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<DIV>Thanks Dave and Dennis for your help and suggestions. I've got the data
file I need. I'll compare the butterworth filter with the Lanczos low pass
filter carefully and choose an appropriate one.<BR></DIV>
<DIV><BR>Best wishes<BR>Shouli Xuan</DIV>
<DIV> </DIV>
<DIV style="FONT-SIZE: 10pt; FONT-FAMILY: Verdana; COLOR: #c0c0c0"
align=left>2019-01-14
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<DIV><STRONG>发件人：</STRONG>Dave Allured - NOAA Affiliate
<dave.allured@noaa.gov></DIV>
<DIV><STRONG>发送时间：</STRONG>2019-01-14 04:33</DIV>
<DIV><STRONG>主题：</STRONG>Re: [ncl-talk] A request for data file and problem on
"bw_bandpass_filter" function</DIV>
<DIV><STRONG>收件人：</STRONG>"Shouli Xuan"<15295538792@163.com></DIV>
<DIV><STRONG>抄送：</STRONG>"ncl-talk"<ncl-talk@ucar.edu></DIV>
<DIV> </DIV>
<DIV>
<DIV dir=ltr>
<DIV dir=ltr>Shouli Xuan,<BR><BR>Documentation for bw_bandpass_filter says it
is optimized for narrow bandwidths. Low pass is far outside the intended
purpose of this function. I looked at the reference paper (David
Russell, 2005) and the source code. It looks like this function may be
unstable if you try to use it as a low pass filter. Therefore you should
find alternate code for low pass Butterworth filter, or else use a different
filter.</DIV>
<DIV dir=ltr><BR></DIV>
<DIV>The source code applies fca and fcb in a symmetric way. This is why
exchanging them gives the same results. However, this is completely
accidental, and outside the specifications for this function. In general
it is a mistake to use function parameters outside of their intended
constraints <SPAN
style="FONT-SIZE: 13px; FONT-FAMILY: verdana,sans-serif; COLOR: rgb(51,51,51)">(fcb
> fca in this case).</SPAN></DIV>
<DIV><BR></DIV>
<DIV>I want to reinforce Dennis's suggestion. Compare any special filter
code with the NCL Lanczos low pass filter. Check the filter responses in
both time and frequency domains, as shown in examples filters_8.ncl and
filters_9.ncl.</DIV>
<DIV><BR></DIV>
<DIV>--Dave</DIV>
<DIV dir=ltr><BR></DIV>
<DIV dir=ltr><BR>
<DIV class=gmail_quote>
<DIV dir=ltr>On Sun, Jan 13, 2019 at 11:46 AM Dennis Shea <<A
href="mailto:shea@ucar.edu">shea@ucar.edu</A>> wrote:<BR></DIV>
<BLOCKQUOTE class=gmail_quote
style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: rgb(204,204,204) 1px solid">
<DIV dir=ltr>
<DIV dir=ltr>
<DIV>[1] DATA: The years have changed but the files are available
at:<BR></DIV>
<DIV><BR></DIV>
<DIV>
<DIV dir=ltr>
<DIV dir=ltr>
<DIV>ftp <A href="http://ftp.cgd.ucar.edu"
target=_blank>ftp.cgd.ucar.edu</A></DIV>
<DIV>anonymous</DIV>
<DIV>your_email</DIV>
<DIV>cd pub/shea/MJO</DIV>
<DIV>mget
*
.<BR></DIV>
<DIV>quit</DIV>
<DIV>==========</DIV>
<DIV>
<DIV><BR></DIV>
<DIV>Note: The file names have changed slightly from those used in the
examples. For example:<BR></DIV>
<DIV><PRE><A href="http://olr.day.anomalies.1980-2005.nc" target=_blank>olr.day.anomalies.1980-2005.nc</A> ==> <A href="http://olr.day.anomalies.1980-2011.nc" target=_blank>olr.day.anomalies.1980-2011.nc</A></PRE></DIV></DIV></DIV>
<DIV>Perhaps, some minor changes in the graphic resources will be needed.
<BR></DIV>
<DIV dir=ltr>==========</DIV>
<DIV dir=ltr>/ftp/pub/shea/MJO<BR>-rw-r--r-- 1 shea cgdcas 458055500 Jan 11
10:08 <A href="http://MODEL.SST.HAD187001-198110.OI198111-201703.nc"
target=_blank>MODEL.SST.HAD187001-198110.OI198111-201703.nc</A><BR>-rw-r--r--
1 shea cgdcas 30699392 Jan 11 10:18 <A
href="http://OLR.12hr_2yrs.wheeler.nc"
target=_blank>OLR.12hr_2yrs.wheeler.nc</A><BR>-rw-r--r-- 1 shea cgdcas
858482293 Jan 11 09:40 olr.day.anomalies.1979-2011.nc.gz<BR>-rw-r--r-- 1
shea cgdcas 304335976 Jan 11 10:10 <A href="http://olr.day.mean.nc"
target=_blank>olr.day.mean.nc</A><BR>-rw-r--r-- 1 shea cgdcas 615513004 Jan
11 10:24 <A href="http://pregpcp19962008.daily.nc"
target=_blank>pregpcp19962008.daily.nc</A><BR>-rw-r--r-- 1 shea cgdcas
350655700 Jan 11 10:23 <A href="http://TRMM.200006-200109.thin.nc"
target=_blank>TRMM.200006-200109.thin.nc</A><BR>-rw-r--r-- 1 shea cgdcas
929887809 Jan 11 09:41 uwnd.day.anomalies.1979-2011.200.nc.gz<BR>-rw-r--r--
1 shea cgdcas 934253770 Jan 11 09:41
uwnd.day.anomalies.1979-2011.850.nc.gz<BR>-rw-r--r-- 1 shea cgdcas 939714739
Jan 11 09:41 vwnd.day.anomalies.1979-2011.200.nc.gz<BR>-rw-r--r-- 1 shea
cgdcas 939092767 Jan 11 09:41
vwnd.day.anomalies.1979-2011.850.nc.gz<BR></DIV>====</DIV>
<DIV>The *gz files will have to be unpacked:</DIV>
<DIV> %> gzip -d *nc.gz</DIV>
<DIV>====</DIV>
<DIV><BR></DIV>
<DIV>FILTERS:</DIV>
<DIV> <A href="http://www.ncl.ucar.edu/Applications/filter.shtml"
target=_blank><B>http://www.ncl.ucar.edu/Applications/filter.shtml</B></A></DIV>
<DIV><BR></DIV>
<DIV>There are several examples of Butterworth filters. <BR></DIV>
<DIV>These included comaprison with Lanczos filters.</DIV>
<DIV><BR></DIV>
<DIV>I do not know why: ".. exchange values of ca and cb .." does not change
the results.</DIV>
<DIV>I don't have time to look right now. <BR></DIV>
<DIV>Perhaps the interface between NCL and the ubderlying fortran code does
an exvhange.</DIV>
<DIV><BR></DIV>
<DIV>re: making the band pass into a low-pass filter. Pergform xome
comparisons withe a Lanczos low pass filter.</DIV>
<DIV><BR></DIV>
<DIV>Good Luck<BR></DIV>
<DIV><BR></DIV></DIV></DIV></DIV><BR>
<DIV class=gmail_quote>
<DIV dir=ltr>On Sat, Jan 12, 2019 at 8:27 PM 15295538792 <<A
href="mailto:15295538792@163.com" target=_blank>15295538792@163.com</A>>
wrote:<BR></DIV>
<BLOCKQUOTE class=gmail_quote
style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: rgb(204,204,204) 1px solid"><U></U>
<DIV
style="BORDER-LEFT-WIDTH: 0px; FONT-SIZE: 10.5pt; FONT-FAMILY: 微软雅黑; BORDER-RIGHT-WIDTH: 0px; BORDER-BOTTOM-WIDTH: 0px; COLOR: rgb(0,0,0); MARGIN: 12px; LINE-HEIGHT: 1.5; BORDER-TOP-WIDTH: 0px"><U></U>
<DIV>Dear NCL experts,</DIV>
<DIV> </DIV>
<DIV>I'm writing to ask if you could send me the data file named "<A
href="http://uwnd.day.850.anomalies.1980-2005.nc"
target=_blank>uwnd.day.850.anomalies.1980-2005.nc</A>" which is used in
the script "bfband_2.ncl". I can't find it in the online data files. In
addition, I'd like to consult you about the function "bw_bandpass_filter"
which applies a Butterworth bandpass filter to time series. The online
documentation says that the cut-off paramaters ca and cb of this function
should meet a condition of ca>cb (fcb>fca). However, when I
exchanged the values of ca and cb, the results remained the same. How
could this happen? In fact, I want to use this bandpass filter as a low
pass filter which need only one cut-off frequency. So which parameter
should I set to arbitrarily large (ca or cb)? Is there anything wrong in
my script? Please help. Thanking in anticipation. (PS: Input and
output data files as well as ncl script I used has been attached to this
email.)</DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV>Sincerely,<BR>Shouli Xuan<BR><A
href="mailto:Email%3A15295538792@163.com"
target=_blank>Email:15295538792@163.com</A></DIV>
<DIV> </DIV>
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<DIV align=left>2019-01-13</DIV>
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