[ncl-talk] Filter, filwgts_lanczos

[Maria Gehne - NOAA Affiliate]

Hi Ed,

if you look at the filter weights example on the NCL website:
http://www.ncl.ucar.edu/Document/Functions/Built-in/filwgts_lanczos.shtml

you can see an example of how to compute the time fca value. You can do
exactly the same for space.

time:

dt = 3 days per time step
            t1 = 50 days  (low frequency cutoff, expressed in time domain)
            t2 = 10 days  (high frequency cutoff, expressed in time domain)

            fca = dt/t1 = 3./50. = 0.06 time steps  (low frequency cutoff)
            fcb = dt/t2 = 3./10. = 0.30 time steps  (high frequency cutoff)


space:

dx = 100m,  x1= 500m (high frequency cutoff), fca = dx/x1 = 100/500= 1/5 = 0.2


Hope this helps,


Maria



On Tue, Oct 27, 2015 at 3:42 PM, yang yang <yang.yang816 at gmail.com> wrote:

> Hi Dennis:
>
> Thanks for your help!
>
> *In *filwgts_lanczos  a parameter, *fca* should be given. That
> is a scalar indicating the cut-off frequency of the ideal high or low-pass
> filter: (0.0 < *fca* < 0.5). In a 1-D spatial data, I want to filter
> out small scale or wavelength waves. For my case, I think that fca should
> be wavelength or wave numbers, instead of frequency. Suppose I have 300
> data with regular 100 m spatial interval. I would to remove
> wavelength smaller than 500 m. How to set fca to do the filtering? Thanks!
>
> Regards
> Ed
>
> On Wed, Oct 28, 2015 at 10:20 AM, Dennis Shea <shea at ucar.edu> wrote:
>
>> A filter has no idea whether the data is temporal or spatial. Most
>> filters do assume constant spacing. If your spatial data is
>> irregularly spaced, you will likely have to interpolate to equal
>> spacing.
>>
>> On Tue, Oct 27, 2015 at 2:51 PM, yang yang <yang.yang816 at gmail.com>
>> wrote:
>> > Dear Sir/Madam:
>> >
>> > In NCL, examples of Filters using filwgts_lanczos  are only in temporal
>> > domain. Now I want to do low-pass for my 1-D spatial data such as
>> dropsonde
>> > data or flight-track data. Does NCL have a similar function as
>> > filwgts_lanczos to do the work for 1-D spatial data? or How to apply
>> > filwgts_lanczos in a 1-D spatial data? Thanks!
>> >
>> > Regards
>> > Ed
>> >
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