WMOD00399

• Title: WMO Training Workshop on Weather Modification for Meteorologists - Lecture Notes
• Date: 12/1/1979
• Weather Modification - Doc Type: Report

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This expiession differs from the preceding one only by the multiplier

1
21 n 2 .

For
the
the

a single spherical scatterer, whose dimensions are
wavelength (f:: 0.1 ). ), the back-scattering cross
sphere's diameter Do by the relationship:
5 2 6
iT =., I K I Do
).
= n - iK) ~s the

small in comparison with
section is connected with

m 2 _ 1
where K = 2 ' and m (
m + 2

This relationship is called Rayleigh's law" The constant characterizing the

refraction depends on temperat~re, wavelength and particle composition. For wave-

lengths > 1 cm and for the whole tempereJture range in the atmosphere

I K ,2::::: 0.93 for water and 0.21 for iCEl.

2.13 For a combination of spherical pa~ticles which are small by comparison
with the wavelength the mean power of the received signal is

sphere's combined refractive index.

p Pt G2e'~h1l"3 II< I 2 D.6
r - - . L
~6 4 Sin (2).2T2) i ~
L
where i ~ designates, as above, the summation over a unit volume.

2.14 Thus, for spherical scatterers which are small compared with the
wavelength, the mean power of the received signal is determined by the radar's
parameters, the distance to them and only two multipliers which depend on the
scatterers themselves,

IK I 2

" D.6
and 'l:' ~

Because of the importance of the second multiplier in meteorological radar a new
value Z is introduced, called the radar reflectivity, and can be determined as
follows: .

~
where ~ designates the summation over a unit volume, and N (D) is the number
of scatterers in the diameter range D to D + d D in the unit volume. N (D)
designates the raindrop size distribution and for snow crystals the drop diameter
distribution obtained after melting. (If such a determination is not made for
snow, the snow's density within the crystals must be taken into account in the
multiplier I K I 2 .)

z = ~ Di 6 = f N ( D )
~

D 6 d D