Estimates of Raindrop Collection 
Efficiencies in Electrified Clouds 
C. B. Moore anp B. VonNEGUT 
Arthur D. Little, Inc. 
Abstract—Observations of thunderstorms 
, Cambridge, Mass. 
from the summit of Mt. Withington, New 
Mexico, with a sensitive 3 cm radar indicate heavy rainfalls from electrified clouds a 
very short time after the initial detection of a radar echo. Estimates of the drop collec- 
tion efficiency necessary to fit the observed 
depending on the asumed liquid water conte 
time sequence give values of 200 to 500% 
nt of the cloud. These deduced values for 
raindrops in electrified clouds are 4 to 10 times greater than the mean collection efficien- 
cies determined by Kinzer and Cobb for drops in the absence of an electric field. It is 
suggested that electrification im clouds may 
greatly enhance the accretion and coales- 
cence processes and thus can be a causative factor in the formation of precipitation. 
INTRODUCTION 
3.2 
During the summer of 1957 a sensitive 3.2-cm 
RAI radar was operated on the summit of Mt. 
Withington, New Mexico, in conjunction with 
a meteorological and atmospheric-electrical ob- 
servatory. The primary purpose of the study was 
to relate the time and location of the first pre- 
cipitation formed to the development of electri- 
cal activity in the stationary clouds growing over 
the mountain. It was found that in general the 
potential gradient within the cloud reversed and 
increased greatly in the negative direction sev- 
eral minutes prior to the appearance of the first 
radar echo. These results have been reported in 
detail elsewhere (Moore and others, 1958, Von- 
negut and others, 1958; 1959). The subject of 
this paper is the short time interval between the 
appearance of a precipitation echo overhead 
and the collection of large raindrops at the 
mountain summit. 
EXPERIMENT 
We designed an experiment to extend the ear- 
her work of Reynolds and Neill [1955] and Rey- 
nolds and Brook [1956] aimed at determining 
the initial precedence of organized electrification 
and precipitation in thunderstorms. We did this 
by increasing the sensitivity of the radar detec- 
tion of precipitation and by making the poten- 
tial gradient measurements in the cloud as close 
as possible to the electrical activity. 
Radar—The primary components of the APQ- 
13A radar we used were made available to us by 
Marx Brook and were those that he and Rey- 
nolds had used in their earlier work. We in- 
creased the sensitivity of the radar for the de- 
291 
tection of initial precipitation by (1) bringing 
the equipment much nearer to the cloud by in- 
stalling it on a mountain summit beneath the 
cloud being studied, (2) changing the antenna 
mount to give a zenith sean, and (3) carefully 
adjusting the radar components. New, low-noise 
1N23E crystals were used so that a measured 
receiver sensitivity of —1l05dbm was attained 
repeatedly. TR tubes with a recovery time of 4 
microsee or less (to within 3db of full sensitivity) 
were used so that precipitation could be detected 
at ranges as small as 2 or 3 km. 
During the summer of 1957 we used the stand- 
ard 76-em (30-inch) diameter parabolic antenna 
furnished with the set, modified only to permit a 
zenith scan. This scan took 8 see and was pre- 
sented as a range-height indication. In addition 
the plane of the scan was rotated slowly about a 
vertical axis, completing one-half revolution 
every 2 min. This arrangement permitted radar 
Inspection of the hemisphere overhead once 
every 2 min. 
In 1958, we replaced the original antenna with 
a larger parabolic reflector 152 em (5 ft) in di- 
ameter. The beam width produced by the new 
antenna was 1.4°, as compared with about 3° 
for the smaller dish. The larger antenna was 
mounted to give either zenith or azimuthal scans 
as desired, thus permitting RHI or PPI presen- 
tation of any echoes. 
Our estimates of the smallest raindrops that 
could be detected in these clouds during 1957 
indicate that the initial precipitation echoes 
could be caused by raindrops with a median di- 
ameter for reflectivity no larger than 100 mi- 
crons at 2 km ranges. 
