[MITgcm-support] staggerTimeStep and advection scheme

[samar khatiwala]

Hi Jean-Michel et al

I am completely baffled by the staggerTimeStep flag.

Several months ago I had asked a question about advection schemes (see
attached email below). Jean-Michel said something about staggerTimeStep
not directly influencing ptracers but influences via changes in advection
velocity. So now I don't know what to make of these lines in
ptracers_integrate:

        IF (staggerTimeStep) iterNb = myIter - 1
        CALL ADAMS_BASHFORTH2( ... iterNb ...)

Has this aspect of the algorithm changed?

I thought I had understood what JMC was saying (see previous thread
below), but looking at the code I am more mystified than ever. In
particular:

1) When and for what quantities is staggerTimeStep used (or should be
   used)?
2) Is it used only for active tracers (T/S) and velocity?
3) Is it only used for advection schemes 77,33,30?
4) Does it get turned off when adams bashforth is used, since my
   understanding is that it is there for internal wave stability.
   (I don't see this being done in the code, though.)
   In which case, why is it used for ptracers?

Thanks very much! Samar

---------- Forwarded message ----------
Date: Tue, 21 Sep 2004 18:35:02 -0400
From: Jean-Michel Campin <jmc at ocean.mit.edu>

Hi Samar,

> 1) Does staggerTimeStep affect the ptracers integration in any way (apart
> of course via changing the dynamics)? I don't see it come in anywhere
> in the code except for dynamical quantities but want to confirm.

staggerTimeStep does not affects the ptracers solution.
However, you should not expect identical ptracers results (with
and without staggerTimeStep) after few iterations, because
of a shift of 1 time step in the flow field that is used to advect
the tracers. So, it might not be very easy to check that it's true.


> 2) Does what you say about using staggerTimeStep with advScheme 77 and 33
> also apply to advScheme 30?

Yes. those 3 advection schemes don't use the adams-bashforth
(you can look in STDOUT file: tempAdamsBashforth & saltAdamsBashforth)
and need a stagger-TimeStep for internal-wave stability.
I don't know what happens when only 1 active tracer is using
the AB-II but not the other (depending on the EOS and the kind
of stratification you have ... ) and it's one of the reasons why
staggerTimeStep is not turned on automatically (+ historical reason).

Hope this helps.

See you,
Jean-Michel