[MITgcm-devel] Re: seaice_evp

[Patrick Heimbach]

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
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