[MITgcm-support] increased total energy (kinetic and potential energy) in mitgcm model

[Jiao, Yang]

I should add that the order of this growth is very small O(10^4) compared to the magnitude of total energy O(10^8), and was mainly contributed by growth in potential energy.


Yang

From: Jiao, Yang [yj10 at my.fsu.edu]
Sent: Saturday, August 17, 2013 7:42 PM
To: MITgcm-support at mitgcm.org
Subject: [MITgcm-support] increased total energy (kinetic and potential energy) in mitgcm model


Hi everyone,


I'm having problem in the total energy (kinetic and potential energy) evolution of my non-hydrostatic models. I was trying to simulate a water flow in a rectangular channel (cross section 24km*1km) with an initial condition, a current satisfying some instability criterion (all variables are y-independent, which is the direction along the channel).


The total energy is supposed to decrease all the time, but when I used a 4800*3*500 (nx*ny*nz) grid points model to simulate the flow and choose different horizontal eddy viscosity parameters (Ah=0.05~0.5m^2/s), the total energy will increase for small Ah values. And I used 1600*3*500  model to simulate a similar initial condition but without satisfying the instability criterion, the total energy had a perfect linear growth during the whole model run, and it didn't change much no matter what Ah I choose in the 0.05~0.5 range.


I used area integration of the cross section of the channel to get the energy using matlab. Kinetic energy is u^2/2 and potential energy is -z*b (negative depth*buoyancy) for each cell. I used 3 second time interval but using smaller values won't help. Adding density into the energy also won't change anything.


Does anyone also have this kind of issue before? Is there any clue on what caused this?


Thanks,

Yang
Graduate student
EOAS, Physical Oceanography
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