[MITgcm-devel] Bug in KPP

[Dimitris Menemenlis]

David, thank you for taking another look at vertical KPP profiles and forfinding and fixing the riwmix bug.

The ghat term is supposed to be active in convectively unstable mixed layers but, if I remember correctly, turned off for purely wind-driven mixing.

Dimitris Menemenlis
818-625-6498
-----Original Message-----
From: David Ferreira <dfer at mit.edu>
Date: Wednesday, May 27, 2009 7:06 am
Subject: Re: [MITgcm-devel] Bug in KPP
To: "MITgcm-devel at mitgcm.org" <MITgcm-devel at mitgcm.org>Reply-To: "MITgcm-devel at mitgcm.org" <MITgcm-devel at mitgcm.org>

Hi Dimitris,
I looked into the code and did some tests with a 1d set-up. The shape of 
the visc/diff
are consistent with the kpphbl, the shear/convective instabilities also 
look fine to me
in terms of vertical location. I still have a small doubt about the 
position of ghat, but I
need to reread the Large et al. to be sure.
Anyway, I'm going to check in the fix for the vertical indices in ri_iwmix.
cheers,
david

Dimitris Menemenlis wrote:
> David, I agree that change below will work and that, assuming KPP code 
> was bug-free before kpp_routines.F version 1.21, this is the correct 
> fix.  If it's not too much trouble, as a further sanity check for 
> vertical mismatch prior to 1.21, could you also check to see that 
> convective or shear instabilities trigger increase in K at the right 
> level and that mixed layer depth increase in K is consistent with hbl 
> depth.
>
> For mixed layer depth, I would run your configuration forward for a 
> few time steps, set KPPwriteState=.true. in data.kpp, dumpfreq=deltat 
> in data, and I would compare the vertical profiles of KPPdiffkz and 
> KPPviscAz to KPPhbl.
>
> For shear or convective instabilities in the interior, as you are 
> unlikely to have any of those in your model configuration after 
> spin-up, I would generate instabilities artificially by adding a bit 
> of salt or cooling a few random points, and then checking that 
> KPPdiffkz and KPPviscAz kick in at the correct level to remove these 
> instabilities.
>
> The above tests are probably (hopefully) superfluous but if they are 
> not too much trouble to run, it would not hurt to have a fresh pair of 
> eyes looking at this issue one more time.
>
> Did Hong get back to you regarding your horizontal viscosity question?
>
> Cheers, Dimitris
>
> On May 26, 2009, at 7:08 AM, David Ferreira wrote:
>
>> I think the problem is in ri_iwmix. vddiff is defined at the 
>> interface (with
>> k=0 corresponding to the air-sea interface), so the lines in ri_iwmix
>> should be changed from:
>> diffus(i,ki,3) = diffusKzT(i,ki)+fcon*diftcon+fRi*dift0
>> to
>> diffus(i,ki,3) = diffusKzT(i,ki+1)+fcon*diftcon+fRi*dift0
>
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