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<br />For artificial ice nucleants such as silver iodide, the
<br />sink terms represent various interactions of AgI with
<br />cloud liquid, rain and water vapor which produce ice
<br />particles. In our formulation most AgI particles act as
<br />deposition or condensation-freezing nuclei. The
<br />microphysical processes modeled are similar to those of
<br />the second generation seeding technique, but the effects
<br />are spatially and temporally restricted by the presence
<br />and concentration of the seeding agent. For dry ice
<br />seeding simulations an appropriate fallout term is
<br />included in the conservation equation, and dry ice
<br />produces ice crystals at a rate governed by the
<br />sublimation rate of the dry ice.
<br />
<br />Third generation seeding treatment can be applied
<br />to a wide variety of real world seeding techniques such
<br />as aircraft drops of silver iodide flares or dry ice pellets,
<br />broadcast seeding in the updraft below cloud base or
<br />the cloud intlow region, aircraft (or rocket) release of
<br />seeding material directly into clouds, and ground-based
<br />generators release of seeding material in the boundary
<br />layer. Third generation seeding techniques can also be
<br />used to simulate the transport of an inert tracer such as
<br />sulfur hexatloride (SF6).
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<br />Figures 2 and 3 illustrate this third generation
<br />technique for silver iodide and dry ice seeding
<br />respectively. There are some basic differences between
<br />these different seeding techniques, and some
<br />similarities. The dry ice falls rapidly through the cloud,
<br />forming cloud ice in the supercooled region of its path.
<br />Typically, only about 15 to 20% of the dry ice sublimes
<br />in the supercooled region of the cloud when it is
<br />introduced in the cloud at the -10" to -15"C level, and
<br />the direct effect of the dry ice (production of ice
<br />crystals) is over in a few minutes. Fm silver iodide the
<br />direct effects typically occur over a longer, but still
<br />relatively short, time period depending on the transport
<br />of the agent to supercooled cloud regions. The ice
<br />crystals produced by the seeding, whether it be by AgI
<br />or dry ice, then go on to produce a variety of indirect
<br />effects as the ice crystals grow and interact with other
<br />cloud particles.
<br />
<br />The accelerated and enhanced formation of
<br />precipitation discussed above are the microphysical
<br />effects of ice-phase seeding. Enhanced ice formation
<br />also results in the release of additional latent heat of
<br />fusion and possible dynamic effects. The induced
<br />dynamic effects may take the form of increased updraft
<br />b.
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<br />Fig. 2. The distributions of seeding agent (silver iodide)for case Hl/Sl at (a) 15 min, (b) 18 min, (c) 21 min, and (d) 24
<br />min, respectively. Cloud areas (J 00% relative humidity) are outlined by a dotted line. Streamlines are dashed lines. The
<br />maximum seeding agent values are (a) 6.79 x 10,10 g g'l, (c) 2.81 X 10-11 g g'l, and (d) 5.32 x 10-12 g g'l, respectively. The
<br />total seeding agent in the domain at these times is 100, 69, 13, and 2%, respectively, of the amount introduced at 14 min.
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