[Wrf-users] Results variability depending on processor count

[Jan Ploski]

Hello,

WRF produces slightly different results depending on how many processors 
are used in a shared memory run (of course, the "slight" differences 
become bigger with integration time). I know that the general explanation 
for this is "rounding errors". What I would like to understand is why the 
rounding errors must appear at all. After all, data can be transmitted 
exactly from one MPI process to another, so why does it matter how many 
MPI processes are involved? I'd be grateful for a brief explanation...

Regards,
Jan Ploski

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Dipl.-Inform. (FH) Jan Ploski
OFFIS
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