[MITgcm-support] Re : Water at the bottom of NA below freezingpoint

[Gus Correa]

Hello MITgcm experts

We've been working on a coupled ocean-atmosphere setup similar to Dror 
Mirzayof's,
although on a lat-lon grid, and using a different mixed layer atmosphere
(Seager et al., Journal of Climate, 1995, p. 1951-1964).

We have the same problem mentioned by Dror Mirzayof and by Jeff Scott,
where a few grid cells next to the ocean bottom get cold in the 
beginning of the run. 
In our case the problem starts on the Southern Ocean,
but eventually the whole ocean bottom gets filled with cold water ( ~ 
-1.8 degrees C).
The ocean bottom heat loss doesn't stop, and on the long run (~1000 years)
the ocean loses about half of its initial heat content.
This seems to prevent a correct heat balance at the surface as well.

Our model is on lat-lon coordinates with a resolution
of 4 degrees by 4 degrees by 24 levels (similar to Jeff Scott's).

Our "data" namelist sets the following parameters
(a bit fortuitously, they were copied from the verification case 
global_ocean.90x40x15):

useOldFreezing=.TRUE.
useRealFreshWaterFlux=.TRUE.
useCDscheme=.TRUE.
useNHMTerms=.TRUE.
hFacMin=.05
hFacMindr=50.0


Most namelist options are not set, so I believe the time step is *not* 
staggered (i.e. default),
and the advection scheme is the default (centered 2nd order, right?).

Questions:

1) Do you have any advice on how to prevent the ocean bottom to lose heat?
2) What is the right advection scheme to use to prevent bottom heat 
loss? (Which namelist options to set?)
3) Do we need to use staggered time step to prevent the bottom cooling? 
(Which namelist option to use?)
4) Should we continue to use shaved bottom cells or not? (And how do we 
cancel shaved cells in the namelist?)
5) Does the "CD" scheme have any impact on this "bottom cooling" 
problem?  Should we continue to use it?
6) Should we change "useOldFreezing" to ".FALSE.",
as forcing the bottom temperature value seems to be rather unphysical?

Thank you for any answers, suggestions, or additional comments.

Gus Correa

-- 
---------------------------------------------------------------------
Gustavo J. Ponce Correa, PhD - Email: gus at ldeo.columbia.edu
Lamont-Doherty Earth Observatory - Columbia University
P.O. Box 1000 [61 Route 9W] - Palisades, NY, 10964-8000 - USA
Oceanography Bldg., Rm. 103-D, ph. (845) 365-8911, fax (845) 365-8736
---------------------------------------------------------------------


Dror Mirzayof wrote:

> Hi
>
> How bad would it be if we force the ocean temperature to be above the 
> freezing point (using useOldFreezing) ?
> According to Martin this is like adding an additional heat source at 
> the bottom. Is this approach appropriate ??
>
> Dror
>
>
> Martin Losch wrote:
>
>> Just a quick comment for Jeff:
>> On 8 Mar 2008, at 19:48, Jeff Scott wrote:
>>
>>> I've also tried different advection schemes (besides 2). 33 and 
>>> especially 77 are not usable, as they radically affect the model's 
>>> temperature structure, being rather diffuse. I've also tried the 
>>> Prather/SOM
>>
>>
>>
>> When you use one of the multidimensional advection schemes (e.g., 33 
>> or 77), make sure that you have "staggerTimeStep=.true.", otherwise 
>> these scheme give very unexpected results during spinup. If you use 
>> staggerTimeStep=.true., then advscheme 2 and 77 (which are both 2nd 
>> order), give similar results in coarse resolution with much smaller 
>> overshoots (according to my experience). As far as I remember, Dror 
>> is using that kind of parameter combination, so his results are 
>> either due to the very small but finite overshoots, that are still 
>> present in 33 or 77, or something completely different.
>>
>> another comment on numerical diffusivity: Thomas Zweigle has carried 
>> out a nice comparison of numerical diffusivities of the various 
>> schemes in a very high resolution (DNS) simulation. Thomas, maybe you 
>> can share a figure (a small jpeg or png please)?
>>
>> Martin
>>
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>
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