Intracloud Flashes Research Articles

The Characterization of Electric Field Pulses Observed in the Preliminary Breakdown Processes of Normal and Inverted Intracloud Flashes

We have studied the parameters of preliminary breakdown (PB) pulses in 395 normal and 319 inverted intracloud (IC) flashes observed in Gifu, Japan, and Ningxia, China, respectively, by using a low-frequency mapping system called fast antenna lightning mapping array (FALMA). These parameters are extracted from the first half of the PB pulses. It is found that compared to normal IC flashes, inverted IC flashes exhibited PB pulses with slower rise times (6.8 vs. 3.1 μs), wider half-peak widths (3.8 vs. 2.5 μs), longer zero-crossing times (26.2 vs. 14 μs), and extended fall times (4 vs. 3.2 μs). We further demonstrated that such discrepancies between normal and inverted IC flashes should not be caused by subjective factors, like noise threshold setting, or objective factors, like signal propagation distance. Based on this analysis, finally, we inferred that the discrepancies should be a reflection of the PB channel properties of normal and inverted IC flashes.

Total lightning signatures in a tornadic thunderstorm over the Pearl River Delta of Southern China

Tornado rises rotating air columns extending from the bottom of cumulonimbus clouds to the ground, often accompanied by strong convection with thunderstorms, hail, and short-term heavy precipitation, which seriously threaten people's lives and property safety. Studying the characteristics of thunderstorm activity during a tornado is crucial for comprehending the atmospheric electrical mechanisms involved in its generation process and developing cooperative methods for tornado warning and forecasting. Based on the VLF/LF total lightning location system and radar echo data, this paper analyzes the spatial-temporal evolution, frequency, polarity, and height of total lightning during a strong tornado (EF3) in the Pearl River Delta region on June 3, 2014. The lightning activity lasted for about 3.5 h, with a total of 41,117 total lightning flashes, of which intra-cloud (IC) flashes accounted for 74.7%, cloud-to-ground (CG) flashes accounted for 21.6%, and narrow bipolar events (NBEs) accounted for 3.7%. Connected-Component Labeling (CCL) algorithm was used to divide the thunderstorm into four stages: initiation, development, maturation, and dissipation, and it was observed that the occurrence of tornadoes was closely related to total lightning activities. The observed characteristics of lightning activity in the tornado are summarized through comparison with other studies.

The impact of aerosols as ice nucleating particle on microphysics and electrification in the cumulus model

Here, a heterogeneous ice nucleation parameterization associated with aerosol acting as ice nucleating particle (INP) was implanted into a two-dimensional numerical cumulus model. To explore the impact of INP on microphysical and electrical processes, a comparison was conducted with the original approach, which employs an empirical formula. Simulation results indicate that INP greatly impacted microphysical evolution in the heterogeneous ice nucleation process, reducing total liquid precipitation amounts and causing a slight precipitation delay, as well as increasing the diameter and mixing ratio of ice crystals and the expansion of the vertical distribution of ice crystals. This led to a notable change in electrification in thunderstorms. The increase of ice crystal diameter was the dominant contributor to the enhancement of electrification using the new parameterization. Additionally, unlike the structure of thunderclouds in a mature stage, which always retains a normal dipole structure adopting the empirical formula, a tripole structure developed a lower positive charge, and the polarity inversion with upper negative and lower positive charges occurred when the new parameterization was adopted. This predominately was the result of abundant ice crystals present below the reversal temperature. The electrification characteristics of thunderstorms may have a close connection with lightning activity. It has been found that charge structure changed significantly in the two cases, with the tripolar charge structure facilitating the production of inverted intra-cloud (IC) flashes and negative Cloud-to-ground (CG) flashes simulated by the new parameterization; additionally, clouds simulated using the empirical formula may be able to develop normal IC lightning and positive CG flashes. Therefore, it will be very meaningful to obtain greater insight into the characteristics of thunderstorms electrification in cumulus model with aerosols.

Study of dynamics-microphysical-lightning activity characteristics in a tropical hailstorm

Based on three-dimensional lightning locations and S-band polarimetric Doppler radar data, a hailstorm that occurred on Hainan Island on June 4, 2021 was analyzed in detail. The analysis revealed that the flash activity of hailstorms is mainly intracloud (IC) flashes, the cloud-to-ground (CG) flashes frequency were less, positive CG flashes (+CG) were domination in the CG activity before hailfall. The flash frequency appeared “lightning jumps” when hailstorms merging with convective cells, ice hydrometeors also rapidly increased in the clouds of the mixed-phase area in this period, which was beneficial for efficient electrification. The Zdr column and evolution of echo top indicated presence of an intense updraft during the mature stage of the hailstorm. Zdr column depth about 4 km close to 20 °C during the mature stage. Near the time of the hailfall, Zdr column was contraction, due to sinking hail generated downdraft within the cloud. Zdr column after hailfall may be produced by the convergence of downdraft and sea breeze. In combination with the spatiotemporal distribution of the hydrometeors in hailstorms, flash activity was most closely related to graupel and it plays a important role for flashes electrification. The hail plays a minimum role in charge separation. Radiation sources were concentrated between 50 dBZ and 55 dBZ. The spatial distribution of radiation sources indicated possible complex charge structure within hail storm clouds.

A Lightning Classification Method Based on Convolutional Encoding Features

At present, for business lightning positioning systems, the classification of lightning discharge types is mostly based on lightning pulse signal features, and there is still a lot of room for improvement. We propose a lightning discharge classification method based on convolutional encoding features. This method utilizes convolutional neural networks to extract encoding features, and uses random forests to classify the extracted encoding features, achieving high accuracy discrimination for various lightning discharge events. Compared with traditional multi-parameter-based methods, the new method proposed in this paper has the ability to identify multiple lightning discharge events and does not require precise detailed feature engineering to extract individual pulse parameters. The accuracy of this method for identifying lightning discharge types in intra-cloud flash (IC), cloud-to-ground flash (CG), and narrow bipolar events (NBEs) is 97%, which is higher than that of multi-parameter methods. Moreover, our method can complete the classification task of lightning signals at a faster speed. Under the same conditions, the new method only requires 28.2 µs to identify one pulse, while deep learning-based methods require 300 µs. This method has faster recognition speed and higher accuracy in identifying multiple discharge types, which can better meet the needs of real-time business positioning.

Machine-learning-based investigation of the variables affecting summertime lightning occurrence over the Southern Great Plains

Abstract. Lightning is affected by many factors, many of which are not routinely measured, well understood, or accounted for in physical models. Several commonly used machine learning (ML) models have been applied to analyze the relationship between Atmospheric Radiation Measurement (ARM) data and lightning data from the Earth Networks Total Lightning Network (ENTLN) in order to identify important variables affecting lightning occurrence in the vicinity of the Southern Great Plains (SGP) ARM site during the summer months (June, July, August and September) of 2012 to 2020. Testing various ML models, we found that the random forest model is the best predictor among common classifiers. When convective clouds were detected, it predicts lightning occurrence with an accuracy of 76.9 % and an area under the curve (AUC) of 0.850. Using this model, we further ranked the variables in terms of their effectiveness in nowcasting lightning and identified geometric cloud thickness, rain rate and convective available potential energy (CAPE) as the most effective predictors. The contrast in meteorological variables between no-lightning and frequent-lightning periods was examined for hours with CAPE values conducive to thunderstorm formation. Besides the variables considered for the ML models, surface variables and mid-altitude variables (e.g., equivalent potential temperature and minimum equivalent potential temperature, respectively) have statistically significant contrasts between no-lightning and frequent-lightning hours. For example, the minimum equivalent potential temperature from 700 to 500 hPa is significantly lower during frequent-lightning hours compared with no-lightning hours. Finally, a notable positive relationship between the intracloud (IC) flash fraction and the square root of CAPE (CAPE) was found, suggesting that stronger updrafts increase the height of the electrification zone, resulting in fewer flashes reaching the surface and consequently a greater IC flash fraction.

A Strong Pulsing Nature of Negative Intracloud Dart Leaders Accompanied by Regular Trains of Microsecond‐Scale Pulses

AbstractWe report the first observations of negative intracloud (IC) dart‐stepped leaders accompanied by regular trains of microsecond‐scale pulses, simultaneously detected by shielded broadband magnetic loop antennas and the radio telescope Low Frequency Array (LOFAR). Four investigated pulse trains occurred during complicated IC flashes on 18 June 2021, when heavy thunderstorms hit the Netherlands. The pulses within the trains are unipolar, a few microseconds wide, and with an average inter‐pulse interval of 5–7 μs. The broadband pulses perfectly match energetic, regularly distributed, and relatively isolated bursts of very high frequency sources localized by LOFAR. All trains were generated by negative dart‐stepped leaders propagating at a lower speed than usual dart leaders. They followed channels of previous leaders occurring within the same flash several tens of milliseconds before the reported observations. The physical mechanism remains unclear as to why we observe dart‐stepped leaders, which show mostly regular stepping, emitting energetic microsecond‐scale pulses.

Observations of Elves and Radio Wave Perturbations by Intense Lightning

AbstractElectromagnetic pulses (EMPs) and quasi‐static electric fields (QE) from powerful lightning heat and ionize the lower ionosphere. The EMP disturbance may appear as an elve at ∼80–95 km altitude, and the QE field as a halo or a sprite at ∼60–80 km altitude. Both are thought to perturb crossing radio signals because of changes to the electrical conductivity of the regions. Here we present an analysis of 63 elves and corresponding radio signal perturbations from an almost stationary thunderstorm system that allows us to untangle some of the dependencies of perturbations on the lightning characteristics. The amplitude perturbations of a VLF‐transmitter signal are characterized as either long‐recovery, early events (LOREs) or as early events. We find that LOREs are related to lightning with high peak currents and bright elves, and that their sign (amplitude increase or decrease) depends on the relative locations of the transmitter, disturbance and receiver. Based on a subset of strokes, lightning with elves has on average ∼3 times the impulse charge‐moment‐change and power in broadband as lightning of similar peak currents without elves. The early events occur without observed elves, sprites or halos. They recover in ∼10–100 s and are observed for both polarities of cloud‐to‐ground lightning and for intracloud flashes. It is proposed that these observations may relate to regions of reduced conductivity caused by an electron attachment/detachment process at lower heights, or by electron enhancements associated with TLEs that are too dim to be detected by the camera.

Three‐Dimensional Broadband Interferometric Mapping and Polarization (BIMAP‐3D) Observations of Lightning Discharge Processes

AbstractFollowing on our earlier single‐station, 2‐dimensional (2D) broadband interferometric mapping and polarization (BIMAP) observations of lightning discharges, we recently deployed two BIMAP stations at Los Alamos National Laboratory to map the lightning sources and their polarization in full 3‐dimensional (3D) space (BIMAP‐3D). The two stations are separated by 11.5‐km and each station consists of four antenna sets (instead of three for the original BIMAP) that form a Y‐shaped array for improved interferometric performance. In this paper, we report the BIMAP‐3D system design, a generalized and analytical 2D interferometry technique for noncoplanar antenna array, a two‐stage 3D mapping technique based on geometric triangulation and baseline‐based differential time of arrival, and a technique to reconstruct the polarization orientation in 3D space by combining the 2D polarization results from the two‐station observations. Along with description of the techniques, we demonstrate and discuss the initial lightning results, including 3D maps for a hybrid intracloud and cloud‐to‐ground flash and for a normal intracloud flash, development of abnormal K‐change leaders, and polarization signatures for a K‐change leader. We find that with the two‐stage 3D mapping techniques, the sources can be located to meters accuracy for a favorable event that occurs between the two stations. We also find the polarization vectors for the example K leader are mostly orthogonal to the leader channel after the full 3D polarization analysis. The main purpose of this paper is to report the BIMAP‐3D techniques and capabilities. Detailed analysis of more specific discharge processes will be reported in later studies.

Three-Dimensional Mapping on Lightning Discharge Processes Using Two VHF Broadband Interferometers

Lightning Very-high-frequency (VHF) broadband interferometer has become an effective approach to map lightning channels in two dimensions with high time resolution. This paper reports an approach to mapping lightning channels in three dimensions (3D) using two simultaneous interferometers separated by about 10 km. A 3D mapping algorithm was developed based on the triangular intersection method considering the location accuracy of both interferometers and the arrival time of lightning VHF radiation. Simulation results reveal that the horizontal and vertical location errors within 10 km of the center of the two stations are less than 500 m and 700 m, respectively. The 3D development of an intra-cloud (IC) lightning flash and a negative cloud-to-ground (-CG) lightning flash with two different ground terminations in the same thunderstorm are reconstructed, and the extension direction and speed of lightning channels are estimated consequently. Both IC and CG flash discharges showed a two-layer structure in the cloud with discharges occurring in the upper positive charge region and the lower negative charge region, and two horizontally separated positive charge regions were involved in the two flashes. The average distance of the CG ground terminations between the interferometer results and the CG location system was about 448 m. Although disadvantages may still exist in 3D real-time location compared with the lightning mapping array system working with the principle of the time of arrival, interferometry with two or more stations has the advantage of lower station number and is feasible in regions with poor installation conditions, such as heavy-radio-frequency-noise regions or regions that are difficult for the long-baseline location system.

Analyzing lightning characteristics in central and southern South America

Lightning data has become an integral part of weather observation and has been shown to be an effective tool for alerting and nowcasting. The Earth Networks Total Lightning Network (ENTLN) is a global lightning detection network established in 2009. ENTLN consists of roughly 1800 broadband electric field sensors globally that detect intra-cloud (IC) and cloud-to-ground (CG) lightning, with over 140 sensors in South America. In this study, we analyze the geographical variation of lightning characteristics such as density, peak current, and polarity, with the goal of better understanding the meteorological processes that produce these thunderstorms. Results indicate significant variation in average peak current per flash, with a maximum occurring throughout Northern Argentina, and a localized strong minimum North of São Paulo, Brazil. This minimum is coincident with a region dominated by negative IC flashes. In Central and Northern Argentina, 40%–50% of CG flashes were observed to be positive. Conventional thunderstorms typically have around 10% positive CG flashes. The large-scale patterns observed in this study support the findings of previous case studies regarding inverted polarity storms, mesoscale convective systems, and transient luminous event production, most of which had more limited scopes.

A Self‐Sustained Charge Neutrality Intracloud Lightning Parameterization Containing Channel Decay and Reactivation

AbstractA self‐sustained charge neutrality parameterization scheme of intracloud (IC) lightning is presented in this paper. The scheme contains the following features: simultaneous development of multiple branches, polarity asymmetry of positive and negative leaders, nonlinear electrical parameters, complete charge neutrality, and channel decay and reactivation. Nonlinear electrical parameters and charge neutrality are the key factors that determinate channel decay and reactivation. To demonstrate the simulation capacity of this scheme, two simulated intracloud flashes by this scheme under the tripole charge structure are chosen for analysis. These two IC flashes show clearly the above features, where reactivation processes that initiated from different branches and the real‐time changes in nonlinear electrical parameters are mainly analyzed. The simulation results are in line with the current knowledge on lightning, which also suggests that the new scheme may become a useful tool to deeply explore the discharge phenomena involving channel decay and reactivation.

Upward Bipolar Lightning Flashes Originated From the Connection of Recoil Leaders With Intracloud Lightning

AbstractThe present work shows high‐speed videos of two upward flashes that started with positive upward leaders and, instead of being followed by negative subsequent return strokes, they were followed by positive subsequent return strokes. In both cases, after the positive leaders developed, recoil leaders (RL) appeared in their decayed branches as would be usual in negative upward lightning flashes. However, in these flashes the negative end of a recoil leader connected to a positive leader of an intracloud (IC) flash nearby. The connection initiated a downward positive leader that re‐ionized the decayed channel of the upward flash all the way to the tower giving origin to a positive subsequent return stroke. This work shows that RL do play an important role in the occurrence of bipolar upward flashes and their interaction with IC flashes can provide explanations for all types of bipolar upward flashes initiated by upward positive leaders.

Relationship Between Convective Storm Properties and Lightning Over the Western Ghats

AbstractX‐band radar observations are integrated with lightning location network observations to investigate the relationship between convective storm properties and lightning over the Western Ghats during a monsoon season (June–September 2014). Convective storms (cells) were identified using an objective‐tracking method and instantaneous lightning strikes within the radar domain were then linked with observed storms. This spatio‐temporal sampling of individual convective cells and lightning has facilitated process‐based study of electrified convection over the Ghats for the first time. Storm and lightning occurrences are typically high during monsoon onset and withdrawal months of June and September, respectively. A spatial correspondence between deep‐intense storms, lightning, and intense convective cores indicated presence of large hydrometeors in the mixed‐phase region of storm supported by strong updrafts and is essential for lightning. The large‐scale instability that peaked during afternoon hours was a key factor in the formation of deep‐intense storms and lightning. Results show that majority of lightning‐producing storms are located on the leeward side as opposed to the windward side. These storms have deeper top‐heights, larger areas and vertically integrated liquid, and an enhanced hail probability than those devoid of lightning. Warm season convection in the study area is accompanied by the preponderance of negative Cloud to Ground (−CG) flashes over positive Cloud to Ground (+CG) lightning. Storms with +CG features exhibited much higher (>2 times) vertical airmass flux in the mid‐troposphere (6–9 km) than storms without +CG features. Furthermore, for majority of +CG storms, intracloud flash occurrences increased significantly above the freezing level.

Positive Cloud-to-Ground Lightning Characteristics in the Eyewall of Typhoon Faxai (2019) Observed by Tokyo Lightning Mapping Array

Although a number of studies have been conducted of the lightning activity in hurricanes and typhoons, little information has been obtained on the 3-dimensional structure of the lightning, or how it is related to the precipitation structures within the storms. Here we utilize observational data from the 3-D Tokyo Lightning Mapping Array (Tokyo LMA), a Japan Meteorological Agency C-band Doppler radar, and the Japanese Lightning Detection Network (JLDN) to conduct a study of the lightning activity during Typhoon Faxai (2019) in comparison with the storm's precipitation structure. This is done for the dissipating stage of the typhoon, when the eyewall was well within the range of the instruments and undergoing a surge in lightning activity. Of particular interest in the surge was the occurrence of numerous positive cloud-to-ground (+CG) lightning flashes. Detailed study of the Tokyo LMA and JLDN data show that, out of 52 flashes during the surge, 29 flashes or 56 % produced positive strokes to ground, an unheard-of number considering that, from the lightning and 3-D radar structures, the storm appeared to be normally-electrified, and under such circumstances would produce negative rather than positive strokes to ground. It also focuses attention on the question of how +CGs are produced in tropical cyclones in the first place. Based on a lack of −CG strokes and the LMA observations showing that the +CG strokes are produced mid-way or toward the end of normal-polarity intracloud (IC) flashes, it appears that the dissipating storm cells have a depleted or horizontally-sheared mid-level negative charge, such that an IC flash propagating into and through upper positive storm charge effectively funnels a steadily increasing amount of positive charge into the mid-level initiation region, eventually causing the positive breakdown of the IC flash to turn downward toward ground, producing a +CG stroke.

Luminosity with large amplitude pulses after the initial breakdown stage in intracloud lightning flashes

Optical data are presented for two intracloud flashes in which there were five events having large amplitude, bipolar electric field change (E-change) pulses and strong VHF emissions. These bright events occurred 18.98–67.33 ms after the initiating event and 10–59 ms after the end of the initial breakdown stage. The three largest events were coincident with WorldWide Lightning Location Network detections of the sort previously associated with terrestrial gamma-ray flashes (TGFs); these pulses had range-normalized E-change amplitudes of 8.73, 8.33, and 3.81 V/m and estimated peak current magnitudes of 262, 250, and 114 kA. The other two events were 3.20 and 1.62 V/m (96 and 49 kA). All five events have bright enhanced luminosity (>10% above background) for durations of 0.7–1.20 ms, similar to durations of their moderate-to-strong VHF emissions. Full-frame peak intensities are factors of 1.82–4.46 times the background level. Maximum cumulative intensity in the camera frame occurs 84–98 μs after the E-change peak in three cases and ranges from −10 μs (before) to +115 μs (after) for all cases. Notably, in all five events the luminosity starts increasing when the E-change and VHF sensors begin detecting oscillations, 135–550 μs before the E-change peak. Pulse locations of each event extend through 3.6 to 4.3 km depth, starting below and ending above the altitude of flash initiation, and the linear path-length of activity with these events is estimated at 8.9–11.3 km within radar reflectivities of 20–40 dBZ. Although these events are not initial breakdown pulses, their luminosity could be visible from satellite-borne instruments and they may be associated with TGFs.

Characteristics of Regular Pulse Bursts Generated From Lightning Discharges

In this work, we studied the waveforms of all lightning discharges from about 15 min. Eighty-three percent of all lightning discharges contain particular waveforms called regular pulse bursts (RPBs), which have regular microsecond-scale electric or magnetic field pulses. Maximum proportion of RPBs occur in middle or rear of lightning discharges. Prior to or after RPBs, there is always a chaotic pulse period. The analysis indicated that RPBs are caused by a secondary discharge in the fractured old breakdown channel, likeness to dart-stepped leader occuring in negative cloud-to-ground discharge (-CG). Four types of RPBs, namely, category of normal RPBs, category of back RPBs, category of symmetric RPBs, and category of reversal RPBs, were sorted in the light of the evolution of the pulse amplitude, interval between neighboring pulses and pulse polarity. In addition, the difference between normal RPBs and back RPBs was considered to be caused by the distance between neighboring charge pockets and the magnitude of the charge in every charge pocket. The symmetric RPBs were considered to be caused by a discharge channel with a large central charge area. Reversal RPBs were considered to be caused by a bending channel or superposition of two or more RPBs. We located some RPBs in a typical intra-cloud flash (IC) in three-dimensional. The analysis showed that the developing velocity of RPBs ranged from approximately 1.2 × 106 m/s to 3.0 × 106 m/s, which slower less than both of the dart leader or dart-stepped leader process from previous studies. And we found it is several meters to dozens of meters that the lengths range of discharge step which between two adjacent pulses.

Modeling initial breakdown pulses of intracloud lightning flashes

In this study 29 initial breakdown pulses (IBPs) from four intracloud (IC) lightning flashes are modeled using data from five or more electric field change (E-change) sites. For each flash the first 5–9 located IBPs are investigated. For each IBP the modeling first extracts the IBP current waveform from the E-change data by matrix inversion and then determines the best channel length and current velocity to match the IBP data. Derived IBP quantities of total charge, charge moment, peak current, peak radiated power, and total energy are calculated. Resulting IBP vertical lengths varied from 27 m to 1300 m; most values were 100–500 m. Current velocities ranged over 4.0–20.0 × 107 m/s, with most values 10–16.5 × 107 m/s. Two of these IC flashes had two “extraordinary” IBPs each with very large E-change amplitude and multiple subpulses; these four extraordinary IBPs had longer current rise times than fall times and charge moments of −3.45 to −20.06C km. Subpulses of classic IBPs were coincident with, and likely caused by, smaller current pulses superimposed on the main IBP current. Overall, most of the 29 IC IBPs had peak current amplitudes <120 kA and total (negative) charge <2C, while the four extraordinary IB pulses had peak currents of 217–359 kA and total charges of −8.4C to −71.7C. The four extraordinary IBPs all have the characteristics of Energetic In-cloud Pulses (EIPs), which are thought to be the radio signals of events producing terrestrial gamma-ray flashes (TGFs). The extraordinary IBPs may have caused double-pulse TGFs and overlapping TGFs.

3D Total Lightning Observation Network in Tokyo Metropolitan Area (Tokyo LMA)

The National Research Institute for Earth Science and Disaster Resilience deployed a lightning mapping array (LMA) in the Tokyo metropolitan area in March 2017. Called the “Tokyo LMA,” it obtains detailed three-dimensional observations of the total lightning activity (cloud-to-ground and intracloud flashes) in storms. The network initially consisted of 8 receiving stations, expanded to 12 stations in March 2018. Real-time total lightning images were first opened on the webpage in Japan. Real-time observations from the Tokyo LMA will be used in nowcasting lightning hazards and mitigating lightning disasters. Archived data will be used to develop lightning prediction techniques and a lightning climatology for the Tokyo metropolitan area.

Ultra-high speed video observations of intracloud lightning flash initiation

This study describes results from video observations of five intracloud flashes located ≤ 20 km from the camera and recorded with 6.1 µs exposure time and 6.66 µs frame intervals. Video data are supported with electric field change (E-change) and VHF measurements, with emphasis on the flash initiating event (IE) and initial breakdown (IB) stage. In four of the five flashes, the IE is accompanied by weak luminosity, ≤ 5% above background, lasting for 300–500 µs. Two of these four IEs were positive Narrow Bipolar Events (NBEs) with VHF powers of 43 and 990 W; these are the first (known) data showing visible light detected with a positive NBE. Two other IEs with weak luminosity had powers of 0.5 and 1 W, and the IE with no detected luminosity had a VHF power of 3 W. A typical IB cluster consists of several narrow pulses and one classic pulse in E-change data (along with many VHF pulses), and each example flash has 2–10 IB clusters in the first 5–50 ms. The luminosity of IB clusters was substantially greater than IE luminosity, ranging from 10 to 40% above background in four examples, while for one flash with 10 IB clusters, the luminosity range was 35–360% above background (average 190%). Luminosity durations of IB clusters were 520–1750 µs with average 1210 µs. For both IEs and IB clusters, increases in the detected luminosity were closely timed with substantial VHF emissions and decreased when VHF emissions weakened.