[gradsusr] difference between field and single point values for the same defined expression

[Helio Camargo]

I´m performing the following expressions over time 1 of a 50 ensemble
member run:

'set t 1 last'
'set e 1 last'
'define ens = tpratesfc.1 '

1) 'define smens = sum(ens,e=1,e=50)'
2) 'define 'smensmsk =  maskout(sum(ens,e=1,e=50),sum(ens,e=1,e=50)-0)'
3) 'define 'smensmskcnst
= const(maskout(sum(ens,e=1,e=50),sum(ens,e=1,e=50)-0),0,-u)'

'set t 1'

For some reason, for t=1, expression 1, yelds small negative values
(smens.gif, attached). Expressions 2 and 3 should set undef to these
negative values and set 0 for these undef grid points respectively. Looking
at the figures smensmsk.gif and smensmskcnst.gif, these operations seem to
work perfectly

And setting lon/lat to a point where the above operations should influence,
is consistent with the figures above:
Lon = 298.442  Lat = -28.8304

*ga-> d smens*
*Result value = -8.33652e-19*

*ga-> d smensmsk*
*Result value = -9.99e+08*

*ga-> d smensmskcnst*
*Result value = 0*

** differences also show expected result*
*> operation 2 - operation 1*
ga-> d smensmsk-smens
Result value = -9.99e+08

*> operation 3 - operation 2*
ga-> d smensmskcnst-smensmsk
Result value = -9.99e+08

*> operation 3 - operation 1*
ga-> d smensmskcnst-ens
Result value = 1.9073e-20

However, making FIELD  diferences between defined expression 3 and defined
expression 1 results "Constant Field=0" (diff_smensmskcnstsmens.gif).

Can anyone explain why this is happening?

Thanks in advance for your patience.
--
Helio
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