RADAR STORM OBSERVATION 
By MYRON G. H. LIGDA 
Massachusetts Institute of Technology 
INTRODUCTION 
Through the use of radar for precipitation detection 
the science of meteorology has acquired an entirely 
new and unique method of weather observation [14]. 
As a result of this use the meteorologist has been pre- 
sented with graphic, dynamic, and up-to-the-second 
depictions of precipitation formations of all types, and 
these in several dimensions. Techniques for analysis of 
radar precipitation echo signals have not yet been com- 
pletely developed, and for this reason radar is presently 
of minor (but rapidly increasing) meteorological im- 
portance. It appears to have vast potentialities both in 
the fields of physical meteorological research and 
weather observation and forecasting, as well as other 
closely allied activities. 
Radar is of interest to the meteorologist in several 
different ways: (1) in the field of hydrometeor detection 
and the study of storm structure, (2) in the observation 
of winds aloft under adverse conditions [85], and (8) in 
certain regions of the world radars occasionally have 
their propagation radically modified by atmospheric 
conditions which meteorologists have been called upon 
to forecast [18, pp. 11-17]. Each of these subjects may 
be treated quite independently of the others, although 
one particular radar system may be involved im all 
three. For a discussion of the second and third of these 
subjects the reader is invited to refer to articles else- 
where in this Compendium.! 
Radar operates on the principle that radio energy is 
scattered and reflected by the dielectric gradients 
which exist at the surface of various objects which, 
when they are detected by radar, are designated as 
targets [49, pp. 1-6]. A pulse radar, as distinguished 
from other types of radar systems, consists of a power- 
ful radio transmitter which emits radio energy in short 
bursts or pulses, a highly sensitive radio receiver which 
detects the back-scattered energy, and indicators of 
various types (called scopes) which present the informa- 
tion visually for use by the operator. Radars discussed 
in this article are of the microwave pulsed type, 
microwave indicating that the operating wave length 
lies between 1 and 20 cm. 
Radar indicates the relative position of a target in 
terms of polar coordinates: elevation angle, azimuth 
or bearing from a given direction, and range. Elevation 
and azimuth angle information is obtained from the 
orientation of the antenna at the instant of detection. 
Range determination is accomplished by timing the in- 
1. Consult ‘‘Instruments and Techniques for Meteorological 
Measurements”’ by Michael Ference, Jr., pp. 1207-1222; and 
“Meteorological Aspects of Propagation Problems” by H. G. 
Booker, pp. 1290-1296. 
terval between the transmitted pulse and the received 
echo. 
Subject to certain restrictions, the information pres- 
ently available to the forecaster or physical meteorolo- 
gist by means of radar observation is as follows: 
1. “Instantaneous” location of all precipitation over 
several thousand square miles horizontally, and at all 
altitudes from a single observing station. 
2. Direction and velocity of precipitation movement. 
3. Qualitative information concerning intensity of 
precipitation. 
4. Heights of cloud bases and tops. 
5. Approximate height of the freezing level. 
6. Information as to whether or not certain storm 
are thunderstorms. 
7. Position, direction, and speed of hurricanes. 
8. Upper-level wind data to high altitudes, with 
great accuracy and under adverse conditions of visi- 
bility and precipitation. 
9. Wind shear when precipitation is falling through 
shear surfaces. 
10. Turbulence within precipitation. 
11. Distribution of fall-velocity for precipitation par- 
ticles at any level above the radar. 
12. Qualitative mformation regarding water-vapor 
and temperature distribution in the vertical. 
Not all of the information contained in the list above 
is yet obtained to a useful degree of accuracy. For 
example, rainfall intensity can be measured only in the 
immediate vicinity of the radar and even then only 
approximately. Considerable effort is being made to 
improve the accuracy of such measurements and to 
remove as many restrictions and limitations from the 
above list as is physically possible. 
THE BACKGROUND OF RADAR STORM 
DETECTION 
About 1940 it became evident to those working on 
the development of radar that it was possible to con- 
struct radar systems with operating wave lengths of 
20 em (1500 me sec!) and shorter. Before these systems 
were completely operational, Ryde [50] of the General 
Electric Laboratories Ltd. in England made a study to 
determine if the performance of such radar systems 
would be affected by precipitation. This information 
was necessary in order to determine whether aircraft 
could escape microwave radar detection by flying in 
clouds, rain, snow, or dust and sand storms. Ryde’s 
calculations indicated that microwave radars would 
receive echoes from precipitation, and the results he 
obtained concerning the relative signal strengths of 
precipitation echoes and aircraft echoes were later 
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