[MITgcm-devel] advection scheme 42 and 81

[Martin Losch]

Hallo,

I am having a problem (one of many) on our Cray CS400 computer ollie with the latest version of the cray compiler (8.7.7). This problem may have to do with a bug in the code (or in the compiler):

verification experiment advect_xz fails for optimization level > 1, i.e. with -O1 it works, but with -O2 (default), it doesn’t, and the model explodes in temperature, so that the run "stops due to EXTREME Pot. Temp”. Some numerical instability develops around grid points (i,k) = ( 2, 16) in timestep 2. The two optimization level imply a lot of smaller differences, but I think it has to do with the scalar optimization level (when it move from “conservative” for -O1 to moderate” for -O2)

in data we have:
tempAdvection = 42,
saltAdvection = 81,
Interestingly, this problem is the same for tempAdvection = 40, 41, 50, 51, 52 but goes away when I change  saltAdvection to a non-prather-scheme (i.e. with saltAdvection=80, the problem is still there, but e.g. 2, 77, 30, 33 makes it go away), also any combination of saltAdvection=81 (or 80) with anything, but the ppm/pqm schemes (40,41,42,50,51,52), seems to work.
In summary, the combination of prather advection with ppm/pqm schemes does not work for default optimization.

I think
1. there shouldn’t be any interaction between the schemes
2. if there’s something wrong with the code (memory access, floating point operations), this interaction should not depend on the (scalar) optimization levels

Do you have any thoughts on this? Any comment is appreciated. I just don’t know what to do here. I could just stop investigating this and wait for the next compiler ...

Martin