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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enumerated in the next section.

5. CRITICAL ISSUES

There are a number of issues that remain the
subject of modeling studies in cloud electrification.
I will list them here without going into great detail.

Charaino Schemes

As noted above, the non inductive charging
mechanism currently is considered by most
atmospheric electricians as being primarily
responsible for charge separation in clouds.
However, the details of this mechanism have not
been worked out to everyone's satisfaction and
there continues to be disagreement between the
results from different laboratories investigating the
mechanism in cold chambers. Models can help in
sorting out these details by applying the
formulations to specific observed thunderstorm
situations and determining which provides the best
results. There has also been a resurgence of
interest in the graupel/cloud water inductive
mechanism as being of some importance in the
development of the lower positive charge center of
storms. This charge center is thought to promote
cloud-to-ground lightning and its origin needs to be
explained. The question of how charging
mechanisms operate is also tied to the next item.

Storm Charoe Structure - Polarity .

Observations over the last 20 years have indicated
that some storms appear to have a charge
structure that is "inverted" with respect to "normal"
storms, i.e., the main charge dipole exhibits a
negative-over-positive structure (an
oversimplification). These storms seem to be
severe, contain large hail, and produce a high
percentage of positive cloud-to-ground lightning
flashes. The Severe Thunderstorm Electrification
and Precipitation Studies (STEPS) in the summer
of 2000 was undertaken to investigate these
storms. Several good case studies were obtained
and several hypotheses are "on the table" to offer
an explanation for the observed charge structures.
Numerical modeling is being used to try to sort out
the processes responsible for the development of
these storm charge structures. This highlights the
necessity of having correct formulations for charge
separation processes, as noted above. Thus far
no model simulations have been successful in
attaining the observed charge structures. This is a
high priority item in the atmospheric electricity
community.

liahtnino Influence on Charoe Structure

While the primary thinking is that charge
separation mechanisms are responsible for the
basic charge structure in thunderstorms, balloon
and aircraft observations indicate that these
structures in mature storms are more complicated
than the simple tripole model. In early-storm
observations, the evidence suggests that charge
structures tend toward the tripole, but not so in
more mature storms. Models with physics-based
lightning schemes have shown that lightning can
cause considerable charge rearrangement within
storms. The question at hand is, does lightning
merely travel through charge regions developed
by charge separation processes, or is the lightning
itself responsible for the developing charge
structure in the mature stages of the storm?
Models are one of the primary means of
addressing this question.

Liohtnina Influence on Trooosoheric Chemistrv

As noted above, lightning is the least well
quantified of the sources of NOx in the
atmosphere. The fact that it is injected direcUy
into the troposphere at mid to upper levels, where
it can influence ozone production, makes
quantifying its production rate of great importance.
Since this production term is being incorporated
within global models with a range of values
spanning an order of magnitude, it is essential that
we determine the production rate more accurately.
Field programs that investigate the Iightning/NOx
connection have been undertaken, but the results
are difficult to put within the context of f1~sh
production rates. Modeling of NO production
using models with explicit lightning physics can
help to better quantify the amount and distribution
of lightning-produced NO within developing storms
and its transport and transformation through
chemical reactions. Such modeling studies,
undertaken within the context of observed storms,
should help refine the production rates used in
models without explicit lightning physics and
ultimately improve our prediction of the ozone
chemistry of the atmosphere.

LiohtninQ Physics - Positive/Neoative Leaders

There is much we do not know about lightning.
Studies using instruments associated with rocket-
triggered lightning have begun to reveal much
about c1oud-to-ground lightning and the
attachment process. What remains less well
understood is the virgin breakdown and in-cloud

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