[MITgcm-devel] seaice adjoint and EVP

[Martin Losch]

Hi Jinlun,

the velocities are quite smooth in the central Arctic, aren't they,  
just along the ice edge I see problems. However, where does the noise  
in the log10(1-area) plots come from? That seems to me to be a  
different issue. I am working on reproducing these problems. Maybe  
I'll find out something down that route.

Martin
On 24 May 2007, at 17:58, Jinlun Zhang wrote:

> It is not just over open water, but also in the central arctic.  
> However, the noise is suppressed with 1s timestep over both open  
> water and pack ice. So I start to think perhaps nothing is wrong,  
> just needing a small timestep.
> Jinlun
>
> Martin Losch wrote:
>
>> I have a new feeble theory for the noise in the evp solver over  
>> open  ocean:
>>
>> heff = 0 over open ocean, therefore seaiceMassU/V = 0.
>> momentum equation in seaice_evp is discretized (in time) as
>> m*duice/dt = -m dphi/dx + tau_air + cd*(uice-uocean) + m*f*vice +   
>> \nabla\sigma
>> m*(uice(n+1)-uice(n))/dt = -m dphi(n)/dx + tau_air(n) - cd*(uice(n 
>> +1)- uocean(n)) + m*f*vice(n) + \nabla\sigma(n),
>> so coriolis is explicit, ice-ocean stress is implicit. if the mass  
>> m  is zero (and zetaMin=0, so that zeta=eta=press = 0 over open  
>> ocen)  this reduces to
>> cd*uice(n+1) = tau_air(n) + cd*uocean(N)
>> so that uice ist a purely diagnostic quantity and not time  
>> stepped.  cd is a function of uice-uocean at the nth time step,  
>> averaged to  center points and the averaged back to velocity points.
>>
>> Dimitris, could that be the problem, somehow I don't think so,  
>> but  you can try by putting a minimum seaiceMassU/V in  
>> seaice_dynsolver.F,  say seaiceMassU = max 
>> (seaiceMassU,SEAICE_rhoIce*0.05)
>>
>> Martin
>>
>> On 22 May 2007, at 18:59, Jinlun Zhang wrote:
>>
>>> Hi Martin,
>>> Yeah, we are sort of stuck, but hey it is very interesting and   
>>> revealing.
>>> I would vote against masking ice velocities over open water   
>>> because, as mentioned earlier, the ice velocities would be wrong  
>>> at  ice edge and the ice velocity discontinuity at ice edge will  
>>> get  into ocean. (o:. We don't do the masking with LSR solver,  
>>> perhaps  we can avoid doing that with EVP.
>>> Jinlun
>>>
>>> Martin Losch wrote:
>>>
>>>> Hi Jinlun,
>>>> the evp-solver is only in place for the C-grid. I don't have  
>>>> the  time  to code the solver for the b-grid now. The b-grid  
>>>> code (for  LSR) is  still working, but I have not kept it up to  
>>>> date, so  there may be a  few thing different other than the  
>>>> different grids.
>>>>
>>>> In general I though that the c-grid is perfect for evp as all  
>>>> the   discretizations fall in place naturally. Only for this  
>>>> \delta  term  one needs to average from center to corner points  
>>>> and vice  versa  (have a look at seaice_calc_strainrates and  
>>>> seaice_evp).  However,  there may be issues with the coriolis  
>>>> terms (commonly a  problem with  the c-grid).
>>>>
>>>> Actually, Elizabeth told us that she masks ice velocities over   
>>>> open  water in CICE.
>>>>
>>>> Now we are a little stuck, aren't we?
>>>>
>>>> Martin
>>>>
>>>> PS. I need to be able to reproduce these results myself (I   
>>>> haven't  been able to, yet), maybe I can debug the stuff this  
>>>> way.  Via email  etc. it's quite demanding (o:
>>>>