Re: [ferret_users] lanczos function - retaining annual cycle signal in monthly time series

[Ansley Manke <ansley.b.manke@xxxxxxxx>]

Hi -
This was a contributed function and I don't have current experience with 
it.  However the reference listed in the Ferret documentation,
Duchon, C. E., 1979: Lanczos filtering in one and two dimensions. J. of 
App. Met., 18, 1016-1022

is available on-line, here,
http://journals.ametsoc.org/doi/pdf/10.1175/1520-0450%281979%29018%3C1016%3ALFIOAT%3E2.0.CO%3B2

I have not studied the paper but I do see that there is discussion in it 
of choices for the number of weights and the cutoff frequencies. We 
would appreciate any examples or guidance that any of you may have to 
offer for using this function.

Ansley

On 9/21/2011 6:02 PM, Torben Mueller wrote:
> Dear Ferret Community,
>
> I want to use the lanczos filter to remove any signal from a time 
> series that is not the annual cycle.
> Regrettably, the documentation for the function is not very clear to 
> me, especially in which units the low and high frequency cutoff points 
> need to be given.
> My time series has monthly data and I just want to keep the annual 
> cycle. Which values should I choose for F1, F2 and N?
>
> Thanks!
>
> http://ferret.pmel.noaa.gov/Ferret/documentation/users-guide/appendix-a-external-functions/LANCZOS/