[ncl-talk] eof function

[Maria Gehne - NOAA Affiliate]

Very likely that's where the difference comes from. Have you checked the
Ferret documentation to see how the EOFs are normalized there? The variance
is different as well. How does Ferret compute the singular value
decomposition?

Again, have you looked at the time series? Do those have a different
amplitude?

In any case for both Ferret and NCL you should be able to reconstruct the
whole data set using the EOFs, PCs, and singular values. Have you tried
doing that?

Maria

On Fri, Sep 4, 2015 at 7:49 AM, Vanúcia Schumacher <
vanucia-schumacher at hotmail.com> wrote:

> The difference between results generated by NCL and Fortran or ferret is
> in the amplitude! Using the same data set!
> I wonder if this difference in amplitude (very low values for NCL) is due
> to normalization inserted in eofunc_wrap function, would it?
>
>
>
>
> ------------------------------
> From: maria.gehne at noaa.gov
> Date: Thu, 3 Sep 2015 16:55:09 -0600
> Subject: Re: [ncl-talk] eof function
> To: vanucia-schumacher at hotmail.com
> CC: ncl-talk at ucar.edu
>
>
> I don't think you attached the figure you meant to attach.
>
> EOFs have no fixed normalization. The value of the patterns (EOFs) and
> principal components (the time series associated with the patterns) depends
> on how you normalize. The conventions on how to normalize differ between
> applications.The only value that is fixed coming out of an EOF analysis are
> the singular values (the percentage of variance of the EOFs).
>
> Are the spatial patterns the same between Fortran, Ferret and NCL, even if
> they don't have the same amplitude? What about the time series? If both
> patterns and time series have a different amplitude maybe that's where the
> normalization is different.
>
> For NCL the website for the EOF function states:
> The returned values are *normalized* such that the sum of squares for
> each EOF pattern equals one. To *denormalize* the returned EOFs multiply
> by the square root of the associated eigenvalue (*aka*, the singular
> value).
>
>
> Check if the Fortran and Ferret EOF patterns are normalized in the same
> way, or not.
>
> Hope this helps,
> Maria
>
> On Thu, Sep 3, 2015 at 2:59 PM, Vanúcia Schumacher <
> vanucia-schumacher at hotmail.com> wrote:
>
>
> The values of EOF (output) are different, as attached (figure)
>
> ------------------------------
> From: vanucia-schumacher at hotmail.com
> To: shea at ucar.edu
> Subject: RE: [ncl-talk] eof function
> Date: Thu, 3 Sep 2015 17:57:45 -0300
>
> The values of EOF output are different, as attached (figure)
>
>
> > Date: Thu, 3 Sep 2015 14:51:19 -0600
> > Subject: Re: [ncl-talk] eof function
> > From: shea at ucar.edu
> > To: vanucia-schumacher at hotmail.com
> > CC: ncl-talk at ucar.edu
> >
> > Different normalizations by the EOFs?
> >
> > On Thu, Sep 3, 2015 at 1:44 PM, Vanúcia Schumacher
> > <vanucia-schumacher at hotmail.com> wrote:
> > > Dear NCL users,
> > >
> > > I calculated the EOF for the Pacific region ({-20:20},{140:290}) using
> the
> > > function of NCL (eofunc_Wrap) and tested with other programs (ferret
> and
> > > fortran ) and the values of EOF using the NCL are very small compared
> with
> > > the other tested outputs.
> > >
> > > Using Fortran and Ferret - values between 0.6 to -0.2
> > > Using NCL function - values between 0.14 to -0.10
> > >
> > >
> > > Could someone explain to me why this happens?
> > >
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