# band_pass_latlon_time

Create band-pass filtered series at each lat/lon grid point.

*Available in version 5.1.0 and later.*

## Prototype

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl" ; These three libraries areautomaticallyload "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl" ;loadedfrom NCL V6.2.0 onward. load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl" ; No need for user to explicitly load. load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/diagnostics_cam.ncl" ; Still need to load this library. function band_pass_latlon_time ( x [*][*][*] : numeric, srate [1] : numeric, bpf [3] : integer, opt [1] : logical ) return_val : array the same size and shape asx

## Arguments

*x*

A three-dimensional variable array. Nominally: (time,lat,lon).

*srate*

Sampling rate. For example: if *srate* refers to "samples per day",
then, *srate* is one of the following [1,2,4,8,12,24].
These correspond to [24,12,6,3,2,1] hour sampling rates.
Other common sampling units are "samples per year" and "samples per month".

*bpf*

An integer array of length 3. The first two elements contain the band pass start and end times. The last element is the number of weights to be used.

*opt*

If *opt*=False, the function will operate under default mode
regardless of any attributes associated with the variable.

If *opt*=True, then attributes associated with *opt*
may be used to alter the default behavior.

*opt*@*detrend*=True: Series will be detrended. Default:*opt*@*detrend*=False.*opt*@*fft*=True: An FFT will be used to perform the filtering. The FFT coefficients will be weighted by the response function returned by**filwgts_lanczos**.Default:

*opt*@*fft*=False.If

*opt*@*fft*=True, then a taper will be applied to the series prior to the FFT. The taper amount can be customized by setting*opt*@*taper*. Default is 0.10 (10%).

## Return value

The values at each grid point will have had the specified band pass filter applied.

## Description

At each time step, the input time series at each grid point
is band-pass filtered using weighted running averages or,
optionally, via an FFT. If the FFT option is activated,
the FFT coefficients will be weighted by the response function
returned by **filwgts_lanczos**.

The user must be aware that the appropriate number of filter weights is a direct function of the band pass period. The shorter the period, the more weights that will be required to sufficiently resolve the desired period.

The input array, *x*, should have sufficient
'temporal padding' so that the filter will have its
desired result over the period of interest. For example, if daily mean
data, *srate*=1, are being analyzed and *bpf(2)*=201, then the
input array should have *bpf(2)*/2 days padding before
and after the time period of focus.

FYI: The US-CLIVAR MJO working group:

http://www.usclivar.org/mjo.phphas suggested a number of diagnostics for evaluating the MJO ( Madden-Julian Oscillation). See their diagnostics website:

http://climate.snu.ac.kr/mjo_diagnostics/index.htmThis inclues the statement: "Intraseasonal (20-100 day) bandpass filtered anomalies are constructed using a 201-point Lanczos filter, which has half power points at 20 day and 100 day periods."

Also,

"No windowing/tapering or de-trending was applied in the calculation of these spectra, since sensitivity tests indicated their application had a negligible impact on the results"

## See Also

**band_pass_latlon_time_cam** (not yet developed),

**band_pass_area_time**,

**band_pass_area_time_plot**,

**band_pass_hovmueller**,

**band_pass_hovmueller_plot**,

**filwgts_lanczos**,

**wgt_areaave**,

**wgt_areaave_Wrap**,

**wkSpaceTime**,

**wkSpaceTime_cam**

## Examples

The following example focuses on the Madden-Julian
Oscillation [MJO]. However, this function is **general
purpose** and can be applied to **any** spatial
time series. It can be used to focus on other
periods of interest.

**Example 1**

Consider daily "olr". Extract information using the 20-to-100 day band pass period and 201 weights as suggested by the WMO CLIVAR Working Group.

; These libraries automatically loaded from NCL 6.2.1 onward ; load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl" ; load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl" ; load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl" ; load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/diagnostics_cam.ncl" f =The edited output (currently) looks like:addfile("...", "r") x = f->OLR ; (time,lat,lon) , ntim=1461 latS = -20. ; select region latN = 20. lonL = 0. ; GM ; =60 Indian Ocean lonR = 360. ; ; =280 East Pacific srate = 1 ; daily means bpf = (/20,100,201/) ; MJO ... suggested by MJO WG [WMO] opt = False ; pass in area of interest bpass =band_pass_latlon_time(x(:,{latS:latN},{lonL:lonR}) \ ,srate, bpf, opt)printVarSummary( bpass )printMinMax( bpass, True)

Variable: bpass Type: float Number of Dimensions: 3 Dimensions and sizes: [time | 1461] x [lat | 17] x [lon | 144] Coordinates: time: [17479080..17514120] lat: [-20..20] lon: [ 0..357.5] Number Of Attributes: 17 band_pass_Nwgts : 201 band_pass_last : 100 band_pass_start : 20