[MITgcm-devel] lab_sea sensitivity between platforms/compilers

[Martin Losch]

Hi there,
Disregard my previous email. it's Friday, and I was not careful  
enough. The problem is associated with deltaC, but not with the MAX  
function. I'll keep digging.

M.

On Jul 17, 2009, at 3:37 PM, Martin Losch wrote:

> Hi there,
>
> while going through the EVP code again for obvious reasons, I could  
> isolate at least some of the sensitivity of the lab_sea experiment:
>
> In the EVP code there is a line
>            deltaCreg = SQRT(MAX(deltaC,SEAICE_EPS_SQ))
> If you replace this line with
>            deltaCreg = SQRT(deltaC)/(SQRT(deltaC)+SEAICE_EPS)
> the number of matching digits increase from 5 to 12 (for cg2d) for  
> the lab_sea/input experiment, between a
> Linux csysl18 2.6.22.19-0.2-bigsmp #1 SMP 2008-12-18 10:17:03 +0100  
> i686 i686 i386
> gfortran (gcc version 4.2.1)
> and a
> Darwin csysm15.dmawi.de 9.7.0 Darwin Kernel Version 9.7.0: Tue Mar  
> 31 22:52:17 PDT 2009; gfortran (gcc version 4.3.1)
>
> Both expressions give about the same results for large DeltaC, but  
> still I would like to find out what would be the correct replacement  
> of the SQRT(MAX(deltaC,SEAICE_EPS_SQ)). Here's what did *NOT* work:
>            deltaCreg = max(sqrt(deltaC),seaice_eps)
>            deltaCreg = sqrt(dmax1(deltaC,seaice_eps_sq))
>            deltaCreg = SQRT(MAX(DBLE(deltaC),DBLE(SEAICE_EPS_SQ)))
>            deltaCreg = SQRT(MAX(SEAICE_EPS_SQ,deltaC))
>
> Any idea, what might be going on? If I cannot solve this problem I  
> would suggest replacing this expression with the more stable one,  
> also in seaice_calc_viscosities (although it will change all results).
>
> Martin
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