[MITgcm-devel] more seaice blues for the adjoint

[Martin Losch]

No, but it's a numerical issure. I did not pay any attention to this  
before and maybe this was no problem in the original code that you  
supplied but if you have zero strain, Delta = 0, and this is replaced  
by some Delta_min (SEAICE_EPS in the code) in order to avoid division  
by zero (and infinite viscosities). P however is still non-zero and  
divergence(stress) end up being (dP/dx, dP/dy) .NE. 0, so that you  
have a forcing of down the slope of P. This does not make sense (why  
would ice spontaneously spread?), so that the pressure is replaced by  
P = P*Delta/max(Delta,SEAICE_EPS), so that in the (rare) case of no  
strain P = zeta = eta = 0 and thus no forcing by stress. Makes sense  
to me (and is really only relevant in idealized test case, just like  
the spurious motion of sigma coordinates vs. z-coordinates in ocean  
models).

Hope I did not misunderstand anything here.

Martin
On 7 Jun 2007, at 13:40, Jinlun Zhang wrote:

> Hi Martin,
> Would this be due to some kind of finite differencing problem?
> Jinlun
>
> Martin Losch wrote:
>> Hi Patrick,
>>
>> I have found (or rather, was pointed to) a problem with the seaice  
>> solvers: The start to move spontaneously (in the absence of  
>> forcing), if the sea ice distribution is NOT uniform.
>> I have implemented a fix but this will cause problems with the  
>> adjoint: I need terms like
>> SQRT(deltaC), which used to be SQRT(MAX(deltaC,SEAICE_EPS_SQ)), so  
>> that the derivate code will be involve 1/sqrt(deltaC). Should I  
>> put this into #ifdefs?
>>
>> Martin
>
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