TROPICAL CYCLONE ELECTRIFICATION STUDIES.
Robert A. Black
Michael L. Black
Christopher E. Samsury
Dr. John Hallett (DRI)
Dr. C. P. R. Saunders (UMIST)
To determine the conditions under which tropical oceanic convection becomes
electrified, and to relate that activity to the evolution of the storm
The electrical activity of any convective system is intimately linked to the
strength of the updraft at and above the melting level. Storms that can loft
appreciable numbers of raindrops > 1 mm in diameter above the melting level
become electrically active, whereas those that don't do so do not. Beyond this
basic level, however, the details of the process(es) by which charge is
separated in convective clouds are poorly understood. Conflicting laboratory
results have indicated that the simultaneous coexistence of millimeter graupel,
supercoooled cloud drops, and vapor-grown ice crystals are required for rapid
charge separation, although the necessary quantity of cloud water and the
temperature required are disputed.
The Hurricane Electrification experiment uses in-situ sampling of cloud and
precipitation particles, vertical and horizontal (parallel to the wings)
electric field, winds, temperatures, and digitized airborne radar, as well as
land-based cloud-to-ground lightning detection to answer these questions. The
first attempts to make these measurements concentrated on simply penetrating
the most active active convection at the highest altitude attainable;
subsequently, we have tried to find storms within 400 km of the U.S. mainland
so that the cloud-to-ground lightning activity in the storms could be
In situ measurements of electric field revealed that the strongest fields
in Hurricanes Tina and Claudette (Figure 1)
occurred in updrafts that contained
multiple condensate phases: graupel, cloud ice, and cloud water (Figure 2); other
areas were either weakly electrified, or had no electrical activity at all.
Particle charge measurements made in cumulus clouds near Kennedy Space Center
showed that under similar microphysical conditions, larger ice particles had
predominantly negative charge so that their more rapid fallout led to an
accumulation of negative charge near the melting level. In the electrified
areas of Hurricane Claudette, the fields became somewhat stonger with
increasing altitude, indicating that the main charge centers were at or above
Advances in microphysical measurement equipment and software have greatly
improved discrimination of ice particle habit and estimation of particle
population statistics with PMS (Particle Measuring Systems, Inc.)
instrumentation. The two-dimensional (2-D) Greyscale particle imaging probes
(Figure 3) offer finer resolution and
depth-of-field information to better
characterize the ice particle size distribution, but with a smaller image
rate than is achieved with the older 2-D mono probes. Compare these images
with those in Fig. 2.
Black, R. A. and J. Hallett, 1995: The relationship between the evolution of
the ice phase hydrometeors with altitude and the establishment of strong
electric fields in Hurricane Claudette. Preprints, Conference on Cloud
Physics, 15-20 January 1995, Dallas, TX. American Meteorological Society,
Black, R. A., J. Hallett, and C. P. R. Saunders, 1993: Aircraft studies of
precipitation and electrification in hurricanes. Preprints, 17'th Conference
on Severe Local Storms and Conference on Atmospheric Electricity, October 4-8,
1993, St. Louis, MO. American Meteorological Society, Boston, MA.
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