Combined Optical And Radio-Frequency Perspectives On A Hybrid Cloud-To-Ground Lightning Flash Observed By The Forte Satellite

We use the coincident optical and radio-frequency (RF) measurements taken by the Fast On-orbit Recording of Transient Events (FORTE) satellite to shed light on common optical signatures recorded by NASA and NOAA lightning imagers during Cloud-to-Ground (CG) lightning. We build flash cluster data for FORTE using the same techniques as the NASA/NOAA instruments to document the optical/RF evolution of an oceanic hybrid -CG flash. The flash began with strong Very High Frequency (VHF)-band emission from a Narrow Bipolar Event (NBE) that initiated a period of normal bilevel intracloud (IC) activity in two vertical layers (8 and 12 km) that lasted for 490 ms. VHF waveforms show step leader activity ahead of the return stroke. All impulsive VHF sources after the stroke come from the lower (8 km) layer. K-changes are noted following the return stroke, but no subsequent strokes are detected. The optical flash began 136 ms after the NBE RF pulse. Twenty-two of the 33 optical groups were dim and occurred during the in-cloud phase of the flash. This activity included both isolated pulses and sustained periods of illumination over tens of milliseconds. Initial cloud pulses accounted for 23% of the total optical radiance from the flash. Illumination during the return stroke contributed 58% of the total radiance, and the K-changes and cloud pulses after the stroke supplied the remaining 19%. These results highlight the benefit of having RF alongside optical lightning measurements for clarifying signatures in the optical data and providing information on their physical origins.Plain Language Summary Lightning has been measured from space for more than 25 years. Most space-based sensors were optical instruments that detect lightning by looking for the cloud-top lighting up. Recent studies have shown that we can measure how lightning moves using this type of instrument, but distinguishing strokes from radiant intracloud processes on a one-to-one basis is still an unsolved problem. Radio lightning measurements provide greater insights into the physical origin of lightning signals, and one satellite flew with both optical and radio lightning sensors. We use data collected by this FORTE satellite to investigate the link between physical lightning processes (CG, in-cloud impulsive events, and K-changes) and their optical signals. We confirm many of the findings from previous studies, but also show that making inferences with optical data alone can be problematic. Only optical and radio measurements, combined, provide a complete picture of what is going on in the lightning flash and how the flash illuminates the cloud.