Cloud-to-ground Flashes Research Articles

Temporal characteristics of positive cloud-to-ground flashes from tropical thunderstorms

The occurrence of positive cloud-to-ground (CG) lightning flashes proves to be a challenging task when compared to the negative CG flash which common happen in tropical region. This due to the positively charged clouds to reach higher altitudes to enable the transfer of electrical charges to the Earth's surface. With several sensors that deployed in the Malacca region (2.314077°N, 102.318282°E) such as the wide band fast and slow antenna, magnetic field sensors, and the electric field mill (EFM-100). This research study focuses on the temporal characteristics of positive CG lightning activity in the tropical region during September and October 2021. A total of 203 positive cloud-to-ground flashes were meticulously analyzed in the context of 16 thunderstorm events. Key findings highlight the consistent initiation of positive cloud-to-ground flashes through initial breakdown (IB) pulses, followed by the development of stepped leaders and subsequent return strokes. Notably, most positive cloud-to-ground events (72%) were observed as single strokes and the maximum multiplicity recorded being four subsequent return strokes. Furthermore, the analysis focuses into the temporal aspects of these lightning events, revealing average durations for various parameters, including rise-time (6.26 μs), zero-crossing (26.43 μs), pulse duration of the first RS (139.76 μs), and the interval from the first IB to the first return stroke (134.19 ms). Additionally, the recorded data from the Electric Field Mill (EFM) instruments together with Constant Altitude Plan Position Indicator (CAPPI) radar demonstrate a notable correlation. Specifically, the electric field (E-field) values exhibit a discernible increase when positive cloud-to-ground lightning events are detected near the lightning sensor.

Observation of tropical positive cloud-to-ground flashes accompanied by chaotic and regular pulse trains

This study investigates the occurrence of chaotic pulse trains (CPTs) and regular pulse trains (RPTs) in tropical positive cloud-to-ground (CG) lightning flashes. These flashes are categorized into four types based on the initial polarity of the initial breakdown (IB) pulses and their relationship to the first return stroke (RS). A total of 71 positive CG flashes from five thunderstorm events were analyzed. The analysis reveals instances of CPTs and RPTs both before and after the first positive RS, along with the occurrence of mixed polarities in RPTs. Variations in IB pulse polarities and the presence of CPTs and RPTs before the first positive RS were observed, contrasting with previous findings in negative CG flashes. All positive CG flashes have been detected when cloud top height occurrences were between 12 and 18 km. In contrast, for negative CG flashes with CPTs and RPTs the cloud top height occurrences were between 5 and 12 km. It is interesting that CPTs and RPTs can be detected during IB process of positive CG flashes at relatively high altitude in the thundercloud. Perhaps due to low pressure at higher altitudes in the cloud, electrical process associated with CPTs and RPTs are easily discharged before the occurrence first positive return stroke. The altitudes of cloud top heights for the inverse polarity of IB pulses were located between 16 and 18 km. This research enhances the understanding of positive CG lightning initiation process and their relationship with CPTs and RPTs, as well as the occurrence of recoil leaders.

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.

Assessing Flash Characteristics in Lightning-Initiated Wildfire Events between 1995 and 2020 within the Contiguous United States

Abstract Twenty-six years of lightning data were paired with over 68 000 lightning-initiated wildfire (LIW) reports to understand lightning flash characteristics responsible for ignition in between 1995 and 2020. Results indicate that 92% of LIW were started by negative cloud-to-ground (CG) lightning flashes and 57% were single stroke flashes. Moreover, 62% of LIW reports did not have a positive CG within 10 km of the start location, contrary to the science literature’s suggestion that positive CG flashes are a dominant fire-starting mechanism. Nearly ⅓ of wildfire events were holdovers, meaning 1 or more days elapsed between lightning occurrence and fire report. However, fires that were reported less than a day after lightning occurrence statistically burned more acreage. Peak current was not found to be a statistically significant delineator between fire starters and non–fire starters for negative CGs but was for positive CGs. Results highlighted the need for reassessing the role of positive CG lightning and subsequently long-continuing current in wildfire ignition started by lightning. One potential outcome of this study’s results is the development of real-time tools to identify ignition potential during lightning events to aid in fire mitigation efforts.

How Much Lightning Actually Strikes the United States?

Abstract The number of cloud-to-ground (CG) flashes over the contiguous United States (CONUS) has been estimated to be from as small as 25 million per year to as many as 40 million. In addition, many CG flashes contact the ground in more than one place. To clarify these values, recent data from the National Lightning Detection Network (NLDN) have been examined since the network is performing well enough to make precise updates to the number of CG flashes and their associated ground contact points. The average number of CG flashes is calculated to be about 23.4 million per year over the CONUS, and the average number of ground contact points is calculated as 36.8 million per year. Knowledge of these two parameters is critical to lightning protection standards, as well as better understanding of the effects of lightning on forest fire initiation, geophysical interactions, human safety, and applications that benefit from knowing that a single flash may transfer charge to the ground in multiple, widely spaced locations. Sensitivity tests to assess the effects of misclassification of CG and in-cloud (IC) lightning are also made to place bounds on these estimates, and the likely uncertainty is a few percent.

Tracking Lightning Through 3D Thunder Source Location With Distributed Acoustic Sensing

AbstractInvestigating lightning is of key significance in understanding the lightning mechanism and mitigating lightning hazards. We reported an experiment of investigating lightning through three‐dimensional (3D) thunder locating using a Distributed Acoustic Sensing (DAS) array in Hefei, China. In this experiment, we recast a 7.7 km long urban telecom optical fiber cable as 3,850 sensors using the DAS technique. From dense DAS recording, we manually identified 101 thunder events during six positive cloud‐to‐ground (CG) lightning flashes within 20 min. The DAS recorded thunder signals are dominated by direct acoustic waves rather than air‐ground coupled surface waves. The thunder events were then located using the arrival times of thunder signals. The locations and amplitudes of thunder events are generally consistent with those from the conventional lightning detection data set and broadband magnetic field. There is likely a correlation between the maximum strength of thunder events and the highest peak current for individual CG flashes. Moreover, the comparison with weather radar observations indicates that lightning usually originated from areas of high reflectivity (e.g., ≥50dBz) with diffusely distributed (from ground surface to ∼5 km altitude) thunder events and extended in a narrow altitude range of 3–5 km to areas with low radar reflectivity.

Evaluate WRF-based Lightning Potential Index (LPI) lightning parameterization over Sri Lanka during second inter-monsoon in 2018

It is important to develop accurate and reliable lightning prediction system that can be contributed towards the safety of life, both concerning forecasting for the public safety and safety of aviation and electrical power. This study aims to evaluate WRF-based Lightning Potential Index(LPI) lightning parameterization and its applicability for predicting lightning over Sri Lanka during the second inter-monsoon. The WRF-ARW model 3.9.1 was used to produce predictions for three lightning events with various physical parameterization schemes with two nested domains with a resolution of 12km and 4km respectively. The model simulated LPI values were evaluated using the Earth Networks Global Lightning (ENGLN) dataset. Results show corresponding lightning simulations were produced with spatial distribution aligned with ground-based lightning data. Results consistently show a high correlation of the LPI index with an hourly CG flash rate over the three cases. Moreover, the WRF model was able to capture the lightning using LPI in Sri Lanka, suggesting that it can be used operationally to predict lightning potential region.

Understanding the Effects of Aerosols on Electrification and Lightning Polarity in an Idealized Supercell Thunderstorm via Model Emulation

AbstractAerosol effects on the lightning intensity and polarity of a continental supercell storm were investigated using a three‐dimensional lightning scheme within the Weather Research and Forecasting model. We find that both intra‐cloud (IC) and cloud‐to‐ground (CG) flashes are enhanced by the increasing number of cloud condensation nuclei (CCN), especially the percentage of positive CG (+CG) strokes peaking at 42%. Electrical characteristics of the storm varied in different aerosol scenarios through microphysical processes. Added aerosols increase the number of cloud droplets and ice‐phase hydrometeors. The greater ice‐crystal concentration and larger graupel size ensure sufficient charge separation, leading to higher charge density and more lightning discharges. In addition, an inverted polarity charge structure with a strong positive‐charge region in the mid‐levels was formed mainly due to the positively charged graupel in the presence of higher supercooled cloud water content. Positive lightning channels originating from this positive‐charge region propagated to the ground, producing more +CG strokes. When the aerosol concentration was low, the charge density in the upper positive‐charge region was much lower due to smaller ice‐particle content. Consequently, there were barely any +CG strokes. Most of the negative CG flashes deposited positive charge in the lower negative‐charge region.

Statistical Characteristics of Thunderstorm Activity in the Middle Reaches of the Yangtze River Basin Based on a Five‐Year Cloud‐To‐Ground Lighting Data Set

AbstractUsing a clustering algorithm based on cloud‐to‐ground (CG) lighting data, 72,974 thunderstorms were identified and tracked in the middle reaches of the Yangtze River Basin from May to September of 2016–2020. Thunderstorms predominantly occur in the southeast region and move to the northeast at a speed of 16–64 km/hr. Most thunderstorms have short durations (98.3%, ≤3 hr) and low CG flash frequencies (90.0%, ≤64). Thunderstorms with longer durations are mainly triggered near the mountains and tend to occur (end) earlier (later) in the afternoon (evening). The peak composite reflectivity (CR) corresponding to CG flashes from all thunderstorms is 50 dBZ. Approximately 70% (20%) of CG flashes occur in convective (stratiform) areas. The first CG flash of a thunderstorm tends to occur in convective areas with a higher CR than that of the last CG flash. The average and maximum CRs of CG flashes increase significantly with thunderstorm duration.

High‐Speed Video Observations of Needles Evolving Into Negative Leaders in a Positive Cloud‐To‐Ground Lightning Flash

AbstractHigh‐speed video records of a single‐stroke positive cloud‐to‐ground (+CG) flash were used to examine the evolution of eight needles developing more or less radially from the +CG channel. All these eight needles occurred during the later return‐stroke stage and the following continuing current stage. Six needles, after their initial extension from the lateral surface of the parent channel core, elongated via bidirectional recoil events, which are responsible for flickering, and two of them evolved into negative stepped leaders. For the latter two, the mean extension speed decreased from 5.3 × 106 to 3.4 × 105 and then to 1.3 × 105 m/s during the initial, recoil‐event, and stepping stages, respectively. The initial needle extension ranged from 70 to 320 m (N = 8), extension via recoil events from 50 to 210 m (N = 6), and extension via stepping from 810 to 1,870 m (N = 2). Compared with needles developing from leader channels, the different behavior of needle flickering, the longer length, the faster extension speed, and the higher flickering rate observed in this work may be attributed to a considerably higher current (rate of charge supply) during the return‐stroke and early continuing‐current stages of +CG flashes.

Analysis of lightning flash rate with the occurrence of flash floods and hailstorms in Peninsular Malaysia

This paper will present five flash flood cases in Johor and two hailstorms from Kuala Lumpur and Melaka reported during lightning activities to determine the relationship between lightning flash rate and rainfall that occurred from 2013 to 2017. The flash floods were observed based on lightning data obtained from Tenaga Nasional Berhad Research Sdn. Bhd. (TNBR) and rainfall data from the Department of Irrigation and Drainage (DID). On the other hand, storm-producing hails were observed based on the lightning flash rate using electromagnetic field measurements and weather radar for their reflectivity value and cloud structure. The detection of a lightning flash, especially the negative narrow bipolar event (−NBE) and positive cloud-to-ground (+CG), could be a possible indicator of a deeper convective storm that could lead to the production of hails. In the 24-h observation of selected days when a flash flood occurred, lightning strikes correlation with rainfall had r-values ranging from 0.6 to 0.9, where the lightning data may enhance flood forecasting accuracy when combined with rainfall. Meanwhile, during the first hailstorms in Kuala Lumpur, the highest −NBEs flash rate detected during the hails appeared where 16 −NBEs were detected in a span of 5 min with reflectivity value between 53 and 55 dBz and cloud top height of 20 km. During the second hailstorms in Melaka, more + CG flashes rather than −NBE were detected during the appearance of the hail, with 64 +CG flashes and five −NBEs with reflectivity values between 60 and 66 dBz and cloud top height of 18.8 km. Based on the high convective strength, cloud top height, and presence of hailstones, −NBE and +CG flash could be possible indicators of storm severity, especially in Malaysia.

Observation and Simulation of Lightning Strikes in an Offshore Wind Turbine Cluster

AbstractBased on the detection data of the Guangdong‐Hong Kong‐Macau lightning location system from 2013 to 2022, this paper analyzes the characteristics of lightning strikes in an offshore wind turbine cluster (WTC) in Guangdong. This is the first time to quantify the lightning strikes difference between the inner and outer wind turbines (WTs) and discuss the impact mechanisms of WTCs on the characteristics of lightning strikes by the simulation results of electric potential distribution. The observation results show that the stroke density, cloud‐to‐ground (CG) flash density, and average peak current within the affected area increase by 10.1%, 11%, and 5.1%, respectively, but the lightning multiplicity changes little. The inner and outer WTs show different characteristics of lightning strikes. The stroke density, lightning multiplicity, and average peak current in the area of outer WTs increase by 21%, 4.1%, and 6.8%, respectively, while in the area of inner WTs decrease by 20.2%, 19%, and 3.9%, respectively. The CG flash density in the area of outer WTs increases by 16.1% but changes little in the area of inner WTs. The simulation results show there is a significant difference in the electric potential distribution between the inner and outer WTs: the distortion effect of the outer WTs on spatial electric potential is stronger than that of the inner WTs at a greater height, but the difference is not obvious at a lower height. Such a difference in electric potential distribution leads to different characteristics of lightning strikes between the inner and outer WTs.

A 3D Interferometer-Type Lightning Mapping Array for Observation of Winter Lightning in Japan

We have developed and deployed a 3D Interferometer-type Lightning Mapping Array (InLMA) for observing winter lightning in Japan. InLMA consists of three broadband interferometers installed at three stations with a distance from 3 to 5 km. At each interferometer station, three discone antennas were installed, forming a right triangle with a separation of 75 m along their two orthogonal baselines. The output of each InLMA antenna is passed through a 400 MHz low-pass filter and then recorded at 1 GS/s with 16-bit accuracy. A new method has been proposed for finding 3D solutions of a lightning mapping system that consists of multiple interferometers. Using the InLMA, we have succeeded in mapping a positive cloud-to-ground (CG) lightning flash in winter, particularly its preliminary breakdown (PB) process. A study on individual PB pulse processes allows us to infer that each PB pulse process contains many small-scale discharges scattering in a height range of about 150 m. These small-scale discharges in a series of PB pulses appear to be continuous in space, though discontinuous in time. We have also examined the positive return stroke in the CG flash and found a 3D average return stroke speed of 7.5 × 107 m/s.

Spatiotemporal patterns of long series of cloud-to-ground lightning in Beijing and its cause

Previous studies on the climatic characteristics of lightning activity in Beijing are sparse due to the lack of long-term, continuous and stable lightning location data. In this paper, the long series of lightning location data during 2009–2018 were applied to reveal the spatiotemporal distribution of cloud-to-ground (CG) lightning in Beijing. The spatial characteristics were low west and high east, and the temporal characteristics were frequent in summer. In the complex terrain areas of northeast Beijing, there was a positive relationship between CG density and altitude, with the correlation coefficient higher than 0.8. The northeastern trumpet-shaped terrain had the forced lifting effect on thunderstorms, forming lightning-intensive areas of 500 km2. In addition, urban heat island (UHI) effect provided the necessary thermal conditions for the development of thunderstorms, forming lightning-intensive areas in the built-up areas, with the highest value of CG flash density exceeding 6 fl/km2. Note that the lightning-sparse areas in the urban center exhibited a distinct barrier effect on the spatial patterns of summertime CG lightning in the built-up areas, which was mainly modulated by the type of synoptic background.

Modeling the Neuruppin ball lightning incident

In 1994, a 370-kA positive cloud-to-ground (CG) discharge 5 km east of Neuruppin, Germany was immediately followed by the production of at least ten ball lightning objects seen by numerous eyewitnesses. We propose a sequence of events based on observations of upward lightning triggered by CG flashes which can explain how a large ground-level electric field (∼1 MV m−1) could be produced by negative leaders extending into a region of positive charge above Neuruppin, resulting in transfer of a large amount of charge horizontally between the CG discharge and the region of high electric field. This model provides a causal link between the large CG discharge east of Neuruppin and the large ground-level fields which were associated with the production of ball lightning. Recommendations follow for producing ball lightning in a large laboratory experiment.

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.

Transient phenomena in positive cloud-to-ground lightning discharges

Positive lightning discharges to ground (+CGs) are relatively rare and considerably less studied than negative ones (-CGs). We present observations of unusual transient phenomena occurring in +CGs and discuss their mechanisms. One of them is a brief electric coupling to a concurrent -CG initiated from a 257-m tall tower located 11 km from the +CG channel. A transient process (stroke) in the -CG flash appears to cause a transient luminosity enhancement (M-component) in the +CG channel. In the course of these essentially simultaneous transients, positive charge is in effect taken from the ground at the position of the tower and injected into the ground at the position of the +CG channel. Recoil leaders reactivating decayed +CG branches near the cloud base are each observed to cause a transient luminosity decrease (dip), as opposed to the expected luminosity increase, in the +CG main channel.

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.

Relationships between cloud-to-ground flashes and ice-phase hydrometeors in a tropical hailstorm

Based on observed lightning locations and S-band polarimetric Doppler radar data, a hailstorm that occurred on Hainan Island was analyzed in detail. The hailstorm initially developed offshore to the southwest of Hainan Island. After the hailstorm made landfall, hail fell on the surface of Hainan Island for 43 min. An analysis revealed that the activity of cloud-to-ground flashes (CGs) peaked in the early mature stage of the hailstorm, followed by four intermittent CG flash active periods. The relationships between the CG flash frequency and the ice-phase hydrometeors retrieved from polarimetric parameters were compared, revealing that the CG flash activity was most closely related to graupel and snow particles in the cloud, whose correlation coefficients reached 0.8 and 0.76, respectively. In combination with the evolutionary features of the hailstorm, we found four independent periods of CG flash activity corresponding to different stages of the hailstorm: the merging of cloud bodies, the landfall of the convective body, the period of hailfall, and the period of orographic uplift. During these periods, the vertical distributions and concentrations of graupel and snow particles were distinct. During the hailfall period, the most significant vertical rate of change in radar echo intensities larger than 40 dBZ was located in the region of 0–10 °C, and this rate of change was the largest of the whole hailstorm.