[MITgcm-support] Setting up viscosities for the model

[kunal madkaiker]

 Dear Team,

I am having slight confusion about setting the horizontal eddy viscosity in
my model. Here, I am only considering Laplacian horizontal viscosity (i.e.
not considering Leith, Biharmonic viscosity, etc). I have a 360x780 grid,
with 3000m x 3000m spatial resolution, length of mesh ~20 deg, width of
mesh ~10 deg and timestep of 120 sec.

Referring to the manual, I find two criterias to be satisfied i.e., CFL
Constraint on Viscosity (Section 2.19.1.7) and Numerical Stability Criteria
(Section 4.3.2.1, eq 4.26). Currently, I am specifying viscAh=5000 m2/s.
[image: image.png]The upper limit for Ah comes to be 18750 > 5000. Also,
[image: image.png]i.e. the stability for horizontal viscosity. For my
setup, it gave a value of 0.53 < 0.6 (solving using Adams-Bashforth). Now,
when I use viscAh=6000, Sl becomes 0.64 > 0.6, but my model simulation went
through without crashing. I am trying to understand how the model still ran.

Also, I believe using viscAhgrid would be a better option. But the manual
doesn't mention how viscAhgrid is being computed/ what's the formula (range
is between 0 and 1). I have set viscAhGridMax=0.5 in data

I am trying to address the following questions:
1. How to compute viscAhgrid? (or do I have to use Sl value as viscAhgrid?)
2. Can both viscAz and Implicitviscocity=.TRUE. be set at the same time
(since KPP needs implicitviscocity=.TRUE). I have kept viscAz=0.001
3. Is there any way to check if the run has been stable by using the U and
V momentum tendencies from dissipation and advection term in the output? Or
any other variable in the output (S,T,U,V,W,eta)?
Note: I am specifically interested in using the Laplacian eddy viscosity.
I am also attaching my data file. Kindly assist how I can improve the setup
further.

Regards
Kunal
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