Changes to the Appearance of Optical Lightning Flashes Observed From Space According to Thunderstorm Organization and Structure

Bulletin of the American Meteorological Society

2021

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Section: Measuring Lightning Flash Extent and Duration With Glmmentioning

confidence: 71%

“…Peterson et al (2020c) plots the group-level structure of GLM flashes at 17 UTC (12 CT), and a large flash can be seen extending behind the convective line in the stratiform region that developed over the previous day.Accepted for publication in Bulletin of the American Meteorological ociety. DOI S 10.1175/BAMS-D-20-0178.1.…”

mentioning

confidence: 99%

Where Are the Most Extraordinary Lightning Megaflashes in the Americas?

Bulletin of the American Meteorological Society

2021

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“…We previously reported cases of highly irregular radiance patterns where the shape of the optical pulse on the CCD imaging array followed the cloud boundaries (i.e., Figures 3 and 4 in Peterson, Deierling, et al, 2017), cases where the radiance pattern extended outward from the edge of the thunderstorm core giving an incorrect impression that these warm boundary clouds were producing lightning (Peterson, Rudlosky, et al, 2017), and cases where optical emissions were blocked from reaching orbit by certain cloud regions, resulting in “holes” in otherwise‐contiguous flash footprints (i.e., Figure 1 in Peterson & Liu, 2013). Unobscured lightning sources have also been proposed as the mechanism responsible for certain lightning “superbolts” (Turman, 1977) that primarily illuminate the edge of the storm (Peterson et al, 2020).…”

Section: Introductionmentioning

confidence: 99%

Modeling the Transmission of Optical Lightning Signals Through Complex 3‐D Cloud Scenes

2020

JGR Atmospheres

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Space-based lightning imagers have shown that complex cloud scenes consisting of multiple tall convective features, anvil clouds, and warm boundary cloud layers are illuminated by lightning in many different ways depending on where the lightning occurs and how energetic it is. Modifications to the optical lightning signals from radiative transfer in the cloud medium can lead to reductions in detection efficiency and location accuracy for these instruments and can also cause some of the optical signals that are detected to have unexpected spatial energy distributions. In this study, we perform Monte Carlo radiative transfer simulations of optical lightning emissions in clouds with complex 3-D geometries to shed some light on the origins of certain irregular radiance patterns that have been recorded from orbit. We show that diffuse reflections off nearby cloud faces can explain lightning signals in nonelectrified clouds, tall clouds can result in poor optical transmission and suppressed radiances that could lead to missed events and that particularly favorable viewing conditions can cause otherwise normal lightning to produce a superbolt that is orders of magnitude brighter than the same flash seen from a different angle. Plain Language Summary Lightning is detected from space using instruments that report rapid changes in cloud brightness from lightning illumination. However, this light can be modified by scattering and absorption in the cloud. Scattering off water drops causes portions of the signal to be diluted in space and delayed in time. What starts off as an impulsive point light source in the cloud may illuminate a region of the cloud-top that is 100 km across with a waveform that persists over significant fraction of a millisecond. Interactions between the optical lightning emissions and the cloud scene are particularly complex when the surrounding clouds do not take on a simple geometric shape. Clouds observed in nature often contain multiple vertical layers including warm boundary clouds and overhanging anvils. Understanding some of the more irregular spatial energy distributions recorded by space-based lightning sensors requires accounting for these complex geometries. In this study, we develop 3-D cloud models that approximate cloud structures found in nature and perform Monte Carlo radiative transfer simulations of how they are illuminated by lightning. In doing so, we confirm the suspected origins of irregular cloud illumination, such as reflections off of nearby cloud faces or particularly favorable viewing conditions allowing normal lightning to appear highly energetic.

“…The most energetic optical pulses recorded by LIS typically occurred in one of two scenarios: "anvil superbolts" where most of the illuminated pixels were located near cloud boundaries outside of the raining area of the storm, and "stratiform superbolts" that almost exclusively illuminated raining stratiform clouds. Peterson et al (2020b) also used continuous Geostationary Lightning Mapper (GLM: Goodman et al, 2013;Rudlosky et al, 2019) observations to examine how lightning characteristics changed over time within a single storm system. These analyses showed that early convection was favorable for large flashes that lacked apparent lateral motion.…”

Section: Superbolts As Normal Lightningmentioning

confidence: 99%

Revisiting the Detection of Optical Lightning Superbolts

Kirkland

2020

JGR Atmospheres

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This study uses Fast On-Orbit Detection of Transient Events (FORTE) satellite observations to identify superbolt-class optical lightning events and evaluate their origins. Superbolts have been defined by Turman (1977, https://doi.org/10.1029/JC082i018p02566) as lightning pulses whose peak optical power exceeds 10 11 W. However, it has been unclear whether superbolts resulted from particular types of high-energy lightning process or whether they were the result of measurement bias. In the latter case, any decently bright lightning process could be recorded as a superbolt if the sensor had a particularly clear sight line to the hot channel without thick clouds diluting the optical signals. Our 12-year analysis of FORTE superbolt detections indicates that the lower optical superbolt energy range (~100 GW) is dominated by normal lightning, but brighter cases are predominantly strong +CG strokes that originate from specific types of storms. Oceanic storm systems, particularly during the winter, and especially those located around Japan are shown to produce these intense superbolts. We suggest that some optical superbolts result from favorable viewing conditions and would not be identified as such by another instrument located elsewhere and that others are associated with a unique set of physics that may merit the "superbolt" distinction. Plain Language Summary In 1977, Turman identified lightning that was 100 times brighter than normal in the Vela satellite data. These pulses radiated between 100 GW and multiple terawatts of optical power at the source. This observation sparked a debate as to whether these "superbolts" were caused by a certain type of powerful lightning or were merely the result of measurement biases. Clouds dilute the optical signals generated by lightning and reduce the optical powers recorded by satellites. If lightning occurs at the edge of the storm, then the light can travel to the space-based sensor at full intensity. Thus, any lightning event could produce a superbolt if the satellite happened to be in a favorable position to see it, and sensors elsewhere might not classify it as a superbolt. This study analyzes FORTE satellite data to garner a better understanding of optical superbolts. We find that weaker superbolts (100 GW) result from both scenarios: Some come from normal lightning, while others are caused by strong +CG strokes that tend to occur in oceanic regions, in the winter, and often near the coast of Japan. The most powerful superbolts (>350 GW), however, predominantly come from strong +CGs and may still merit the "superbolt" distinction.