WMOD00399

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

- 3:5 -

2.3 The target area for PEP is expected to be of the order of 104 km2, with
the t~tal experimental area, which will include control areas being approximately
5xlO 4km 2. The required raingauge network density has not yet been determined, but
preliminary estimates indicate that about 50 instruments will be needed for the total
area. The problem is treated in detail by Woodley (commencing p. of this report)
and the final requirements for PEP will be determined during the third phase of the
site selection process.

2.4 In view of the above, the radar precipitation measurement programme
should be one of the basic observational tasks of PEP.

2.5 Below we shall consider the main principles for radar measurement of
areal precipitation.

2.6 One of the main relationships on which the radar method for precipitation
measurement is based is the radar equation which relates the power of the received
signal with the target's back-scattering cross ~ection. Consider first the radar
return from a point target. If the emission from the radar has a pulse power Pt ,
and is isotropic, it will fall over a small area At at a distance r from the radar
with a power of:

p =
IT

Pt At
4rr2

2.7 The radar antenna is del.igned to focus the energy into a narrow beam
which increases the power of the emission falling on the area in comparison to the
isotropic case to

I~ 0" =

G Pt At

4rr2

where G is a dimensionless multiplier called the antenna ga1n factor along the axis
of the beam pattern.

2.8 If the target scatters the received emission isotropically then the
power returning to the antenna, with an effective area Ae, would be equal to

= --

=

G Pt At Ae
(4rr2)2

Pr

P IT Ae

4,rr2

2.9 The antenna gain factor is connected with its aperture by the
relationship

4 r Ae

G

:A 2

from which it follows that

Pr

G2 :Aa
= Pt (47r~4

. At