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

[Martin Losch]

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.

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.

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.

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