WMOD00289 ‎(3)‎

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

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Observations by Rodi (1981) and two-dimensional cloud modeling studies
by Kopp et al. (1980) in support of HIPLEX have indicated that organized
internal motions as well as turbulence must be invoked to explain the spread
of seeding material or ice crystal plumes inside a convective cloud. Rodi
(1981) found that the standard deviation of the plume width reached 1 km
in about 1 000 s, in agreement with earlier findings. HIPLEX-1 seeded clouds
had ice crystals well distributed throughout them by 10 min or so after treat"
ment time. Unfortunately, while turbulence was distributing the ice crystals,
it was also hastening dissipation of the smaller clouds by mixing ambient air
into them.
There is some evidence that precipitating ice was more abundant in seeded
than in unseeded clouds 8 min after treatment time (Fig. 2), but the differ-
ence is not statistically significant (Table V). The later links in the physical
hypothesis relating seeding to rain at cloud base are not distinguishable in
the HIPLEX-1 statistical results (Fig. 3, Tables IV and V). It had been ex-
pected, for example, that AWe 8, the cloud water concentration measured
by the cloud physics airplane 8 min after treatment time, would show a de-
crease in the seeded clouds as the cloud water was taken up by the artificial-
ly produced precipitation embryos. However, no statistically significant dif-
ference was detected, apparently because mixing with dry air around the
clouds, rather than depletion by growing ice crystals, dominated the evolu-
tion of the liquid water.

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0.00

0.05

0.10 0.15
Pica (r')

0.20

0.25

Fig. 2. Cumulative distribution of PIC 8, precipitating ice concentration 8 min after treat-
ment time, in unseeded and seeded HIPLEX-1 clouds (after Cooper et al., 1981).