# [ncl-talk] vertical wavenumber FFT

Dennis Shea shea at ucar.edu
Thu Feb 26 07:58:55 MST 2015

```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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