[MITgcm-support] global simulations and resolution

[Martin Losch]

Hi Baylor,

thanks for your suggestions. I use KPP in all cases (and GM). But I now 
think (after some more testing and reading a little) that my problem is 
really connected to surface boundary conditions, in particular the 
combination of restoring BC and vertical resolution. I have repeated 
the experiments where I now scale the restoring time scales with the 
ratio of the surface layer thickness and the results are far more 
agreeable than before. I have also found indications that this is the 
way to do it in Large etal 1997 JPO, "Sensitivity to surface BC and 
vertical mixing ..." (something like that).

Thanks again,

Martin

On Sep 7, 2005, at 3:34 PM, Baylor Fox-Kemper wrote:

> Hi Martin,
>   What are you doing about surface mixing?  I think folks at GFDL had 
> some luck with Richardson number Criteria in tuning similar problems.
>   Where is the deep water (AABW) forming?  How are the surface 
> conditions there?  You might contact Bob Hallberg at GFDL for tips, he 
> spent a lot of time getting the Hallberg model to work around the 
> ACC...
>   Cheers,
>    -Baylor
>
>
> 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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