beam_profile_overlap: Calculate overlap between a vertical profile ('vp') and the vertical radiation profile emitted by the radar

Description

Calculates the distribution overlap between a vertical profile ('vp') and the vertical radiation profile of a set of emitted radar beams at various elevation angles as given by beam_profile.

Usage

beam_profile_overlap(
  vp,
  elev,
  distance,
  antenna,
  zlim = c(0, 4000),
  noise_floor = -Inf,
  noise_floor_ref_range = 1,
  steps = 500,
  quantity = "dens",
  normalize = TRUE,
  beam_angle = 1,
  k = 4/3,
  lat,
  re = 6378,
  rp = 6357
)

Value

A data.frame with columns distance and overlap.

Arguments

vp: A vp object.
elev: Numeric vector. Beam elevation(s), in degrees.
distance: Numeric. The distance(s) from the radar along sea level (down range) for which to calculate the overlap, in m.
antenna: Numeric. Radar antenna height, in m. Default to antenna height in vp.
zlim: Numeric vector of length two. Altitude range, in m
noise_floor: Numeric. The system noise floor in dBZ. The total system noise expressed as the reflectivity factor it would represent at a distance noise_floor_ref_range from the radar. NOT YET IMPLEMENTED
noise_floor_ref_range: Numeric. The reference distance from the radar at which noise_floor is expressed. NOT YET IMPLEMENTED.
steps: Numeric. Number of integration steps over altitude range zlim, defining altitude grid size used for numeric integration.
quantity: Character. Profile quantity (dens or eta) to use for the altitude distribution.
normalize: Logical. If TRUE, normalize the radiation coverage pattern over the altitude range specified by zlim.
beam_angle: Numeric. Beam opening angle in degrees, typically the angle between the half-power (-3 dB) points of the main lobe.
k: Numeric. Standard refraction coefficient.
lat: Numeric. Radar latitude. Defaults to latitude in vp.
re: Numeric. Earth equatorial radius, in km.
rp: Numeric. Earth polar radius, in km.

Details

This function also calculates the overlap quantity in the output of integrate_to_ppi.

Overlap is calculated as the Bhattacharyya coefficient (i.e. distribution overlap) between the (normalized) vertical profile (vp) and the (normalized) radiation coverage pattern as calculated by beam_profile(). In the calculation of this overlap metric, NA and NaN values in the profile quantity specified by quantity are replaced with zeros.

The current implementation does not (yet) take into account the system noise floor when calculating the overlap.

In the ODIM data model the attribute /how/NEZ or /how/NEZH specifies the system noise floor (the Noise Equivalent Z or noise equivalent reflectivity factor. the H refers to the horizontal channel of a dual-polarization radar). In addition, the attribute /how/LOG gives "security distance above mean noise level (dB) threshold value". This is equivalent to the log receiver signal-to-noise ratio, i.e. the dB above the noise floor for the signal processor to report a valid reflectivity value. We recommend using NEZH + LOG for noise_floor, as this is the effective noise floor of the system below which no data will be reported by the radar signal processor.

Typical values are NEZH = -45 to -50 dBZ at 1 km from the radar. LOG is typically around 1 dB.

Need to evaluate beam by beam the returned signal relative to a uniform beam filling of at least NEZH + LOG If returned signal is lower, the gate is below noise level.

Examples

Run this code

# Read the polar volume example file
pvolfile <- system.file("extdata", "volume.h5", package = "bioRad")

# Read the corresponding vertical profile example
pvol <- read_pvolfile(pvolfile)

# let us use this example vertical profile:
data(example_vp)
example_vp

# Calculate overlap between vertical profile of birds and the vertical
# radiation profile emitted by the radar
bpo <- beam_profile_overlap(
  example_vp,
  get_elevation_angles(pvol), seq(0, 100000, 1000)
)

# Plot the calculated overlap:
plot(bpo)