[ncl-talk] PSD vs. Frequency plot ...

[u.utku.turuncoglu at be.itu.edu.tr]

Hi Dennis,

Thanks for your help. Yes, i found the SPECX_ANAL script and i'll start to
implement a new one to calculate PSD.  Thanks again.

Regards,

--ufuk

>  x(t) - data values as a function of time
>     N - number of time steps
>
> f      - frequency
> df    - frequency interval , f-df/2 to f+df/2
> A(f) - periodgram estimates (FFT); amplitudes at each frequency .
>          FFTs have some normalization associated with them (eg: 1/N)
> k     - wave number
> A(k) = A(1/f)
>
> Periodogram:  P(k) = A^2(k)/2
> Total_Power = SUM[ A^2(k) ]/2
>
> Power Spectral Density (PSD) :
>                                      The density of power in the interval
> 'df'
>
> ===
> There is an undocumented function in 'shea_util.ncl.' This does everything
> the builtin function 'specx_anal' does. It illustrates the spectral
> analysis process step-by-step.
>
> function SPECX_ANAL(X[*]:numeric, iopt[1]:integer, jave[1]:integer,
> pct[1]:numeric)
>
> You can see the function via:
>
> %> less "$NCARG_ROOT/lib/ncarg/nclscripts/csm/shea_util"
>
> or, use an editor (say, vi) to extract the code and make a local function
>
>    SPECX_ANAL -> SPECX_UFUK
>
> You can then change whatever you want.
>
> ---
> I wrote a draft, then found the following and modified it a bit:
>    http://th-www.if.uj.edu.pl/zfs/gora/timeseries09/lecture02.pdf
>
> On Wed, Aug 17, 2016 at 1:36 AM, Ufuk Utku Turuncoglu (BE) <
> u.utku.turuncoglu at be.itu.edu.tr> wrote:
>
>> Hi,
>>
>> I just wonder that is there any exampleNCL script to plot time series in
>> PSD (Power Spectral Density) vs. Frequency? I know that there are some
>> NCL functions such as specx_anal but it gives variance not PSD. FFT
>> functions might be used but i am not sure. Any suggestion might help.
>>
>> Regards,
>>
>> --ufuk
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>