[MITgcm-devel] more seaice blues for the adjoint

[Martin Losch]

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zeta*2*delta, if delta has been replaced by delta_min, this give non  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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