[MITgcm-devel] seaice adjoint and EVP

[Jinlun Zhang]

Martin,
I would think that the noisy log10(1-area) means velocities are not 
smooth in central arctic. We would likely see that if we make a log plot 
of velocity.
Jinlun

Martin Losch wrote:

> 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:
>>>>>
>
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-- 

Jinlun Zhang
Polar Science Center, Applied Physics Laboratory
University of Washington, 1013 NE 40th St, Seattle, WA 98105-6698

Phone: (206)-543-5569;  Fax: (206)-616-3142
zhang at apl.washington.edu
http://psc.apl.washington.edu/pscweb2002/Staff/zhang/zhang.html