Principal Investigator: Robert A. Black
Collaborating scientist(s):
Michael L. Black
Christopher E. Samsury
Dr. John Hallett (DRI)
Dr. C. P. R. Saunders (UMIST)
Objective: To determine the conditions under which tropical oceanic convection becomes electrified, and to relate that activity to the evolution of the storm circulation.
Rationale: 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.
Method: 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 monitored.
Accomplishment: 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 the aircraft. 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.
Key reference:
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, Boston, MA.

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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