WMOD00289 ‎(3)‎

• Title: Augmentation of Rainfall from Summer Cumulus Clouds
• Date: 2/5/1983
• Weather Modification - Doc Type: Report

11

100

11111 11111111 III
I
r---------J
I
rJ
Unseeded-...j
r--J
I

80
~
~
~
w 60
~
f0-
e::(
...J
:::J
~ 40
:::J
U
20

o
0.01

0.10

1.0
AER (mm km2)

10.0

100.0

Fig. 3. Cumulative distribution of AER, aircraft estimated rainfall, in unseeded and
seeded HIPLEX-1 clouds (after Cooper et aI., 1981).

The appearance of natural precipitating ice at the sampling altitude in un-
seeded HIPLEX-1 clouds further masks differences between seed and no-
seed cases. There is some evidence that the natural ice formed at higher levels
and was carried down by downdrafts (Rodi, 1981). There is also evidence of
differences in subsequent growth mechanisms, with aggregation of ice crys-
tals predominating in seeded clouds and riming in unseeded clouds (A.B.
Super, private communication, 1982).

FUTURE OUTLOOK

Although the HIPLEX-1 results have not demonstrated net rainfall in-
creases due to seeding, they do support the physical hypothesis. HIPLEX-1
has shown beyond any doubt that dry ice seeding produces ice crystals in
supercooled cumulus clouds and that some of the ice crystals grow to pre-
cipitation embryo size in about 5 min, as theory predicts. If the artificial
embryos remained in a favorable growth environment, say in a multicelled
cloud, they would grow into graupel or snowflakes. However, natural preci-
pitation forms in large clouds in a time frame comparable to that for artifi-
cial precipitation, so the artificial embryos would face competition. There-
fore, a seeding hypothesis for significant rainfall increases at the ground must
be more complex than the straightforward one enunciated for HTPLEX-l.
Some persons have suggested that dynamic effects related to latent heat re-
lease play a role in the total impact of seeding upon rainfall in the Great