The FFTPACK5 is an open source library of Fast Fourier
Transforms.  The download and reference pages are
available at:
http://www2.cisl.ucar.edu/docs/fftpack5


1) What does WRF use from the package

The WRF code makes use of only of a few of the routines:

The Stochastic Backscatter scheme:
CFFT1I
CFFT1B

The Spectral Nudging option:
RFFTMI
RFFTMF
RFFTMB

The polar filtering capabaility for a global domain:
DFFT1I
DFFT1F
DFFT1B