[MITgcm-support] [EXTERNAL] MITgcm- Regarding horizontal and vertical viscosity

[kunal madkaiker]

Thank you Dimitris. This helps.

Kunal

On Mon, Oct 12, 2020 at 8:32 PM Dimitris Menemenlis <menemenlis at jpl.nasa.gov>
wrote:

> You can use your sensitivity experiments to select an optimized set of
> horizontal and vertical viscosities, and other empirical model parameters,
> for your model configuration as is done here:
> https://journals.ametsoc.org/mwr/article/133/5/1224/67525/Using-Green-s-Functions-to-Calibrate-an-Ocean
>
> On Oct 11, 2020, at 8:36 PM, kunal madkaiker <kunal.madkaiker02 at gmail.com>
> wrote:
>
> Dear All,
>
> I need some help in setting up my model viscosities. Few model studies
> done in Arabian Sea and N. Indian ocean suggest that horizontal viscosity
> can be in the range of 5.E3 to 5.E4 m2/s whereas vertical viscosity can be
> set as 1.E-3 m2/s in this region. (My model setup has delT=120s,
> delX=delY=3000m)
>
> After going through the formulae in mom_calc_visc.F, I found 'viscAh=5.E3'
> computes to be 'viscAhgrid=0.26' (for consideration, selecting
> viscAhgrid=0.2), which satisfies stability and CFL criteria (Manual,
> Section 4.3.2.1, eq. 4.24, 4.26). Also, using viscAz=0.03, which satisfies
> eq. 4.28 for min delZ of 5m. I find this setup to be most stable (KE
> oscillations are the least)
>
> Now, manual suggests to setup viscAz=0.003, which is 1 order less than my
> computed value. Also, considering Munk eq. 4.27, ratio of (Ah/Mw^3)
> computes to be in order of 4x10^-13. So, for Mw=3000 m (delx/dely), Ah
> becomes 0.0127 m2/s, which is again an order less.
>
> Can anyone shed some light on this? Incase second combination is more
> robust, I feel viscAhgrid value will be too small (Ah itself is in order of
> 1.E-2)
>
> After doing some preliminary experiments, I observed the below mentioned
> points:
> 1. Increasing vertical viscosity definitely subdues oscillations in the
> KE, but same may not be said for horizontal viscosity.
> 2. Reducing vertical viscosity is making the surface currents much
> stronger.
>
> Kindly assist.
>
> Regards
> Kunal
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