[MITgcm-support] global simulations and resolution

[Samar Khatiwala]

Hi Martin

What is your momentum time step? I am guessing its pretty small (like 
3600 s).
If so, the rather large tracer/momentum ratio in your integration (for 
async
timestepping) could really distort the equilibrium solution. I have run 
the
2.8 deg 'OCMIP' configuration to equilibrium and found that the final 
solution
(after synchronous integration) differed markedly from the asynchronous
solution. I am not claiming this is the explanation for what you are 
seeing.

It would actually be interesting to compare the solutions I have with 
yours.
I can send you the data files if you want (or you can send me the 
m-file you
used to make your pretty pictures).

Samar

On Sep 7, 2005, at 3:33 AM, Martin Losch wrote:

> Hi there,
>
> I am running a 4x4 degree global simulation (almost identical to the 
> verification experiment global_ocean.90x40x15) in parallel to a 2x2 
> degree global simulation (180x80x23). These simulations differ mainly 
> in their respective horizontal and vertical resolution. Of course, I 
> had (initally) friction parameters adjusted, so that viscAh=5e5 in the 
> 4x4-run became 5e4 in the 2x2 run.
> Both experiments use trenberth winds, ncep heat flux (short wave and 
> the rest), SST and SSS restoring to Levitus, GM, KPP, no seaice, no 
> Arctic ... All runs are integrated for 3000 years with asynchronous 
> time stepping (deltaTtracer=172800 for the 4x4 and 43200 for the 2x2 
> runs).
> The solutions are broadly similar in terms of circulation (the 4x4 run 
> is more sluggish, but I can tune the friction parameters so that they 
> give similar ACC transports, for example). What is really different 
> are the water masses, and I cannot see a way to make them agree more 
> closely:
> broadly speaking, in the 2x2-run: when compared to Levitus data, the 
> Southern Ocean (south off the ACC) is too cold and too salty. The deep 
> ocean (below 3000m) is far too cold. The 4x4-run is too cold and too 
> fresh below 3000m, but in the Southern Ocean theta is nearly OK, but 
> salinity is too low. (for pictures see 
> http://mitgcm.org/~mlosch/run2x2_ts.pdf and 
> http://mitgcm.org/~mlosch/run4x4_ts.pdf)
>
> I would like to know, how this difference can be explained, that is, 
> how I can tune either model (preferably the 2x2 degree model, because 
> is too cold) to be similar to the other. Is there any experience with 
> this out there? Is it possible that the vertical resolution in 
> combination with surface restoring is responsible for this difference 
> (the 4x4-top layer is 50m thick, whereas the  2x2-top layer only 
> 10m.)?
> Any suggestion is greatly appreciated.
>
> Martin
>
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