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

2022

Self Cite

Lightning processes generate a diverse collection of optical pulses from current traversing the lightning channels. These signals are then broadened spatially and temporally via scattering in the clouds. The resulting waveforms measured from space with instruments like the photodiode detector (PDD) on the Fast On‐orbit Recording of Transient Events (FORTE) satellite have a variety of shapes. In this study, we use coincident optical and Radio‐Frequency measurements to document the properties of optical PDD waveforms associated with different types of lightning, estimate delays from scattering in the clouds, and comment on how pulse shape impacts optical lightning detection. We find that the attributes of optical pulses recorded by the PDD are consistent with prior studies, but vary globally and with event amplitude. The most powerful lightning tends to be single‐peaked with faster rise times (median: ∼100 µs) and shorter effective widths (median: ∼400 µs) than normal lightning. Particularly dim events, meanwhile, include cases of broad optical waveforms with sustained optical emission throughout the PDD record, which the pixelated FORTE Lightning Location System (LLS) instrument has difficulty detecting. We propose that this is due to the optical signal being divided between individual LLS pixels that are each, individually, not bright enough to trigger. We use PDD waveforms and Monte Carlo radiative transfer modeling to demonstrate that increasing the temporal and spatial resolution of a pixelated lightning imager will make it more difficult to detect these broad/dim pulses as their energy becomes divided between additional pixels/integration frames.

Section: Resultsmentioning

confidence: 99%

FORTE Measurements of Global Optical Lightning Waveforms and Implications for Optical Lightning Detection

2022

Self Cite

“…Low‐altitude sources are found to be particularly common in winter and/or oceanic regions outside of the tropics, as well as over the La Plata basin, where large MCSs produce abundant stratiform lightning, including megaflashes. We previously showed that an individual large stratiform flash can produce hundreds of FORTE RF events with impulsive VHF sources clustered in a layer between 4 and 8 km altitude (Peterson et al., 2021a). Regions with frequent stratiform lightning are thus expected to have source altitude profiles with significant contributions from both high‐altitude convective sources and low‐altitude stratiform sources.…”

Section: Discussionmentioning

confidence: 99%

“…We used it previously to identify interesting flashes in Peterson et al. (2021a, 2021b), and to generate event and flash statistics in Peterson (2022a, 2022c).…”

Section: Methodsmentioning

confidence: 99%

FORTE Measurements of Global Lightning Altitudes

2022

Self Cite

While multiple lightning detection systems provide geographical locations of lightning events across the globe, robust lightning altitude measurements on a global scale have proven elusive. Space‐based platforms have an advantageous viewing geometry for making these measurements, but prior studies with the Fast On‐orbit Recording of Transient Events (FORTE) satellite were limited to a few thousand events. In this study, we apply the same technique for calculating source altitude from the previous efforts to a large catalog of hundreds of thousands of global FORTE in‐cloud lightning events that were coincident with flashes geolocated by its lightning imager between 1997 and 2003. We use this new data set to document global variations in lightning altitude. As in previous studies, we find that FORTE primarily resolves sources from the upper (positive) charge layer at ∼11 km altitude in normal thunderstorms. However, sources are also recorded from other charge layers in the storm and from leaders developing between layers. In particular, we note a pronounced increase in source altitude in the first 20 ms of FORTE flashes from the negative leader developing upward into the upper positive charge layer. Regions known for wintertime and/or stratiform lightning have increased contributions from low‐altitude sources, while tropical regions particularly around Panama and the Maritime Continent have the greatest concentrations of high‐altitude sources.

“…This data set is documented at length in Peterson et al. (2021a) and the components of the data set that we will use in this study are described in the following sections. See Jacobson et al.…”

Section: Methodsmentioning

confidence: 99%

“…Whenever a specified number of PDD events is recorded (i.e., a multiple of 10) during a specified time interval, the PDD will turn off until the next GPS-derived 1 Hz signal. We have discussed PDD dropping out after precisely 20 triggers in both FORTE flashes that we previously analyzed in detail in Peterson et al (2021aPeterson et al ( , 2021b due to this filter. It will only activate in periods with high lightning rates (including high event rates from individual flashes), and the instrument should be able to recover at the next GPS second, with only triggers in the middle of the flash being missed.…”

Section: Forte Pdd Eventsmentioning

confidence: 98%

Combined Optical and Radio‐Frequency Perspectives on the Time Evolution of Lightning Measured by the FORTE Satellite

2022

Self Cite

We use a cluster feature data set for the Fast On‐orbit Recording of Transient Events (FORTE) satellite that combines detections from its pixelated Lightning Locating System (LLS), photodiode detector (PDD) and Radio‐Frequency (RF) instrumentation to generate statistics describing the frequency and timing of lightning events detected by each instrument during lightning flashes. Coincident observations from the same vantage point allow us to directly compare flash details that can be resolved by the wide Field of View (FOV) instruments relative to the pixelated LLS—whose design is based on NASA's Lightning Imaging Sensor. We find that both the PDD and RF system typically generate more detections than the lightning imager (mean: 1.5 PDD events per LLS group, 2 RF events per LLS group) from pulses that are either not sufficiently bright in the optical band (in the case of RF) or that lack the optical energy density (in the case of the PDD) required to trigger one of the pixels on the LLS imaging array. This includes additional activity before the first LLS group or after the final LLS group. These FORTE results demonstrate that certain lightning processes would be better resolved by wide‐FOV optical and RF instruments than lightning imagers. Current/future space‐based missions that use/plan to use similar instruments will improve our understanding of flash evolution by resolving details missed by lightning imagers.