RAINDROP COLLECTION EFFICIENCIES IN ELECTRIFIED CLOUDS 
(equivalent diameter) raindrops is computed to 
be about 11 m sec’. On subtracting the mini- 
mum fall time through the clear air beneath the 
cloud from the observed total time interval, we 
estimate that the maximum fall time within the 
cloud is 140 see and 65 sec for two echoes on 
August 18, 1957, and 180 see on August 16, 1957 
for the lowest part of the echo. Evaluating (6) 
for these days from the minimum radius de- 
teetable to the observed size on collection, for 
the maximum time of fall within the cloud, we 
get 
cL = 55 gm m”“, August 13, 1957, 10h 00m 
MST (3-mm drops estimated) 
cL = 12.5 gm m*%, August 13, 1957, 10h 15m 
MST (second echo) 
cL = 46 gm m™, August 16, 1957, 11h 16m 
MST 
(We let 7min = 50 microns for this computation. 
The median cloud-drop diameter for reflectivity 
was estimated to be about 70 microns.) 
Discussion OF CALCULATIONS 
From the soundings for these days, the maxi- 
mum liquid water content possible (assuming 
no dilution) from the cloud base (at about 3.5 
km) to the lowest portion of the first echo (at 
4.5 km) is about 2.0 gm m™ before the first pre- 
cipitation is formed. This calculation then sug- 
gests that drops in these clouds may have net 
collection efficiencies in excess of 200% after the 
appearance of a high potential gradient within 
the cloud. Use of the more realistic mean liquid 
water content in the layer from the cloud base 
to the initial echo leads to values in excess of 
400% for the computed collection efficiencies. 
It should be noted the simplifying assumptions 
have been made that no dry air is entrained into 
the cloud and that evaporation beneath the 
cloud has not decreased the raindrop radius. 
Our estimated values of the collection efficiency 
would be still higher had these factors been 
taken into account. 
A solution of (6) for various assumed initial 
drop diameters is given below using the fre- 
quently observed 3-mm drop diameter as the 
final size and three minutes as the drop fall time 
within the cloud. The computed reflectivities Z 
which would arise from these initial sizes are 
shown in Table 3 for raindrop liquid water con- 
centration of 1 gm m”™. 
From this it can be seen that the computed 
297 
TaBLeE 38—Computed collection  efficiency—water 
content products required and cloud reflec- 
tivity as a function of the assumed 
initial drop diameter 
Assumed initial drop | 
diameter | Required cL | Z at detection 
‘ : mm i. ah ea - 7 ae - 
0.07 | 4.5 1 
0.1 4.4 Dates 
0.3 3.8 750 
0.5 | 3h2 350 
1.0 D2) | 2500 
5 | 1.6 | 4000 
2.0 | eal 20000 
high value of the cL product is not a critical 
function of the initial drop size chosen but the 
reflectivity and hence the detectability of the 
cloud is very sensitive to drop size. Therefore, 
even if the initial drop size used in the calcula- 
tion were in error by a factor of 10 (so that the 
threshold of detection for the radar was 1/1000th 
as sensitive as we believe it is) the computed 
collection efficiency would only be decreased to 
unity. We believe that the sensitivity of the 
radar approached the design performance so that 
we place some credence in these indicated high 
values of raindrop collection efficiency. 
Consider these results from another point of 
view. Under these conditions in New Mexico it 
has been observed that the rate of rainfall be- 
came ‘torrential’ (50 mm hr” or greater) two 
minutes or so after the first drops arrived at the 
summit and only four to eight minutes after the 
echo was first detected. High rates of rainfall so 
soon after detection of the echo indicate that 
these first echoes did not rise from a low con- 
centration of very large drops but from many 
drops in the cloud. It follows from the low initial 
reflectivity of the echo at detection (and the sub- 
sequent rapid increase in intensity) that these 
first drops must have been small in size but grew 
very rapidly. 
These deduced collection efficiencies are quite 
remarkable if they are to be believed. The ob- 
servations of which we are quite sure are the 
initial altitudes and the repeated short elapsed- 
time intervals between the detection of a radar 
echo in these electrified clouds and the collec- 
tion of 8-mm raindrops on the mountain summit 
in a gush of rain. If our estimates of the thresh- 
old drop size for detection were in error by a 
factor of 10 (so that we could not detect rain- 
drops less than 1 mm in diameter at a 2-km 
