[MITgcm-support] surface treatment and resolution dependent instability?

[Jody Klymak]

Hi all,

Just trying to educate myself…

I am running a doubly-periodic domain with some the only external forcing a body force applied to keep the flow moving. The domain is 4000 m deep, with dz=10 m.  Hydrostatic simulations...

I had as my surface conditions:

rigidLid=.TRUE.,
implicitFreeSurface=.FALSE.,
exactConserv=.FALSE.

and this ran fine at 100-m horizontal resolution, and 1000-m.  

Without changing anything about the simulations, except moving to 2000-m, the solution would blow up after a few time steps.  In particular I noticed that `dynstat_eta_mean` would start increasing (or decreasing) before any of the CFL criteria seemed to be going bad.   With that clue in mind I found that:

rigidLid=.FALSE.,
implicitFreeSurface=.TRUE.,
exactConserv=.FALSE.

seems stable.

1) Is this instability something thats known?  It seems non-intuitive to me that as dx,dy gets larger the instability grows.
2) What are the consequences of rigid lid versus implicit free surface? Is there a good primer on choosing these?  I’m ashamed to say that I have simply plugged along w/ the same values for years w/o really questioning them.

Thanks a lot,   Jody

--
Jody Klymak    
http://web.uvic.ca/~jklymak/