35.2.2.5 Summary of namelist parameters

In summary, the namelist parameters for the VMHS scheme, which are set in the namelist /ncdiff/, are the following:

1.

D_b = depthbot is the maximum depth over which the vertically averaged Richardson number is computed. The default is depthbot = 1000m. In regions shallower than depthbot, the lower limit on the vertical average defaults to the local model depth.

2.

D_t = depthtop is the minimum depth over which the vertically averaged Richardson number is computed. The default is depthtop = 100m. In regions where the depth is shallower than depthtop, the scheme returns a diffusivity equal to the value athkdf input to the namelist.

3.

diffmin is the minimum diffusivity returned from the scheme. The default is diffmin = 10⁶ cm²/sec.

4.

diffmax is the maximum diffisivity returned from the scheme. The default is $diffmax = 2 \times 10^{7} cm^{2}/sec$. Note that the effective value of the maximum diffusivity employed is constrained by the steep slope conditions discussed in Section 34.1.9.

5.

diffint sets the frequency, in model time steps, on which the nonconstant diffusivity will be updated. The default is diffint = 5.

6.

$vmhs\_alpha$ is a tuning parameter which scales the diffusivity. The default is $vmhs\_alpha = 0.015$.

7.

$vmhs\_rate\_limit$ defines what is meant by a large growth rate when defining the length scale. The default is $vmhs\_rate\_limit = 1.4 \times 10^{-6} sec^{-1}$

8.

$ijvmhs\_maxlength$ is the maximum value of the search which is conducted in the north, south, east, and west directions for determining the width of the baroclinic zone. The default is 10 latitude and longitude grid boxes.

The last three namelist parameters are the only ones not also used by the Held and Larichev scheme (Section 34.2.1).