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band_pass_area_time

Create a time series of area averages; band-pass filter the resulting area-averaged time series and calculate other statistics.

Available in version 5.1.0 and later.

Prototype

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"      ; These three libraries are automatically
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl"       ; loaded from 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"

	function band_pass_area_time (
		x [*][*][*] : numeric,  
		srate   [1] : numeric,  
		bpf     [3] : integer,  
		wgty    [*] : numeric,  
		opt     [1] : logical   
	)

	return_val  :  array [3][dimsizes(time)] 

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.

wgty

A one-dimensional array containing the latitudinal weights.

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@Nrun: If opt=True and this attribute is present, then this attribute will specify the number of values to be used in the running average of variance. This should be in units of days and it should be an odd number. The function will actually use srate*opt@Nrun in the running average. Default: 101

Return value

The return variable will contain time series from three different operations:

  1. the 0-th component will contain the band pass filtered areal averages.
  2. the 1-th component will contain the raw areal averages.
  3. the 2-th component will contain running variances [101 days by default].
The following attributes will be associated with the returned variable.
band_pass_start=bpf(0)
band_pass_last=bpf(1)
band_pass_Nwgts=bpf(2)
var_0, var_1, var_2= string identifiers

Description

At each time step, weighted areal averages are computed using wgt_areaave. The resulting time series is band-pass filtered using Lanczos weights (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 (samples per day), are being analyzed and bpf(2)=201, then the input array should have 100 (=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.php
has suggested a number of diagnostics for evaluating the MJO ( Madden-Julian Oscillation). See their diagnostics website:
     http://climate.snu.ac.kr/mjo_diagnostics/index.htm
This 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_area_time_cam (not yet developed),
band_pass_area_time_plot,
band_pass_latlon_time,
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 MJO (Madden-Julian Oscillation) information using the 20-to-100 period and 201 band pass weights as suggested by the WMO CLIVAR Working Group.

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     = addfile("...", "r") 
  x     = f->OLR                 ; (time,lat,lon) , ntim=1461

  latS  =  -5.              ; select region
  latN  =   5.
  lonL  =  60.              ; Indian Ocean
  lonR  = 280.              ; East Pacific

  srate = 1                 ; daily means
  bpf   = (/20,100,201/)    ; MJO ... suggested by MJO WG [WMO] 
  wgty  = latRegWgt(x&lat, "double", 0) 
  opt   = False
                            ; pass in area of interest
  mjo_area = band_pass_area_time (x(:,{latS:latN},{lonL:lonR} \
                                  ,srate, bpf, wgty({latS:latN}), opt)

  printVarSummary( mjo_area )
  printMinMax( mjo_area, True)
The edited output (currently) looks like:
       Variable: mjo_area
       Number of Dimensions: 1
       Dimensions and sizes:  [var | 3] x [time | 1461]
       Coordinates:
                  time: [17479080..17514120]
       Number Of Attributes: 18
        units :       W/m2
        long_name :   filtered: weighted area avg: Daily OLR
        band_pass_start :     20
        band_pass_last :      100
        band_pass_Nwgts :     201
        var_0:                band pass
        var_1:                raw areal means
        var_2:                local variances
       
        filtered: weighted area avg: Daily OLR: min=-6.58817   max=5.37212