Combined Optical and Radio‐Frequency Perspectives on a Hybrid Cloud‐To‐Ground Lightning Flash Observed by the FORTE Satellite

Abstract

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.

Combined Optical and Radio‐Frequency Measurements of a Lightning Megaflash by the FORTE Satellite

“…We also noted in Peterson et al. (2021) that the PDD stops reporting after a specific number of triggers are reported on typical flash time scales. As this case occurred just 2 months after the previous hybrid CG flash case (thus, the FORTE configuration was largely the same), this 20‐trigger maximum noted previously will be a limitation in the present study as well.…”

“…Unlike Peterson et al. (2021), this flash did not start with a powerful TIPP waveform from a narrow bipolar event. Any IC events that could have occurred before first LLS light were too weak to trigger TATR.…”

“…This study and Peterson et al. (2021) both use TATR observations from late 1999 when it was configured to measure lowband (26–48 MHz) waveforms over a 0.4 ms record and set to trigger autonomously. In its autonomous mode, TATR monitored the RF power in eight RF channels that were each 1 MHz wide.…”

“…Design considerations for the full FORTE cluster feature data set are described at length in Peterson et al. (2021) and we will discuss only the optimal data structure for FORTE analyses here (shaded boxes in Table 1 from Peterson et al., 2021). LLS events are used to define “group” features that represent contiguous regions on the LLS CCD array that are illuminated during the same 2.47 ms integration frame.…”

“…While these optical‐only capabilities are useful for analyzing lightning activity, a comprehensive view of lightning physics is achieved when optical and radio frequency (RF) measurements are combined to describe the same flash. We previously used coincident optical and VHF measurements taken by the fast on‐orbit recording of transient events (FORTE) satellite to identify the physical processes responsible for the optical signals recorded by its pixelated lightning imager during a hybrid CG flash (Peterson et al., 2021). In this study, we use the same approach to investigate the combined‐phenomenology evolution of a horizontally expansive lightning megaflash (American Meteorological Society AMS, 2021) observed over the open ocean near the Canary Islands.…”