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Ground-based
vs. Aircraft Seeding
The
question frequently arises: Which of these two means of delivery of
seeding materials is most effective? The answer requires more
research, and it also depends on the local situation. One
may describe this complex situation in terms of cloud
physics,
terrain, and meteorology (including local airflow and its effect on the
transport and diffusion (T&D) of seeding materials).
Despite
this complexity, one major variable is at the heart of the decision of
"air vs ground" - the availability of supercooled liquid water (SLW).
This availability is treated in detail on our "How Seeding Works"
page,
which explains that SLW is the "fuel" needed to for seeding to
work. In the current context, it is imperative that
enough seeding material must reach regions with substantial SLW;
otherwise seeding will not be effective. Therefore the two
delivery methods must be judged by how well they meet this
criterion. The ensuing discussion relates only to winter
seeding
of orographic (mountain) clouds to augment snowpack; warm-season
convective cloud seeding over flat terrain is usually done with
aircraft
only.
SLW Facts
Research has shown that in mountainous regions that
are targets for winter seeding to augment snowpack, SLW is
largely confined to within 3000 feet (1 km) above ground level (AGL).
Also, SLW is usually found above the windward slopes and crests of the
mountains. This location is the result of the forced uplift
of moist air by the mountain slopes.
Atmospheric conditions are usually stable in winter, resulting in
relatively small
vertical air motion from the forced uplift over mountains.
More
unstable conditions sometimes arise, especially over ranges near
oceans, such as in California. During such conditions,
vertical motion is greater than normal and convective clouds will
develop. If they grow large and strong enough, these clouds
form thunderstorms (usually in warmer months).
In winter, however, convection is usually less vigorous and is imbedded
in the typical stratiform (layered) cloud mass caused by lift over the
mountains. These vertical motions can transport seeding
materials
higher than the usual 3000 foot AGL limit.
Even during stable conditions, atmospheric phenomena known as gravity waves can
occur. These waves produce substantial vertical motion that
can in turn generate large amounts of SLW at unusually high altitudes
(greater than 3000 feet AGL). Mountain ranges
themselves can induce atmospheric gravity waves, directly
over the mountain crest or just downwind. Unfortunately,
gravity waves are highly transient in time and space, so their
existence is difficult to predict.
Implications of SLW
Facts and Other Factors for Choice of Seeding Method
Given the character of SLW as described above, how
best to
achieve T&D that will routinely reach the SLW regions with
seeding
material? This is no simple task, regardless of delivery
mechanism. Several review articles in the scientific
literature
state that achieving
adequate T&D is probably the most difficult problem facing
winter cloud seeding.
This T&D is influenced by winds,
turbulence, and cloud microphysics, but the delivery mode
also
determines where seeding materials will be transported and in what
concentrations.
The differences between these delivery modes are explored in
Page 2.
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