A Comparison of the Optical Pulse Characteristics of Intracloud and Cloud-to-Ground Lightning as Observed above Clouds

Abstract

The time-resolved optical waveforms at 777.4 nm and electric-field changes produced by intracloud and cloud-to-ground lightning flashes were measured above clouds from a U2 airplane (flying at a height of 20 km) at the same time that ground-based measurements of lightning were obtained from a mobile laboratory and a regional lightning location network. The U2 optical pulse trains are examined for variability both within and between flashes. The optical pulse characteristics of cloud-to-ground flashes are further subdivided into first strokes, subsequent strokes, and intracloud components (k-changes). Descriptive statistics on these pulse categories have been compiled for 25 visually confirmed cloud-to-ground flashes (229 optical pulses) and 232 intracloud flashes (3126 optical pulses). The pulse shapes and intensities of intracloud and cloud-to-ground flashes as viewed from above cloud are shown to exhibit remarkably similar waveshapes, radiances, and radiant energy densities. The median radiance at cloud top is approximately 7 ? 10?3 W m?2 sr?1, and the median energy density is 3 ? 10?6 J m?2 sr?1. A simple physical model is used to estimate, for comparative purposes, the radiance and energy density of the original light source within the cloud. First stroke optical pulses are seldom the most radiant or energetic pulses produced by ground discharges as seen from above the clouds. The intracloud components of cloud-to-ground flashes typically produce the optical pulses with the largest peak radiance within a cloud-to-ground flash; however, subsequent strokes are more likely to have the largest energy densities and most complex pulse shapes. On average, intracloud flashes have almost twice as many optical pulses as ground discharges. There is often significant pulse structure variation within and between individual flashes. Because of this variation, multiple stroke cloud-to-ground flashes are difficult to distinguish uniquely from intracloud flashes solely on the basis of their optical signature above cloud. Single stroke cloud-to-ground flashes, however, appear to have a unique single pulse optical signature. The relevance and implications of these pulse characteristics for lightning mapping from satellite-based optical sensors is addressed.