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Due to the weak radiation generated by positive leaders, our understanding of how positive leaders are initiated and reach the ground remains limited. This study investigated positive cloud‐to‐ground (CG) lightning induced after the long intracloud lightning based on high‐speed video and fast antenna mapping results. Below the cloud base in the field of view, re‐breakdowns of decayed negative branches occurred consecutively beneath the horizontal negative channel in the form of bidirectional leaders. These bidirectional leaders advanced along the same channel, and eventually, the third reached the ground, generating a positive return stroke (RS) with a peak current of 157 kA. Following the RS, new negative discharges emerge adjacent to the vertical return stroke channel, persisting and propagating to form a continuing current (CC) lasting over 200 milliseconds. Plain Language Summary Positive cloud‐to‐ground flash attracts more attention than its negative counterpart because it is more hazardous. Previous studies have confirmed the close relationship between intracloud (IC) lightning and positive cloud‐to‐ground (+CG) lightning. Downward positive leaders forming +CG can originate from branching of intracloud lightning or even from negative polarity leader channels. However, due to the weak radiation from positive leaders during their development within the cloud, common lightning detection devices, such as LF/VLF fast antennas and VHF antennas, find it very difficult to detect positive leaders. Consequently, our understanding of how positive leaders initiate within the cloud and reach the ground remains limited. In this study, we comprehensively utilized lightning channel mapping results and high‐speed video to investigate a case of +CG. We found that the occurrence of downward positive leaders around the negative leader channel is facilitated through multiple side breakdowns, events that continue even after the return stroke. Therefore, we propose that the multiple positive side breakdowns beneath the reactivated negative leader channel are responsible for generating positive leaders from the negative lightning channel. These side discharges play a crucial role in the generation of +CG lightning. Key Points A positive cloud‐to‐ground lightning flash was analyzed based on synchronous high‐speed video and lightning mapping results Downward positive leader initiated, reached ground via three bidirectional side discharges beneath horizontal negative leader channel Side negative breakdowns from vertical return stroke channel and reactivation of decayed negative branches were found

We present time‐correlated ultra‐high‐speed video camera and electromagnetic field measurements of the attachment processes in a natural negative cloud‐to‐ground stroke. The video camera frame exposure time and pixel resolution were 740 ns and 0.91 m/pixel, respectively. The common streamer zone (CSZ) was first observed 2.52 µs preceding the first frame showing the return stroke (RS) in progress, when the upward and downward leader‐tips were 9.8 m apart. In the next frame, the two leaders were observed to have propagated toward each other within the CSZ, with their tips being 0.91 m apart. Our observations show with unprecedented precision/clarity that (a) the slow front in the field waveform is associated with the CSZ, and (b) the “proper” start of the RS is marked by the onset of the fast transition in the field waveform which occurs at the completion of the attachment processes (when the upward and downward leaders have merged). Plain Language Summary Detailed observations of how cloud‐to‐ground lightning attaches to ground remain very difficult to obtain. This is because the associated processes occur on a microsecond‐scale (or faster), making it challenging to perform time‐correlated multi‐instrument measurements. In this study, we present time‐correlated observations of ultra‐high‐speed video and electromagnetic fields, which provide new insights regarding the lightning attachment processes. These processes comprise the so‐called breakthrough phase of lightning attachment when two oppositely charged leaders (one from the thundercloud and one from ground or ground‐based object) merge with each other. Key Points Time‐correlated multi‐sensor observations (with camera exposure time of 740 ns) of the natural lightning attachment processes were made The slow front portion of the field waveform is associated with the time‐evolution of the common streamer zone Fast‐transition onset in the field waveforms occurs when the upward and downward leaders fully merge (“proper” start of return stroke)

Four years of Indian Institute of Tropical Meteorology lightning location network lightning observation data are used to determine the seasonal and spatial (over different geographical locations) distribution of the ratio of intra‐cloud (IC) lightning to cloud‐to‐ground (CG) lightning in thunderstorms over the Indian subcontinent. The ratio is high (6–10) in the northwestern parts and low (0.5–3.5) in the northeastern parts. No prominent latitudinal variation of the IC to CG ratio exists, but a climatological seasonal variability exists over all regions. In the pre‐monsoon season (March–May), the mean ratio is observed to be 3.81 with a standard deviation of 0.79, and during the monsoon season (June–September), a value of 3.04 with a standard deviation of 0.50. Although convective available potential energy is the regulating factor, little dependency has been found between the ratio of IC to CG lightning (IC:CG ratio) and the total flash rate (f), as well as with cold cloud depths. The ratio is observed to be proportional to the total flash rate as f0.61. The cold cloud depth is most prominently linked with the regional and seasonal IC:CG ratio. The implication of these observed results has the importance of separating CG lightning flash from total and can be used in numerical models to give a proper prediction of CG lightning in hazard mitigation. Plain Language Summary Pre‐monsoon thunderstorms exhibit more intra‐cloud (IC) discharge than monsoonal thunderstorms; hence, the IC:cloud‐to‐ground (CG) ratio is also high in pre‐monsoon. In this paper, we have shown that CG lightning is approximately 20% of total lightning in pre‐monsoon whereas 25% of total lightning in monsoon all over the Indian region. A stronger vertical updraft associated with high convective available potential energy enhances the cold cloud depth. It may expand the mixed phase region, which can broaden and uplift the size of the upper positive charge center inside a thunderstorm. In contrast, the middle negative charge center remains at the same temperature level. Therefore, this process may enhance IC discharge between the upper positive charge center and the middle negative charge center, increasing the IC:CG ratio of a thunderstorm. Key Points The mean intra‐cloud:cloud‐to‐ground (IC:CG) ratio remains high in the Pre‐monsoon season compared to the Monsoon season over the Indian land mass The high cold cloud depth associated with stronger updrafts expand the mixed‐phase region and increases the IC flash rate and IC:CG ratio High flash rate associated with high IC flash occurrences is also responsible for a high IC:CG ratio

Forty‐five unconnected upward leaders (UULs) occurred in 19 downward negative flashes are analyzed. Each observed UUL is initiated by a downward stepped leader before a new strike point is struck. For each UUL, several parameters are determined when possible mainly by using high‐speed images: inception height, inception time prior to return stroke (RS), horizontal distance from the flash's strike point, two‐dimensional (2D) distance between the nearest downward leader branch tip and the UUL's inception point at its inception time, 2D length, and 2D average propagation velocity. Their values range from 40 to 503 m (number of samples: 45), <0.1 to 1.32 ms (38), 20 m to 1.3 km (38), 99 to 578 m (21), 0.48 to 399 m (45), and 5.79 to 33.8 × 104 m s−1 (22), respectively. 86% (19/22) of the velocities are smaller than 1.7 × 105 m s−1. No UUL with an inception time prior to RS greater than 0.5 ms is initiated from a structure lower than 300 m. Those UULs with inception heights lower than 300 m seldom exhibit lengths longer than 50 m and only can be initiated by flashes within approximately 600 m, while those higher than 400 m can even reach several hundred meters and be initiated by flashes over 1 km away. The maximum distances for the downward leaders to attract the UULs with inception heights from 100 to 200 m, 200 to 300 m, and over 400 m are approximately 350 m, 450 m, and 600 m, respectively. Key Points Cases and statistical analysis of unconnected upward positive leaders Observation of lightning discharges striking a city with many tall structures The data can help us estimate the effect of downward leader on upward leader

Lightning-generated nitrogen oxides (LNO x ) have a major influence on the atmosphere and global climate change. Therefore, it is of great importance to obtain a more accurate estimation of LNO x . The aim of this study is to provide a reference for the accurate estimation of the total LNO x in the mainland of China based on cloud-to-ground lightning (CG) location data from 2014 to 2018. The energy of each CG flash was based on the number of return strokes per CG flash, the peak current of each return stroke, and the assumed CG breakdown voltage. The energy of intracloud lightning (IC) was based on the estimated frequencies of IC and the assumed energy of each IC flash. Combining the energy of lightning and the number of nitric oxide (NO) molecules produced by unit energy ( ρ no ), the total LNO x production in the mainland of China was determined. The LNO x in the mainland of China estimated in this study is in the range (0.157–0.321) × 10 9 kg per year [Tg(N) yr −1 ], which is on the high end of other scholars’ works. Negative cloud-to-ground lightning (NCG) flashes produce the most moles of NO x , while positive cloud-to-ground lightning (PCG) flashes produce the least total moles of NO x . The breakdown voltage of PCG is greater than that of IC or NCG, while the latter has a greater output of LNO x .

The effect of aerosols on lightning has been noted in many studies, but much less is known about the long-term impacts in air-polluted urban areas of China. In this paper, 9-year data sets of cloud-to-ground (CG) lightning, aerosol optical depth (AOD), convective available potential energy (CAPE), and surface relative humidity (SRH) from ground-based observation and model reanalysis are analyzed over three air-polluted urban areas of China. Decreasing trends are found in the interannual variations of CG lightning density (unit: flashes km−2day−1) and total AOD over the three study regions during the study period. An apparent enhancement in CG lightning density is found under conditions with high AOD on the seasonal cycles over the three study regions. The joint effects of total AOD and thermodynamic factors (CAPE and SRH) on CG lightning density and the percentage of positive CG flashes (+CG flashes/total CG flashes × 100; PPCG; unit: %) are further analyzed. Results show that CG lighting density is higher under conditions with high total AOD, while PPCG is lower under conditions with low total AOD. CG lightning density is more sensitive to CAPE under conditions with high total AOD.

This study investigates the characteristics of negative cloud‐to‐ground (CG) lightning flashes across varying peak current (PCs) ranges by combining 3D location data from 2,597 negative CG flashes with radar observations. The flashes were classified into five PC bins: −25 kA < PC < 0 kA, −50 kA < PC < −25 kA, −75 kA < PC ≤ −50 kA, −100 kA < PC ≤ −75 kA, and PC ≤ −100 kA. Key parameters analyzed include initiation altitude, reflectivity at initiation points, vertical reflectivity cores, lightning extension spaces, horizontal distances between initiation and grounding points (HD), and time differences between the first‐detected radiation source and the first return stroke (TD). Results reveal that while initiation environments were broadly similar across PC ranges, large‐PC flashes tended to initiate at lower altitudes (1–3 km). These flashes form first return strokes faster (the median TD for current bins below −75 kA is less than 44 ms, whereas for bins above −75 kA the median TD is greater than 57 ms) after shorter horizontal propagation (The median HD for current bins below −75 kA is around 1.0 km, while for bins above −75 kA the median HD exceeds 1.2 km), with smaller pre‐return‐stroke extension areas. Despite this, large‐PC flashes usually develop more extensive total channel areas. These findings underscore the distinct environmental conditions that facilitate the initiation and propagation of large‐PC negative CG flashes during thunderstorms. Plain Language Summary Few studies have examined how the characteristics of cloud‐to‐ground (CG) lightning flashes relate to their peak currents. This study investigates the correlation between initiation altitudes, lightning channel extension, and peak currents, as well as the environmental conditions associated with negative CG flashes. By categorizing flashes based on their peak currents, we provide new insights into the differences between large‐peak‐current and low‐peak‐current negative CG flashes, particularly in how they initiate and propagate. Our findings show that while large‐peak‐current negative CG flashes occur in similar overall environments as smaller‐peak‐current flashes, they tend to start at lower altitudes. Additionally, large‐peak‐current flashes exhibit faster formation of the first return stroke after propagating shorter horizontal distances, typically displaying more compact horizontal extension prior to the first return stroke. However, subsequent channel development generally leads to more extensive total channel areas compared to low‐peak‐current flashes. These results help clarify the specific electrical conditions that promote the occurrence of large‐peak‐current negative CG flashes. Key Points Large‐peak‐current negative cloud‐to‐ground (CG) flashes typically initiate at lower altitudes when the vertical reflectivity center is near the ground A large‐peak‐current negative CG flash typically strikes the ground through a near‐vertical channel shortly after initiation A large‐peak‐current negative CG flash is likely initiated by a compact, high‐density negative charge region

Remote-sensing techniques are currently the only means of collecting information to monitor the atmospheric dynamics of lightning from the regional to the national scales, allowing for the generation of homogeneous and long time series. Attempts to characterize the impacts of atmospheric discharge in Brazil presuppose an understanding of spatial and temporal lightning patterns. Despite the high frequency of lightning and significant disturbances caused in highly populated regions such as the metropolitan region of Rio de Janeiro (MRRJ), these phenomena are not well characterized when using a long-term contemporaneous dataset. Accordingly, this work focuses on the spatial and temporal variability of cloud-to-ground lightning in the metropolitan region of Rio de Janeiro, an area affected by a high level of atmospheric discharge every year. We performed a statistical analysis of lightning data taken from a lightning location system for the 16-year period of 2001–2016 and analysed characteristics such as polarity, peak currents, geographic distributions and diurnal, intra- and inter-annual variability. Extremely high levels of high activity were observed from 258,794 cloud-to-ground lightning events recorded over the analysed period and for 64.3% events occurring in summer, 20.5% events occurring in spring and 12.9%, and 2.3% events occurring in autumn and winter. The discharge events were predominantly negative (93.54% of the total). Peak levels of electrical activity were observed from roughly 18:00 to 19:00 local time, when there is more potential energy available for convection. The results of the spatial analysis reveal that most lightning observed over Rio de Janeiro derived from the orographic effect, which spurs the formation of convective storms along the southern part of the slope.

Lightning‐generated nitrogen oxides (LNOx) have an impact on the concentration of ground‐level ozone which acts as a toxic air pollutant, thereby negatively influencing human health and the environment. To understand the impact of LNOx on ground‐level ozone, we simulated four thunderstorm events in Tehran using the WRF‐Chem model. As observations of LNOx are not available, we evaluated the temporal distribution of the simulated ground‐level ozone concentration against an air quality monitoring station. We also compared the simulation results against the spatial distribution of the total column ozone from the Ozone Monitoring Instrument. WRF‐Chem performs well in the simulation of ground‐level ozone concentration, with the best performance for an event with the highest lightning activity (correlation coefficient of 0.91). The analysis of the spatial distribution of the observed and simulated total column ozone also indicates the good performance of WRF‐Chem. Hourly variation in the simulated LNOx during lightning activity is compared against both hourly variation in the observed ground‐level ozone and the number of lightning for four thunderstorm events. There is an agreement between the simulated LNOx and the observed ground‐level ozone during lightning for two events, with correlation coefficients of 0.55 and 0.57. LNOx emissions enhance ozone production in the middle to upper troposphere, which can subsequently contribute to an increase in ground‐level ozone by transport, vertical mixing, and chemistry. In addition, the initiation of chemical processes in response to cloud‐to‐ground lightning strikes may contribute to an increase in both LNOx and the concentration of ground‐level ozone. Plain Language Summary Lightning‐generated nitrogen oxides (LNOx) contribute to higher ground‐level ozone, which acts as a harmful air pollutant for humans and the environment. We applied the WRF‐Chem model to simulate four thunderstorm events in Tehran, aiming to understand the impact of LNOx on ground‐level ozone concentration. Overall, WRF‐Chem performs well in the simulation of ground‐level ozone, particularly for an event with the highest lightning activity. The comparison of the spatial distribution of the observed and simulated total column ozone also indicates the good performance of WRF‐Chem. By comparing the simulated LNOx against the observed ground‐level ozone, we found that WRF‐Chem performs well for two thunderstorm events. The produced ozone in the middle to upper troposphere due to LNOx emissions can be transported downward and increase ground‐level ozone. Cloud‐to‐ground lightning strikes also contribute to an increase in both LNOx and ground‐level ozone pollution, which can negatively influence human health and the environment. Key Points The simulated ground‐level ozone concentration is consistent with the observation The initiation of chemical processes in response to cloud‐to‐ground lightning contributes to an increase in both LNOx and surface ozone LNOx influence tropospheric ozone production, which can contribute to an increase in ground‐level ozone

Using a 10‐year data set of cloud‐to‐ground lightning (CG) flashes, the climatological characteristics of ±CG flashes with a large peak current (>75 kA or <−75 kA) (±LPCCGs) are for the first time obtained in China’s most populous areas in terms of density distribution, seasonal variation, and monthly and diurnal evolution. The results show that the distribution of +LPCCG flashes has evident seasonal variations, while the activity center of −LPCCG flashes always remains in Southwest China. The diurnal evolution of +LPCCG flash percentages in southern China’s Mainland shows an opposite trend to that in the north. Furthermore, the peak current distributions of +LPCCG flashes in most of China’s mainland are found to be distinct from those shown in other regions of the world. Additionally, Southwest China is found to be an important and special area for LPCCG flash activity in China. Not only is it the density center of ±LPCCG flashes, but the peak current distribution of −LPCCG flashes in this region also shows an evidently different pattern from those in other regions. The diurnal evolution of the ‐LPCCG/+LPCCG flash frequencies in Southwest China also shows a different pattern from that in other regions. Plain Language Summary This study exhibits the climatological characteristics of CG with a large peak current (LPCCG) in China’s most populous areas for the first time. The results show that LPCCG flashes in northern and southern China have some significantly differences in dominant polarity, seasonal and diurnal variations. Furthermore, the peak current distributions of +LPCCG flashes in most of China’s mainland are found to be distinct from those shown in other regions of the world. Southwestern China is an important and special area for LPCCG flash activity in China because not only is it the density center of both negative and positive LPCCG flashes, but the peak current distribution of −LPCCG flashes and diurnal evolution of ±LPCCG flashes in this area are different from that in other areas of China. Key Points For the first time, the climatology of large‐peak‐current CG (LPCCG) flashes in China’s population concentration area was analyzed Significant numbers and unique diurnal trends of LPCCG flashes in Southwest China were revealed LPCCG flash activities in northern and southern China showed some significant differences in seasonal and diurnal variations