[MITgcm-devel] another bug in growth.F ?

[Jinlun Zhang]

Martin Losch wrote:

> Hi Jinlun,
> thanks for you input. I really enjoy this discussion!
>
> For clarification: I use the same advection for HEFF, HSNOW, and  
> AREA. That is run47 has 1st order upwind for all three variables,  
> while run41 has 2nd order central differences scheme (and not  
> flooding algorithm). All runs use a little bit of diffusion (the  
> default values of DIFF1=0.004), which is probably not good for run47.

Hi Martin,

Good the ice and snow advection is consistent. When you use 1st order 
upwind, perhaps you do not have to use any diffusion (if you have used 
any) since it is quite diffusive.

>
>
> run48 uses only 10% of the snow precipitation, but uses flooding  
> (it's just like 45). Are you saying that this should not reduce the  
> ice amount? One source of the ice is flooded snow in the flooding  
> algorithm in the current version of growth.F (http://dev.mitgcm.org/ 
> cgi-bin/viewcvs.cgi/MITgcm/pkg/seaice/growth.F? 
> rev=1.34&only_with_tag=MAIN&content-type=text/vnd.viewcvs-markup).  
> One of my problems was, that the huge amounts of snow that you see in  
> run40 (160m in some places, no flooding, no advection) are turned  
> into ice by flooding and lead to ice thicknesses beyond my  
> expectation. Either there is too much snow to begin with, or  
> something is wrong in the handling of snow and not enough snow is  melted.

I was looking at wrong figure and had mistakenly thought that when you 
reduced snow, ice was pretty much gone. But actually ice was only slight 
reduced with run48, not really bad. Sorry for the mistake.
I don't know what is wrong with run40, but I wonder if this huge snow 
depth also occurs in the Arctic. If that is the case in Arctic also, 
then definitely something is really wrong with the model.

>
> 1D tests: As far as I understand the physics of ice formation: Ice  
> forms because the atmospheric heat flux cools the ocean surface below  
> freezing. Ice continues to grow as long a the atmospheric surface  
> flux continues to cool the ocean. In the presence of ice this  
> atmospheric heat is "diffused" (conducted) through the ice according  
> to the net conductivity. In the absense of snow this conductivity  
> should be SEAICE_iceConduct (XKI in budget.F). If the ocean provides  
> heat from below by upward transport of warmer waters (by vertical  
> convection), then this heat flux can balance the atmospheric heat  
> flux and stop the ice from growing. When you equate these fluxes  
> roughly at equilibrium: Qocean = conductivity*(Tair-Tsurfocean)/hice  
> you get the ice thickness that follows form this balance hice =  
> conductivity*(Tair-Tsurfocean)/Qocean.
> Hypothetically I should be able to modify this "equilibrium  
> thickness" by playing with the conductivity (or Qocean or the  
> temperature difference). However I find that the model parameter  
> XKI=SEAICE_iceConduct has no impact on hice (I use 1e-6 instead of  
> 2!). That's puzzling, isn't? For the thsice package, the  
> corresponding parameter does have an impact.

Your reasoning sounds ok. But I am not sure why ice is so insensitive to 
XKI in equilibrium. It does not make sense. Note that XKI is a physical 
term that is likely determined by lad experiments. Better not use a 
different number. Would be interesting to see how sensitive ice 
thickness/extent is to XKI in real simulations.
Jinlun

>
> Martin
>
>
> On 5 Dec 2006, at 18:17, Jinlun Zhang wrote:
>
>>
>> Martin Losch wrote:
>>
>>> Jinlun,
>>> thanks for your opinion. The thsice thermodynamics are basically   
>>> Winton's (2000) model, but we have not yet fully sorted out the   
>>> advection part.
>>> I have now a run47 with SEAICEadvScheme = 1 (1st order upwind,  too  
>>> smooth) and no flooding, and and another one (run48) which is  just  
>>> like run45 but with only a 1/10th of the snow fall, just to  see 
>>> what  happens, see
>>> http://mitgcm.org/~mlosch/run47.png
>>> http://mitgcm.org/~mlosch/run48.png
>>> As expected is run47 closest to what we expect. But run48 is not  
>>> too  bad either, too little snow (of course) and as a consequence  
>>> too  little ice. So either there is too much snow/precip in the   
>>> atmospheric forcing, or there is something not kosher in the snow   
>>> parameterizations. As the problems are similar with thsice I  would  
>>> agree that the forcing may be the problem ... I have to try  and 
>>> find  different precipitation fields.
>>
>>
>> Marin,
>> Yeah run47.png looks pretty good. The advection works ok. But I  
>> wonder what ice advction you are using, 2nd order or 1st order? The  
>> one I installed is 2nd order. Ideally, the snow advection should be  
>> exactly the same as the ice advection so ice and snow won't devorce  
>> with each other.
>> It is not right with run48 that when the snow is turned off, ice is  
>> gone. Some thing is wrong here.
>>
>>>
>>>
>>> I have also made another observation: I tried to run the  different  
>>> thermodynamics without any dynamics in a 1D case. I  expect (and 
>>> JMC  agrees with me) that for constant air temperature  (say 
>>> -30degC), ice  thickness should grow until there is some  
>>> equilibrium thickness, when  the remaining heat flux out of the  
>>> ocean is balanced by the diffusive  flux of heat through the ice.  I 
>>> assume that the diffusion is  controlled by "SEAICE_iceConduct"  for 
>>> seaice and kice for thsice. The  equilibrium thickness can  roughly 
>>> be estimated by hequil =  conductivity*(Tair-Twater)/ heatflux.
>>> I have only succeded yet in reaching some equilibrium thickness  
>>> with  thsice (with an unrealistic value of kice=1e-6 instead of  2). 
>>> For  growth, this only works if I turn on some precipitation  
>>> (snow).  Without snow HEFF is completely independent of  
>>> SEAICE_iceConduct,  which I don't think is right.
>>
>>
>> I don't understand this equil. ice thickness )-:. As said above,  
>> without snow-without ice thing or ice not working right without  snow 
>> does not make sense to me.You might want to check with  Thorndike 
>> (199?) for a toy model of equil. ice thickness.
>> Jinlun
>>
>>>
>>> M.
>>>
>>> On 5 Dec 2006, at 03:39, Jinlun Zhang wrote:
>>>
>>>> Martin,
>>>>
>>>> I would vote run45.png for best performance except that the  
>>>> summer  ice is slightly overestimated. I would not vote run41.png  
>>>> because  of its weird snow distribution. The snow pattern should  
>>>> generally  follow the ice pattern (could mean a problem with ice  
>>>> advection). I  don't know why the snow gets so thick with  
>>>> run40.png, the precip  forcing could be way off. But obviously  
>>>> snow advection helps a lot.  Snow flooding, if it overestimates  
>>>> ice, then turn it off, not big  deal (since what we do is to make  
>>>> the fields look like  observations). As for thsice, I don't know  
>>>> what is going on. But  for any ice thermodynamics that involves  
>>>> ice salinity (if thsice  uses ice salinity), there might be a  
>>>> singularity in the formulation  (I had such feeling before, but I  
>>>> could be wrong).
>>>>
>>>> Jinlun
>>>
>
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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