[MITgcm-support] online calculation of energy flux

[Ryan Abernathey]

Robert and Sonya,

This is an issue that came up at the recent ECCO meeting. Many people would
probably like to have such kinetic energy diagnostics from MITgcm. Although
the energy equation is easy to write down, diagnosing all of the terms in a
way that is consistent with the model discretization can be very difficult.
Many people have probably tackled it in the past and come up with a "good
enough" solution, no standard solution has emerged.

Peng's suggestion is correct, but the problem is that mean(up) is not
currently an available diagnostic. That would be fairly trivial to add,
especially if you already know how to diagnose it from offline output. The
current pressure diagnostics are filled in model/src/dynamics.F, which
could be modified to include the pressure flux term.

More generally, if you want a complete energy budget, you will also want
the advective transport of kinetic energy, which involves tracking "triple
correlation" terms such as mean(uv^2). This is similar to the problem
encountered in the diagnosis of tracer variance budgets. Several people
(including Jean Michel, myself, and Liam Brannigan) have recently become
interested in adding such diagnostics to the model. Nonlinear advection
schemes can make this difficult for tracers, but perhaps the momentum
advection is actually simpler.

The biggest issue I see in closing the kinetic energy budget is the
dissipation term (epsilon). If you are doing energy budgets for internal
waves, presumably this term is a leading order one for you. A robust method
for diagnosing dissipation (consistent with the numerics and LES options
such as Smagorinsky) would be a great contribution.

I personally wish the MITgcm community could come together to produce a
definitive answer to this problem. But of course, it is not a high priority
for most people, and it is a very difficult numerical problem. Plus it is
possible that the necessarily diagnostics would significantly slow down the
code execution. Whatever solution you come up with, you should definitely
share it with the community, perhaps through the MITgcm_contrib repository,
for the benefit of the next grad student who is faced with this simple
sounding but actually very deep task.

Best,
Ryan


On Fri, Jan 31, 2014 at 11:45 AM, Sonya Legg <sonya.legg at noaa.gov> wrote:

>  Hi Robert,
>
> You should state that it's the product u'p' that you want, and you want to
> do it online so that you can calculate time-averages. Otherwise you might
> get a response that u is already output, as is p, so why do you need to add
> anything?
>
> Sonya
>
>
> On 1/31/2014 11:39 AM, Robert Nazarian wrote:
>
> Hi,
>
> I'm trying to add an online evaluation of u' and p' to the source code to
> calculate the energy flux. Is there a particular diagnostic that would be
> best to do this in? If not, is there a particular subroutine that's ideal
> to write such a diagnostic? Previously, I did these calculations offline
> but am hoping to incorporate it into the code itself.
>
>  Thanks,
> Rob
>
>   Robert Nazarian
>
> Program in Atmospheric and Oceanic Sciences
>
> Princeton University
>
> rn2 at princeton.edu
>
>
> _______________________________________________
> MITgcm-support mailing listMITgcm-support at mitgcm.orghttp://mitgcm.org/mailman/listinfo/mitgcm-support
>
>
>
> _______________________________________________
> MITgcm-support mailing list
> MITgcm-support at mitgcm.org
> http://mitgcm.org/mailman/listinfo/mitgcm-support
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mitgcm.org/pipermail/mitgcm-support/attachments/20140131/4e644aba/attachment-0001.htm>