[MITgcm-support] the weird Temperature diffusion

[Jody Klymak]

Hi,

> The question hear is why the middle water temperature change rate are so different with the top layer and bottom layer? Because the diffusion parameters are the same through the depth, the result makes no sense to me. Any suggestion is highly appreciated.


If dT/dz>0, Kpp essentially sets the diffusivity to 10 m^2/s, so the 90-m diffusion time scale is something quick like 800 s.  

For the other runs, if you have nonHydrostatic = .FALSE. your only temperature change will be due to the diffusion KzT=1e-5, so your time scale will be more like 9x10^8 s, i.e. it'll take a long time to mix!  (or, 80 tides = 6m, which agrees w/ your plots).  

What I don't understand is your run 9-7-4 with an unstable stratification, but no KPP. If you actually did have nonHydrostatic=.TRUE. as you indicated, this run should have had no trouble convecting, and you should see huge changes in the temperature.  Are you sure you had nonHydrostatic =.TRUE. for this run?  If you did, and this is how it behaved, you might consider adding a bit of noise to the initial T to give the convective instability something to grow off of.  Though it'd sure be a testament to the numerics if 80 tidal cycles of running on an unstable stratification didn't set off some sort of instability!

Cheers,   Jody

--
Jody Klymak    
http://web.uvic.ca/~jklymak/




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