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Re: [ferret_users] difficulty comparing variables because of time axis mismatches

Hi Keith,

> I am sometimes unable to get ferret to compute TEMP[d=2]-TEMP[d=1]
> where the datasets have different time axes.

ferret can be sensitive here depending on your input files.  I usually
do one of three things, depending on the problem:

1) take out the time dimension by specifying the time step exactly
(can be different for each file),  e.g.

     let tdiff = temp[d=2,l=1] - temp[d=1,l=12]

   This is what you need in your case to compare 2 different times.

2) if your time axes are properly defined (i.e. ferret prints the
   correct date and time) you can put one variable onto the grid of the other:

     let tdiff = temp[d=2] - temp[d=1, gt=temp[d=2]]

   ferret may interpolate in time to achieve this, but it should
   properly match the dates of both files before subtracting.

3) if the time axes are really identical but ferret somehow doesn't
   recognize this, assign them forcibly, index-by-index:

     let tdiff = temp[d=2] - temp[d=1, gt=temp[d=2]@asn]

   No interpolation is done, index N is subtracted from index N.

In your case the variable MOC is messing things up because of the axis
order. if I only take out the temp variable with

ncks -v TEMP A.nc At.nc

I can read the resulting file with ferret 6.8.4 without errors.  Does
your model data really begin on january 1 of the years 52 AC and 102
AC?  If not, you may have a mistake in your time axis definition in
your model.

Kind regards,
      Hein Zelle


Dr. Hein Zelle
Senior consultant meteorology & oceanography

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