RAINDROP COLLECTION EFFICIENCIES IN ELECTRIFIED CLOUDS 
Weather Bureau and the Office of Naval Re- 
search.) This airplane was also used to obtain 
time-lapse motion pictures of the cloud tops and 
to make temperature and humidity soundings. 
To determine the nature of the precipitation, 
the airplane made several flights through the 
cloud according to radioed instructions from the 
radar observer. 
OBSERVATIONS 
During a typical summer day in the moun- 
tains of New Mexico, the early morning sky is 
usually cloudless. Under the influence of solar 
heating small Cumulus clouds begin to form over 
the mountains and after a few hours these clouds 
grow in size and become thunderstorms. Figure 
3 shows how the cloud grows with time and the 
sequence of cloud electrification and radar echo 
formation on August 13, 1957. On this day an 
isothermal stable layer about 200 m thick at 
about 7 km limited the early convection; thus, 
several episodes of cloud activity were studied. 
The sequence seemed to be as follows: A new 
cell rose at about 4 m sec™*. When the top rose 
beyond 6 km or so, negative charge concentra- 
tions appeared in the lower portion of the cloud. 
In two minutes or so, precipitation was detected 
by radar as a cup echo above the upper radio- 
sonde, which still reported fair-weather polarity. 
The precipitation echo spread, and the vigorous 
updraft ceased. The electrification then vanished. 
In five minutes heavy rain and small hail fell to 
the surface. In 15 minutes or so the rainfall di- 
minished, a new updraft appeared, and the entire 
sequence was repeated. 
In situations where the wind speeds aloft were 
low, we found that quite commonly the first ra- 
dar echo in a developing Cumulus is in the form 
of a hollow inverted cup that changed in appear- 
ance quite rapidly. Initially the cup echo is dis- 
tinetly hollow. However, in a matter of several 
minutes it increases in reflectivity and fills in. 
As the echo grows and moves downward, virga 
makes its appearance beneath the cloud and on 
occasions when the cloud is directly overhead 
rain falls on the mountain summit. 
Cook in his instrumented P-38 airplane played 
an important part in our study by making ob- 
servations directly over the tops of the growing 
cloud. On several occurrences we were lucky 
enough to observe simultaneously an initial, hol- 
low echo and the echo of his airplane as he flew 
a few feet above the top of the visual cloud. 
When this happened, we determined that he was 
293 
TasBLe 1—Characteristics of the APQ-138A 
radars used on Mt. Withington 
Item | 1957 1958 
Magnetron frequency | 9875 me | 9363.8 me 
Nominal wavelength 3.2cm | 3.2 cm 
Peak power 40 kw | 40 kw 
Receiver sensitivity —103 to} —105_ to 
—105 —107 
| dbm dbm 
Ratio: Measured sys- 1/30 1/3 
tem sensitivity /Com- 
puted theoretical | 
sensitivity 
Antenna diameter 76 em 152 em 
Beam width to !g power) 3° 1eaS 
points 
Pulse duration ly usec | 0.65 psec 
nominal 
Pulse repetition rate 850 sec"! 850 sec"! 
TR tube recovery time | about 3.5 usec 
to within 3db of full 4 psec 
sensitivity 
of the order of 500 to 700 m above the initial 
echo and established that in these cases the ini- 
tial echo was well below the cloud top. Although 
we made several attempts we were unsuccessful 
in directing Cook into the initial hollow echo. 
Passes made through some of these clouds during 
later stages in their development showed that 
the precipitation was liquid water even when it 
was considerably above the melting level. 
The phenomenon of the first precipitation echo 
in the form of an inverted cup was observed 
over quite a temperature range. In some cases 
the temperature at the top of the echo was as 
low as —17°C, while in other cases the tops of 
the echo were below the freezing level. 
Rapid appearance of rain from clouds in New 
Mexico—One of the most striking features of the 
thunderstorms in New Mexico is the rapidity 
with which the clouds frequently develop and 
the great speed with which electrification, rain, 
and lightning make their appearance (see Fig. 
4 and 5). As we have pointed out in earlier work, 
the available evidence suggests that the first rain 
is formed by a coalescence process. It is therefore 
of some interest to make estimates of the collec- 
tion efficiencies of the drop growth process in 
these clouds. 
Regarding Figure 4, closely associated with a 
strong updraft at 09h 58m MST, the potential 
gradient within the base of the cloud went off 
seale at —20 V em™. At 10h 00m MST, a hollow 
precipitation echo appeared 2 km away at about 
6 km altitude. By 10h 02m MST the echo had 
