WMOD00379

• Title: Potential Opportunities for Precipitation Augmentation in the Eastern Tennessee Valley
• Date: 2/1/1988
• Weather Modification - Doc Type: Report

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due to marked decreases on days when cloud tops were colder than ~40 OCt
Interestingly, modeling studies of the effects dynamic seeding would have on
growing cumulus in the Midwest indicate clouds with summit temperatures between
-10 oC and -40 oC would be most amenable to precipitation increases (Ackerman
and Sun, 1985).

Model results for soundings in the vicinity of the eastern TVA region have shown
that, of the 3 months under investigation, 6nly May offers any potential for
dynamic enhancement of clouds for precipitation increases. Results for May show
that almost all days observed to have convection have some dynamic growth poten-
tial. That would amount to about 15 days per year, mostly between May 10 and
May 31. However, the potential appears to occur only in the smaller convective
clouds (r < 2 km). The expected additional growth is about 1.5 km. Using 10 km
as a minimal natural cloud top based on radar studies, this amounts to about a
15 percent increase in cloud top.

The real potential for streamflow augmentation by seeding of convective clouds
for dynamic effects cannot be estimated at this time. Given that only 50 per-
cent of the precipitation that falls in May runs off, that convective clouds are
present on only half the days, and only the smaller clouds appear to offer
potential, the advisability of seeding convective clouds in May for preclpita-
tion increases needs further study. .

3.7 Phase II Program

Based on the Phase I results, Phase II should be considered. A one-time 3-month
field program would be run from March 1 to May 30, 1989. 'The major objectives
to be accomplished in Phase II would be:

1. Determine the frequency of occurrence, areal distribution and con-
centration of supercooled liquid water and ice in clouds over the eastern
TVA region during March, April and May.

2. Determine the relationship between supercooled liquid water and the kine-
matic and dynamic properties of the cloud systems. Determine the natural
precipitation processes occurring, and make estimates of the cloud precipita-
tion efficiency.

3. Establish the wind flows over the target area and the transport and
dispersion of aerial and ground released tracers during cloudy and clear
conditions to determine the potential effectiveness of aerial and ground
release treatment methods.

.4. Through in situ studies, determine the persistence and concentrations of
supercooled liquid water, the cloud droplet size distributions and their evo-
lution, and thus the dynamic seeding potential of convective clouds over the
eastern TVA region.

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