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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.

VHF, LF, and ELF lightning events, thunderstorms, and surface electric fields related to sprites were observed simultaneously during the winter of 2004/2005 in Hokuriku, Japan. The analysis of these observations enables us to investigate the relationship among sprites, lightning characteristics, and thunderstorm structure just before sprite genesis. Typical winter sprite parent thunderstorms had a mesoscale cloud area with small, embedded convective cells. Positive charges responsible for sprites tend to reside in the upper part of the thunderstorms; only a few positive charges were assumed to be located in the lower part. The total amount of positive charges removed by a sprite‐producing flash from the upper and lower parts of the thunderstorms were estimated to be approximately 100 C and as large as 300∼400 C, respectively. Active thunderstorms with lightning accompanied by transient currents tended to generate simple sprites; more complex sprites were excited by lightning with continuing currents, which were generated by a few active thunderstorms and thunderstorms with precipitating stratiform clouds. VHF sources related to sprites can be found in the range of 5 to 72 km. The range of displacement between a sprite element and the corresponding positive cloud‐to‐ground lightning discharge or the first VHF source was 6∼30 km, and the bottom of the sprite bodies was located between 66 and 74 km. On the basis of these results, we deduced that the complexity of sprite morphology might be attributed to the differences in lightning characteristics. Key Points Relationship among winter sprite, lightning, and thunderstorm

Terrestrial gamma‐ray flashes (TGFs) are short bursts of intense gamma radiation associated with lightning discharges. Although thousands of TGFs have been observed from space, TGFs detected at ground level, known as downward TGFs, are still very limited, and their relationship with lightning discharge processes remains elusive. Here we report a special type of strong negative lightning stroke, termed energetic compact stroke (ECS), in winter thunderstorms in Japan, and provide strong evidence that ECSs are consistently associated with downward TGFs. Based on this relationship, we successfully identified three new downward TGFs by the observations of ECSs. Further, 12 out of 19 (63%) of downward TGFs analyzed in this paper were associated with ECSs, indicating that ECSs are the major source of downward TGFs in winter thunderstorms in Japan. These findings open up the possibility of remotely monitoring a large fraction of downward TGFs with simple lightning observations. Plain Language Summary Terrestrial gamma‐ray flashes (TGFs) are the most intense natural sources of gamma‐ray emissions on Earth. It is well established that TGFs are coincident with lightning discharges, but it is still largely unclear what lightning discharge processes can produce TGFs and under what circumstances. In this paper, we provide strong evidence that a special type of negative lightning stroke, referred to as “energetic compact stroke” (ECS), in winter thunderstorms is consistently associated with downward TGFs. This finding makes it possible for future studies to investigate TGF mechanisms by simply observing ECSs. We also found that a large portion, 63% in this study, of downward TGFs were associated with ECSs. This unexpectedly high percentage opens up the possibility of remotely monitoring downward TGFs over wide areas simply through the detection of lightning. Key Points A special type of lightning stroke named “energetic compact stroke” (ECS) is always coincident with downward TGFs Three new downward TGFs were identified by the observations of ECSs More than 60% of downward TGFs in winter thunderstorms in Japan are associated with ECSs

Gamma‐ray glows are observational evidence of relativistic electron acceleration due to the electric field in thunderclouds. However, it is yet to be understood whether such relativistic electrons contribute to the initiation of lightning discharges. To tackle this question, we started the citizen science “Thundercloud Project,” where we map radiation measurements of glows from winter thunderclouds along Japan's sea coast area. We developed and deployed 58 compact gamma‐ray monitors at the end of 2021. On 30 December 2021, five monitors simultaneously detected a glow with its radiation distribution horizontally extending for 2 km. The glow terminated coinciding with a lightning flash at 04:08:34 JST, which was recorded by the two radio‐band lightning mapping systems, FALMA and DALMA. The initial discharges during the preliminary breakdown started above the glow, that is, in vicinity of the electron acceleration site. This result provides one example of possible connections between electron acceleration and lightning initiation. Plain Language Summary Thunderstorms are natural particle accelerators. The strong electric field inside thunderclouds accelerates relativistic electrons, which emit gamma rays via interaction with the atmosphere. High‐energy photons generated in this process have been observed as radiation enhancements called gamma‐ray glows. Winter thunderclouds along the sea of Japan are an ideal target for monitoring glows because their altitudes are usually sufficiently low for the generated gamma‐ray photons to reach the ground. We started a new citizen science “Thundercloud Project” in this area, where we distributed radiation detectors to citizen supporters to observe glows and to reveal their relationship with the aerological condition and lightning discharges. On 30 December 2021, five of those sensors detected a glow from a single thundercloud. Two of them recorded a sudden termination of the glow coinciding with a lightning flash, which was monitored by our two radio mapping systems of FALMA and DALMA. The initial discharges of the flash started at a location about 1.6 km above the glow region with an unusually fast downward progression. This paper is the first report of our citizen science project. We discuss the possibility that accelerated electrons contribute to the initiation of lightning discharges. Key Points We started the citizen science “Thundercloud Project,” a multi‐point observation campaign of gamma‐ray glows from thunderstorms On 30 December 2021, five radiation monitors detected a 2‐km‐long size gamma‐ray glow, which suddenly terminated with a lightning flash Two radio mapping systems of lightning identified the initiation of the discharges, which started at a location above the glow region

A lightning location system consisting of multiple ground-based stations is an effective means of lightning observation. The dataset from CNLDN (China National Lightning Detection Network) in 2016–2022 is employed to analyze the temporal and spatial lightning distributions and the differences between +CG (positive cloud-to-ground lightning) and −CG (negative cloud-to-ground lightning) strokes in China. On the annual scale, lightning activity is most prevalent during the summer months (June, July, and August), accounting for 72.6 % of the year. Spring sees more lightning than autumn, and winter has only a small amount in southeastern coastal areas. During the day, the frequency of lightning peaks at 15:00–17:00 CST (China standard time) and is lowest at 8:00–10:00 CST. For the period with high CG stroke frequency (summer of a year or afternoon of a day), the proportion of +CG strokes and the discharge peak current are relatively small. Winter in a year and morning or midnight in a day correspond to a greater +CG stroke proportion and discharge current. Spatially, low latitudes, undulating terrain, the seaside, and humid surfaces are favorable factors for lightning occurrence. Thus, the southeast coastland has the largest lightning stroke density, while the northwest deserts and basins and the western and northern Tibetan Plateau, with altitudes over 6000 m, have almost no lightning. The proportion of +CG strokes and the peak current are low in the southern region with high density but diverse in other regions. The Tibetan Plateau causes the diversity of lightning activity in China and lays the foundation for studying the impact of surface elevation on lightning. Results indicate that the +CG stroke proportion on the eastern and southern Tibetan Plateau is up to 15 %, larger than the plain regions. The peak current of −CG strokes is positively correlated with altitude, but +CG strokes show a negative correlation, resulting in a large difference in peak current between +CG and −CG on the plain and a small difference on the plateau.

Cloud-to-ground (CG) lightning substantially impacts human health and property. However, the relations between U.S. lightning activity and the Madden-Julian Oscillation (MJO) and El Niño-Southern Oscillation (ENSO), two predictable drivers of global climate variability, remain uncertain, in part because most lightning datasets have short records that cannot robustly reveal MJO- and ENSO-related patterns. To overcome this limitation, we developed an empirical model of 6-hourly lightning flash count over the contiguous U.S. (CONUS) using environmental variables (convective available potential energy and precipitation) andNational Lightning Detection Network data for 2003–2016. This model is shown to reproduce the observed daily and seasonal variability of lightning over most of CONUS. Then, the empirical model was applied to construct a proxy lightning dataset for the period 1979–2021, which was used to investigate the summer MJO-lightning relationship at daily resolution and the winter-spring ENSO-lightning relationship at seasonal resolution. Overall, no robust relationship between MJO phase and lightning patterns was found when seasonality was taken into consideration. El Niño is associated with increased lightning activity over the Coastal Southeast U.S. during early winter, the Southwest during winter through spring, and the Northwest during late spring, whereas La Niña is associated with increased lightning activity over the Tennessee River Valley during winter.

We have studied the vertical negative leader progression features and the charge structures of 13 typical winter lightning flashes that produced downward terrestrial gamma‐ray flashes (TGFs). All these flashes started at altitudes below 1.5 km with an initial downward negative leader that propagates at a speed ranging from 1.3 to 4.5 × 106 m/s, followed by a strong negative return stroke called “energetic compact stroke” (ECS). After the ECS, usually there exists a radio quiet period lasting more than 10 ms. Interestingly, for more than half of the cases, soon after the resumed activities, an upward negative leader occurred at a position close to the lightning initiation point. Most of TGF lightning occurred under a main negative charge layer at the height of around 2 km. This negative charge layer is usually featured with a thickness of less than 2 km and a horizontal extension of more than 10 km.

Annual maps of cloud-to-ground lightning flash density have been produced since the deployment of the National Lightning Detection Network (NLDN). However, a comprehensive national summary of seasonal, monthly, and weekly lightning across the contiguous United States has not been developed. Cloud-to-ground lightning is not uniformly distributed in time, space, or frequency. Knowledge of these variations is useful for understanding meteorological processes responsible for lightning occurrence, planning outdoor events, anticipating impacts of lightning on power reliability, and relating to severe weather. To address this gap in documentation of lightning occurrence, the variability on seasonal, monthly, and weekly scales is first addressed with NLDN flash data from 2005 to 2014 for the 48 states and adjacent regions. Flash density and the percentage of each season’s portion of the annual total are compiled. In spring, thunderstorms occur most often over southeastern states. Lightning spreads north and west until by June, most areas have lightning. New England, the northern Rockies, most of Canada, and the Florida Peninsula have a small percentage of lightning outside of summer. Arizona and portions of adjacent states have the highest incidence in July and August. Flash densities reduce in September in most regions. This seasonal, monthly, and weekly overview complements a recent study of diurnal variations of flashes to document when and where lightning occurs over the United States. NLDN seasonal maps indicate a summer lightning dominance in the northern and western United States that extends into Canada using data compiled from GLD360 network observations. GLD360 also extends NLDN seasonal maps and percentages into Mexico, the Caribbean, and offshore regions.

Lightning flashes play a key role in the global electrical circuit, serving as markers of deep convection and indicators of climate variability. However, this field of research remains challenging due to the wide range of physical processes and spatiotemporal scales involved. To address this challenge, this study utilizes the Lightning Differential Space (LDS), which maps lightning stroke intervals onto a parameter space defined by their temporal and spatial derivatives. Using data from the Earth Networks Total Lightning Network (ENTLN), we analyze the Number Distribution LDS clustering patterns across specific seasons in three climatically distinct regions: a tropical rainforest region (Amazon), a subtropical marine environment (Eastern Mediterranean Sea), and a mid-latitude continental region (Great Plains in the U.S.). The LDS reveals a robust clustering topography composed of “allowed” and “forbidden” interval ranges, which are consistent across regions, while shifts in cluster position and properties reflect the underlying regional meteorological conditions. As an extension of the LDS framework, we introduce the Current Ratio LDS, a new diagnostic for identifying flash initiation by mapping the ratio of peak currents between successive strokes into the LDS coordinate space. This space reveals a spatiotemporal structure that enables a clearer distinction between local and regional scales. It also reveals a distinct cluster, suggesting a possible teleconnection between remote strokes, spanning tens to hundreds of kilometers. Together, the Number Distribution LDS and the novel Current Ratio LDS provide a scalable, data-driven framework for analyzing and interpreting large datasets of CG lightning activity. This approach strengthens the ability to characterize multiscale lightning behavior, offers a framework for evaluating model representations of stroke and flash processes, and provides a basis for developing diagnostics relevant to operational monitoring and forecasting of lightning activity.

Cloud-to-ground (CG) lightning data and the ECMWF ERA-Interim reanalysis dataset are analyzed to gain insight into the spatiotemporal distribution and synoptic background of winter-season CG flashes between December 2010 and February 2020 in China. We identify three Winter Lightning Frequent Areas (WLAs): the southwest side of the Yunnan-Guizhou Plateau (WLA1), the east side of the Yunnan-Guizhou Plateau (WLA2), and the Poyang Lake Plain (WLA3). The CG lightning flashes most frequently occur at local midnight and have a monthly peak in February. The CG lightning in WLA1 is mostly generated in non-frontal weather; however, the lightning in WLA2 and WLA3 mostly occurs in frontal systems. The frontal circulation situation is divided into four typical types: transversal trough after high pressure, low vortex, confrontational convergence, and asymptotic convergence. In all typical weather patterns, the lightning occurs downstream of a 500 hPa trough and is accompanied by a southwesterly low-level jet. The convective parameters of winter thunderstorms differ greatly from those of summer thunderstorms. The maximum convective available potential energy (MCAPE) and K-index (KI) are more useful metrics than convective available potential energy (CAPE) and Showalter index (SI) during winter. This study further deepens the understanding of the distribution characteristics of winter CG lightning in China, which motivates further research to improve the ability of winter thunderstorm prediction.