[MITgcm-support] Eta calculation on different grid sizes

[Hadar Berman]

Hi Matt,

Thanks for your quick reply and sorry for the delay. 
As far as I can tell, I am implementing the OBCS in perfect timing. To check this I output the results in every time step and see the differences in the boundary are zero. I think the differences arise from the normalisation of the cg2d_b (or RHS of the continuity equation) by cg2dNorm. Before the normalisation, cg2d_b is exactly the same in both models (the nested one and the full domain), and after it, the iterative procedure produces (for some reason I do not understand) results that are slightly different. When comparing these two models, should I stop the normalisation? And if so, what is the best way to do so?
Thanks again for your help,
Hadar.

On May 18, 2018, at 9:45, mitgcm-support-request at mitgcm.org wrote:

> 
> 
> Hi Hadar
> 
> Its a Boussinesq model so you can have action at a distance. 
> 
> However most likely what you are seeing is that your new OBCS are not perfectly balanced and you have radiated barotropic waves throughout the domain. Barotropic waves move very fast so you will see these small changes in one time step. They may settle down, but if you don?t handle the nesting well undesired effects will continue. And unfortunately dealing with OBCS isn?t trivial 
> 
> Matt
> 
> 
>> On May 17, 2018, at 6:04 AM, Hadar Berman <hadarberman at gmail.com> wrote:
>> 
>> Hi everyone,
>> 
>> I am trying to reduce the resolution of my model. As a first step I only cut the domain to a part of it, so that I now have two running models - (1) the full domain model and (2) only the northern part of the domain. Both models use the same grid size and the bathymetry, initial conditions, surface forcing are exactly the same. The only difference is that the full domain model now drives the boundary of the northern domain one.
>> Running these two models for only a few time steps, I expected to have small differences close to the boundary, but not in the interior. Instead, I can see small differences between the new model and the old one in the temperature, velocities etc after only one time step in the entire water column. I started to look into the code to check what causes this, and found that EtaN in solve_for_pressure.F and cg2d.F is computed using the global maximum of the RHS (rhsMax) (I probably should have mentioned that it is a linear free surface hydrostatic model in both cases).
>> 
>> I was wandering, first - can this be the case, that eta changes as a function of the domain I choose, and if so, why? second - is there a flag to use so that eta calculations would not be affected by the size of the domain I choose?
>> 
>> Thanks,
>> Hadar.
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> 

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