[MITgcm-support] The criteria of cg3dMaxIters

[Jody Klymak]

On 15 Aug 2017, at 9:29, Saeid Esmaeilpour wrote:

> Jody,
> my model is about The simulation of currents Circulation from surface 
> to
> depth in indian Ocean,  and I selected  nonHydrostatic=.TRUE., cause I
> think the model resolve the Nonlinear terms with nonHydrastatic=True 
> (am I
> wrong ?).  If I select nonHydrastatic=True, in this case how much 
> should I
> set for cg3MaxIters? and If select nonHydrastatic=False, how much for 
> that?
> basically for the simulation of currents Circulation and at 2-degree I 
> have
> to take non Hydraustatic=false?

`nonHydrostatic=.True.` solves the non-hydrostatic equation for dw/dt, 
which essentially means the pressure needs to be determined globally 
from a Poisson equation, and hence the 3-D solver. `.False.` means that 
pressure is solved using the hydrostatic approximation (if you don’t 
know what this is, Google plus a basic GFD course would help).

This really has nothing to do with non-linearity until you start 
resolving scales where the hydrostatic approximation breaks down 
(basically dw/dt is no longer << rho’g / rho_0).

You really only need non-hydrostatic approximation for fine-scale runs 
looking at internal waves, dense overflows, convection, or other 
turbulent phenomena.  None of which will show up at 2-degree resolution.

Cheers,   Jody




>
> Thanks again,
>     Saeid
>
> On Tue, Aug 15, 2017 at 11:59 AM, Jody Klymak <jklymak at uvic.ca> wrote:
>
>> Hi Saied,
>>
>> cg3dMaxIters is only used if nonHydrostatic=.True.. You aren’t 
>> resolving
>> any non-hydrostatic physics at 2-degree resolution, so I would 
>> suggest
>> setting nonHydrostatic=.False. Your model runs will be *much* faster.
>>
>> Cheers, Jody
>>
>> On 15 Aug 2017, at 8:43, Saeid Esmaeilpour wrote:
>>
>> Hi Dear users,
>> could someone please explain to me what are the following PARM02?
>> when I increase cg3dMaxIters from 50 to 400 (e.g,. cg3dMaxIters=400) 
>> the
>> run speed becomes very slowly. how can I select this value? what's 
>> the
>> criteria for selecting this number? My domain is 510*210 with 20layer 
>> and
>> also my resolution is 2 degree (3704m).
>>
>> # Elliptic solver parameters
>> &PARM02
>> cg2dMaxIters=1000,
>> cg2dTargetResidual=1.E-13,
>> cg3dMaxIters=400,
>> cg3dTargetResidual=1.E-13,
>>
>> Thanks for any help,
>> Saeid
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