[MITgcm-support] Small timestep - rapid overadjustment

[Chris Horvat]

Martin,

I've been using the Smagorinsky viscosity for the most part, viscC2Smag =
1, which is low for oceanic problems, as I understand, which maybe the
reason for the issue. I'll try switching to vischAhgrid ~.5 to see if this
makes a difference.

Chris


On Mon, Jan 7, 2013 at 3:48 AM, Martin Losch <Martin.Losch at awi.de> wrote:

> Hi Chris,
>
> I don't know enough about your configuration, but the only time dependent
> stability parameter that I can think of is for the horizontal viscosity:
> viscAh*deltaT/deltaX^2. Maybe this has some effect (not on the stability,
> as you decrease you time step, but maybe on the solution). Did you try
> using viscAhgrid (value of <1, see mom_calc_visc.F or the online
> documentation for more details) instead of viscAh?
>
> Martin
>
> On Jan 4, 2013, at 8:33 PM, Chris Horvat <horvat at fas.harvard.edu> wrote:
>
> > Hi,
> >
> > I'm running a reentrant ocean channel configuration (120km by 40km), and
> OBCS is setting temperatures on the north and sound boundaries in order to
> force a temperature gradient. Initial conditions interpolate between the
> boundaries. It is an Arctic simulation, so the deep water is warmer than
> the top water. (Temp. profile at http://imgur.com/4RZ0X)
> >
> >
> > Here's the problem. If I set my timestep to be too large (~3600 s),
> numerical instability kills me in a few steps. If I set smaller (~600 s), I
> can get much further before I run into a similar numerical error. (maybe 2
> mo.s), and the first thing that happens, as expected, is the interior
> begins to relax to match the imposed gradient. (seen at
> http://imgur.com/Ofo5Q)
> >
> > So my thought is, setting deltaT = 30s would help... but instead what I
> see is an incredibly unrealistic change in the temperature profile. The
> gradient is immediately wiped out in favor of extreme gradients on either
> side of the channel with weak interior gradient. (Seen at
> http://imgur.com/Od1lM)
> >
> > Could someone give me some guidance as to what might be causing this?
> >
> > Thanks!
> >
> > Chris
> >
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