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

[Martin Losch]

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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