion coefficients are large. In such cases, implicit vertical mixing must be enabled (implicit_vertical_mix = .true.) to avoid severe restrictions on the model time step.

If implicit vertical mixing is chosen, the parameter aidif governs the time-centering of the implicit scheme. The bottom_drag coefficient is used to compute bottom drag. To simulate geothermal heating at the bottom of the ocean, a constant heat flux can be applied below a fixed depth in the ocean. A heat flux of zero turns off this option.

CESM1 Notes


Constant coefficients Up: Running POP Previous: Pressure gradient options   Contents