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

[kunal madkaiker]

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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