[ncl-talk] Fourier decompose
Gus Correa
gus at ldeo.columbia.edu
Fri Feb 22 15:31:55 MST 2019
Just follow the very clear script outline that Dennis sent to you.
And where he wrote:
>> ; do something with coef *<== *YOUR* responsibility*
take responsibility and do it.
Maybe, create a 4D array of dimensions (time, 2, k_lon, k_lat), (or if you
prefer (2,time,k_lon,k_lat) ),
where k_lon, k_lat are the corresponding wavenumber index dimension sizes,
and for each time record,
save the output of fft2df in the corresponding (time, :, :, :) array
section/slice.
You may want to read the fft2df documentation to understand what you're
doing, and create the array
to store what the function returns (complex numbers, separated in real and
imaginary parts) with the correct size:
http://www.ncl.ucar.edu/Document/Functions/Built-in/fft2df.shtml
On Fri, Feb 22, 2019 at 3:37 PM Soma Roy <somaroy892 at gmail.com> wrote:
> Then how can I proceed?
> Please give me a hint..
>
> Soma
>
> On Sat, Feb 23, 2019, 00:47 Gus Correa <gus at ldeo.columbia.edu wrote:
>
>> If what you want is the *spatial* 2D Fourier Transform
>> (" fourier decomposition ... over lat lon", as stated in your original
>> email),
>> i.e., if you're interested in the 2D wavenmuber components (not in the
>> time frequencies)
>> you have to do it separately for each time record, as Dennis explained.
>>
>> You can apply the 2D Fourier Transform function to the 2D array formed by
>> the reshaped
>> lat,lon coordinates (into a 1D array) concatenated over the time
>> dimension,
>> because all you need to apply a 2D Fourier transform is a 2D numerical
>> array,
>> regardless of the meaning of the array components.
>> However, the result won't be the " fourier decomposition ... over lat
>> lon" that you said you want.
>>
>> I hope this helps,
>> Gus Correa
>>
>>
>>
>> On Fri, Feb 22, 2019 at 1:24 PM Soma Roy <somaroy892 at gmail.com> wrote:
>>
>>> Best idea will be use nlat*nlon as total grid points using reshape and
>>> reorder the data so that input to the fft2df will be 2 dimensional (ngrids,
>>> ntim).
>>>
>>> Coef will then give you output as 2 x ngrids x (ntim/2+1).
>>>
>>> Soma
>>>
>>> On Fri, Feb 22, 2019, 21:32 Soma Roy <somaroy892 at gmail.com wrote:
>>>
>>>> Glad to know.
>>>> Many thanks Dennis.
>>>>
>>>> Soma
>>>>
>>>> On Fri, Feb 22, 2019, 21:09 Dennis Shea <shea at ucar.edu wrote:
>>>>
>>>>> *Please do not send direct emails to people. Only to ncl-talk.*
>>>>>
>>>>> If "X" is the data then how can we use it?
>>>>>
>>>>> X (ntime, nlat, nlon).
>>>>>
>>>>> Output result matrix should be same as X.
>>>>> ==============
>>>>>
>>>>> *As noted in the documentation, *the function is prototyped as
>>>>> 2-dimensional
>>>>>
>>>>> *function fft2df ( x [*][*])*
>>>>>
>>>>> As I said, you must use a do loop. It is user (your) responsibility to
>>>>> do any book keepind [savinh]
>>>>>
>>>>> ntim = 8
>>>>> nlat = 6
>>>>> mlon = 8
>>>>> x = *random_normal* <http://www.ncl.ucar.edu/Document/Functions/Built-in/random_normal.shtml>( 10, 5, (/ntim,nlat,mlon/) )
>>>>> xNew = new( *dimsizes*(x), "float")
>>>>>
>>>>> do nt=0,ntim-1
>>>>> ; ANALYSIS
>>>>> coef = *fft2df* (x(nt,:,:_) ; *coef: [2] x [6] x [5] ; [2] x [nlat] x [mlon/2+1]*
>>>>>
>>>>> ; do something with coef *<== *YOUR* responsibility*
>>>>>
>>>>> xNew(nt,:,:) = *fft2db* <http://www.ncl.ucar.edu/Document/Functions/Built-in/fft2db.shtml> (coef) ; SYNTHESIS
>>>>>
>>>>> end do
>>>>>
>>>>> Please do not use this if you do not know how to use 2D FFT output.
>>>>> ncl-talk can not become your programmer.
>>>>>
>>>>> Again, as noted in the function documentation, a readable introduction to 2D-FFT is at:
>>>>> *http://www.dspguide.com/pdfbook.htm* <http://www.dspguide.com/pdfbook.htm>
>>>>> ===
>>>>> As noted in the documentation, NCL uses* FFTPACK5 <https://www2.cisl.ucar.edu/resources/legacy/fft5> *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 <http://www2.cisl.ucar.edu/resources/legacy/fft5/documentation#rfft2f.html>
>>>>>
>>>>> fft2db:
>>>>> http://www2.cisl.ucar.edu/resources/legacy/fft5/documentation#rfft2b.html <http://www2.cisl.ucar.edu/resources/legacy/fft5/documentation#rfft2b.html>
>>>>> *
>>>>>
>>>>>
>>>>> On Fri, Feb 22, 2019 at 8:06 AM Soma Roy <somaroy892 at gmail.com> wrote:
>>>>>
>>>>>> If "X" is the data then how can we use it?
>>>>>>
>>>>>> X (ntime, nlat, nlon).
>>>>>>
>>>>>> Output result matrix should be same as X.
>>>>>>
>>>>>> Soma
>>>>>>
>>>>>>
>>>>>> On Fri, Feb 22, 2019, 20:24 Dennis Shea <shea at ucar.edu wrote:
>>>>>>
>>>>>>> Please look at NCL's function documentation
>>>>>>>
>>>>>>>
>>>>>>> *http://www.ncl.ucar.edu/Document/Functions/Built-in/fft2df.shtml
>>>>>>> <http://www.ncl.ucar.edu/Document/Functions/Built-in/fft2df.shtml>
>>>>>>> <http://goog_952811495>*
>>>>>>> *http://www.ncl.ucar.edu/Document/Functions/Built-in/fft2db.shtml*
>>>>>>> <http://www.ncl.ucar.edu/Document/Functions/Built-in/fft2db.shtml>
>>>>>>>
>>>>>>> You will have to loop over each time step.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Fri, Feb 22, 2019 at 12:09 AM Soma Roy <somaroy892 at gmail.com>
>>>>>>> wrote:
>>>>>>>
>>>>>>>> Hello,
>>>>>>>> I have a time series data (ntime, nlat, nlon) in spatial domain. I
>>>>>>>> want to calculate fourier decomposition of the same over lat lon.
>>>>>>>> How to do it using ncl? Is there any function available for the
>>>>>>>> above?
>>>>>>>>
>>>>>>>> Kindly inform.
>>>>>>>>
>>>>>>>> Thank you,
>>>>>>>> Soma
>>>>>>>> _______________________________________________
>>>>>>>> ncl-talk mailing list
>>>>>>>> ncl-talk at ucar.edu
>>>>>>>> List instructions, subscriber options, unsubscribe:
>>>>>>>> http://mailman.ucar.edu/mailman/listinfo/ncl-talk
>>>>>>>>
>>>>>>> _______________________________________________
>>> ncl-talk mailing list
>>> ncl-talk at ucar.edu
>>> List instructions, subscriber options, unsubscribe:
>>> http://mailman.ucar.edu/mailman/listinfo/ncl-talk
>>>
>> _______________________________________________
>> ncl-talk mailing list
>> ncl-talk at ucar.edu
>> List instructions, subscriber options, unsubscribe:
>> http://mailman.ucar.edu/mailman/listinfo/ncl-talk
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.ucar.edu/pipermail/ncl-talk/attachments/20190222/004066eb/attachment.html>
More information about the ncl-talk
mailing list