[MITgcm-support] Setting up viscosities for the model

[kunal madkaiker]

Attaching CFL and stability criteria of horizontal viscosity from MITgcm
documentation

On Fri, Oct 2, 2020 at 6:01 PM kunal madkaiker <kunal.madkaiker02 at gmail.com>
wrote:

> 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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