Re: filters

From: Dennis Shea (shea AT XXXXXX)
Date: Tue May 25 2004 - 16:56:41 MDT

  • Next message: Chad Bahrmann: "Pres/Height vs Time"

    >
    >Does NCL have a function to run a specific filter on time series data
    >(ex: Butterworth)? For example, if I want to keep all low-frequencies
    >of 10 years or greater, how could I do that? Previously I have only used
    >wgt_runave.
    >

    NCL does not have a Butterworth filter. NCL does have a function

        http://ngwww.ucar.edu/ngdoc/ng/ref/ncl/functions/filwgts.html
        
    That will allow the user to specify low/high/band-pass frequencies
    and generate a user specified number of weights.

    The more weights ... the better the filter characteristics.
    The response functio is returned as an attribute which
    you may wish to plot.

    The returned weights may then be input to wgt_runave for application.

    ---
    

    An alternative is to perform and fft on the data and set the desired frequencies to 0.0

    This can create problems [Gibbs phenomena] due to the abrupt cut-off.

    See "Digital Filters" by Hamming

    --- Butterworth filter routine written in fortran are available and could be included in future NCL releases.

    _______________________________________________ ncl-talk mailing list ncl-talk@ucar.edu http://mailman.ucar.edu/mailman/listinfo/ncl-talk



    This archive was generated by hypermail 2b29 : Wed May 26 2004 - 09:45:37 MDT