WMOD00311

• Title: Harold D. Orville Symposium - Forty Years of Modeling Clouds and Weather Modification
• Date: 4/26/2003
• Weather Modification - Doc Type: Report

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formation and the influence of timing and other factors
on the potential for successful hail suppression.

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IV. HYGROSCOPIC SEEDING SIMULATIONS

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Hygroscopic seeding seeks to increase the
efficiency of the warm rain process. As such, most
early studies of hygroscopic seeding tended to neglect
the ice phase and deal only with warm rain processes.
Detailed microphysical models were the preferred
vehicle for these studies, with Farley and Chen (1975)
being an example of IAS efforts in this area. That study
stressed the importance of the drop breakup process in
achieving a positive seeding effect in a one-dimensional
time-dependent model simulation of the salt-seeded
case of Biswas and Dennis (1972). More recent studies
employing detailed microphysics by various other
researchers have addressed critical aspects of
hygroscopic seeding such as the size of the salt particles
used in seeding, the amount of seeding material, and the
time and location of seeding. Examples of this work
include Tzivion et al. (1994), Reisin et al. (1996a,b)
and Yin et al. (2000). These studies have also extended
the detailed treatment to include the ice phase to
examine the influence of hygroscopic seeding on ice
formation. Unfortunately, the axisymmetric framework
employed in these studies is not suitable for
investigations of dynamic effects.

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Initial simulations of hygroscopic seeding by lAS
staff using first and second generation treatments with
parameterized microphysical schemes were probably
based on unrealistic representations of likely effects.
They tended to predict rain increases for cases of
cellular convection, and precipitation decreases for a
quasi-steady updraft case (Orville and Kopp, 1974).

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Results of hygroscopic seeding experiments using
third generation seeding techniques described in Kopp
(1994) and Kopp et al. (1996) show positive effects as
the initial response to seeding with indications of
complex interactions in later stages which can either
enhance or retard subsequent cloud and precipitation
development. This finding, together with the general
finding from field studies that hygroscopic seeding
apparently induces dynamic effects which extend
beyond the lifetime of the initial cloud being treated,
prompted longer term simulations in subsequent
studies.

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The variable nature of the results produced by
these extended simulations led to a simplified concept
to evaluate the effects of hygroscopic seeding (Orville
et al., 1998). This involved the simulation of extreme
cases, one an extreme continental case with the
autoconversion process disabled and the other an
extreme maritime case with auto conversion active as
soon as cloud formed. Figures 16 and 17, derived from

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material in Kopp et at. (1998) and Wang (2003),
provide justification for this approach. Note that the
results of salt flare seeding simulations lie somewhere
between the two extremes, with a positive seeding
effect indicated for the moderate case in Fig. 16
whereas the strong case shown in Fig. 17 indicates
decreases in both rain and hail as a result of
hygroscopic seeding.

This simple concept that the difference in these
extreme cases represents perfect seeding of a
continental cloud facilitates the study of a wide variety
of cases without considering the details of hygroscopic
seeding. Wang (2003) reports on the application of this
simple concept for a variety of cases using two- and

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Fig. 16. Top panel - horizontal distribution of
surface rainfall for moderate ND case. Bottom
panel - total surface rain versus time. The solid line
is the extreme maritime case, the dotted line is for
the extreme continental case. The dashed and do/-
dash lines depict the results of hydroscopic flare
seeding.

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