[MITgcm-support] Energy diagnostics?

[Klymak Jody]

Hi Jeff,

>
> Several years ago Alistair and I worked out how to calculate this,  
> i.e. ,  given complications due to Adams-Bashforth time stepping,  
> and we able to diagnose under what conditions/run parameters the  
> MITGCM conserves KE (albeit fairly limited testing of MITGCM  
> options). PE is a bit more messy and I gave up at some point,  
> perhaps others have had more patience.
>


I expect that it would be helpful for me to know how the time-stepping  
affects the energy balance. I was hoping to avoid such subtleties: I'm  
willing to accept some step-by-step errors so long as they don't bias  
the energy balance too much.  I also don't expect the energy balance  
to be perfect, but it would be nice if the errors were small compared  
to my explicit dissipation (which is driven by breaking waves over  
topography, and thus quite large).

PE is a pain principally because it is so large, yet the reference PE  
gets advected around so you can't just do anomalies. I have an  
acceptable balance in Matlab (by outputting every time step).  I'll  
write it up and post when I'm done, and those who are better at  
numerical methods can help me get the dt's and dz's right.

Thanks,  Jody


On Feb 17, 2009, at  8:51 AM, Jeffery Scott wrote:

>
> Hi Jody,
> However, we did this all offline, so I'm not sure how much help it  
> would be, but happy to share, have some notes written up...
>
> Jeff Scott
>
>
> On Feb 16, 2009, at 3:51 AM, Martin Losch wrote:
>
>> Hi Jody,
>>
>> I am not aware of energy flux diagnostics [(E+P)u] in the code. I  
>> agree with you that for closing the budget they need to be computed  
>> correctly (at each time step and then averaged), horizontal  
>> averaging will also be an issue.
>>
>> Martin
>>
>> On Feb 15, 2009, at 10:25 PM, Klymak Jody wrote:
>>
>>>
>>> Hi all,
>>>
>>> Before I spend a couple of days doing this, has anyone gone  
>>> through the effort of calculating energy diagnostics?
>>>
>>> I'm running barotropic flow over an obstacle (all in 2-D).  If I  
>>> take every timestep I can get a decent energy budget:
>>>
>>> (d/dt)\int E dV = \oint (E+P) u \cdot dA + \int \epsilon dV
>>>
>>> where \epsilon is specified and numerical dissipation.  However (E 
>>> +P)u changes so much over my output timesteps the residual when  
>>> compared w/ dE/dt is unrealistically large.      Averaging the  
>>> linear and non-linear flux over the dump interval should give a  
>>> much better estimate.
>>>
>>> Has anyone done this?  If not, I'll share my code.
>>>
>>> Thanks,  Jody
>>>
>>> --
>>> Jody Klymak
>>> http://web.uvic.ca/~jklymak/
>>>
>>>
>>>
>>>
>>>
>>>
>>>
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>>
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--
Jody Klymak
http://web.uvic.ca/~jklymak/