The filter response of filwgts_lancos?

Hi,all

   I've been using "filwgts_lancos" to do a low pass filter,

   
    nwt = 7
    fca = 0.2 ; cutoff at 5 yr period
    ihp = 0 ; use low pass
    nsigma = 1. ; "nsigma = 1. is common"
    wgt = filwgts_lancos (nwt, ihp, fca, -999., nsigma)
    wgt = wgt/sum(wgt) ; normalize wgts
    ann_5yr_filter = wgt_runave (ann_pre, wgt, 0) ; compute running ave
    jja_5yr_filter = wgt_runave (jja_pre ,wgt, 0)
   
   I also use some codes as following to do a visual examination of the filter response
     ; test wgt
     wks = gsn_open_wks ("ps","Lancos_low_pass_filter_cutoff_0.2")
     res3 = True
     res3_at_tiMainString = "ihp="+ihp+" fca="+fca+" nwt="+nwt
     plotu = gsn_csm_xy (wks,wgt_at_freq, wgt_at_resp,res3)

but the plot of the filter function puzzled me, Why the wgt_at_resp (amplitude) converge to the zero when
wgt_at_freq nearly equal to 0.4? I suggest that wgt_at_resp should be zero when wgt_at_freq equal to cutoff freq (0.2).

I have no access to the earlier reference ,
Reference: C. Duchon
J. Applied Meteorology; August,1979;
pp 1016-1022
Lancos Filtering in One and Two Dimensions

 Could anyone give me a short explainantion or send me a referece? Thanks !

Good Luck

Lin

LEO_ARIES
2007-01-12

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