[ncl-talk] question regarding Butterworth filter

Dave Allured - NOAA Affiliate dave.allured at noaa.gov
Mon Jun 15 17:43:40 MDT 2015


The documentation for this filter says that it is optimized for narrow
bandwidths.  The description goes on to mention certain shortcuts in its
calculations.  Your settings describe a low pass filter with a very wide
pass band, far to one end of the signals in your time series.  This is not
what is meant by "narrow bandwidth".

This is just speculation, but I think your settings may be breaking key
assumptions within this filter's code implementation.  You may have chosen
a filter function that is badly matched to your particular problem.
Perhaps, look at some of the other available NCL functions.  HTH.


On Mon, Jun 15, 2015 at 11:51 AM, Daniel Barandiaran <dbarandiaran at gmail.com
> wrote:

> Hi everyone,
> I was trying out the bw_bandpass_filter function and found some unexpected
> results. The gist of the problem is that the end of the time series being
> filtered approaches a smoothed zero, even if the time series is doing
> something else entirely (see attached figure, consider bold blue line and
> shaded blue area). I imagine this is due to the internal workings of the
> filter itself, but regardless of that, does anyone have any ideas on
> "fixing" this?
> Some details: I have fca set to 1/60., or 5 years, and fcb set arbitrarily
> large, i.e. 1/100000., so the filter essentially behaves as low pass. I am
> also making sure to not give the function any missing values.
> --
> Danny Barandiaran
> Ph.D. Candidate
> Department of Plants, Soils, and Climate
> Utah State University
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