[MITgcm-devel] Re: seaice_evp

[Martin Losch]

Hi Patrick,

I don't know what to do about this. All we need here is that P =  
(almost) const in the case of small deltaC (not strain). Maybe we can  
talk about this (Skype)
Martin

On 7 Jun 2007, at 18:43, Patrick Heimbach wrote:

>
> Martin,
>
> the term
>    SQRT(deltaC)/deltaCreg
> must be getting awfully small.
> If I replace it by
>    MAX( SQRT(deltaC)/deltaCreg , 1. _d -12 )
> the adjoint is back to "stable".
> I just picked the E-12 randomly. Not sure what a good scale is.
>
> -Patrick
>
>
>
> On Jun 7, 2007, at 12:28 PM, Patrick Heimbach wrote:
>
>>
>> Hmm, I see.
>> So you suggest avoiding the lines
>>             deltaC = SQRT(deltaC)
>>             deltaZ = SQRT(deltaZ)
>> altogether
>> since deltaC and deltaZ are needed only once to evaluate
>> pressC, pressZ and do instead (e.g. for pressC):
>>             IF ( deltaC .GT. 0. _d 0 ) THEN
>>                pressC = press0(I,J,bi,bj)*SQRT(deltaC)/deltaCreg
>>             ELSE
>>                pressC = 0.
>>             ENDIF
>>
>> I actually just tried this for the adjoint and results are even  
>> worse.
>>
>> -Patrick
>>
>>
>>
>> On Jun 7, 2007, at 11:55 AM, Martin Losch wrote:
>>
>>> Hi Patrick,
>>>
>>> the plan was to have pressC = press0*DeltaC/MAX 
>>> (DeltaC,SEAICE_EPS), effectively.
>>> You suggestion gives
>>> pressC = press0 if DeltaC > 0 and
>>> pressC = 0 if deltaC = 0, which is not the same.
>>> I am not sure if that's OK, but why not do it as you suggest for  
>>> ALLOW_AUTODIFF?
>>>
>>> Sorry about the multiple updates. I consider all of this bug  
>>> fixes ...
>>>
>>> Martin
>>>
>>>
>>> On 7 Jun 2007, at 17:39, Patrick Heimbach wrote:
>>>
>>>>
>>>> Hi Martin,
>>>>
>>>> your change of yesterday in seaice_evp.F messed up the adjoint  
>>>> again.
>>>> I had thought that you'd implement changes such as to leave  
>>>> computation
>>>> unchanged in case of ALLOW_AUTODIFF.
>>>> Changing to
>>>>             IF ( deltaC .GT. 0. _d 0 ) deltaC = SQRT(deltaC)
>>>> makes gradient rise by 10 orders of magn.
>>>>
>>>> I understand using the former lines
>>>>             deltaC = SQRT(MAX(deltaC,SEAICE_EPS_SQ))
>>>> will cause deltaC to be non-zero where it wasn't before.
>>>> That's reasonable, but the above should be treated differently  
>>>> then:
>>>>             IF ( deltaC .GT. 0. _d 0 )
>>>>      &           deltaC = SQRT(MAX(deltaC,SEAICE_EPS_SQ))
>>>>
>>>> In this case your argument (Delta = 0 gets replaced by SEAICE_EPS)
>>>> doesn't hold anymore since it Delta remains zero where it was  
>>>> before,
>>>> and unless SEAICE_EPS is chosen badly, it should work.
>>>>
>>>> What speaks against implementing it this way?
>>>>
>>>> -Patrick
>>>>
>>>> PS:
>>>> Could we reduce the code and reference output update turnaround
>>>> from maybe daily to weekly...?
>>>
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>>
>> ---
>> Dr Patrick Heimbach | heimbach at mit.edu | http://www.mit.edu/~heimbach
>> MIT | EAPS, 54-1518 | 77 Massachusetts Ave | Cambridge, MA 02139, USA
>> FON: +1-617-253-5259 | FAX: +1-617-253-4464 | SKYPE: patrick.heimbach
>>
>>
>> _______________________________________________
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>> http://mitgcm.org/mailman/listinfo/mitgcm-devel
>
> ---
> Dr Patrick Heimbach | heimbach at mit.edu | http://www.mit.edu/~heimbach
> MIT | EAPS, 54-1518 | 77 Massachusetts Ave | Cambridge, MA 02139, USA
> FON: +1-617-253-5259 | FAX: +1-617-253-4464 | SKYPE: patrick.heimbach
>
>
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