[ncl-talk] Fourier decompose
Soma Roy
somaroy892 at gmail.com
Mon Feb 25 10:48:11 MST 2019
So if we use fft2df, spatial extent will be reduced in output matrix?
I can save coef., split coef. into real and imaginary part..but that is not
the issue I am looking for.
Kindly inform me.
Soma
On Mon, Feb 25, 2019, 14:26 Soma Roy <somaroy892 at gmail.com wrote:
> As per the below;
>
> 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
>
> We can see that longitude is reduced from 8 to 5..
>
> Can we place time dimension on that place keeping latxlon as total grid points in the 2 nd dimension?
>
> Otherwise at the time of spatial plot, no.of grids will be short due to the reduction along longitude.
>
> Kindly inform me about the above issue.
>
> Let me know if I am thinking wrong in it.
>
>
> Thanking you,
>
> Soma
>
>
>
>
> On Sat, Feb 23, 2019, 04:02 Gus Correa <gus at ldeo.columbia.edu wrote:
>
>> 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
>>>>
>>> _______________________________________________
>> 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/20190225/341cfe46/attachment.html>
More information about the ncl-talk
mailing list