<div dir="ltr">Hi Chang,<div><br></div><div>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!</div><div><br></div><div>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.</div><div><br></div><div>Hope this helps,</div><div>Maria</div></div><div class="gmail_extra"><br><div class="gmail_quote">On Tue, Oct 6, 2015 at 10:39 AM, Xi Chang <span dir="ltr"><<a href="mailto:xi.chang01@gmail.com" target="_blank">xi.chang01@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><font size="2"><span style="background-color:rgba(255,255,255,0)">Hi Maria,</span></font><div><font size="2"><span style="background-color:rgba(255,255,255,0)"><br></span></font></div><div><font size="2"><span style="background-color:rgba(255,255,255,0)">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?</span></font></div><font size="2"><span style="background-color:rgba(255,255,255,0)">the steps that i do are:</span></font><div><font size="2"><span style="background-color:rgba(255,255,255,0)">1. filter data using forward and backward fourier to retain k=1</span></font></div><div><font size="2"><span style="background-color:rgba(255,255,255,0)">2. bandpass this filtered data to retain only freq at 1/16day</span></font></div><span class="HOEnZb"><font color="#888888"><div><font size="2"><span style="background-color:rgba(255,255,255,0)"><br></span></font></div></font></span><div><span class="HOEnZb"><font color="#888888"><font size="2"><span style="background-color:rgba(255,255,255,0)">Chang<br></span></font></font></span><div><div class="h5"><div><font size="2"><span style="background-color:rgba(255,255,255,0)"><br><br>On Tuesday, 6 October 2015, Maria Gehne - NOAA Affiliate <<a href="mailto:maria.gehne@noaa.gov" target="_blank">maria.gehne@noaa.gov</a>> wrote:<br></span></font><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div dir="ltr"><font size="2"><span style="background-color:rgba(255,255,255,0)">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?</span></font><div><font size="2"><span style="background-color:rgba(255,255,255,0)"><br></span></font></div><div><font size="2"><span style="background-color:rgba(255,255,255,0)">Maria</span></font></div></div><div class="gmail_extra"><font size="2"><span style="background-color:rgba(255,255,255,0)"><br></span></font><div class="gmail_quote"><font size="2"><span style="background-color:rgba(255,255,255,0)">On Tue, Oct 6, 2015 at 12:31 AM, Xi Chang <span dir="ltr"><<a>xi.chang01@gmail.com</a>></span> wrote:<br></span></font><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><font size="2"><span style="background-color:rgba(255,255,255,0)">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?</span></font><div><div><font size="2"><span style="background-color:rgba(255,255,255,0)"><br></span></font></div><div><font size="2"><span style="background-color:rgba(255,255,255,0)"><br></span></font></div><div><font size="2"><span style="background-color:rgba(255,255,255,0)"><br><br>On Tuesday, 6 October 2015, Dennis Shea <<a>shea@ucar.edu</a>> wrote:<br></span></font><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><font size="2"><span style="background-color:rgba(255,255,255,0)">Are you looking a 'complex demodulation" ???<br><br>If so, that is in the 6.3.1 release:<br> <a href="http://www.ncl.ucar.edu/future_release.shtml" target="_blank">http://www.ncl.ucar.edu/future_release.shtml</a><br><br>Click 'demod_cmplx' =><br><a href="http://www.ncl.ucar.edu/Document/Functions/Contributed/demod_cmplx.shtml" target="_blank">http://www.ncl.ucar.edu/Document/Functions/Contributed/demod_cmplx.shtml</a><br><br>See Examples at the bottom of <a href="http://www.ncl.ucar.edu/Applications/spec.shtml" target="_blank">http://www.ncl.ucar.edu/Applications/spec.shtml</a><br><br><br>On Mon, Oct 5, 2015 at 4:32 PM, Xi Chang <<a>xi.chang01@gmail.com</a>> wrote:<br>> Hi NCL,<br>><br>> I have daily data 10 years. I'd like to filter the data at frequency (1/16)<br>> day. To do so what the most appropiate frquency ranges that i have to use<br>> ?, is it domething like fa=1/15 and fb=1/17 ?<br>> I aware if the band is too narrow the filtered signals are fake /artificial<br>> (i tesed with ncl butterwooth and the filtered results are weird) ? is there<br>> any idea how to do that appropriately? do i need to use hourly data instead<br>> of daily?<br>><br>> Thanks<br>><br>> _______________________________________________<br>> ncl-talk mailing list<br>> <a>ncl-talk@ucar.edu</a><br>> List instructions, subscriber options, unsubscribe:<br>> <a href="http://mailman.ucar.edu/mailman/listinfo/ncl-talk" target="_blank">http://mailman.ucar.edu/mailman/listinfo/ncl-talk</a><br>><br></span></font></blockquote></div></div><font size="2"><span style="background-color:rgba(255,255,255,0)"><br>_______________________________________________<br>ncl-talk mailing list<br><a>ncl-talk@ucar.edu</a><br>List instructions, subscriber options, unsubscribe:<br><a href="http://mailman.ucar.edu/mailman/listinfo/ncl-talk" rel="noreferrer" target="_blank">http://mailman.ucar.edu/mailman/listinfo/ncl-talk</a><br></span></font><br></blockquote></div></div></blockquote></div></div></div></div><div class="HOEnZb"><div class="h5"><br>On Tuesday, 6 October 2015, Maria Gehne - NOAA Affiliate <<a href="mailto:maria.gehne@noaa.gov" target="_blank">maria.gehne@noaa.gov</a>> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">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?<div><br></div><div>Maria</div></div><div class="gmail_extra"><br><div class="gmail_quote">On Tue, Oct 6, 2015 at 12:31 AM, Xi Chang <span dir="ltr"><<a>xi.chang01@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">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?<div><div><div><br></div><div><br></div><div><br><br>On Tuesday, 6 October 2015, Dennis Shea <<a>shea@ucar.edu</a>> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Are you looking a 'complex demodulation" ???<br>
<br>
If so, that is in the 6.3.1 release:<br>
<a href="http://www.ncl.ucar.edu/future_release.shtml" target="_blank">http://www.ncl.ucar.edu/future_release.shtml</a><br>
<br>
Click 'demod_cmplx' =><br>
<a href="http://www.ncl.ucar.edu/Document/Functions/Contributed/demod_cmplx.shtml" target="_blank">http://www.ncl.ucar.edu/Document/Functions/Contributed/demod_cmplx.shtml</a><br>
<br>
See Examples at the bottom of <a href="http://www.ncl.ucar.edu/Applications/spec.shtml" target="_blank">http://www.ncl.ucar.edu/Applications/spec.shtml</a><br>
<br>
<br>
On Mon, Oct 5, 2015 at 4:32 PM, Xi Chang <<a>xi.chang01@gmail.com</a>> wrote:<br>
> Hi NCL,<br>
><br>
> I have daily data 10 years. I'd like to filter the data at frequency (1/16)<br>
> day. To do so what the most appropiate frquency ranges that i have to use<br>
> ?, is it domething like fa=1/15 and fb=1/17 ?<br>
> I aware if the band is too narrow the filtered signals are fake /artificial<br>
> (i tesed with ncl butterwooth and the filtered results are weird) ? is there<br>
> any idea how to do that appropriately? do i need to use hourly data instead<br>
> of daily?<br>
><br>
> Thanks<br>
><br>
> _______________________________________________<br>
> ncl-talk mailing list<br>
> <a>ncl-talk@ucar.edu</a><br>
> List instructions, subscriber options, unsubscribe:<br>
> <a href="http://mailman.ucar.edu/mailman/listinfo/ncl-talk" target="_blank">http://mailman.ucar.edu/mailman/listinfo/ncl-talk</a><br>
><br>
</blockquote></div>
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