RADAR STORM OBSERVATION 
these distances are considerable even with respect to 
the heights of thunderstorms. 
In the determination of rainfall intensity by radar, 
it must be assumed that the reflectivity per unit vol- 
TasiE I. Beam WiptH Between HAuF-Powsrr Pornts 
(in ft) 
Angular Width of Beam 
Range (miles) 
Oa oj 4° 
5 320 920 1,840 
25 1,610 4,600 9,210 
50 3,230 9,210 18,400 
100 6,450 18,400 36,900 
ume is constant i the volume of space contained by 
the limits of the beam and the pulse length. Because 
of the beam width of existing radar equipment, this con- 
dition can rarely be fulfilled except very close to the 
radar where the lmear distance across the beam is 
small. Even then, reasonably uniform raimfall condi- 
tions must exist. 
Distance of the Storm, R. This term is included in the 
equation to provide for range attenuation. If the storm 
completely fills the beam, range attenuation increases 
with the square of the distance. If the target consists 
of a pomt, such as an airplane, range attenuation in- 
creases with the fourth power of the range. Most in- 
vestigators, in making rainfall intensity measurements 
by radar, endeavor to make their observations as close 
as possible to the radar for two basic reasons: (1) to in- 
sure complete beam filling by relatively uniform rain or 
snow, and (2) to reduce range attenuation, thus 1n- 
creasing the signal strength of the received echo [6]. 
OBSERVATIONAL VERIFICATION OF RADAR 
STORM-DETECTION THEORY 
Exact verification of the theory (or of equation (1)) 
is very difficult because of several factors. The value of 
P, must be measured by averaging the power of very 
weak signals (about 10 to 10-2 w) which are fluctuat- 
ing rapidly. Moreover, the precipitation echo signal is 
not received continuously from a given point in the 
storm but at intervals equal to the transmitted pulse 
repetition frequency. Techniques have been developed 
to measure P,, however, and they will be discussed in 
the section on signal analysis. A more fundamental 
difficulty lies in the problem of determining the number 
and size of the drops within the illuminated portion of 
the beam in order to calculate the reflectivity. This 
must necessarily be a sampling process which may be 
performed either on the surface or aloft [65, pp. 70-76]. 
Surface sampling, while easier to accomplish, may not 
be representative of conditions aloft in the atmosphere 
where the radar beam may be directed. 
One greatly desired meteorological use of radar is the 
determination of rainfall intensity over a wide area, or 
at various points distant from the radar. It is evident 
from the theory that the only radar factors closely re- 
lated to rainfall intensity are the reflectivity per unit 
volume of the storm and the attenuation [9]. Reflec- 
1269 
tivity 1s generally the more useful. Establishing the 
relationship, however, is a formidable undertaking, re- 
quiring assumed drop-size distribution, particle shapes 
and spatial distributions, as well as power measure- 
ments of weak, rapidly fluctuating signals. Several in- 
vestigators [32, 34, 42] have reported empirical relation- 
ships between precipitation intensity and echo-signal 
power, but only at limited ranges and under certain 
specific conditions of precipitation. 
OBSERVATION OF WEATHER PHENOMENA ON 
RADAR SCOPES 
Types of Radar Scopes. Radar targets are usually pre- 
sented visually by means of cathode ray tubes or 
scopes. There are about a dozen methods of presenta- 
tion, depending upon the type and use of the radar, 
but only four or five of these are of more than casual 
interest to the meteorologist for storm-detection pur- 
poses [49, pp. 160-175]. A brief discussion of these will 
be given to assist the reader in understanding later de- 
scriptions of radar storm displays. 
The A Scope. This scope is the simplest and most 
widely used display; its appearance is illustrated in 
Fig. 2. From it the operator learns several things about 
AIRCRAFT & 
NEARBY ECHOES PRECIPITATION 
aati Dene ECHOES 
wal A i 
SIGNAL 
INTENSITY 
—> 
le) 25 50 75 
RANGE (MILES) 
Fie. 2.—Diagram illustrating the appearance of A scope of 
SCR 615B (S-band) radar, showing characteristics of various 
types of echo signals. Range information is virtually useless 
without elevation and azimuth angle information, neither of 
which is shown on this type of presentation. 
a target: its straight-line distance from the radar, the 
approximate intensity of its echo signal, and not in- 
frequently its nature. Target echo signals are usually 
displayed as vertical deflections from a horizontal base 
line. The amount of vertical deflection is proportional 
to the power of the echo signal. The distance of the 
target from the radar is indicated by the relative posi- 
tion of the deflection from one end of the base line, 
usually the left. There are usually several strong echoes 
present at close range at all azimuths; these result from 
the presence of buildings or uneven terrain in the im- 
mediate vicinity of the radar. These echo signals are 
sometimes called “ground clutter.” 
The A scope is also useful for target identification. 
An experienced operator can detect differences in echo- 
signal characteristics which are sufficient to inform him 
whether the target is an aircraft, ship, buildings or ter- 
rain, or precipitation. Precipitation gives a very dis- 
tinctive echo signal because of its rapidly fluctuating 
character which is caused by the changing interference 
pattern established by the precipitation from pulse to 
pulse. 
