[MITgcm-devel] seaice

[Martin Losch]

Hi Jinlun,
thanks for the quick reply. However, I still have questions, see below:
On Feb 12, 2006, at 12:36 AM, Jinlun Zhang wrote:

>
> Martin Losch wrote:
>
>> Hi there,
>> I have (again) questions about the seaice-pkg. I am working on a   
>> 2x2*cos(phi) deg quasi global (no arctic) model approximately 30  
>> to  50km resolution in the Weddell Sea (and Ross Sea). I am  
>> forcing with  the CORE fields (with daily winds, tair and  
>> humidity), but my  problems probably not connected to the forcing.  
>> I use the exf-pkg and  kpp and seaice (SEAICE_MULTILEVEL undefined).
>> My basic problem is that there is not enough sea-ice in the  
>> Weddell  Sea (and the Ross Sea, but that's a different story). One  
>> of the  reasons is that the ACC "breaks" through the South Scotia  
>> ridge (the  extension of the Antarctic pensinsula) into the  
>> Weddell Sea and  directly imports warm water instead of staying  
>> further north and  introducing the warm water further east into  
>> the Weddell Gyre. As a  consquence (or a cause?) there is no  
>> Weddell Gyre to speak of and too  little ice (the distribution is  
>> OK, but the thickness is too small  towards the north-east of the  
>> Weddell Sea).
>
> Hi Martin,
>
> The MITgcm sea ice model  is the one I used/tested based on POLES  
> forcing (http://psc.apl.washington.edu/POLES/model_forcings/ 
> ModelForcings.html) for Arctic and NCEP/NCAR forcing for both polar  
> oceans. Some tuning involved. If you got too little ice in the  
> Weddell/Ross sea, it is likely that the CORE forcing (dynamic or  
> thermal) is significantly different.
I don't expect perfect seaice in the southern ocean, but something  
that is reasonable to first order.
>
>> The funny thing is that, when I turn off the seaice-model, things   
>> look much better (except that the surface is now too cold, which  
>> I  would have expected in the absence of ice growth): I have a  
>> strong  Weddell Gyre and the ACC stays completely north of the  
>> Weddell Sea as  it should. The same is true, when I use ncep heat  
>> fluxes and surface  field restoring (as in global_ocean.90x40x15).  
>> So topography (which  was my first guess) cannot be the reason for  
>> this behavior, but it is  most likely the seaice that makes the  
>> Weddell Gyre disappear and lets  the ACC into the Weddell Sea.  
>> Candiates are:
>> 1. not likely: buoyancy fluxes: I don't see how the buoyancy  
>> fluxes  can cause this problem. I have SEAICE_EXTERNAL_FLUXES  
>> defined, so  that exf does all the work over open water.
>
> If you found that surface stress does not affect the ocean cir.,  
> then it could be due to buoyancy fluxes.
But what can go wrong in the buoyancy fluxes that causes the Weddell  
Gyre disappear? The fluxes are modified according to ice formation  
and melt, so that in winter the ocean does not loose enormous amounts  
of heat and doesn't gain enormous amounts in summer. In fact the  
oceanic stratification is not too bad with seaice, just too warm near  
the surface, because the ACC flushes the Weddell Sea.
>
>> 2. more likely: momentum forcing: But ... I tried a physically   
>> sensible stress formulation (seaice_concentration*ocean_ice_stress  
>> +  (1-seaice_concentration)*ocean_air_stress, see ostres.F with   
>> SEAICE_TEST_ICE_STRESS_1 defined); and I tried using only   
>> ocean_air_stress as precomputed by exf_bulkformulae (no influence  
>> of  ice on stress, the default). Both give similar results, which  
>> I don't  understand.
>
> Try to just use 0*ocean_ice_stress +  (1-0)*ocean_air_stress and  
> see what happens. This would exclude ice dynamics effect on ocean  
> cir. if you suspect that ice dynamics is the trouble maker.

I'll try that tomorrow (so far I just didn't do anything in ostres.F,  
basically commented out the enitre routine.).
>
>>
>> In order to understand what I am doing when I mess up the code, I   
>> need to know what the different fields are. As far as I can see,  
>> the  seaice-velocities UICE and VICE have three time levels, with  
>> UICE(:,:, 1,:,:) being the most current, and UICE(:,:,2,:,:) being  
>> the previous  timestep and level 3 probably some intermediate step/ 
>> auxiliary  variable used in lsr.F. But what are UICEC and VICEC,  
>> in what sense  are they different from the first time-level of  
>> UICE/VICE?
>>
>> The SEAICE_ORIGINAL_BAD_ICE_STRESS in ostres.F uses a mixture of  
>> UICE (:,:,1,:,:), UICE(:,:,3,:,:) and UICEC to compute the  
>> stresses, which  I do not understand at all. Which is the ice- 
>> velocity to use for  computing ocean-seaice stresses?
>
> UICE(1) is the one,  ice velocity, others are all intermediate  
> quantities for numerical reasons (or stability). Since UICEC is  
> used in the dynamics solver, it shows up in ostres.F for an exact  
> calculation of stress.

Yes, but what is UICEC exactly? Or rather, what is the correct, exact  
stress? The computations in between the #ifdef  
SEAICE_ORIGINAL_BAD_ICE_STRESS in ostres.F? Unfortunately I don't  
understand what's going on there, (please excuse my ignorance about  
sea-ice models, I should probably start reading about sea-ice models,  
but) what are these terms is ostres.F:
> DWATN(I,J,bi,bj)*(COSWAT*(GWATX(I,J,bi,bj)-UICE(I,J,1,bi,bj))
>      &          -SINWAT*(GWATY(I,J,bi,bj)-VICEC(I,J,bi,bj)))
This I can understand: tau = c_d*f(|u_ocean-u_ice|) * Rotation *  
(u_ocean-u_ice), the rotation is currently zero. But this is what I  
think should be the stress between ice and ocean. (In fact, I cannot  
find that term in dynsolver.F, just c_d*f(|u_ocean-u_ice|) * Rotation  
* u_ocean, so not the relative velocity, but I am probably missing  
something?), but what are the following terms?
> -( COR_ICE(I,J,bi,bj)*GWATY(I,J,bi,bj)-COR_ICE(I,J,bi,bj)*VICEC 
> (I,J,bi,bj))
-f*(v_ocean-v_ice), why do you need a coriolis term in the stress  
formulation?
> (UICE(I,J,1,bi,bj)-UICE(I,J,3,bi,bj))*AMASS(I,J,bi,bj)/SEAICE_DT*TWO,
0.5*(du_ice/dt)*rho_ice*h_ice? What is the physical meaning of this?

I have to admit that so far I have only used the first term (what the  
lay-man in me believes is the correct stress, see the current version  
of ostres.F with SEAICE_TEST_ICE_STRESS_1
defined.), maybe that is my problem?

still puzzled,

Martin