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# kf_filter

Extract equatorial waves by filtering in the Wheeler-Kiladis wavenumber-frequency domain.

## Prototype

```load "\$NCARG_ROOT/lib/ncarg/nclscripts/contrib/kf_filter.ncl"

function kf_filter (
x      [*][*] : numeric,
obsPerDay [1] : integer,
tMin      [1] : numeric,
tMax      [1] : numeric,
kMin      [1] : numeric,
kMax      [1] : numeric,
hMin      [1] : numeric,
hMax      [1] : numeric,
waveName  [1] : string
)

return_val [dimsizes(x)] :  float or double
```

## Arguments

x

Data to be filtered. Time should be the left dimension and longitude should be the right dimension. For example: x(time,lon).

obsPerDay

Number of time steps per 24 hour period.

tMin
tMax

Range of cut-off periods (must be positive).

kMin
kMax

Range of wavenumbers, can be positive (eastward) or negative (westward).

hMin
hMax

Range of equivalent depths to use.

waveName

Name of dispersion curve to use. The following names are recognized: "ER", "Kelvin", "MRG", "IG0", "IG1", "IG2" (IG0 and MRG are the same). Specifically:

1. ER: Equatorial Rossby Wave
2. Kelvin: Kelvin Wave
3. MRG: Mixed Rossby-Gravity
4. IG0: Inertia-Gravity [n=0]
5. IG1: Inertia-Gravity [n=1]
6. IG2: Inertia-Gravity [n=2]

## Description

At each point, the data are space-time bandpass filtered following Wheeler and Kiladis (1999). The data are first detrended with dtrend and tapered with taper in time, and then they are filtered using 2-dimensional FFT (fft2df, fft2db). The filter bounds can simply be rectangular (as in Wheeler and Kiladis's MJO filter), or they can bounded by the dispersion curves of the shallow water equatorial waves. At this time, other filter shapes (e.g., the TD filter from Kiladis et al. 2006) are not supported.

The input data should be periodic (global) in longitude. In addition, filtered anomalies near the temporal ends of the dataset should generally be ignored. The longer the periods filtered for, the more data should be ignored at the ends.

References:
Wheeler, M., and G. N. Kiladis, 1999: Convectively coupled equatorial waves: Analysis of clouds and temperature in the wavenumber frequency domain. J. Atmos. Sci., 56, 374-399.

Kiladis, G. N., C. D. Thorncroft, and N. M. J. Hall, 2006: Three-dimensional structure and dynamics of African easterly waves. Part I: Observations. J. Atmos. Sci., 63, 2212-2230.

Acknowledgement: This function was donated by Carl Schreck, III [SUNY at Albany].