[MITgcm-devel] lab_sea sensitivity between platforms/compilers

[Jean-Michel Campin]

Hi Martin,

I don't have a good method to check this.
I was thinking of dumping intermediate state to disk and
restarting from there. painful.

Cheers,
Jean-Michel

On Sun, Jul 19, 2009 at 10:22:00AM +0200, Martin Losch wrote:
> Hi Jean-Michel,
>
> I might be chasing a Friday afternoon blackout, but I ran lab_sea  
> without the dynamics solver, and got very similar numbers on for my  
> different platforms.
> Turning off the thermodynamics did not have the same effect. But it may 
> very well be a coincidence for my particular combination of  
> platforms/compilers.
>
> How do you normally test these things?
>
> Martin
>
> On Jul 17, 2009, at 11:48 PM, Jean-Michel Campin wrote:
>
>> Hi Martin,
>>
>> Are you sure that this lab_sea "sensitivity" to compiler
>> is not comming from the thermodynamics part ?
>> If I look at the results of ifort/ifc (or pgi) test, compared
>> with g77/gfortran, I still get a good results for
>> global_ocean.cs32x15.icedyn (10 or 11 digits for cg2d)
>> but all the lab_sea are failing with only 5 digits.
>>
>> I don't feel enough courage now to look in details in seaice_growth.F
>> but do we still apply instantaneous melting when SST > t_freeze ?
>> and do we still start from 1.m ice thickness everywhere
>> in all those lab_sea test ?
>>
>> Cheers,
>> Jean-Michel
>>
>> On Fri, Jul 17, 2009 at 10:00:16PM +0200, Martin Losch wrote:
>>> Hi David (and others),
>>>
>>> your are right, SQRT(deltaC)/(SQRT(deltaC)+SEAICE_EPS) is order one  
>>> or
>>> zero (if deltaC=0), the correct expression would have been
>>>>> deltaCreg = SQRT(deltaC)+SEAICE_EPS
>>> to avoid divisions by zero.
>>> And consequently, when I do that, then the sensitivity returns.
>>> I found that when I fix zetaC to a small value (say 1.), the  
>>> sensitivity
>>> goes away, but since zetaC is a factor that scales everything, it  
>>> just
>>> means that the entire stress term is scaled down by orders of  
>>> magnitude
>>> (normally zetaC is order 1e10 and more), and the dynamics solver  
>>> gives
>>> nothing much. All that means, is that the dynamics solver is very
>>> sensitive; and I still cannot figure out why (the same is true for  
>>> LSR of
>>> course), rats.
>>>
>>> Martin
>>>
>>> On Jul 17, 2009, at 4:32 PM, David Ferreira wrote:
>>>
>>>> Hi Martin,
>>>>> In the EVP code there is a line
>>>>>           deltaCreg = SQRT(MAX(deltaC,SEAICE_EPS_SQ))
>>>>> If you replace this line with
>>>>>           deltaCreg = SQRT(deltaC)/(SQRT(deltaC)+SEAICE_EPS)
>>>> It doesn't look like these 2 are equivalent.
>>>> Also, what about something like :
>>>>
>>>> IF deltaC >= SEAICE_EPS_SQ
>>>>  deltaCreg =SQRT(deltaC)
>>>> else
>>>>  deltaCreg = SEAICE_EPS
>>>> end
>>>>
>>>> it adds a "IF" but remove a "MAX" ... Not sure which one costs more
>>>> david
>>>>
>>>>
>>>>
>>>>> the number of matching digits increase from 5 to 12 (for cg2d) for
>>>>> the lab_sea/input experiment, between a
>>>>> Linux csysl18 2.6.22.19-0.2-bigsmp #1 SMP 2008-12-18 10:17:03 +0100
>>>>> i686 i686 i386
>>>>> gfortran (gcc version 4.2.1)
>>>>> and a
>>>>> Darwin csysm15.dmawi.de 9.7.0 Darwin Kernel Version 9.7.0: Tue Mar
>>>>> 31 22:52:17 PDT 2009; gfortran (gcc version 4.3.1)
>>>>>
>>>>> Both expressions give about the same results for large DeltaC, but
>>>>> still I would like to find out what would be the correct  
>>>>> replacement
>>>>> of the SQRT(MAX(deltaC,SEAICE_EPS_SQ)). Here's what did *NOT* work:
>>>>>           deltaCreg = max(sqrt(deltaC),seaice_eps)
>>>>>           deltaCreg = sqrt(dmax1(deltaC,seaice_eps_sq))
>>>>>           deltaCreg = SQRT(MAX(DBLE(deltaC),DBLE(SEAICE_EPS_SQ)))
>>>>>           deltaCreg = SQRT(MAX(SEAICE_EPS_SQ,deltaC))
>>>>>
>>>>> Any idea, what might be going on? If I cannot solve this problem I
>>>>> would suggest replacing this expression with the more stable one,
>>>>> also in seaice_calc_viscosities (although it will change all
>>>>> results).
>>>>>
>>>>> Martin
>>>>> _______________________________________________
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>>>>> http://mitgcm.org/mailman/listinfo/mitgcm-devel
>>>>
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