[MITgcm-support] rhoConst and rhoNil

[Martin Losch]

Hi Andy,

what is the default value for rhoNil? 999.8kg/m^3, I think. In the 
Boussinesq approximation you replace \rho*\vec{u} in the momentum 
equations by \rho_{0}\vec{u}, where \rho_{0}=rhoConst should be a 
reference density, that is something like the mean density or some 
other representative value. If you are willing to accept that 
999.8kg/m^3 is this representative density of your fluid then, there is 
no reason setting rhoConst in data. I usually set it to 1035kg/m^3, but 
that's not really the mean density (of the ocean) either, so I would 
say: no, there is little reason not to leave rhoConst unset. It 
shouldn't matter whether you scale your equation with 999.8/1000/1035. 
I am sure that Jean-Michel can think of a reason, though?

Martin

On Jun 14, 2005, at 7:12 PM, Andrew Eichmann wrote:

> * Martin Losch <mlosch at awi-bremerhaven.de> wrote on Tue, Jun 14, 2005 
> at 05:31:17PM +0200
>
> Thanks, Martin.
>
> It appears that if rhoConst is left unset it is assigned the value of
> rhoNil (in ini_parms).  Is there any reason not to leave rhoConst
> unset?  To reduce previous boolean expression, is there any general
> reason to set rhoConst?
>
> -A
>
>
>> Andy,
>> as far as I remember, rhoNil concerns only the equation of state (and
>> therein only the linear EOS), whereas rhoConst is the constant
>> reference density that the momentum equations are divided by. changing
>> rhoConst should not change the result (except for round off errors),
>> whereas changing rhoNil will.
>>
>> Martin
>> On Jun 14, 2005, at 4:58 PM, Andrew Eichmann wrote:
>>
>>>
>>> Anybody care to explain the relationship between rhoConst and rhoNil?
>>> I understand rhoNil to be the reference density, presumably to
>>> calculate/output the density anomaly....  This is for purposes of
>>> documenting the rotating tank example, and it's not discussed in the
>>> manual nor in any comments I could find.
>>>
>>> Thanks,
>>> -- 
>>> Andrew Eichmann
>>> MIT-PAOC
>>> Rm 54-1623
>>> 77 Massachusetts Ave.
>>> Cambridge, MA 02139
>>> _______________________________________________
>>> MITgcm-support mailing list
>>> MITgcm-support at mitgcm.org
>>> http://mitgcm.org/mailman/listinfo/mitgcm-support
>>
>> _______________________________________________
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>
> -- 
> Andrew Eichmann
> MIT-PAOC
> Rm 54-1623
> 77 Massachusetts Ave.
> Cambridge, MA 02139
> _______________________________________________
> MITgcm-support mailing list
> MITgcm-support at mitgcm.org
> http://mitgcm.org/mailman/listinfo/mitgcm-support