Cloud‐to‐ground lightning in Austria: A 10‐year study using data from a lightning location system

Cloud-to-ground (CG) lightning data have been analyzed for the years 2001–09 for North America, which includes Alaska, Canada, and the lower 48 U.S. states. Flashes recorded within the North American Lightning Detection Network (NALDN) are examined. No corrections for detection efficiency variability are made over the 9 yr of the dataset or over the large geographical area comprising North America. There were network changes in the NALDN during the 9 yr, but these changes have not been corrected for nor have the recorded data been altered in any way with the exception that all positive lightning reports with peak currents less than 15 kA have been deleted. Thus, the reader should be aware that secular changes are not just climatological in nature. All data were analyzed with a spatial resolution of 20 km. The analyses presented in this work provide a synoptic view of the interannual variability of lightning observations in North America, including the impacts of physical changes in the network during the 9 yr of study. These data complement and extend previous analyses that evaluate the U.S. NLDN during periods of upgrade. The total (negative and positive) flashes for ground flash density, the percentage of positive lightning, and the positive flash density have been analyzed. Furthermore, the negative and positive first stroke peak currents and the flash multiplicity have been examined. The highest flash densities in Canada are along the U.S.–Canadian border (1–2 flashes per square kilometer) and in the United States along the Gulf of Mexico coast from Texas through Florida (exceeding 14 flashes per square kilometer in Florida). The Gulf Stream is “outlined” by higher flash densities off the east coast of the United States. Maximum annual positive flash densities in Canada range primarily from 0.01 to 0.3 flashes per square kilometer, and in the United States to over 0.5 flashes per square kilometer in the Midwest and in the states of Louisiana and Mississippi. The annual percentage of positive lightning to ground varies from less than 2% over Florida to values exceeding 25% off the West Coast, Alaska, and the Yukon. A localized maximum in the percentage of positive lightning in the NALDN occurs in Manitoba and western Ontario, just north of North Dakota and Minnesota. When averaged over North America, first stroke negative median peak currents range from 19.8 kA in 2001 to 16.0 kA in 2009 and for all years, average 16.1 kA. First stroke positive median peak currents range from a high of 29.0 kA in 2008 and 2009 to a low of 23.3 kA in 2003 with a median of 25.7 kA for all years. There is a relatively sharp transition from low to high median negative peak currents along the Gulf and Atlantic coasts of the United States. No sharp transitions are observed for the median positive peak currents. Relatively lower positive peak currents occur throughout the southeastern United States. The highest values of mean negative multiplicity exceed 3.0 strokes per flash in the NALDN with some variation over the 9 yr. Lower values of mean negative multiplicity occur in the western United States. Positive flash mean multiplicity is slightly higher than 1.1, with the highest values of 1.7 observed in the southwestern states. As has been noted in prior research, CG lightning has significant variations from storm to storm as well as between geographical regions and/or seasons and, consequently, a single distribution for any lightning parameter, such as multiplicity or peak current, may not be sufficient to represent or describe the parameter.

Characteristics of lightning related to wildfire ignitions in Catalonia

The physical and geographical characteristics of over 216 million cloud-to-ground lightning flashes recorded during the first decade (1989–98) of operation of the National Lightning Detection Network (NLDN) covering the entire continental United States are presented. These characteristics include the total cloud-to-ground flash density, the positive flash density, the percentage of positive flashes, the first stroke negative and positive peak currents, and the multiplicity for negative and positive flashes. All analyses were done with a spatial resolution of 0.2° corresponding to an approximate resolution of 20 km. Flash densities were not corrected for detection efficiency; the measured values are presented. The maximum measured flash density is found to exceed 9 flashes km−2 across Florida in the Tampa–Orlando–Cape Canaveral corridor, near Fort Myers, and between Lake Okeechobee and the Atlantic Ocean. The mean monthly flash count peaks in July at approximately 5.5 million flashes. Positive flash density maxima, greater than 0.4 flashes km−2 occur in southern Florida; Houston, Texas; and along the Texas–Louisiana border. A broad region of relatively high positive density also occurs throughout the Midwest. The mean monthly positive flash count peaks in June and July at approximately 240 000 flashes in each month. The annual mean percentage of lightning that lowered positive charge was highest in the upper Midwest, exceeding 10% or 20% throughout most of the region. High percentages are also characteristic along the West Coast. The annual percentage of positive lightning has increased from 3% in 1989 to approximately 9% in 1998. The authors believe the increase is the result of improved sensor detection capability in the past decade. The mean monthly percentage of positive lightning flashes ranged from 4% in August to 17% in December for the decade. The annual median negative peak current ranged from 30 kA in 1989, decreasing steadily to about 20 kA in 1998. The annual median positive peak current ranged from 55 kA in 1989 decreasing to about 22 kA in 1998. The annual median peak negative and positive currents have approximately the same value since 1995, the first year after the NLDN upgrade. The monthly median first stroke peak currents for the decade peak in the winter and reach a minimum in May (positive current) and July (negative current). The mean monthly negative multiplicity for the decade ranges from 2.1 in February to 2.5 from June to October. The mean monthly positive multiplicity is approximately 1.2 throughout the year. The diurnal variation of the maximum flash rate over land was examined and found to peak during 1200–2000 local time (LT) with an exception for the upper Midwest, which peaked during 2000–0400 LT. Over water surrounding the continental United States, the lightning flash rate peaks primarily in the morning hours from 0400 to 1200 LT.

The present lightning location system of the Finnish Meteorological Institute has been in operation since 1998. We present statistics from the 10‐year period 1998–2007. The main emphasis of this study is to give a comprehensive analysis of the development of the location system. Our results complement previous studies by providing 10‐year lightning statistics from high latitudes in Europe (60–70°N). The changes in the detection efficiency have also been discussed. Our results indicate that in Finland (1) the median (average) peak currents in 2007 were −11.3 (−16.0) kA and +13.3 (+20.7) kA for negative and positive flashes, (2) the percentage of positive flashes in 2007 was 17%, (3) the average multiplicity for negative (positive) flashes in 2007 was 2.1 (1.2), (4) the median semimajor axis of the 50% location error ellipse in 2007 was 600 m (for 92% of flashes it was below 5 km), and (5) the average number of sensors reporting a flash in 2007 was 6.3. The network has experienced several configuration changes, leading to a more stable and better performing system, but these technical changes have also caused significant changes in the statistics of the measured lightning data. Clearly, lightning statistics over a 10‐year period cannot be handled without considering the impact of technical changes on the detection network.

Ground truth data of atmospheric discharges in correlation with LLS detections

Lightning is a main cause of electric utility outages and equipment damage throughout the world. The authors propose a flexible and reliable lightning detection and location system in order to take protective measures against such lightning damage. The system is based on standard PC hardware and software. They discuss several tropical lightning stroke characteristics found with the help of the described equipment. They show that lightning flash duration and inter-stroke interval as the negative lightning peak current measured by the station are similar to the data collected by alternative lightning location systems. They also discover parameters of the so called isolated pulse and irregular negative stroke. Good agreement of determined stroke parameters with data found in the literature demonstrates the capabilities of the presented lightning counter station, both as a lightning detection and location network unit as well as a lightning physics investigation system.

The National Lightning Detection Network (NLDN) underwent a major upgrade during 2002–03 that increased its sensitivity and improved its performance. It is important to examine cloud-to-ground (CG) lightning distributions before and after this upgrade because CG characteristics depend on both measurement capabilities and meteorological variability. This study compares preupgrade (1996–99, 2001) and postupgrade (2004–09) CG distributions over the contiguous United States to examine the influence of the recent upgrade and to provide baseline postupgrade averages. Increased sensitivity explains most of the differences in the pre- and postupgrade distributions, including a general increase in total CG and positive CG (+CG) flash densities. The increase in +CG occurs despite the use of a greater weak +CG threshold for removing ambiguous +CG reports (post 15 kA versus pre 10 kA). Conversely, the average +CG percentage decreased from 10.61% to 8.65% following the upgrade. The average +CG (−CG) multiplicity increased from 1.10 (2.05) before to 1.54 (2.41) after the upgrade. Since true +CG flashes rarely contain more than one return stroke, explanations for the greater than unity +CG multiplicities remain unclear. Postupgrade results indicate that regions with mostly weak peak current +CG flashes now exhibit greater average +CG multiplicities, whereas regions with mainly strong +CG flashes now exhibit smaller average +CG multiplicities. The combination of NLDN performance, meteorological conditions, and physical differences in first −CG return strokes over saltwater produce maxima in −CG multiplicity and peak current over the coastal waters of the southeast United States.

Although positive lightning flashes to ground are not as frequent as negative flashes, their large amplitudes and destructive characteristics make understanding their parameters an important issue. This study summarizes the characteristics of 103 positive cloud‐to‐ground (+CG) flashes that have been recorded using high‐speed video cameras (up to 11,800 frames per second) in three countries together with time‐correlated data provided by lightning location systems (LLS). A large fraction of the +CG flashes (81%) produced just a single stroke, and the average multiplicity was 1.2 strokes per flash. All the subsequent strokes in multiple‐stroke +CG flashes created a new ground termination except one. The geometric mean of 21 interstroke time intervals was 94 ms, which is about 1.5 times larger than the average interstroke interval in negative CG flashes (∼60 ms); 75% of the +CG flashes contained at least one long continuing current (LCC) ≥ 40 ms, and this percentage is significantly larger than in the negative flashes that produce LCCs (approximately 30%). The median estimated peak current (Ip) for 116 positive strokes that created new ground terminations was 39.4 kA. Positive strokes with a large Ip were usually followed by a LCC, and both of these parameters are threats in lightning protection. The characteristics presented here include the multiplicities of strokes and ground contacts, the percentage of single‐stroke flashes, the average interstroke time interval, the durations of the continuing current, and the distributions of Ip, the total flash durations, and the 2‐D leader speeds.

This paper presents a comparison among the mean monthly distributions of the number of cloud‐to‐ground (CG) flashes, the percentage of positive CG flashes and the peak current of negative and positive CG flashes obtained in Brazil for the period from 1999 to 2004, with the same distributions observed by similar networks for long time periods in other countries. From a correlation analysis, it was found that the mean monthly distributions of the number of CG flashes are very similar, even though the period of larger lightning activity along the year in Brazil, a tropical country, is longer than in the other temperate countries. The mean monthly distributions of the percentage and the peak current of positive CG flashes are also very similar, while the mean monthly distribution of the peak current of negative CG flashes in Brazil differs from the other countries. This difference is related to a significant decrease in the mean negative peak current in Brazil in the months of August and September. Apparently, the decrease is related to the injection in the atmosphere of large amounts of smoke from fires in these months, since the results also show a significant correlation between the monthly distribution of the number of fires and the negative peak current.

Seasonal, regional, and storm-scale variations of cloud-to-ground (CG) lightning characteristics in Florida are presented. Strong positive CG (+CG) and negative CG (−CG) flashes (i.e., having large peak current) are emphasized since they often are associated with strong storms, structural damage, and wildfire ignitions. Although strong −CG flashes are most common during the warm season (May–September) over the peninsula, the greatest proportion of strong +CG flashes occurs during the cool season (October–April) over the panhandle. The warm season exhibits the smallest +CG percentage but contains the greatest +CG flash densities, due in part to more ambiguous +CG reports (15–20 kA). The more frequent occurrence of ambiguous +CG reports helps explain the unusually small average +CG peak current during the warm season, whereas strong +CG reports (>20 kA) appear to be responsible for the greater average warm season +CG multiplicity. The −CG flash density, multiplicity, and peak current appear to be directly related, exhibiting their greatest values during the warm season when deep storms are most common. A case study examines the atmospheric conditions and storm-scale processes associated with two distinct groups of storms on 13–14 May 2007. Although these groups of storms form in close proximity, several factors combine to produce predominately strong +CG and −CG flashes in the northern (south Georgia) and southern (north Florida) regions, respectively. Results suggest that heat and smoke very near preexisting wildfires are key ingredients in producing reversed-polarity (+CG dominated) storms that often ignite subsequent wildfires.

In the early days, the risk of lightning strikes was described by the average number of thunderstorm days or thunderstorm hours, where a thunderstorm day is defined as an "Observational day during which thunder is heard at the station" [1]. On the basis of long-term records of thunderstorm days by the meteorological services, maps showing the so-called isoceraunic level were produced for the individual countries, from which the regional thunderstorm hazard could be obtained [2]. The first attempts to locate lightning discharges date back to the 1920s. W. Watt in [3] describes the considerations and experimental observations made at that time, which made it possible to identify thunderstorms and lightning discharges as the main cause of the observed electromagnetic disturbances in the early days of long-range radio communication.In the beginning, the problem of electromagnetic disturbances in radio communication was the driving force for research activities related to lightning electromagnetic fields and their source location. Only since the middle of the 1980s lightning detection has been applied in the field of thunderstorm observation for meteorological applications, in the field of lightning risk assessment and in fault analysis for power utilities. These new applications also placed significantly higher demands on the performance of the lightning location systems (LLS), especially with regard to detection efficiency and location accuracy. Lightning detection has thus evolved from a real-time thunderstorm observation tool to the most precise detection of each individual electrical discharge in lightning both in cloud-to-ground (CG) flashes and discharges within the thundercloud, often referred to as intracloud lightning (IC).In a typical thunderstorm, the number of IC discharges exceeds the number of CG discharges many times with an average ratio of IC/CG discharges being in the range of 5-10, but this ratio can be much higher in individual thunderstorms. As CG lightning poses the greatest danger to humans and property, at the beginning of the development of LLS, the focus was on the best possible and reliable detection of CG lightning. Only in recent years, the trend is increasingly moving towards the detection of total lightning. New lightning data have recently become available through the detection of lightning discharges by the Geostationary Lightning Mapper (GLM) on the satellites GOES West and GOES East [4]. Compared to land-based LLS, optical detection from space has the advantage that lightning activity is monitored over continents and oceans with approximately the same detection quality. The main limitation of lightning detection from space is the lack of differentiation between CG and IC discharges and the comparatively limited detection accuracy due to the spatial resolution of the optical sensor in the range of 4-5 km looking to earth surface from a distance of about 36,000 km. In this chapter, we will focus on ground-based systems only which are employing Magnetic Direction Finding (MDF) and/or Time of Arrival (TOA) technique, as data from these systems are used for many applications from lightning risk management to severe storm forecast.

We report upward lightning observations in São Paulo city, SP, Brazil and compare them to data from Lightning Location System (LLS). These data are provided by 4 different networks from different technologies: BrasilDAT, RINDAT, Worldwide Lightning Location Network (WWLLN), and Earth Networks Global Lightning Network (ENGLN). Several upward flashes were observed from 2012-2014 using GPS time-stamped optical sensors and electric field measurements. These upward flashes were initiated from tall towers located at Jaragua Peak (70 and 130 m) and along Paulista Avenue (23 towers with heights between 50 and 220m). Time-correlated analyzes allowed to evaluate the network detection efficiency, intracloud (IC) / cloud-to-ground (CG) misclassification percentage, and location accuracy of the 4 different LLS. We will also show how different upward flash physical processes (leader initiation, recoil leaders, return strokes) are detected and classified by the distinct LLS technologies. Preliminary results show that in general, recoil leaders were not detected; on the other hand, several cases of return strokes and some cases of M-component were detected by one or more LLS networks.

Pekan is located at east of peninsular Malaysia which experience very high cloud to ground (CG) lightning density. The lightning strike hazards can be properly managed by using a lightning location system (LLS). In this paper, a system to monitor the lightning strikes and thunderstorm has been installed and setup. Pekan Lightning Detection System (PLDS) has a combination of Magnetics Direction Finder and Time-of - Arrival sensor. The PLDS provides both real-time and historical lightning data. The real-time data include the time of lightning strokes, their location and an estimate of the peak current.

Time-intervals between negative lightning strokes and the creation of new ground terminations

Assessment of the Lightning Performance of overhead distribution lines based on Lightning Location Systems data