# [ncl-talk] vertical wavenumber FFT

Xi Chang xi.chang01 at gmail.com
Thu Feb 26 08:52:48 MST 2015

```Thank you Dennis,

Maybe a quick question for this: Has anyone ever computed vertical
wavenumber from data which has vertical level in pressure instead of
geometric altitude? I know that we can just simply do FFT or 2DFTT in
vertical direction but how to deal with vertical pressure level and to
interpolate them with equally same space (every 1 km) ? I still cant find a
way to deal with this.

Chang

On Thu, Feb 26, 2015 at 3:58 PM, Dennis Shea <shea at ucar.edu> wrote:

> Not sure what information you have available.
>
> [1] not geometric altitude but if you have temperature and surface
> geopotential, you could use the hydrostatic equation.
>      https://www.ncl.ucar.edu/Document/Functions/Built-in/hydro.shtml
>
> [2] then interpolate (int2p_n) the results of [1] to fixed geopotential
> height
>      https://www.ncl.ucar.edu/Document/Functions/Built-in/int2p_n.shtml
>
> ===
> Else ... maybe ...
>
> https://www.ncl.ucar.edu/Document/Functions/Built-in/stdatmus_p2tdz.shtml
>
> On Thu, Feb 19, 2015 at 1:37 PM, Xi Chang <xi.chang01 at gmail.com> wrote:
>
>> Hi Gehne, and NCL users,
>>
>> Thanks for the hints, I knew this function, but I have following question
>> prior of using the ezfft:
>>
>> 1. How can I convert the vertical level in my data (which is in pressure)
>> to altitude (geometry) in order to get the vertical wavenumber with units
>> km? I dont have any idea for this using ncl.
>>
>> 2.  do I need to interpolat the vertical level (km) to equally space (per
>> 1 km each) ?
>>
>> 3. Is there any function or hints that you may purpose if I want to get
>> the spectrum as a function as vertical wavenumber and frequency ?
>>
>> Thank you for your experience and hints.
>>
>> Dr. Chang.
>>
>> On Tue, Feb 17, 2015 at 5:08 PM, Maria Gehne - NOAA Affiliate <
>> maria.gehne at noaa.gov> wrote:
>>
>>> Hi Chang,
>>>
>>> the function ezfftf should do that.
>>>
>>> https://www.ncl.ucar.edu/Document/Functions/Built-in/ezfftf.shtml
>>>
>>> It works for multidimensional arrays as well as one dimensional arrays,
>>> for more than one dimension you need to reorder so that the pressure levels
>>> are the right most dimension. So if x = (time,lev,lat,lon) you may need to
>>> transform y = x(time,lat,lon,lev).
>>>
>>> As with all FFT applications you probably need to taper the values you
>>> are trying to transform so that the end points are zero and you are
>>> essentially transforming a periodic sequence.
>>>
>>> Hope this helps,
>>>
>>> Maria
>>>
>>> On Sun, Feb 15, 2015 at 3:35 PM, Xi Chang <xi.chang01 at gmail.com> wrote:
>>>
>>>> Hallo NCL,
>>>>
>>>> is there any trick or function in ncl to decompose the data in pressure
>>>> level to vertical wavenumber (m) based on FFT filter?
>>>>
>>>> Thank you
>>>> Chang
>>>>
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>>
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