The Time Evolution of Optical Lightning Flashes

The composition and time evolution of lightning are examined using the Lighting Imaging Sensor (LIS). Frame-by-frame optical lightning measurements are clustered into features whose radiant energy, horizontal footprint, and timing may be analyzed statistically. A LIS series feature is used to describe distinct periods of near continuous illumination that persists over multiple LIS frames. Series are integrated into the LIS clustering hierarchy between the group and flash level. An average series illuminates 40% of the flash footprint while accounting for 20% of the flash radiance, and just 1% of the flash duration. LIS flashes typically contain optical emissions that are exceptionally radiant and may persist over multiple frames. Series features cluster these bright optical pulses, allowing their number and time separation to be quantified in each flash. This optical multiplicity averages 1.7 for flashes with at least one particularly radiant group. Multigroup series most often occur early in the flash duration with 13% to 18% at first light. Series are typically separated by 100ms or more in multiseries flashes. Bright series, by contrast, typically occur in rapid succession, with at most a few dozen milliseconds between them. Because series are optical features, they may result from any physical process that produces strong optical emissions. The statistics presented herein support the idea that series may originate from multiple physical processes. Plain Language Summary Satellite lightning imagers record lightning flashes at 500 frames per second. We use the Lightning Imaging Sensor (LIS) on the Tropical Rainfall Measuring Mission (TRMM) satellite to describe the time-evolution of lightning from first light through the end of the flash. The radiant energy of the flash may appear as isolated flickers, or it may occur as a stream of nearly continuous illumination coming out of the top of the cloud. We organize these sustained optical pulses into features called "series" and examine their energy and timing in the flash. Flashes and series are comprised of mostly dim optical emissions, but a small number of pulses substantially increase the energy of the flash. These bright "groups" and the "bright series" that contain them describe physical lightning processes that produce large amounts of light. The frequency of bright series and the amount of time between subsequent bright series agree with ground-based measurements of return strokes and physical processes known as K-changes. Thus, lightning imagers may observe a combination of physical lightning processes.