WMOD00311

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

theories came as a result of the experiments of
Reynolds et al. (1957) and is classified as a
noninductive process (one in which the ambient
electric field plays no role). In their experiments.
Reynolds et al. found that there is substantial
charge transfer between vapor-grown ice crystals
and graupel particles that collide in a cloud of
supercooled water droplets that are riming the
graupel particle. They also found that when the
supply of supercooled riming droplets was
removed the charge transfer between the
interacting ice crystals and graupel particles was
reduced by orders of magnitude. From these
experiments it was clear that the mixture of
graupel. ice crystals, and supercooled droplets
was necessary for significant charge transfer to
take place. While the exact nature of the charge
transfer at the micro-scale has not, to this day,
been resolved. considerable additional laboratory
work has been carried out. and this noninductive
riming mechanism currently holds the favored
position as the mechanism thought to be
responsible for primary electrification in
thunderstorms.
Thus, over a period of half a century, several
different theories related to the separation of
charge and the electrification of thunderstorms
were developed. The question as to which one(s)
are of primary importance was (and remains) the
key question facing scientists in the field of
thunderstorm electricity. Throughout the period of
theoretical and laboratory work related to charge
separation, observational studies were also
undertaken. Much of the character of
thunderstorms' electrical nature was revealed by
these studies. but observational instruments and
techniques were not adequate to make the crucial
determination as to the efficacy of the various
proposed mechanisms. With the advent of digital
computers and the development of models of
cloud growth a new avenue of approach was
opened.

2. THE LEGACY

In light of the theories of cloud electrification
noted above. some initial attempts at calculations
related to charge distributions were undertaken.
The VonneguUGrenet convective hypothesis was
the first to be tested. Phillips (1967a, b, c) and
Ruhnke (1970) used simple. quasi-static
calculations of the charge accumulation within
clouds subject to conductivity differences between
clear and cloudy air. Despite the simplicity
employed in the calculations, Ruhnke's results, in
particular, turned out to be reasonably accurate for

non-precipitating clouds. However, none of these
efforts included any consideration of cloud
dynamics or microphysics. Since all theories of
charge separation involved either or both of these
. considerations, the calculations were of limited
utility. Dr. Orville, recognizing this,. began the
process of incorporating electrical effects within
the context of dynamic cloud models.
Dr. Orville and his students had been
developing two-dimensional (20) models of storm
and precipitation development for several years.
In the summer of 1969, M. H. Smith traveled to
Rapid City from the Univ. of Manchester, England
to work on the problem of incorporating electrical
effects into cloud models. The result of that
collaboration was an unpublished report to the
Office of Naval Research for Project THEMIS
(Smith and Orville. 1970) that outlined the
requirements for merging electricity with cloud
growth in a multi-dimensional, dynamic cloud
model. The premise of the report was the
question, "Given a numerical cloud model.... what
considerations are necessary to include electrical
effects in the model?" One of the primary
motivations for merging electrical effects with an
existing dynamic and microphysical model was the
recognized need for a platform to test various
charge separation mechanism, in particular the
VonneguUGrenet convective theory, although the
report stated that, "Possibly something can be said
about sedimentation theories also...".
According to Smith and Orville. the general
considerations for merging electrical effects into
the model were deemed to include:

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1) addition of electrical terms to the equations of
motion (possible effects on dynamics) and
terminal velocity calculations,
2) assurance of charge conservation.
3) the inclusion of small and large ions (positive
and negative) as well as neutral aerosols.
4) the addition of diffusion and conduction of
ions to cloud particles,
5) consideration of electrical effects on
collection efficiencies and their effect on rain
and hail production,
6) point discharge at the surface under strong
electric fields.
7) vertical variation in small ion profiles and
altitude-dependent recombination
coefficients, and
8) an accounting for conduction currents above
the cloud.

To account for these items Smith and Orville listed
a number of variables and parameters that needed

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