Lightning processes generate a diverse collection of optical pulses depending on how current traverses the lightning channels. These signals are then broadened spatially and temporally via multiple scattering in the clouds. The resulting optical 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 (RF) 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 generally consistent with prior studies, but vary across the globe and with event amplitude. The brightest lightning tends to be single-peaked with faster rise times (median: ~100 µs) and shorter effective widths (median: ~400 µs). Dim events also include cases of broad optical waveforms with sustained optical emission throughout the PDD record, which the pixelated FORTE LLS instrument has difficulty detecting. We propose that this is due to the optical signal being divided between individual pixels that are each, individually, not bright enough to trigger the LLS. 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.

We use a cluster feature dataset for the Fast On-orbit Recording of Transient Events (FORTE) satellite that combines detections from its pixelated lightning imager (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 (LIS). 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.

Space-based optical lightning sensors including the Lightning Imaging Sensor (LIS) and Geostationary Lightning Mapper (GLM) are pixelated imagers that detect lightning as transient increases in cloud-top illumination. Detection requires optical lightning emissions to escape the cloud-top to space with sufficient energy to trigger a pixel on the imaging array. Through scattering and absorption, certain clouds are able to block most light from reaching the instrument, causing a reduction in Detection Efficiency (DE). We use cases of radiant lightning emissions that illuminate large cloud-top areas to examine scenarios where clouds block light in only certain pixels on the imaging array. In some cases, these anomalies in the spatial radiance distribution from the lightning pulse leads to “holes” in the optical lightning flash where certain pixels fail to trigger, entirely. Such holes are identified algorithmically in the Tropical Rainfall Measuring Mission (TRMM) satellite LIS record over the southern Continental United States, and the microphysical properties of the coincident storm region are queried. We find that holes primarily occur in tall (IR Tb < 235 K) convection (87%) and overhanging anvil clouds (10%). The remaining 3% of holes occur in moderate-to-weak convection or in clear air breaks between stormclouds. We further demonstrate how an algorithm that assesses the spatial radiance patterns from energetic lightning pulses might be used to construct an optical transmission gridded stoplight product for GLM that could help operators identify clouds with a potentially-reduced DE.

Two years of Geostationary Lightning Mapper (GLM) science data are used to document the brightest lighting flashes observed on the Americas continent. The most radiant optical lightning emissions – termed “superbolts” – were first identified by our Vela satellite constellation in the 1970s (Turman, 1977) and are defined in terms of peak optical power. GLM is an integrating sensor that, instead, measures the total optical energy from a lightning pulse. While GLM might not correctly classify short-duration energetic superbolts, its top lightning cases certainly fall in the superbolt category, and the wealth of GLM measurements over its stationary hemispheric field of view provide an unmatched sample of extraordinarily bright lightning. While radiant bolts in excess of 100x the optical energy of typical lightning are ubiquitous across the Americas and result from many types of lightning processes, we find the most radiant cases (>1000x) are concentrated in the central United States and in the La Plata basin in South America. Coincident Earth Networks Global Lightning Network (ENGLN) observations reveal that these extremely bright emissions usually result from +CG strokes with high peak currents in long horizontal flashes outside of the convective core. Single cases of these megaflashes might produce multiple superbolts over their durations.

Space-based lightning imagers have shown that complex cloud scenes that consist 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 three-dimensional geometries to shed some light on the origins of certain irregular radiance patterns that have been recorded from orbit. We show that reflections off nearby cloud faces can explain lightning signals in non-electrified 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 direction.

We use the coincident optical and radio-frequency measurements taken by the 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 clustering techniques as GLM and document the optical / RF evolution of an oceanic hybrid -CG flash over its 656 ms duration. The flash began with strong VHF emission from a Narrow Bipolar Event (NBE) that initiated a period of normal bilevel intracloud (IC) activity in two vertical layers (8 km and 12 km) that lasted for 490 ms. VHF waveforms show step leader activity ahead of seawater attachment in the return stroke. All impulsive VHF sources after the stroke come from the lower (8 km layer) only. 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. 22 of the 30 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 a further 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.