Where Are the Most Extraordinary Lightning Megaflashes in the Americas?

2022

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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

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“…The impulsive VHF sources that we could identify were clustered in a single vertical layer between 4 and 8 km altitude (Peterson et al., 2021a). While megaflashes occur in many regions, the La Plata basin is one of the primary Americas hotspots (Peterson, 2021; Peterson & Stano, 2021)—along with the Continental United States, which also has an increased share of low‐altitude sources in Figure 6a. We have also recorded long horizontal lightning over the Mediterranean Sea (Peterson, Rudlosky, & Deierling, 2017) (Figure 6b) and showed that the other midlatitude oceanic regions with increased low‐altitude source fractions in Figure 6 are locations with long horizontal flashes (Figure 5 in Peterson, Deierling, et al., 2017).…”

Section: Resultsmentioning

confidence: 99%

FORTE Measurements of Global Lightning Altitudes

2022

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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.

“…When a flash is terminated for violating one of these thresholds, it will be marked with a “degraded” quality flag in the operational GLM data and any subsequent events/groups will define a new and independent flash feature. This causes cases of long horizontal lightning megaflashes (Lyons et al., 2020; Peterson, 2021a; Peterson, Lang, et al., 2020) to be split into multiple (often tens of) “flash” features in the GLM LCFA data with all but the final emissions along each branch being designated as degraded quality.…”

Section: Methodsmentioning

confidence: 99%

The Illumination of Thunderclouds by Lightning: 4. Volumetric Thunderstorm Imagery

Mach

2022

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Optical instruments such as the Geostationary Lightning Mapper (GLM) detect lightning based on transient changes in cloud illumination. The horizontal location of lightning is determined from the coordinates of the pixels on the imaging array illuminated during the flash. However, the vertical position of the lightning pulses (approximated by GLM “groups”) below the cloud top cannot be routinely measured from a single space‐based instrument. In our prior work, we have developed a machine learning algorithm that can infer optical source altitude for a given pulse based on how the optical energy is distributed across the group footprint and the local Advanced Baseline Imager Cloud‐Top Height (CTH). In this fourth part of our thundercloud illumination study, we leverage these source altitudes to generate volumetric GLM imagery of a Colombia thunderstorm. We find that 3D versions of the current GLM meteorological imagery products (that describe thunderstorm kinematics) and thundercloud imagery products (that depict how the flashes appear from space) provide additional insights into lightning activity in the thunderstorm that are lost in the vertical integration used to generate the current 2D GLM gridded products. This new volumetric imaging capability provides a more comprehensive picture of where lightning occurs in the storm, how its physical characteristics vary across three‐dimensional space, and how its optical emissions interact with surrounding the cloud medium.