[MITgcm-support] obcs adjoint issue (I need dynamics.F help)

[chris hill]

FYI

  If you do the same thing twice in the forward code it should not 
create double sensitivities in _correct_ adjoint code.
  e.g.

  x=a
   :
  x=a

  will transform into reverse code something like

  dx=1

  da=da+dx
  dx=0
   :
  da=da+dx

  remembering to set terminal variables in a statement to 0 is a key 
piece of the AD transformation that the compiler does.

Chris
Martin Losch wrote:
> Hi Matt,
> 
> I have no time to look into this right now, but I suggest, that you try 
> it yourself: use exp4 and see whether keeping removing the extra calls 
> to obcs_apply_uv change the result (by running ./testreport -q -t exp4). 
> If removing a call does change the result, then this call is necessary. 
> (I believe exp4 does not use the vector invariant moment equations, but 
> you can put that in for your tests and have an extra personal 
> verifcation experiment for that case, too).
> 
> Are you sure that the odd structions in adgv does not have other origins?
> 
> Martin
> On 9 Nov 2006, at 02:41, Matthew Mazloff wrote:
> 
>> Hello,
>>
>> I have been using the open boundary controls.  The sensitivities for 
>> the velocity normal to the boundary appear unphysical.  (I am using a 
>> northern boundary so for my case V is the normal vel.)  I have tracked 
>> down what is causing this.  It is that on the open boundary the 
>> adjoint variable of gV has an odd structure.  And because dynamics.F 
>> calls OBCS_APPLY_UV twice, this odd structure gets added to the 
>> sensitivities twice.  Calling  OBCS_APPLY_UV( bi, bj, k, gU, gV, 
>> myThid ) is rather benign in the forward integration, U and V on the 
>> open boundary get written over anyway.  In the adjoint integration, 
>> however, every call matters.  Could someone more expert in the 
>> dynamics.F routine please check if all these calls are necessary.  And 
>> I'd really appreciate it if they could also fill me in as to why they 
>> are there.
>>
>> Thanks,
>> Matt
>>
>> ps> I'm using vectorInvariantMomentum and implicitViscosity
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