WMOD00311

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

Subsequent to the development of this
physics-based representation of lightning for cloud
models, other groups embraced the idea and
improved upon it. Solomon and Baker (1996)
used the concepts from Helsdon et al. (1992) to
devise a scheme for their 1 D axisymmetric model.
Their improvement was to add the contribution of
the channel charge to the electric field calculation,
which influenced whether the flash was intracloud
or cloud-to-ground. Mazur and Ruhnke (1998)
developed a lightning scheme that determined
propagation based on the total electric field at the
leader tip (including the influence of charge on the
channel) and determined termination based on the
potential of the leader tip relative to the potential of
the initiation point rather than using the electric
field. Their lightning model was developed outside
the context of a cloud model, however.
MacGorman et al. (2001) expanded on concepts
from Helsdon et al. by adding to their 3D model a
randomization of the location of the initiation point
around the maximum electric field region, addition
of a consideration of the ambient space charge
distribution in determining the propagation
direction in regions with low electric field, and
branching based on the number of grid points
adjacent to the developing channel that support
continued propagation as determined by the
field/charge criteria. Mansell (2000) and Mansell
et al. (2002), using the same 3D model as
MacGorman et al. (2001), implemented a
probabilistic dielectric breakdown scheme for
lightning that developed branched channels using
the electric field to determine the probability of
continued propagation. The scheme is
computationally intensive, but produces realistic
looking channels and charge tran~fers.
The one limitation that pervades all of these
lightning simulation schemes is that the channel is
required to propagate between model grid points,
so channel geometry is dependent on model grid
spacing and channel "tortuosity" is limited to fixed
angles. The next step in the evolution of lightning
schemes is the removal of this grid point
dependence for channel propagation. Sus (2001)
and Helsdon and Sus (2001) developed a scheme
that accomplished this goal by looking at the
physics of the electron avalanche process at the
tip of the developing leader. By choosing a
random electron at the leader tip that fits certain
criteria, random angles of propagation are
obtained. Branches occur when more than one
electron meets the propagation criteria. Also, the
leader is assumed to develop in 50-m segments,
removing the dependence on the grid spacing.
Figure 6 shows the channel resulting from the

12

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

... .....0.... ".0.'

r=~ -==-~
1 0 ...r-m

,t:::>-'

~ --
.1/ 1-..... -......... .........

. D..

N

8

Figure 6 - Lightning channel (darker) with
branches (light) superimposed over a charge
distribution (dashed-positive, and solid-negative
charge).

application of this new scheme in a 2D simulation.
The removal of the grid point dependence. makes
the channel (and branch) geometry much more
realistic. The scheme is also computationally
intensive and is currently being adapted to the 3D
SEM.
With respect to charging processes within
thunderclouds, our effort at the IAS constitutes the
primary work being done in this area. While early
modeling work focused on the convective
mechanism, recent observational studies (French
et al., 1996; Ramachandran et al., 1996, among
others) have indicated that non inductive charge
transfers during collisions between ice
crystals/snow and graupel in a supercooled riming
environment are the most likely explanation for
thunderstorm electrification. Regarding laboratory
experiments related to noninductive charging,
there have been two somewhat conflicting sets of
results - those of Takahashi (1978, reconfirmed
by Takahashi, 1999) and those of Saunders and
colleagues at the Univ. of Manchester (e.g.,
Saunders et al., 1991, although there have been
numerous additional papers regarding laboratory
work related to the Manchester experiments over
the last decade). Both sets of experiments show
the magnitude and sign of the charge transfer

40