[Mitgcm-support] detatfreesurf == deltatracer

[mitgcm-support at dev.mitgcm.org]

Its not obvious to me that setting dt_fs to tracer dt is
getting to the full answer. For the 4 degree runs
the equatorial grid spacing is about 400km and so the
dt=1200 secs we were using doesn't even allow time
for a free surface wave to travel 1 dx (which 
would require roughly 2000 secs). The result will be
big ssh changes such that the relation ssh/H is small 
is violated. This is presumably what leads to the blow up.

However, I can't see how the switch to dt = dttracer
is argued numerically. What is needed is a dt such
that ssh/H always stays small everywhere I think?
Should we add a ssh/H test that traps this error
for non NL freesurf cases? Based on that quantity
we could maybe actually choose deltatfreesurf 
automatically and avoid having everyine ask what it
should be and why!

Chris

> Added new parameter: deltaTfreesurf
> 
> Previously, the free-surface equation was intergrated forward
> synchronously with the momentum equations. It is more consistent
> to use the tracer time-step. This increases the number of
> iterations required but strengthens the damping.
>
> We *SHOULD* make the default time-step equal to the tracer time-step.
> However, we don't for backward compatibility. At some point in the
> future we need to change the default behaviour.
>
> It turns out that the reason for the "reduced stability" encountered
> in large-scale runs seems to be related to excess variability in
> the free surface which in turn happens when the waves aren't damped.
> Using a longer time-step fixes this.