[MITgcm-support] global simulations and resolution

[Martin Losch]

Samar,

I was afraid of that, but are the water masses really affected that 
much by the asynchronous time step, that would be terrible.

the matlab scripts are all available in 
verification/global_ocean.90x40x15/diags_matlab

Martin
On Sep 7, 2005, at 3:54 PM, Samar Khatiwala wrote:

> 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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