Why the POTEN_ENRG is so big?

General scientific issues regarding ROMS

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WEIQILI
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Joined: Tue Sep 22, 2015 3:09 pm
Location: East China Normal University

Why the POTEN_ENRG is so big?

#1 Post by WEIQILI » Fri Aug 05, 2016 1:05 pm

Hi guys:
I have some questions.
I don't know why the POTEN_ENRG is so big.Which variable can influence the pontiential energy?
Here is a part of my log file.


STEP Day HH:MM:SS KINETIC_ENRG POTEN_ENRG TOTAL_ENRG NET_VOLUME
C => (i,j,k) Cu Cv Cw Max Speed

0 0 00:00:00 3.953632E-04 1.122384E+06 1.122384E+06 9.544185E+14
(278,015,26) 1.865972E-03 0.000000E+00 0.000000E+00 7.170992E-01


What's problem??

Please help me thank you

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kate
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Location: CFOS/UAF, USA

Re: Why the POTEN_ENRG is so big?

#2 Post by kate » Sat Aug 06, 2016 9:44 pm

Is it a problem? You can look at diag.F to see how it is computed. I just checked three domains of mine which have potential energy around 2.e4. Does your solution look weird in some other way? That's what I would focus on.

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shchepet
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Re: Why the POTEN_ENRG is so big?

#3 Post by shchepet » Mon Aug 08, 2016 1:51 am

Kate, this actually exposes a long outstanding problem: how useful ROMS diagnostic of PE?
And what would be the most meaningful definition of PE in principle? Yes, the computation of
PE diagnostic was there in diag.F from the very beginning inherited from SCRUM and SPEM,
yet nobody was wiling to revise it: model simply prints a number at every time step, and
everybody ignores it because you cannot say anything about how good or bad it is.
This is totally different from KE where everybody can quickly see that if KE does beyond
a reasonable number of something of order of ~3.E-3 or so (depending on configuration)
then something is very wrong.

PE is not APE, and APE cannot be defined easily in a primitive-equation model with the
exception of a layered model. Yet we did not even try to make a step in this direction.

In shallow water (barotropic case) PE is same as APE and it horizontal integral of zeta^2/2
(provided that zeta itself integrates to zero)), which is quite useful: PE is of the same order
of magnitude as KE; it is a quadratic interval; KE+PE is (should be) conserved and can be
used do judge model performance.

In 3D case PE is defined as integral of rho*z over the entire volume. This is a very
large number. The physically interesting part is only a very small portion of it --
perturbation over a large constant value. If rho = rho0 and zeta = 0, then formally
computing it using ROMS procedures already results in a large constant value.

Now if rho = rho0 and zeta /= 0, doing so again recovers barotropic APE (provided
that zeta horizontally integrates to zero). So removing the large constant recoveres
barotropic APE.

Now if rho /= rho0, then I would argue that one should further remove contribution due
to bulk compressibility effect, because leaving it in results in second large constant
number: rho = rho0 + rho1(z) + rho^prime, where integrals of z*rho0 and z*rho1(z) under
condition of zeta = 0 should be excluded because they are not dynamically relevant (i.e.,
they are "unavailable" parts of PE).

...something of this sort.

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