[MITgcm-support] Momentum budget using flux form diagnostics

[Andrea Cimatoribus]

Hi Ana,
I know it's not a very insightful answer, but I would just try to see if 
you can close the budget.

The formula, for the hydrostatic case:
[du/dt] = - gravity * ( ETAN(i) - ETAN(i-1) ) / DXC
         + Um_dPHdx
         + Um_Advec   (+ Um_Cori : but only if using CD-Scheme )
         + Um_Diss    (+ Implicit vertical viscosity tendency)
         + Um_Ext
         + AB_gU

I have been asking about this some time ago, if you search with my name 
you will find more details in the archives.
You can easily check if du/dt matches TOTUTEND. In the vector-invariant 
formulation, you can close the budget to very good approximation 
(relative error << 1e-3), even averaging only few time steps, so you 
will immediately know if there is something missing for the flux form. I 
guess it will work, but certainly I don't have much insight in these 
numerical aspects.

Hope it helps,
Andrea

Andrea Cimatoribus
postdoctoral researcher
EPFL ENAC IIE ECOL
https://people.epfl.ch/andrea.cimatoribus

On 12/01/18 06:35, Ana Paula Berger wrote:
> Dear all,
> 
> 
> I am trying to do the momentum budget for a model that is run in the 
> flux form.
> 
> I have been saving and using the outputs available in the flux form, for 
> example for the zonal momentum:
> 
> ADVx_Um, ADVy_Um, ADVrE_Um, VISCx_Um, VISCy_Um, VISrE_Um, VISrI_Um (all 
> divided by the respective RAW * DRF * hFacW).
> 
> I also have the TOTUTEND / timestep to compare with the sum of those 
> terms, plus the Coriolis term which I calculated.
> 
> My question is how to do the budget in the flux form, not using the 
> vector invariant form outputs (I have been finding help for that but not 
> much related to the flux form).
> 
> I am not sure it can be done the same way. If you run the model in the 
> flux form can you use the vector invariant outputs to check the momentum 
> budget? If anyone ever used the flux form outputs, could you give me any 
> advice on what am I missing or what else should I include in the sum?
> 
> 
> Thanks in advance.
> 
> 
> Ana Berger.
> 
> 
> -- 
> Ana Paula Berger - Oceanographer, PhD Candidate.
> +61 0474 833 811
> 
> 
> 
> 
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