Performs a two-dimensional discrete backward Fourier transform (Fourier synthesis).
function fft2db ( coef [*][*] : numeric )
A three dimensional array containing the real and imaginary parts. Generally, this would be the array created by fft2df.
input: coef(2,:,:) where real part: coef(0,:,:) imaginary part: coef(1,:,:)
This function computes the discrete backward Fourier transform of a real periodic array. This transform is also known as Fourier synthesis, transforming from spectral to physical space.
A call to fft2df followed by a call to fft2db (or vice-versa) reproduces the original array within roundoff error.
Missing values are not allowed. For efficiency reasons, no checking for missing values [i.e., _FillValue] is performed. Hence, if missing values are present, the results will be erroneous.
An old but but very readable reference is:
Peter Bloomfield Fourier analysis of time series : An introduction New York : John Wiley and Sons , 1976NCL uses FFTPACK5 developed by Paul Swarztrauber and Richard Valent. The specific Fortran subroutines used are:
fft2df: http://www2.cisl.ucar.edu/resources/legacy/fft5/documentation#rfft2f.html fft2db: http://www2.cisl.ucar.edu/resources/legacy/fft5/documentation#rfft2b.htmlPlease note that Fortran is column major while NCL (like C/C++) is row-major.