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In this study we investigate convective environments and their corresponding climatological features over Europe and the United States. For this purpose, National Lightning Detection Network (NLDN) and Arrival Time Difference long-range lightning detection network (ATDnet) data, ERA5 hybrid-sigma levels, and severe weather reports from the European Severe Weather Database (ESWD) and Storm Prediction Center (SPC) Storm Data were combined on a common grid of 0.25° and 1-h steps over the period 1979–2018. The severity of convective hazards increases with increasing instability and wind shear (WMAXSHEAR), but climatological aspects of these features differ over both domains. Environments over the United States are characterized by higher moisture, CAPE, CIN, wind shear, and midtropospheric lapse rates. Conversely, 0–3-km CAPE and low-level lapse rates are higher over Europe. From the climatological perspective severe thunderstorm environments (hours) are around 3–4 times more frequent over the United States with peaks across the Great Plains, Midwest, and Southeast. Over Europe severe environments are the most common over the south with local maxima in northern Italy. Despite having lower CAPE (tail distribution of 3000–4000 J kg−1 compared to 6000–8000 J kg−1 over the United States), thunderstorms over Europe have a higher probability for convective initiation given a favorable environment. Conversely, the lowest probability for initiation is observed over the Great Plains, but, once a thunderstorm develops, the probability that it will become severe is much higher compared to Europe. Prime conditions for severe thunderstorms over the United States are between April and June, typically from 1200 to 2200 central standard time (CST), while across Europe favorable environments are observed from June to August, usually between 1400 and 2100 UTC.

As lightning-detection records lengthen and the efficiency of severe weather reporting increases, more accurate climatologies of convective hazards can be constructed. In this study we aggregate flashes from the National Lightning Detection Network (NLDN) and Arrival Time Difference long-range lightning detection network (ATDnet) with severe weather reports from the European Severe Weather Database (ESWD) and Storm Prediction Center (SPC) Storm Data on a common grid of 0.25° and 1-h steps. Each year approximately 75–200 thunderstorm hours occur over the southwestern, central, and eastern United States, with a peak over Florida (200–250 h). The activity over the majority of Europe ranges from 15 to 100 h, with peaks over Italy and mountains (Pyrenees, Alps, Carpathians, Dinaric Alps; 100–150 h). The highest convective activity over continental Europe occurs during summer and over the Mediterranean during autumn. The United States peak for tornadoes and large hail reports is in spring, preceding the maximum of lightning and severe wind reports by 1–2 months. Convective hazards occur typically in the late afternoon, with the exception of the Midwest and Great Plains, where mesoscale convective systems shift the peak lightning threat to the night. The severe wind threat is delayed by 1–2 h compared to hail and tornadoes. The fraction of nocturnal lightning over land ranges from 15% to 30% with the lowest values observed over Florida and mountains (∼10%). Wintertime lightning shares the highest fraction of severe weather. Compared to Europe, extreme events are considerably more frequent over the United States, with maximum activity over the Great Plains. However, the threat over Europe should not be underestimated, as severe weather outbreaks with damaging winds, very large hail, and significant tornadoes occasionally occur over densely populated areas.

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.

Within the Charlotte, North Carolina, to Atlanta, Georgia, megaregion (Charlanta), the Atlanta metropolitan area has been shown to augment proximal cloud‐to‐ground (CG) lightning occurrence. Although numerous studies have documented this “urban lightning effect” (ULE) with regard to CG lightning, relatively few have investigated urban effects on distributions of total lightning (TL). Moreover, there has yet to be a study of the ULE using TL observations from the Geostationary Lightning Mapper (GLM). In an effort to fill this gap, we investigated spatial distributions of TL around the cities of Atlanta, GA, Greenville, SC, and Charlotte, NC, using GLM data collected during the warm seasons of 2018–2021. Analyses reveal augmentation of TL intensity and frequency over the major cities of Atlanta and Charlotte, with a diminished urban signal over the smaller city of Greenville. This work also demonstrated the potential efficacy of the emerging satellite‐based TL climatology in ULE studies. Plain Language Summary Studies using ground‐based lightning detection networks have revealed an “urban lightning effect” (ULE) around major cities. In 2016, the U.S. launched a weather satellite with a unique lightning mapping instrument. This study, possibly for the first time, demonstrated the ability to utilize space‐based observation of total lightning to detect the ULE within the Charlotte, North Carolina, to Atlanta, Georgia, urban corridor. The study also paves the way for future ULE analyses as the satellite lightning data record lengthens. Key Points The urban lightning effect (ULE) is detectable in Geostationary Lightning Mapper total lightning observations The ULE is most discernible in the larger metropolitan areas of the Charlotte, NC, to Atlanta, GA, urban corridor The emerging Geostationary Lightning Mapper data set enables a new generation of urban lightning studies as the record lengthens

In this paper, we present the first high‐speed video observation of a cloud‐to‐ground lightning flash and its associated downward‐directed Terrestrial Gamma‐ray Flash (TGF). The optical emission of the event was observed by a high‐speed video camera running at 40,000 frames per second in conjunction with the Telescope Array Surface Detector, Lightning Mapping Array, interferometer, electric‐field fast antenna, and the National Lightning Detection Network. The cloud‐to‐ground flash associated with the observed TGF was formed by a fast downward leader followed by a very intense return stroke peak current of −154 kA. The TGF occurred while the downward leader was below cloud base, and even when it was halfway in its propagation to ground. The suite of gamma‐ray and lightning instruments, timing resolution, and source proximity offer us detailed information and therefore a unique look at the TGF phenomena. Plain Language Summary This study provides the very first simultaneous observations of a downward‐directed terrestrial gamma‐ray flash (TGF) together with its associated cloud‐to‐ground lightning flash using a high‐speed camera in addition to gamma‐ray and radio measurements. The camera, running at 40,000 frames per second, allowed us to check the characteristics of the downward leader, the development stage of the lightning flash, and the luminosity variations in coincidence with TGF production. Key Points Simultaneous recordings of a downward‐directed terrestrial gamma‐ray flash (TGF), high‐speed video images, and radio emissions TGF events occurred while the leader was already branching below cloud base and even when it was halfway in its propagation to ground Energetic downward‐directed TGFs were associated with fast downward leaders that produced high return stroke peak currents

Ground‐based and satellite‐based lightning locating systems are the most common ways to detect and geolocate lightning. Depending upon the frequency range of operation, LLSs may report a variety of processes and characteristics associated with lightning flashes including channel formation, leader pulses, cloud‐to‐ground return strokes, M‐components, ICC pulses, cloud lightning pulses, location, duration, peak current, peak radiated power and energy, and full spatial extent of channels. Lightning data from different types of LLSs often provide complementary information about thunderstorms. For all the applications of lightning data, it is critical to understand the information that is provided by various lightning locating systems in order to interpret it correctly and make the best use of it. In this study, we summarize the various methods to geolocate lightning, both ground‐based and satellite‐based, and discuss the characteristics of lightning data available from various sources. The performance characteristics of lightning locating systems are determined by their ability to geolocate lightning events accurately with high detection efficiency and with low false detections and report various features of lightning correctly. Different methods or a combination of methods may be used to validate the performance characteristics of different types of lightning locating systems. We examine these methods and their applicability in validating the performance characteristics of different LLS types. Key Points Ground‐based and satellite‐based lightning data are complementary in nature Validation is critical to properly utilizing LLS data in applications Various validation techniques can be used to evaluate the performance of LLSs

Terrestrial gamma ray flash (TGF) observations made by the Atmosphere‐Space Interaction Monitor (ASIM) have demonstrated that these TGFs are accompanied by a prominent optical pulse from a hot leader channel. It is hard to confidently resolve the true sequence of the events in the source region due to temporal proximity of the involved processes. Here we report a bright long duration TGF together with its associated optical recordings showing clear temporal separation between the TGF and the optical pulse. In this observation the optical pulse is clearly distinct and subsequent relative to the TGF. The corresponding lightning discharge occurred at the very end of the TGF. We conclude that the current surge inside the lightning leader channel cannot be responsible for generation of this TGF. The current surge that produced the associated optical pulse can itself be conditioned by the TGF and may be responsible for the TGF termination. Plain Language Summary TGFs observed from space are found to be associated with current surges in lightning leader channels. These current surges emit radio waves and can be detected with lightning detection networks. They also produce optical pulses which can be observed by the optical sensors on board of the space satellites. The fact that TGFs have usually short duration does not allow to define the real sequence of events in the source region due to timing uncertainties. In this paper we report a unique observation of a rare coincidence of a long duration TGF accompanied by an optical pulse and a high peak current lightning detection. Duration of the TGF is one order of magnitude larger than the overall observational uncertainty, which allows us to reliably discern the TGF and the accompanying current pulse in the leader channel. We could confidently conclude that the TGF was generated first, in the very end of the TGF the current surge in the leader channel occurred, and the optical pulse was produced. The appearance of the current surge close to the end of the TGF can indicate that the current surge is conditioned by the TGF, and, reciprocally, it could condition the TGF termination. Key Points Accompanying optical pulse is subsequent to TGF TGF precedes the current surge in the leader channel and cannot be generated by this current surge TGF may be terminated by the current surge in the leader channel

The U.S. National Lightning Detection Network (NLDN) underwent a complete sensor upgrade in 2013 followed by a central processor upgrade in 2015. These upgrades produced about a factor-of-5 improvement in the detection efficiency of cloud lightning flashes and about one additional cloud pulse geolocated per flash. However, they also reaggravated a historical problem with the tendency to misclassify a population of low-current positive discharges as cloud-to-ground strokes when, in fact, most are probably cloud pulses. Furthermore, less than 0.1% of events were poorly geolocated because the contributing sensor data were either improperly associated or simply underutilized by the geolocation algorithm. To address these issues, Vaisala developed additional improvements to the central processing system, which became operational on 7 November 2018. This paper describes updates to the NLDN between 2013 and 2018 and then focuses on the effects of classification algorithm changes and a simple means to normalize classification across upgrades.

The existence of mesoscale lightning discharges on the order of 100 km in length has been known since the radar-based findings of Ligda in the mid-1950s. However, it took the discovery of sprites in 1989 to direct significant attention to horizontally extensive “megaflashes” within mesoscale convective systems (MCSs). More recently, 3D Lightning Mapping Arrays (LMAs) have documented sprite-initiating lightning discharges traversing several hundred kilometers. One such event in a 2007 Oklahoma MCS having an LMA-derived length of 321 km, has been certified by the WMO as the longest officially documented lightning flash. The new Geostationary Lightning Mapper (GLM) sensor on GOES-16/17 now provides an additional tool suited to investigating mesoscale lightning. On 22 October 2017, a quasi-linear convective system moved through the central United States. At 0513 UTC, the GLM indicated a lightning discharge originated in northern Texas, propagated north-northeast across Oklahoma, fortuitously traversed the Oklahoma LMA (OKLMA), and finally terminated in southeastern Kansas. This event is explored using the OKLMA, the National Lightning Detection Network (NLDN), and the GLM. The NLDN reported 17 positive cloud-to-ground flashes (+CGs), 23 negative CGs (−CGs), and 37 intracloud flashes (ICs) associated with this massive discharge, including two +CGs capable of inducing sprites, with others triggering upward lightning from tall towers. Combining all available data confirms the megaflash, which illuminated 67,845 km², was at least 500 km long, greatly exceeding the current official record flash length. Yet even these values are being superseded as GLM data are further explored, revealing that such vast discharges may not be all that uncommon.

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.