[MITgcm-support] L4rdt

[Martin Losch]

Hi there,

I need help (not that this is anything new): I cannot reproduce my  
experiments after a recent update and I cannot figure out why. The new  
runs are substatially more energetic (factor 2), especially way below  
the surface and near the topography; eventually they blow up after  
15months of integration with a CFL violation near the bottom/tile-egde- 
corner/open boundaries. As an illustration I have attached the monitor  
variable ke_mean for kemean_run06.png (the old run) and  
kemean_run06_new.png (with new code). I use adv-scheme 33 (no explicit  
diffusion), viscA4grid, no_slip_bottom and sides, kpp, seaice, obcs.

I still have my old exectuables, but I need some new functionality  
(pseudo-timestepping for LSOR), so I need the new code. Plus I have  
made a few changes (new types of OBCS) so that makes tracking down the  
problem (or simply reverting to old code) even more difficult.

My first candiate was this 4times lower L4rdt, but this does not help  
much; it reduces the monitored kemean a little: see attached files  
kemean_run06.png (the old run) and kemean_run06a.png (the new run but  
with old L4rdt).
I noticed (very) small differences in my results between checkpoint61o  
and checkpoint61p (with mom_calc_visc.F still held at 61o) which I  
cannot explain (but they seem to be small, maybe this KPP fix in  
61p?), but all later checkpoints give the same results as 61p. My old  
runs are close to checkpoint61m.

Now my question: In the list of changes in doc/tag-index I cannot find  
anything that I would expect to make my integrations more energetic,  
except for this L4rdt business in mom_calc_visc.F (and I have kept  
this routine at checkpoint61o). Does anyone have an idea what might  
have changed that causes so drastic differences.

Martin


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On Aug 3, 2009, at 9:41 AM, Martin Losch wrote:

> Sorry for the confusion,
>
> 1. I was talking about L4rdt (I was just omitting the 1/deltatmom  
> part, because it's always the same), which applies only when you use  
> viscA4grid and/or viscA4max, as far as I can see.
> 2. Reverting to old code does not fix my problem, so it's something  
> else.
> 3. I do not think that anybody other than us reads the doc/tag-index  
> (also the  comment in there just says "fix L4rdt", but not with what  
> effect)
>
> I'll keep searching ...
>
> Martin
>
> On Aug 3, 2009, at 2:04 AM, David Ferreira wrote:
>
>> Dimitris Menemenlis wrote:
>>> David, does necessity to pump up viscA4grid also carry over to  
>>> viscA4GridMax?
>>>
>>> Specifically, for the CS510 configuration we use
>>> viscC4Leith=1.5,
>>> viscC4Leithd=1.5,
>>> viscA4GridMax=0.5,
>>>
>>> Recently I have noticed a little more grid scale noise than before  
>>> but it is hard
>>> to separate this from other modifications to code and model  
>>> configuration
>>>
>>> Do I need to increase  viscA4GridMax by about 30%, as per Martin's  
>>> email,
>>> to recover previous set up?
>> Dimitri,
>> looks like the changes in L4rdt also apply to viscA4GridMax.
>> But I'm not sure about those 30% of Martin: he was talking about  
>> L4, not L4rdt.
>> In fact, I'm not even sure that L4 is used in mom_cal_visc.F.
>> david
>>
>>
>>
>>
>>>
>>> Thanks, Dimitris
>>>
>>> On Jul 31, 2009, at 1:02 PM, David Ferreira wrote:
>>>
>>>> Martin,
>>>> I put a note in the tag-index about the L4rdt fix !
>>>> Surely, everybody reads it regularly...
>>>> Anyway, the viscosity at the corner points were 4 times those at  
>>>> the
>>>> center points, so
>>>> the "effective" viscosity was larger than intended. And indeed,  
>>>> with the
>>>> new code, it will
>>>> probably be necessary to pump up a bit viscA4grid.
>>>> That said, I don't think that L4 was changed by the code  
>>>> modification,
>>>> only L4rdt.
>>>> Are you sure there is not something else going on ?
>>>> Cheers,
>>>> david
>>>
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>>
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