[MITgcm-support] Setting up viscosities for the model

[kunal madkaiker]

Hi Martin,

Thank you for clarifying that for me. I will certainly implement the
suggestions made by you and see.

Regards
Kunal

On Fri, 2 Oct 2020, 20:52 Martin Losch, <Martin.Losch at awi.de> wrote:

> Hi Kunal,
>
> viscAhGrid is a non-dimensional parameter that basically derives from the
> stability criterion. You can see in the header of
> pkg/mom_common/mom_calc_visc.F how it is used to compute the viscosity. The
> main point is that it should be < 1 to ensure stability (independent of
> grid spacing, because it is scaled by the grid spacing).
>
> viscAh and viscAhGrid can both be specified together, but if you want to
> replace viscAh by viscAhGrid (which I always do) the relation is like this:
> viscAh = viscAhGrid * 0.25 * L**2 / deltaT (where L is the grid spacing in
> meters), or viscAhGrid = 4*deltaT* viscAh/L**2
>
> 2. yes, viscAz (or viscAr) serves as the background viscosity for KPP
> 3. in the monitor output (only if monitorFreq > 0.) in the stdout you’ll
> find the maximum CFL number (advcfl_uvel_max, etc)
>
> specifying a bottom drag is inconsistent with no_slip_bottom=.True.
> (either one of the other)
> I would use Leigth instead of Smagorinsky viscosity (but you don’t need to
> have nonzero values for all of the options, you can choose).
> replace globalFile by useSingleCpuIO =.TRUE.,
> Try to use a delZ without jumps, the rule of thumb is to have dz(k+1) <=
> 1.4*dz(k)
> for constant grid spacing I find it more convenient to replace delX by
> dxSpacing:
>  dxspacing=2891.9791,
>  dyspacing=2892.8394,
>
> Hope that helps,
> Martin
>
> > On 2. Oct 2020, at 14:31, 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.
> > The upper limit for Ah comes to be 18750 > 5000. Also,
> > 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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