CLOUD-TO-GROUND LIGHTNING AND TROPICAL CYCLONES
Principal Investigator:
Christopher E. Samsury
Collaborating scientist(s):
Robert A. Black
Michael L. Black
Richard E. Orville (Texas A&M)
Objective:
To identify the location and characteristics of cloud-to-ground (CG)
lightning with respect to a hurricane's center, to explain why
certain regions of the hurricane are more electrically-active than
others, and to determine the universality of past results.
Rationale:
Until recently, very little research had been done specifically on the
topic of CG lightning in hurricanes. With the aid of the National
Lightning Detection Network (NLDN) and/or other detection systems,
we now have a unique opportunity to examine whether CG lightning occurs in
tropical storms and hurricanes and what regions are most favorable for
electrification. Many aspects of charge separation
and cloud microphysical processes remain poorly understood. The further
analysis and gathering of cloud-to-ground lightning data along with aircraft
measurements will help us to better understand hurricane
electrical properties and hurricane structure. WSR-88D (NEXRAD) radars
along the United States coastline also allow us to view and study the
electrification of landfalling storms when aircraft data are unavilable.
Method:
CG lightning data from the NLDN is examined for hurricanes that track near
the United States coast and radar data from the NOAA P-3 research aircraft
and the National Weather Service WSR-57 and WSR-88D systems are used.
Recent research flights by the NOAA P-3 gathered microphysical and
electrical measurements that are also analyzed.
To minimize the subjectivity in the determination of which lightning
flashes are/are not associated with a storm, a finite area centered on the
storm is defined that best approximates the area of the storm's
precipitation structure. The CG lightning data associated with 1995
Hurricanes Erin, Felix, and Opal has been assembled and analyzed with the
assistance of Dr. Richard E. Orville at Texas A&M University. In addition,
the NLDN data for Hurricanes Bertha (1996) and Fran (1996) are examined
with NEXRAD data to further determine the relationship of radar
reflectivity with cloud-to-ground lightning.
Accomplishment:
Research into lightning in hurricanes (Samsury and Orville 1994; Molinari et.
al 1994; Samsury et al. 1995) has shown that when lightning does occur it
most often happens in the convective outer rainbands that are seen in many
tropical cyclones (frequently on the eastern side of the storm)
(Figure 1). While the eyewall has stronger
updrafts than rainbands, CG lightning is relatively scarce in the eyewall
region. Although it has been
documented by Black and Hallett (1986) that the eyewall lacks the conditions
necessary for much electrification (little or no supercooled water in the
presence of ice/graupel/etc.), it is thus far unclear why the outer rainbands
are so much more likely to have CG lightning. Moreover, the lightning
studies noted above have identified a large variability in many of the
lightning characteristics between different storms. In 1996, the Hurricane
Electrification experiment was flown into Hurricane Fran as part of the
Hurricane Research Division field program. The aircraft measurements
from this experiment in Fran along with the extensive radar reflectivity
data avilable for several storms of the past few years will undoubtedly
lead us to a better understanding of the mechanisms responsible for
hurricane electrification.
Key reference:
Samsury, C. E., and R. E. Orville, 1994: Cloud-to-ground lightning in tropical cyclones: A
study of Hurricanes Hugo (1989) and Jerry (1989). Monthly Weather Review, 122,
1887-1896.
Molinari, J., P. K. Moore, V. P. Idone, R. W. Henderson, and A. B. Saljoughy, 1994:
Cloud-to-ground lightning in Hurricane Andrew. Journal of Geophysical Research, 99, D8,
16665-16676.
Samsury, C. E., M. L. Black, and R. E. Orville, 1995: The relationship of
cloud-to-ground lightning with radar reflectivity and vertical velocity in Hurricanes Bob
(1991) and Emily (1993). 21st Conference on Hurricanes and Tropical Meteorology, Miami, FL,
257-259.
Last modified: 9/12/96