Re: storm track bandpass filter

From: Jeffrey Yin (jyin AT XXXXXX)
Date: Mon Dec 06 2004 - 10:28:14 MST

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    Hi Shih-Yu,

    Yes, you can use the function "filwgts_lancos" to calculate weights for a
    Lanczos filter, then the function "wgt_runave" to calculate a weighted
    running average using the filter. For information on using these
    functions, see the web pages:

    http://ngwww.ucar.edu/ngdoc/ng/ref/ncl/functions/filwgts.html
    http://ngwww.ucar.edu/ngdoc/ng/ref/ncl/functions/wrunave.html

    Typically, what I do is use an 8-day high pass filter on daily mean
    variables to retain variability on approximately 2-8 day time scales for
    my storm track analysis. So, first I create the weights for a 15-point,
    8-day high pass filter with NCL code such as the following:

    befaft = 7 ; number of points to use before and after current point
    numwts = 2*befaft + 1 ; total number of weights
    ihp = 1 ; use high pass filter
    fca = 0.125 ; cutoff at 8 day period
    fcb = -999. ; don't use second cutoff
    nsigma = 1. ; "nsigma = 1. is common"
    wts = filwgts_lancos(numwts,ihp,fca,fcb,nsigma)

    Then, if I have daily 300 mb height data in the variable Z300 with
    dimensions Z300(time,lat,lon), I filter the data using code such as:

    filt = wgt_runave_Wrap(Z300(lat|:,lon|:,time|0:2*befaft+daysinmonth,wts,0)
    Z300_filt = filt(:,:,befaft:befaft+daysinmonth-1)

    noting that I need befaft=7 days before and after the days for which I
    want filtered data.

    Let me know if you have more questions.

    Jeff

    On Thu, 2 Dec 2004, Shih-Yu Lee wrote:

    > Hi,
    >
    > Is NCL has any build in function that could do bandpass filter? Is
    > anybody can tell me how to do storm track analysis? The only thing I
    > have is the geopotential height field in netcdf file. How can I do storm
    > track analysis?
    >
    > Thanks
    >
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