[MITgcm-devel] more seaice blues for the adjoint

[Jinlun Zhang]

Hi Martin,
Before your fix, is it occurring with both LSR and EVP C-grid codes? 
Also, would it be possible for you to try the B-grid that I put in 
originally (if it is still one of the options)? It is OK for not trying 
if you don't have time. I remember I tested this thing with B-grid years 
ago without finding similar problem, but I am not sure now after so many 
years without dealing with it again. I just hope this is not one of the 
differences between B and C. Your explanation certainly indicates that 
this thing does not depend on grid. But a test on B-grid would clear any 
residual doubts.
Thanks, Jinlun


Martin Losch wrote:
> Chris,
> when there is no strain (in the absence of any forcing), the ice 
> should not move, right? Unless you expect it to spread like a pile of 
> sand with time, but I don't know what the time scale for that would 
> be, very slow.
> If delta = 0, we reset it to delta_min, because we have to devide by 
> it zeta = press/(2*delta), this effictively makes zeta <= zeta_max = 
> press/(2*delta_min), after capping zeta, we recompute press_replace = 
> zeta*2*delta, if delta has been replaced by delta_min, this give non 
> zero pressure and thus a forcing due to internal stress. That's the 
> numerical issue. I am not shure that I solved it very well (the 
> adjoint breaks), but this way the ice does not move in the absence of 
> forcing.
>
> Martin
> On 7 Jun 2007, at 18:05, chris hill wrote:
>
>> Martin,
>>
>>  Quick ice question.
>>
>> Is it just a numerical issue i.e. is it clear what the underlying 
>> equations are, since ice is neither a classical fluid or a classical 
>> solid. What time scale does it spread on?
>>
>> Chris
>> Martin Losch wrote:
>>> 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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