[MITgcm-support] Bug in external_fields_load when using periodicExternalForcing

[Christopher L. Wolfe]

Hi all,

I appear to have found a bug in the part of external_fields_load which 
calculates interpolation weights and which records to load when using 
periodicExternalForcing. The bug arises due to round-off errors incurred 
when the variables are converted to integers using INT and NINT. It can 
cause the model to crash when it goes looking for "impossible" record 
numbers and also result in the interpolation weights being calculated 
incorrectly. As an example of what can go wrong, consider

deltaTclock = 86400 (one day)
externForcingCycle = 31536000 (one 365-day year)
externForcingPeriod = 2628000 (1/12 of a 365-day year)

For mod(myIter,365) = 10, 11, ..., 14 this gives intime0 = 13, which is 
"impossible" given that there should be only 12 records. This causes the 
model to crash since Iftm(n)-Ifprd*(intime0(n)-1) .eq. 0 when 
mod(myIter,365) = 10, thus causing the model to go looking for record 
number 13. Simply duplicating record 1 as record 13 will prevent the 
crash, but now the interpolation weights will be calculated incorrectly. 
In particular, for mod(myIter,365) = 10, 11, ..., 14 aWght < .5 when it 
should be greater than 0.5.

I've attached a diff which appears to fix these problems for the case 
above, but I haven't extensively tested it with different values of 
deltaTclock, externForcingCycle, etc. Also, I've introduced a SAVE 
statement to make it easier to figure out whether a new record should be 
loaded. This works in my case, but it will no doubt break TAF. It 
shouldn't be hard to rewrite the routine to avoid the use of the SAVE 
statement.

Cheers,
Christopher

-----------------------------------------------------------
Dr. Christopher L. Wolfe                  858-534-4560
Climate, Atmospheric Science, and Physical Oceanography
Scripps Institution of Oceanography, UCSD  clwolfe at ucsd.edu
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