[MITgcm-support] Question about closing heat budget

[Jacob Wenegrat]

Hi Martha,

Thanks for your advice. I was running with a linear free surface, so that was not the issue.

I think what I've realized is that by using, for example, d/dx(ADVx_TH) I was actually calculating d/dx(u*T) which of course = ud/dxT + Td/dxU.

Looking at gad_advection.F (lines 492-496) it appears the tendency term should be calculated as:

(af(i+1, j)-af(i,j)-tracer(i,j,k,bi,bj)*(uTrans(i+1,j)-uTrans(i,j)), where af is the advective flux that is later output to ADVx_TH.

I can roughly close the budget now offline, but I think probably what makes the most sense for my purpose is to add a complete tracer advective flux divergence diagnostic as a user-defined field.

Thanks,
Jacob

>The TOTTEND term only works with a linear free surface.  Otherwise, you
>should output snapshots of ETAN and THETA and use these to calculate the
>tendency term yourself.
>Martha


>On Wed, May 4, 2016 at 10:45 PM, Jacob Wenegrat <jwenegrat at stanford.edu 
<http://mitgcm.org/mailman/listinfo/mitgcm-support>>
>wrote:

>/Hello All, />//>/First off let me thank everyone who has contributed to the MITgcm and the />/mailing list. It's a great tool, and I've gotten quite a bit of help />/already digging through the mailing list archive. />//>/I am currently running a simple reentrant channel model of a front, and />/finding that while the momentum budget closes nicely following previous />/advice found in the archive, closing the heat budget is proving more />/challenging. />//>/To date I've followed the instructions posted here for closing the heat />/budget: />/http://mitgcm.org/download/daily_snapshot/MITgcm/doc/Heat_Salt_Budget_MITgcm.pdf 
/>/. />//>/Specifically, considering only subsurface grid cells (and with no />/shortwave fluxes): />//>/TOTTTEND/86400 = ADV_Tend + KPP_Tend + DIFF_Tend />//>/Where ADV_tend = d/dx ADVx_TH + d/dy ADVy_TH + d/dz ADVr_TH (accounting />/for cell volume in calculating this). />/KPP_Tend = d/dz KPPg_TH (accounting for cell vol). />/DIFF_Tend = d/dx DFxE_TH + d/dy DFyE_TH + d/dz DFrE_TH+ d/dz DFrI_TH />/(accounting for cell volume) />//>/I find that this balance works well at the beginning of my simulation, />/works quite poorly in the middle (when the flow goes baroclinically />/unstable), and then works reasonably well again towards the end as the />/front spins down, although it is not completely closed. />//>/Specific questions are: />/1) I am using tempAdvScheme: 33, which I gather from the documentation has />/associated diffusion which can be difficult to diagnose. Does the />/associated diffusive flux from this end up within any of the above />/diagnostics? Might this be the cause of the large residuals after the />/formation of sharp frontal regions? />//>/2) The tendency term appears to be a small residual between the />/across-channel (y) and vertical flux divergences. As the features sharpen />/to near grid-scale, might the finite differencing (and later interpolation />/back to the tracer grid points) be causing problems? />//>/More generally any suggestions of avenues to pursue would be greatly />/appreciated. I'm not sure how the mailing list deals with attachments, 
so I />/haven't included my data file, but can easily do so if useful. />//>/Thanks, />/Jacob />//>/-- />/Jacob Wenegrat />/Postdoctoral Fellow />/Dept. of Earth System Science />/Stanford University />//>/Email: jwenegrat at stanford.edu 
<http://mitgcm.org/mailman/listinfo/mitgcm-support> />/Web: http://web.stanford.edu/~jacob13 <http://web.stanford.edu/%7Ejacob13> />//>//>//>/_______________________________________________ />/MITgcm-support mailing list />/MITgcm-support at mitgcm.org 
<http://mitgcm.org/mailman/listinfo/mitgcm-support> />/http://mitgcm.org/mailman/listinfo/mitgcm-support />//


-- 
Martha W. Buckley
marthabuckley at gmail.com 
<http://mitgcm.org/mailman/listinfo/mitgcm-support>
http://sites.google.com/site/marthabuckley/home
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-- 
Jacob Wenegrat
Postdoctoral Fellow
Dept. of Earth System Science
Stanford University

Email: jwenegrat at stanford.edu
Web: http://web.stanford.edu/~jacob13

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