A.2 Reflectivity
It is convenient to say things in atmosphere reflect the radiation, but backscattering is the
accurate physical term. It is measured in Z (radar reflectivity factor) and often expressed in
dBZ (radar reflectivity).
The famous radar equation from classical optics says
Z =
Pr
2
LCK
• P is the measured average power (in Watts) of several samples at the radar
• r is the range to the bin
• L is attenuation
• C is radar (hardware) constant
• K is the refractive index and depends on the dielectric properties of the particle
For meteorologists
Re flectivity =
Watts_Re ceived × Distan ce_Squared
Attenuation × Hardware_Constan t × Rain_or_Snow_Constan t
For cloud physics, Z is a sum
ΣN
i
D
i
6
where
N
i
is the number of particles with diameter
D
i
per
unit volume in the atmosphere. That means, that one droplet with diameter 4 mm (0.16 in)
gives 4096 times as much energy as a 1 mm (0.04 in) droplet. And that we can't know if
there is one droplet of 2 mm (0.08 in) or 64 droplets of 1 mm (0.04 in).
Z varies between 0.001 and 10,000,000. To get understandable numbers, we use a decibel
scale:
dBZ = 10log
Zmm
6
mm
−3
1mm
6
mm
−3
The following figure shows typical values for various phenomena in the atmosphere. You can
see that reflectivity strength alone is not enough for target identification.
Appendix A – Basic Radar Meteorology
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