CLOUD-TO-GROUND LIGHTNING AND TROPICAL CYCLONES

Principal Investigator: Robert A. Black
Collaborating scientist(s):

Micheal L. Black

John Molinari (SUNY-Albany);

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 WP-3D research aircraft and the National Weather Service WSR-57 and WSR-88D systems are used. Recent research flights by the NOAA WP-3D 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. New research has indicated that eyewall GC lightning events tend to occur prior to and during strengthening periods. These periods appear to coincide with the occurrence of updraft greater than 8-10 ms-1 (Black and Hallett, 1997). 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:
Black, R. A., and J. Hallett, 1997: On the electrification of the hurricane. Submitted to Journal of Atmospheric Science.

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.

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Last modified: 11/19/97