TROPICAL CYCLONE ELECTRIFICATION STUDIES.
Principal Investigator:
Robert A. Black
Collaborating scientist(s):
-
Michael L. Black
- 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 contain updraft stronger than 8 ms-1 above
the melting level become electrically active, whereas those that don't have such updrafts 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 proportions of cloud
water, graupel, vapor-grown ice, 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, 1997: On the electrification of the hurricane. Submitted to J. Atmos. Sci
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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