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

[Dimitris Menemenlis]

So I wonder if you need KPP turned on with such high resolution?

> On Apr 2, 2020, at 11:06 AM, Senja Walberg <senja.w at gmail.com> wrote:
> 
> Greetings,
> 
> I have been conducting simulations of a lake and using KPP for vertical mixing. I encountered an issue of noise in derived |grad_h T| fields and traced it to checkerboarding in KPPdiffT and KPPviscA. Based on a 2004 discussion between Dimitris and Martin I found that the horizontal checkerboarding could be fixed by toggling on the KPP_SMOOTH_DENS flag in KPP_OPTIONS.h. However, I am still encountering vertical banding. Attached is a snapshot of this banding from a simplified lake simulation.
> <KPPviscA_y_Support.jpg>
> The lake is stratified between ~25 degrees at the top and ~10 degrees at the bottom with the pycnocline at 15 m. I am using dz=0.5 m and dx=100m.
> 
> I have tried turning on KPP_SMOOTH_VISC, KPP_SMOOTH_DIFF, and ALLOW_KPP_VERTICALLY_SMOOTH, as well as increasing both horizontal and vertical viscosities and diffusivities, and have had no luck getting rid of this banding.
> 
> I'm currently experimenting with changing the vertical resolution to see what effect that has, but would like to not be constrained to a coarse vertical grid if there is some fix to this.
> 
> Does anybody know what's causing this issue or how it can be resolved? 
> 
> Regards,
> Senja
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