Re: how to choose the number of weight nwt

> But there are some mistake because i use a 241-weight
> 20-200 Lanczos filter

> if anyone can tell me how to filter out it in using NCL
> build-in function or has any other method for the band-pass filter

The built-in function is filwgts_lancos().
<http://www.ncl.ucar.edu/Document/Functions/Built-in/
filwgts_lancos.shtml>
There are a lot of discussion on time filter in the ncl-talk archive.
<http://www.ncl.ucar.edu/Support/ncl_talk.shtml>

Assume you have long daily data,

dt = 1. ; data intervals (day)
nwgt = 241 ; num of weights
fca = dt/200. ; cut off freq
fcb = dt/20. ; cut off freq fca < fcb < fnq = 0.5 (Niquist frequency)
nsigma = 1 ; sigma factor, commonly 1

w = filwgts_lancos(nwgt,2,fca,ncb,1)

will give you the weights.

In fact, I never used this function but should give correct weights.
(The above could be wrong.)
I wrote similar functions without knowing its existence.

; --- Off topic ---
; BTW, why Lancos rather than Lanzcos?
; Six letters reminds me of the limitation of f77.
; --- end of off topic ---

If you are not familiar with time filtering,
I recommend to take a look at the reference
pointed on the manual page.

Duchon (1979) J. Appl. Meteor.

and the lecture notes by Prof. Hartmann (Lecture 522).
<http://www.atmos.washington.edu/~dennis/>

-----
Dr Takeshi Enomoto
eno_at_jamstec.go.jp
Earth Simulator Center, JAMSTEC

_______________________________________________
ncl-talk mailing list
ncl-talk_at_ucar.edu
http://mailman.ucar.edu/mailman/listinfo/ncl-talk
Received on Tue Mar 27 2007 - 18:08:51 MDT