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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to be added to the model. The principle
dependent variables were:

1) large and small ion concentrations,
2) hydrometeor charge densities, and
3) the electric potential.

The secondary variables (dependent on the
primary variables) were:

1) net total charge density,
2) the electric field,
3) the conductivity, and
4) current density.

The parameters that would be required were:

1) ion production rates and recombination
coefficients (altitude dependent),
2) ionic diffusion coefficients
3) initial profiles of electrical variables,
4) neutral particle concentrations, and
5) hydrometeor number concentrations.

Although the information in this report was not
published in the open literature, it became the
basis for the first attempt to include electrical
effects in a cloud model. Prior to and shortly after
the preparation of the Smith and Orville report, two
graduate students, J. E. Pringle and T. D.
Stechmann, were recruited from the Electrical
Engineering Dept. to undertake this modeling
effort (both receiving M. S. degrees in Electrical
Engineering for their work).
Pringle (1971) undertook the initial inclusion
of electrical variables and parameters using the
20, slab-symmetric model of Orville (1965) for
non-freezing conditions (rain and cloud water only)
with the intent of comparing convective and
sedimentation theories. With respect to the
additional variables and parameters listed above,
all were included except: 1) recombination among
large ions was ignored, 2) there was no charge
transfer between cloud and raindrops, and 3)
when attachment of ions to hydrometeors was
included it was specified (as a function of the rate
of change of the radius of the particle with a fixed
percentage of the existing ions involved) rather
than calculated from theory. Also, no influence of
electrical effects on collection processes was
attempted. The spacing used in the simulations
was 500 m on a 10-km high by 5-km wide grid,
with initial time steps of 30 s. In all, three cases
were run. The first simulated small ions only with
no attachment to hydrometeors. The second was
the same, but with the addition of large ions. The

third arbitrarily prescribed the attachment of up to
40% of the existing positive ions to cloud droplets
and negative ions to rain. The results from the
first two cases showed only minor changes to the
electrical state of the atmosphere, from which
Pringle concluded that the convective mechanism
was not of primary importance (the conclusion was
correct, but the reason was not). The third case,
with ions arbitrarily attached to cloud and rain,
showed the development of relatively strong fields
(to nearly 25 kV/m for the case presented) and
large net charges. From this Pringle concluded
that sedimentation theories (inductive and/or
noninductive) might better explain cloud
electrification and also noted that at fields about
twice the value quoted above there was a small,
but noticeable electrical effect on the downdraft.
Following on the research of Pringle (1971)
was that of Stechmann (1972). Stechmann's work
included the addition of ice particles to the model
(cloud ice and "hail"), and the neutralization of
charged rain and hail through: 1) the accretion of
oppositely charged cloud water and ice within the
cloud, and 2) the attachment of positive ions below
cloud base using a rudimentary parameterization.
The addition of the ice phase and the neutraliza-
tion processes resulted in a considerable increase
in the complexity of the calculations, in particular
in accounting for the collection of positive ions by
precipitation below cloud base. With these
modifications the simulation showed electric fields
on the order of 8 kV/m (with 30% of positive ions
attached to cloud particles and 50 % of negative
ions attached to precipitation), a factor of 3 less
than those obtained by Pringle. An example of the
results for: a) the total charge density, and
b) vertical electric field component obtained by
Stechmann, is shown in Fig. 1 at 89 min simu-
lation time (positive values are partially shaded).
Despite the shortcomings of the model, the
expected charge dipole and electric field structure
were produced (although this was guaranteed by
the prescribed method of hydrometeor charging).
The first, and only, appearan.ce of electrical
model simulations in the reviewed literature
bearing Dr. Orville's name as a co-author was
Pringle et al. (1973). This was an update of the
third class of simulations done by Pringle (1971),
after some modifications to the model not involving
electrical effects. The results were modified
somewhat with maximum electric fields attaining
10 kV/m, requiring a larger percentage of
attachment of ions to reach these values. The
major conclusions from these efforts were that:
1) the fallout of charged precipitation is important
in charge separation, 2) convection of ionic charge

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