# Re: cross-corelation coherence

From: Xi Chang <xi.chang01_at_nyahnyahspammersnyahnyah>
Date: Wed Oct 30 2013 - 05:29:54 MDT

Great!! Thanks Maria and Dennis!

Cheers,
Xi

On Tue, Oct 29, 2013 at 6:11 PM, Maria Gehne <mgehne@ucar.edu> wrote:

> Hi Xi,
>
> I don't think it is very difficult, maybe a little tedious (because you
> probably want this for multiple latitudes and heights). The paper gives you
> an excellent, very detailed description of how to compute the cross-spectra
> and you can use existing ncl routines for most of the calculations.
>
>
> 1) choose two latitudes and heights that you want to compare (see the
> paper you mentioned, page 921)
>
> 2) this gives you two arrays of size (nt,nx), the Z1 and Z2 in the paper
>
> 3) use ezfftf on both of these (as Dennis already mentioned). ezfftf works
> on the right-most dimension if there is more than one so you will end up
> with two arrays of size (2,nt,nk), where the last one is the number of
> zonal wavenumbers
>
> 4) choose which zonal wavenumber you want, this leaves you with 4 time
> series S1, S2, C1, C2 for the Fourier coefficients at that wavenumber
>
> 5) to get CO(tau) you can use the ncl function esccr: CO =
> esccr(S1,S2,mxlag)+ esccr(C1,C2,mxlag)
>
> 6) similarly you can use that for the quadrature as well: QD =
> esccr(S1,C2,mxlag)- esccr(C1,S2,mxlag)
>
> 7) use these to compute the coherence and phase at the different lags (as
> in equations 1c and 1d in the paper)
>
>
> Hope this helps,
> Maria
>
>
>
>
> On 10/29/2013 09:18 AM, Xi Chang wrote:
>
> I think, it's still difficultto use "ezfftf" in this case, since this
> method needs to find a coherence (based on "le Fourier coefficient")
>
> ==> let's say for k=1 --> Cohr= (CO^2 + QD^2) ^1/2
>
> is it possible to use "specxy_anal" in this case,, however I have no idea
> to retain a specific wavenumber in this case.
>
> Xi
>
>
> On Tue, Oct 29, 2013 at 3:22 PM, Dennis Shea <shea@ucar.edu> wrote:
>
>> To my knowledge, this has not been done. Pretty specialized!
>>
>> Use ezfftf to perform the Fourier analysis at each
>> time, level and latitude. This will provide the Fourier coefficients
>> [his S1(t) and C1(t)]. You specify the desired wave number, k.
>>
>> http://www.ncl.ucar.edu/Document/Functions/Built-in/ezfftf.shtml
>>
>>
>> On 10/29/13 7:13 AM, Xi Chang wrote:
>>
>>> Hallo NCL folks,
>>>
>>> Anyone here has ever used the cross-spectral correlation or cross
>>> corelation coherence (Randel 1987) using NCL? maybe you could share the
>>> main idea/function underlying this diagnostic tool?
>>>
>>> Thanks,
>>> Xi
>>>
>>>
>>>
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>
>
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>
>
> --
> Maria Gehne
> Postdoctoral Fellow
> NCAR/CGD/CAS
> P.O. Box 3000
> Boulder, CO 80307
>
>

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Received on Wed Oct 30 05:30:11 2013

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