[ncl-talk] Lanczos Filtering

Tue Dec 5 09:14:08 MST 2017

Dear NCL users,I am very new on NCL and I am trying to filter my rainfall data using Lanczos Filter (bandpass 10-90 day filtering). My question concerns the processing of the weights. I would like to know 
1-is there any relationship between the number of weights to choose and the cutoff frequencies?2-is there any practical criterion to choose the number of weights? I know that this number influences the loss of data.Below is my code:
----------------------------------------------------------------------------------------------------------------------------------------------------------------------;import the file
gpcp=addfile("gpcp_Sahel.nc","r")
rr=gpcp->precip
;filter parameters
ihp=2 ;bandpass
sigma=1.0 ; Lanczos sigma
nWgt    = 81  ; number of weights -> means 40 data lost at the beginning and 40 data lost at the end.                      
fca     = 1./90.            ; start freq ;highpass= 90 days
fcb     = 1./10.                        ; last  freq; lowpass= 10 days
wgt     = filwgts_lanczos (nWgt, ihp, fca, fcb, sigma )
printVarSummary(wgt)
xBPF    = wgt_runave_n_Wrap ( rr, wgt, 0, 0)      ; 10-90 day bp performed across time dimension(0) of rr (meaning the time dimension)
 ---------------------------------------------------------------------------------------------------------------------------------------------------------------------


Cheers,

GUIGMA
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