Weather Mod Critical Issues Report

• Project Type: General OWC
• Project Name: Weather Modification
• Title: Critical Issues in Weather Modification Research
• Date: 1/1/2004
• Water Conservation - Doc Type: Final Report

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survive the transit through the warm subcloud layer, the available SL W is depleted by the
formation of greater numbers of smaller ice particles (smaller hailstones) that are more
likely to melt during descent. This concept is known as beneficial competition,

Beneficial competition is produced by the introduction of additional hail embryos
to the flanking cells of a hailstorm. In theory this would lead to more numerous and
smaller hailstones, which would melt more, or perhaps entirely during their descent. A
risk is that too few embryos could be added to some inefficient storms and more
hailstones could be produced.

Another concept, early rainout, is based upon the initiation of the ice-phase
precipitation process earlier in the lifetimes of supercooled convective clouds. For
example, if ice-phase hydrometeors can be made to form when cloud top temperatures
are _SoC rather than -ISoC, precipitation can form earlier in the clouds' lifetimes. When
this is made to happen within the flanking line, several positive effects may result.

First, precipitation falls from what would have otherwise have been rain-free
cloud base, possibly in areas oflow-level storm inflow, This could impede or retard the
moisture flux into the storm, which in turn could lessen the condensate (and eventually
SL W) in the stronger updrafts.

Second, conversion of SL W to ice in the smaller turrets reduces the net SL W
available for hail growth in the larger turrets, where updrafts are stronger and more
conducive to the growth of larger hailstones.

Third, the earlier release of latent heat fuels the buoyancy of the smaller, less
vigorous turrets,

Fourth, the total area receiving precipitation from the storm may be increased,
while the intensity and amount of precipitation produced within the main storm core may
be slightly lessened,

Early rainout is theoretically achieved by the same seeding strategy as that used
for beneficial competition. In successful early rainout modification, the precipitation
falls from the cells before ingestion into the mature main cell. If the ice hydrometeors
produced by seeding do not grow large enough to precipitate from the rain-free cloud
base, the number of nascent hail embryos has been increased, aiding beneficial
competition,

The term trajectory lowering is born of the notion that maximum hailstone growth
occurs at higher, colder altitudes, where supercooling is very significant. Such being the
case, trajectory lowering would logically slow and/or lessen hail development. One
could say that early rainout is in fact also trajectory lowering, More complete knowledge
of the optimum hail growth regions begs for the deployment of polarimetric radars.

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