Optical Observations of Lightning from a High-Altitude Airplane

Abstract

Lightning has been observed from above cloud top by using satellites, balloons, rockets, and high-altitude airplanes, each of which provides a unique perspective and holds the potential for gaining new understanding of lightning phenomena. During the 1980s extensive optical observations of lightning have been made from a NASA U-2 airplane with a goal toward placing a lightning sensor in geostationary orbit. Analysis of these U-2 measurements suggest that most of the light generated within a cloud escapes, and that the optical energy of lightning measured from above clouds is not significantly different than the measurements made from below of discharges to ground. Near-infrared optical measurements were made of nearly 1300 optical pulses produced by 79 lightning flashes. The median source estimate of peak flash radiance is approximately 108 W with a dynamic range of less than three orders of magnitude. Of these 79 flashes, 90 percent produced peak radiant energy densities of 4.7 ?J m?2 sr?1 or greater, relative to the full field of view of the instrument. The median pulse rise time and full width at half maximum are 240 and 370 ?s, respectively. We interpret these slow optical rise times and broad pulse widths as primarily a result of multiple scattering within the cloud. The spectral characteristics in the near-infrared of the neutral emission lines observed from above clouds are found to be very similar to ground-based measurements.