[MITgcm-support] KPP blues

[Martin Losch]

Hi there,

remember my ptracer problem? Well, I could trace it down to .... KPP 
(what else). Apparently, in my setup KPP produces boundary layer 
depths, vertical viscosities and diffusivities with a strong 
checkerboard structure (but only where KPP is active). T and S get 
mixed and vertically (and by advection horizontally) homogenized over 
time so that there is little visible effect, because there is only 
mixing of similar values. But the biological tracers react on short 
timescales and thereby retain the checkerboard structure, but you are 
probably not interested in my biology model.

However, the KPP checkerboard structure is a problem that I would like 
to solve. It only appears for high vertical resolution (6 to 40 m in 
the upper 180 m). When I decrease the vertical resolution to 20-30m in 
the upper 180 m this problem goes away! (Now this is  completely 
independent of any biology!). I have to admit, that my vertical grid 
spacing in the case of high resolution increases dramatically (but 
smoothly) below 180 m, but can that be the problem? Here is my delR:
delR = 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 7, 7, 9, 11, 16, 25, 41, 
67, 106, 153, 200, 239, 265, 281, 290, 295, 297, 299, 299, 300, 300, 
300, 300, 300, 300, 300, 300,

Apart from changing the vertical resolution, what can I do? Is it 
possible to change KPP-parameters to achieve a more stable behavior of 
KPP? Are there any limits with respect to KPP, comparable to a CFL 
criterium (which, by the way, is alway OK in my setup) that I did not 
obey? Any ideas?

Martin

PS. I can remember that when I turned on KPP in global_ocean.90x40x15 
(a few months ago), it made the model blow up very quickly, with a nice 
checkerboard pattern of everything in the Southern Ocean. Maybe this is 
related?


Martin Losch // mailto:mlosch at awi-bremerhaven.de
Alfred-Wegener-Institut für Polar- und Meeresforschung
Postfach 120161, 27515 Bremerhaven, Germany
Tel./Fax: ++49(471)4831-1872
http://www.awi-bremerhaven.de/People/show?mlosch