[MITgcm-support] KPP scheme and background viscosities

[Abbas Dorostkar]

Hi all,
I have simulated basin-scale internal waves in a lake using the hydrostatic
version of MITgcm on a 400x400 horizontal grid spacing. The smallest
vertical grid spacing is 0.5 m. A staggered baroclinic time-stepping is used
for the tracer equation. The tracer advection scheme is a 3-DST so I set the
horizontal eddy diffusivity to 1E-7. The horizontal eddy viscosity is
constant with the value of 1. The vertical eddy viscosities and
diffusivities are computed by the KPP scheme.

The model is not sensitive to the background vertical viscosity in the range
of 1E-7 to 1E-4. However, the background viscosity of 1E-3 reduces the
root-mean-square error between the model and field data by 20% over the
simulation which uses a value of 1E-5.  Also, the model also does not show
sensitivity to the background vertical diffusivity ranging 1E-7 to
1E-5.  However,
using higher values such as 1E-4 gives very poor error statistics.  Does
anybody have in any inputs?

The model gets unstable if I use background vertical viscosity larger than
1E-3 unless I use smaller time step. I was wondering if MITgcm has a viscous
limitation controlled by the vertical eddy viscosity (ViscAz) such that
(deltaT)*(ViscAz)/(deltaZ)**2 < 1.

Thanks in advance for your ideas

Abbas
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