[MITgcm-devel] heff_max...more sea ice issues
Jinlun Zhang
zhang at apl.washington.edu
Mon Dec 18 13:53:50 EST 2006
Hi Matt,
When the model crashes, is it the ice that crashes, or the ocean
crashes? Generally, thick ice won't cause crashes (or make the ocean
more stable) because it moves slower and it does not create big buoyancy
change. So change heff_max generally won't help much if the model would
eventually blow up. And it is not a good idea to set heff_max low
because if would create lots of side effect (buoyancy). One sensitivity
study is to turn off ice dynamics and run the model for sufficiently
long to see what happens. Another thing is to make sure the calculation
of ocean surface stress is robust?
Jinlun
Matthew Mazloff wrote:
> Hello,
>
> My upper layer is 10m thick. When heff_max > 5m the model crashes.
> (This only happens with dynamics on (LSR_ERROR = 1e-3 in my
> calculations so not very accurate). With dynamics off the ice never
> reaches this thickness.) I am modeling the Southern Ocean, I am sure
> Arctic modelers must have this problem to an even greater degree.
> What is other people's experiences with thick ice and stability?
> Should an effective thickness capping be implemented?
>
> Thanks
> -Matt
>
>
> fyi: I tried the free drift sea ice hack (i.e. setting uc ~ uvel in
> seaice_advdiff.F) and the model crashed. The results were
> odd....lots of numerical noise in AREA and, it appeared, no
> significant advection.
>
>
>
>
> On Dec 6, 2006, at 3:45 PM, Martin Losch wrote:
>
>> 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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