[MITgcm-support] staggerTimeStep and advection scheme

[Jean-Michel Campin]

Hi Samar,

I understand your point, and I will try to clarify this
staggerTimeStep issue:

There are advection schemes that requires AB-2 for stability
of the advection term itself (2,3,4), and some others that
already incorporate the time evolution (Direct Space and Time,
77,30,33) and don't need the AB-2 timestepping.

How it works ?
You select an advection scheme and the model decides to use AB-2 or not.
If  a) AB-2 is not use for Theta,
and b) Theta affects the density (depending on your EOS) and the pressure,
and c) more than 1 level
you NEED to turn on staggerTimeStep in order to prevent instabilities 
 in the internal-wave solution.
And the same things for Salinity.

This was the subject of (at least) 2 series of emails
that Martin started (>> 1 year ago), and more recently
last September.

Now regarding this specific question:
> In particular, if the primary purpose is
> to deal with the internal wave stability criterion, can this flag be safely
> turned OFF when using adams-bashforth timestepping? I realize this may not
> have a simple or short answer, but for people like me who are clueless
> about numerical methods, could you suggest a "rule of thumb" for when
> this flag should be used and when not.
> 
In this case, stagger-timestep can be turned off safely. However,
you may be able to use a longer time step with this flag ON (especially
when the internal-wave stability criteria is the one that limits
your time step), even when T & S are using AB-2.
This is the case in all atmospheric configs that we are running.

Hope this will help.

Cheers,

Jean-Michel