[MITgcm-devel] (not so) funny things happen in seaice_lsr and pickups

[Matthew Mazloff]

Hi Martin,

I've had similar things.  I now run with Ian Fenty's code (_if) and  
it's ok.  One thing this code has is a bunch  of checks in  
seaice_growth_if.F of the nature:

                   HEFF(I,J,2,bi,bj)  = MAX(0. _d 0, HEFF(I,J, 
2,bi,bj)  )
                   HSNOW(I,J,bi,bj)   = MAX(0. _d 0, HSNOW 
(I,J,bi,bj)   )
                   AREA(I,J,2,bi,bj)  = MAX(0. _d 0, AREA(I,J, 
2,bi,bj)  )

to make sure things never go negative due to the advection scheme.   
He told me he put this in to stabalize the model.  Not sure if that's  
why I can run now, but using _if code did make a difference for me

-Matt



On Feb 18, 2009, at 10:21 PM, Martin Losch wrote:

> Hi all,
>
> just to let you know that we are experiencing problems with the LSR  
> sea ice solver on the C-grid: At unpredictable points of the  
> integration, it appears to become instable and blows up. I have not  
> been able to isolate this in all cases, because a small issue with  
> pickups hampers this:
>
> Apparently, starting from pickup is NOT exact. We have tried the  
> famous 2+2=4 test with our 8CPU job on our SX8 (cc to Olaf, who's  
> been mostly involved in this) and found no difference between the  
> cg2d output (and other output). However, when we run an experiment  
> for a longer time, the same test fails, e.g., 2160+2160 != 4320 (we  
> can provide plots if required). I assume that this is expected,  
> because double precision is not more than double precisioin and in  
> the cg2d output (and other monitor output) there are always only 15  
> digits, and we don't know about the 16th one, correct? Anyway, this  
> tiny pickup issue hinders me from approaching the point of model  
> crash with pickups, because after starting from a pickup, the model  
> integrate beyond the problem and crashes (sometimes) at a much  
> later time. This is to say, that the problem in seaice_lsr (the  
> problem only appears when the C-LSR solver is used) very sensitive;  
> the code crashes without any warning from one time step to the  
> other. A while ago, in a different case I was able to get close  
> enough the point of crashing to do some diagnostics, but its almost  
> impossible to identify, why the model explodes. I am assuming that  
> for random pathological cases one or more matrix entries are nearly  
> zero, which then prevents the solver from converging.
>
> Any comments? Any similar experience?
>
> I run this code in so many different configurations, and I have  
> these problems only very seldom/randomly, so I am a little at a  
> loss where I should continue looking, so any hint is appreciated.
>
> Martin
>
> _______________________________________________
> MITgcm-devel mailing list
> MITgcm-devel at mitgcm.org
> http://mitgcm.org/mailman/listinfo/mitgcm-devel