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

[Martin Losch]

Hi Dimitris,
I have tried this:
> CML          IF(FICE(I,J,bi,bj).GT.ZERO) THEN
>           IF ( AREA(i,j,2,bi,bj).GT.0. _d 0
>      &        .AND. atemp(i,j,bi,bj).LE.273.16 _d 0 )  THEN
> C FREEZING, PRECIP ADDED AS SNOW
>            HSNOW(I,J,bi,bj)=HSNOW(I,J,bi,bj)+SEAICE_deltaTtherm*
>      &            PRECIP(I,J,bi,bj)*AREA(I,J,2,bi,bj)*SDF
>           ELSE
> C ADD PRECIP AS RAIN, WATER CONVERTED INTO equivalent m of ICE BY 1/ 
> ICE_DENS
>              SEAICE_SALT(I,J,bi,bj)=SEAICE_SALT(I,J,bi,bj)
>      &            -PRECIP(I,J,bi,bj)*AREA(I,J,2,bi,bj)*
>      &            SEAICE_deltaTtherm/ICE_DENS
>           ENDIF
to make snow a function of air temperature instead of FICE (ice  
growth rate?), but that does not change the results very much. (after  
10 years, more tomorrow)

However, what seems to be more important is my little flooding  
algorithm. Turning that off decreases HEFF by almost a a factor of 2  
at the cost of a having too much snow (>5m in 10years) in isolated  
places again (all without advection of snow).

I don't know what to say. I'll wait until tomorrow to have a closer  
look after 100years of integration, but a first look implies that the  
snow alogithm is still not working the way we expect it to work and  
the flooding algorithm takes all the snow and converts it into ice.  
Which leads us back to the original problem of too much snow in the  
first place. (I am using the latest code).

M.

On 28 Nov 2006, at 20:29, Martin Losch wrote:

> Hi Dimitris,
>
> since I have been doing all this sort of "by the way" I am no  
> longer absolutely sure, what exactly run34 is. But most likely it  
> is with growth.F pre 1.29, and with flooding turned on, but NO  
> advection of snow (the last two I know for sure). So the flooding  
> algorithm takes care of the snow over open water, but in a pretty  
> simple way, thermodynamically speaking. run38 is the same with  
> growth.F 1.34
>
> This forward sensitivity is frightening. I am not sure if I am as  
> optimistic about it as you are, but I have nothing to offer in  
> terms of explanations. I observe in my 100y runs that the ice  
> thickness builds up within the first 10years and then stays nearly  
> stationary (with seasonal cycle of course). The ice cover does not  
> retreat very far in summer. Does the non-linear effect that you  
> describe explain such an "equilibrium" at higher mean ice thicknesses?
>
> I have not had the time to get my head around the thermodynamics in  
> growth, but your suggestion (making snow/rain depend on forcing  
> fields) sounds good to me. Currently I have not time to play with  
> this (maybe starting again next Friday)
>
> Martin
>
> On 28 Nov 2006, at 17:08, Dimitris Menemenlis wrote:
>
>> Martin, I also notice increased ice thickness around Antarctica in  
>> the high-res cubed sphere integration and increased sea-ice  
>> extent, much more summer ice extent than observed.
>>
>> What is you run34?  In terms of CVS repository,
>> http://mitgcm.org/cgi-bin/viewcvs.cgi/MITgcm/pkg/seaice/growth.F
>> which version of growth.F did you use: pre 1.29, 1.29 with  
>> flooding and advection turned on, 1.30, or 1.34?  So far as I have  
>> looked in my own tests, results from 1.30 and 1.34 are not that  
>> different.  But then your run34 does not have snow over open  
>> water, so it cannot be pre 1.29?
>>
>> Optimistically, the too-much ice in run38 may be a bug in the NCEP/ 
>> CORE forcing fields rather than in growth.F.  That is, too much  
>> precipitation is converted into snow, which extracts heat from  
>> ocean when it melts.  The effect would be highly non-linear since  
>> more ice/snow extent means higher albedo, which leads to cooler  
>> ice/ocean surface temperature, which in turn leads to more  
>> precipitation being converted to snow, since in present treatment  
>> of precipitation, rain to snow conversion depends on thermodynamic  
>> ice growth (rain) or melt (snow).
>>
>> Optimistically again, the high tangent linear sensitivity noted by  
>> Patrick and also in the verification experiments that you report  
>> is also due to above effect.  Incidentally, with growth.F prior to  
>> 1.30, the verification/lab_sea domain is at all times 100% covered  
>> with snow (but not ice).  So low forward sensitivities prior to  
>> 1.30 are almost certainly for wrong reason.
>>
>> One possible way of reducing the forward sensitivity (I have not  
>> yet tried it) would be to remove the snow/rain dependence on ice  
>> growth rate and instead make it depend on forcing field, e.g.,  
>> surface air temperature and fresh water freezing point, as is done  
>> in pkg/thice.  Should I give this a try?
>>
>> Dimitris
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