vad_fit: Velocity Azimuth Display
In paocorrales/rvad: Velocity Azimuth Display of Meteorological Radars

Description Usage Arguments Details Value See Also Examples

View source: R/VAD.R

Approximates the horizontal components of the wind from radial wind measured by Doppler radar using the Velocity Azimuth Display method from Browning and Wexler (1968).

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vad_fit(radial_wind, azimuth, range, elevation, max_na = 0.2,
  max_consecutive_na = 30, r2_min = 0.8, outlier_threshold = Inf,
  azimuth_origin = 90, azimuth_direction = c("cw", "ccw"))

`radial_wind`	a vector containing the radial wind.
`azimuth`	a vector of length = length(radial_wind) containing the azimuthal angle of every radial_wind observation in degrees clockwise from 12 o' clock.
`range`	a vector of length = length(radial_wind) containing the range (in meters) asociate to the observation.
`elevation`	a vector of length = length(radial_wind) with the elevation angle of every observation in degrees.
`max_na`	maximum percentage of missing data in a single ring (defined as the date in every range and elevation angle).
`max_consecutive_na`	maximun angular gap for a single ring.
`r2_min`	minimum r squared permitted in each fit.
`outlier_threshold`	threshhold for removing outliers in standard deviation units
`azimuth_origin`	angle that represents the zero azimuth in degrees counterclockwise from the x axis.
`azimuth_direction`	direction of the azimuth angle.

The algorithm can work with sigle volume of data scanned in PPI (Plan Position Indicator) mode. The radial wind must not have aliasing. Removing the noise and other artifacts is desirable.

vad_fit() takes vectors of the same length with radial wind, azimuth angle, range and elevation angle and computes a sinusoidal fit for each ring of data (the observation for a particular range and elevation) before doing a simple quality control.

First, it checks if the amount of missing data (must be explicit on the data frame) is greater than max_na, by default a ring with more than 20 data is descarted. Second, rejects any ring with a gap greater than max_consecutive_na. Following Matejka y Srivastava (1991) the default is set as 30 degrees. After the fit, the algorithm rejects rings whose fit has a r2 less than r2_min. It is recommended to define this threshold after exploring the result with r2_min = 0.

Rings that fail any of the above-mentioned checks return NA.

A data frame with class rvad_vad that has a plot() method and contains 7 variables:

height: height above the radar in meters.
u: zonal wind in m/s.
v: meridional wind in m/s.
range: distance to the radar in meters.
elevation: elevation angle in degrees.
r2: r squared of the fit.
rmse: root mean squeared error calculated as the standar deviation of the residuals.