Re: About Lanczos filter in NCL

For the wavenumber filtering, you can use the function kf_filter:
http://www.ncl.ucar.edu/Document/Functions/User_contributed/kf_filter.shtml

That function will filter in both longitude and time. However, you will
still run into the problem of n/2 being the maximum period for which you
can filter. Without knowing which paper you're referring to, I can't know
how they did it. One possibility is to pad the data with climatology (zero
anomalies) and filter the result. See Wheeler & Weickmann (2001) for
details:
http://journals.ametsoc.org/doi/abs/10.1175/1520-0493(2001)129%3C2677:RTMAPO%3E2.0.CO%3B2

   Carl

On Thu, Sep 6, 2012 at 6:55 PM, waqar younas <vickyqau@gmail.com> wrote:

> Hi guys,
> I am trying to filter 20-90 days variabilities from the forecast data of
> GEM model in NCL (version 6.1.0). This data consists of 90 days forecast. I
> am trying to use the MJO-Clivar codes on forecast data (these codes are set
> for observation which is continuous for year, 365 days). My main problem is
> that if i want to filter the 20-90 days variabilities from 90 days, the
> maximum period i can get is 45 days (statistically the maximum range is n/2
> if you n days). But i found a paper, in which the author did the same thing
> but using FORTRAN. He also mentioned that Fourier filter is applied to
> these 90 days of data, which is wave number 1-5. Now i do not know how to
> control these wave numbers in the filter? Moreover, the lanczos filter is
> set for observed data which have 365 days so that wgt for lanczos filter is
> 201. When i use it for 90 days, it says the size of wgt cannot be greater
> than 90 so i use 90 instead of 201. The filter still running but i do not
> know what is significance of wgt = 90 instead of 201? Is it still retaining
> the 20-90 days variabilities?
> Thanks.
>
> --
> Best Regards
>
> Waqar Younas
> PhD Candidate
> Natural Resources and Environmental Studies (NRES)
> University of Northern British Columbia (UNBC)
> Canada
>
>
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>

-- 
Carl J. Schreck III, PhD
Postdoctoral Research Associate
Cooperative Institute for Climate and Satellites (CICS-NC)
North Carolina State University and NOAA's National Climatic Data Center
151 Patton Avenue
Asheville, NC 28801
Tel: 828-257-3140
carl@cicsnc.org
http://monitor.cicsnc.org/mjo/
http://www.atmos.albany.edu/student/carl/

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Received on Fri Sep 7 06:16:56 2012