Re: How to remove interdecadal variation by using NCL

From: Dennis Shea <shea_at_nyahnyahspammersnyahnyah>
Date: Wed, 25 Oct 2006 06:44:32 -0600 (MDT)

> I'd like to remove decadal and interdecadal variation
>(periods over 8 years) on the data through harmonic analysis.
> Is it right to use fourier_info function of NCL?
> Please let me know how to remove that variation

[1] No. It is not appropriate to use harmonic analysis.
    This assume the data are cyclic. Obviously, any
    time series is not cyclic.

[2] The classic approach is to use a filter. In the
    case you describe, it would be a low pass filter.
    The filter consists of weights which would be
    passed over the data via "wgt_runave"

[3] You can design your own filter using "filwgts_lancos"

    The filter design is a trade-off between "cut off frequency" and
    the number of weights. You did not state the sampling interval.
    Then use the weights as input to wgt_runave. If you have (say)
    51 weights, then you will "lose" 25 points at each end of the series.

[4] There is another approach: (a) taper the data; (b) forward fft;
    (c) set coefficiebts to 0.0, or weight the coefficients;
    (d) backward fft. This 'sounds' better BUT you will get
    unexpected behavior at the beginning and end of the
    reconstructed [filtered] series.

Good luck
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Received on Wed Oct 25 2006 - 06:44:32 MDT

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