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

Wed Dec 6 15:45:44 EST 2006

Hi Jinlun,

in the end, we want to be able to use a flux-limited 3rd order  
advection scheme, it's just not yet possible.

Unfortunately my model stops at 80degN. I really need to set up a  
truly global ocean model. I do assume though, that there is something  
fishy with the precipitation in the CORE data set. Would be  
interesting to see if some-one else has a similar experience.

In general, though, the ice extend is too large in winter (in  
particular in the Drake Passage, isn't it?). That 's the only thing  
that's still worrying me.

XKI: I am only playing with this parameter to find out how the model 
(s) behave(s). In practice I will always use something around 2. XKI  
only appears in the denominator in budget.F, so I don't quite see  
what it does. I guess I have to dig up Hibler79/80 and have a look at  
the thermodynamics, right?

M.

On 6 Dec 2006, at 18:34, Jinlun Zhang wrote:

>
>
> 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
>
>
>
>
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