[ncl-talk] bandpass filter issue

Tue Oct 6 12:58:03 MDT 2015

Hi Chang,

I would probably try a larger frequency window for the bandpass filter. As
you were saying, the narrow bandpass can induce artificial signals in your
data. The other thing that could be causing issues is the type of window
you use when you filter in Fourier space. If you simply keep the Fourier
coefficients you are interested in and set all others to zero you will see
some evidence of the Gibbs phenomenon in your filtered data. In general it
is better to use a window that is equal to 1 at the frequencies you want to
keep and then goes to zero over a range of frequencies. The squared cosine
usually works well for me. The same holds for filtering in wavenumber!

If you use a larger frequency range and are not convinced that what you are
seeing are the waves you are interested in, I would suggest doing and EOF
analysis on your filtered data to extract the mode with the most variance
in the filtered data. In this case it would be better to start with a much
larger range of frequencies.

Hope this helps,
Maria

On Tue, Oct 6, 2015 at 10:39 AM, Xi Chang <xi.chang01 at gmail.com> wrote:

> Hi Maria,
>
> Yes, i actually want to analyze 16-day planetary waves  (k=1) waves using
> daily data. But in this case, i dont need to decompose it into eastward and
> westward propgating waves. I tried to bandpass filter the data, but since
> the frequency bands are too narrow (1/15) and (1/17).. the results are bad.
> any suggestion? does it mean that i need higher temporal resolution?
> the steps that i do are:
> 1. filter data using forward and backward fourier to retain k=1
> 2. bandpass this filtered data to retain only freq at 1/16day
>
> Chang
>
>
> On Tuesday, 6 October 2015, Maria Gehne - NOAA Affiliate <
> maria.gehne at noaa.gov> wrote:
>
>> Can you explain in more detail what you are trying to do? Are you trying
>> to only look at phenomena that have a period of 16 days?
>>
>> Maria
>>
>> On Tue, Oct 6, 2015 at 12:31 AM, Xi Chang <xi.chang01 at gmail.com> wrote:
>>
>>> Thanks for the reply. No i dont use it, since what i need is how to
>>> extract/filter the daily data having frequency 1/16 day. is it possible to
>>> only use bandpass filter?
>>>
>>>
>>>
>>>
>>> On Tuesday, 6 October 2015, Dennis Shea <shea at ucar.edu> wrote:
>>>
>>>> Are you looking a 'complex demodulation" ???
>>>>
>>>> If so, that is in the 6.3.1 release:
>>>>   http://www.ncl.ucar.edu/future_release.shtml
>>>>
>>>> Click 'demod_cmplx'  =>
>>>> http://www.ncl.ucar.edu/Document/Functions/Contributed/demod_cmplx.shtml
>>>>
>>>> See Examples at the bottom of
>>>> http://www.ncl.ucar.edu/Applications/spec.shtml
>>>>
>>>>
>>>> On Mon, Oct 5, 2015 at 4:32 PM, Xi Chang <xi.chang01 at gmail.com> wrote:
>>>> > Hi NCL,
>>>> >
>>>> > I have daily data 10 years. I'd like to filter the data at frequency
>>>> (1/16)
>>>> > day. To do so what the most appropiate  frquency ranges that i have
>>>> to use
>>>> > ?, is it domething like fa=1/15 and fb=1/17 ?
>>>> > I aware if the band is too narrow the filtered signals are fake
>>>> /artificial
>>>> > (i tesed with ncl butterwooth and the filtered results are weird) ?
>>>> is there
>>>> > any idea how to do that appropriately? do i need to use hourly data
>>>> instead
>>>> > of daily?
>>>> >
>>>> > Thanks
>>>> >
>>>> > _______________________________________________
>>>> > 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
>>>
>>>
> On Tuesday, 6 October 2015, Maria Gehne - NOAA Affiliate <
> maria.gehne at noaa.gov> wrote:
>
>> Can you explain in more detail what you are trying to do? Are you trying
>> to only look at phenomena that have a period of 16 days?
>>
>> Maria
>>
>> On Tue, Oct 6, 2015 at 12:31 AM, Xi Chang <xi.chang01 at gmail.com> wrote:
>>
>>> Thanks for the reply. No i dont use it, since what i need is how to
>>> extract/filter the daily data having frequency 1/16 day. is it possible to
>>> only use bandpass filter?
>>>
>>>
>>>
>>>
>>> On Tuesday, 6 October 2015, Dennis Shea <shea at ucar.edu> wrote:
>>>
>>>> Are you looking a 'complex demodulation" ???
>>>>
>>>> If so, that is in the 6.3.1 release:
>>>>   http://www.ncl.ucar.edu/future_release.shtml
>>>>
>>>> Click 'demod_cmplx'  =>
>>>> http://www.ncl.ucar.edu/Document/Functions/Contributed/demod_cmplx.shtml
>>>>
>>>> See Examples at the bottom of
>>>> http://www.ncl.ucar.edu/Applications/spec.shtml
>>>>
>>>>
>>>> On Mon, Oct 5, 2015 at 4:32 PM, Xi Chang <xi.chang01 at gmail.com> wrote:
>>>> > Hi NCL,
>>>> >
>>>> > I have daily data 10 years. I'd like to filter the data at frequency
>>>> (1/16)
>>>> > day. To do so what the most appropiate  frquency ranges that i have
>>>> to use
>>>> > ?, is it domething like fa=1/15 and fb=1/17 ?
>>>> > I aware if the band is too narrow the filtered signals are fake
>>>> /artificial
>>>> > (i tesed with ncl butterwooth and the filtered results are weird) ?
>>>> is there
>>>> > any idea how to do that appropriately? do i need to use hourly data
>>>> instead
>>>> > of daily?
>>>> >
>>>> > Thanks
>>>> >
>>>> > _______________________________________________
>>>> > 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
>>>
>>>
>>
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