[MITgcm-support] more KPP blues, maybe related?

[Dimitris Menemenlis]

Martin, the checkerboard KPP pattern is something that I have also
struggled with in the past.  In my case it appeared in tropics, where we
had the highest resolution.  I had traced it back to a positive feedback
in KPP physics (large stratification reduces mixing coefficient in turn
enhancing stratification, and vice-versa) that amplified numerical noise
in the model.  That effect seemed to be stronger than mechanisms in the
model (horizontal diffusion, advection, etc.) that tend to smear out 2DX
noise.

To get rid of this, I have included a bunch of grid-scale filter in
pkg/kpp.  The filters operate on the KPP fields not the dynamic
variables.  In my experience these filters got rid of checkerboard
pattern completely without changing large scale averages of the DYNVARS.

The filters are turned on in the KPP_OPTIONS.h file.
The default is:

C o When set, smooth zonal shear meridionally and
C   meridional shear zonally with 121 filters
#define KPP_SMOOTH_SHSQ
#undef KPP_SMOOTH_DVSQ
 
C o When set, smooth dbloc KPP variable horizontally
#define KPP_SMOOTH_DBLOC
 
C o When set, smooth all KPP density variables horizontally
#undef KPP_SMOOTH_DENS
#ifdef KPP_SMOOTH_DENS
#  define KPP_SMOOTH_DBLOC
#endif
 
C o When set, smooth vertical viscosity horizontally
#undef KPP_SMOOTH_VISC
 
C o When set, smooth vertical diffusivity horizontally
#undef KPP_SMOOTH_DIFF
 
C o Switch on vertical smoothing, the number of sweeps is set in the
namelist
#undef ALLOW_KPP_VERTICALLY_SMOOTH
 

which is good enough to get rid of checkerboard pattern in the tropics. 
I have less experience with checkerboard pattern at high latitudes.  But
you can experiment with

#define KPP_SMOOTH_DVSQ

In general the SHSQ term operates in tropics while the DVSQ term
operates at high latitudes.  If KPP_SMOOTH_DVSQ is not sufficient to get
rid of your problem, you can try adding

#define KPP_SMOOTH_DENS

and possibly also

#define KPP_SMOOTH_DIFF

On your second question I am running global_ocean.90x40x15 for 5000
years with KPP turned on and I have not noticed any problems, but maybe
it's because I have not looked closely.

D.

-- 
Dimitris Menemenlis <menemenlis at jpl.nasa.gov>
Jet Propulsion Lab, California Insitute of Technology
MS 300-323, 4800 Oak Grove Dr, Pasadena CA 91109-8099
tel: 818-354-1656;  fax: 818-393-6720