[MITgcm-support] [EXTERNAL] Banding, Checkerboarding in KPP Viscosities

[Martin Losch]

Senja,

I reset the KPP_OPTIONS.h to the default version in pkg/kpp to see the full drama.
I can see that after about 16h of integration, the model becomes dynamically instable (more motion) and the described patterns appear, but they also disappear again when the forcing becomes weaker (right?)

The phases with the vertical grid scale noise in the viscosities is associated with interersting surface field patterns (e.g. temperature and KPPdbloc, which is gravity*drho/rho). In my opinion this has got to do something with the way KPP handles the surface forcing. On the other hand, your surface forcing is fairly smooth and I don’t see how that should lead to this noise. There are so many thresholds and if statements/min/max function in the kpp-code that I find it hard to trace down the source of these instabilities.

I have tried your configuration wih GGL90 instead of KPP. The surface forcing by wind is quite similar in that mixed layer model code, but the noise never appears. So if you don’t have to use KPP I would recommend to use GGL90 for your simulations.

Martin

> On 7. Apr 2020, at 21:40, Martin Losch <Martin.Losch at awi.de> wrote:
> 
> OK, got it to run, but how long does it take until these patterns appear?
> 
> I am not at all certain that this matters: I assume that salinity is zero all the time, but I would suggest either setting a diffusive advection scheme for salinity (saltAdvScheme = 33) or turn salinity off (saltStepping = .FALSE.). With the default advection scheme (2) and the tiny diffusivities, there may be unexpected results.
> 
> Martin
> 
>> On 7. Apr 2020, at 19:18, Senja Walberg <senja.w at gmail.com> wrote:
>> 
>> Thanks Martin for moving this back to the support board. 
>> 
>> I haven't looked at all the fluxes and will output those next to see. I also haven't played with the time step, but I've changed the vertical grid spacing (dz). I'm using a 15 s timestep, when dz=1m I don't get any banding or artifacts. I've tried dz=0.5m and dz=0.25m and the banding is gridscale for both of those cases. 
>> 
>> My case is small enough to share, I've attached it in a zip file. Just run 'genbins.py' to get all the binary files and do the usual steps compiling to set up my configuration.
>> 
>> Cheers,
>> Senja
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