Ground Flash Research Articles

The interaction of ice crystals with hailstones in wet growth and its possible role in thunderstorm electrification

Charge separation during rebounding collisions of ice crystals on hail particles is a probable mechanism of thunderstorm electrification. Modelling of laboratory results successfully predicts the charge structure in thunderclouds under most conditions. An exception is the intense positive ground flashes observed in mid‐continental severe storms which contain large hailstones that are generally in wet growth. It is difficult to see how interacting ice crystals may separate from the wet surface of a hailstone without sticking, although some studies have reported charge separation during this process. Our laboratory experiments show that, during ice crystal interactions with a simulated hailstone in wet growth, a small but significant charge transfer occurs during ice crystal interactions. We present evidence to show that the hailstone surface is not uniformly wet and consists of dry zones, particularly at the edges, where ice crystals are most likely to impact, and hypothesize that a small number of ice crystals rebounding off these zones may give rise to the observed charging current. While this may explain wet growth charging in laboratory experiments, we cannot directly extend the findings to natural thunderstorms where hailstones tend to rotate and gyrate as they fall, but they may experience different growth regimes within the cloud.

Characteristics of Preliminary Breakdown Pulses in Positive Ground Flashes during Summer Thunderstorms in Sweden

This paper presents the characteristics of the preliminary breakdown pulses (PBP) in 51 positive ground flashes recorded during 2014 summer thunderstorms in Sweden. Electric field measurements were conducted remotely using a broadband antenna system (up to 100 MHz) for a recording length of 1 s with 200 ms trigger time. In the analysis, PBP trains were observed in 86% of the cases. Based on the number of trains preceding the first return stroke, the PBP were classified into single and multiple train PBP. Characteristics of the first PBP train were determined and based on the initial polarity of the pulses, three types of PBP were identified. Characteristics of the subsequent PBP trains in the multiple train PBP were also analyzed and they were compared with the first PBP train. Based on the conceptual charge cloud configuration, we found that the inverted dipole is consistent with our observation. We also found that PBP in positive ground flashes during summer thunderstorms in Sweden are weak since the average ratios of the PBP peak to the first return stroke peak lie only between 0.21 and 0.26. Possible reasons for no detection of PBP and the different types of PBP observed were also discussed.

On the characteristics of positive lightning ground flashes in Sweden

In this study the stroke characteristics of positive cloud-to-ground flashes in Sweden were obtained from the electric field records measured from 14 thunderstorms. The electric fields were measured with nanosecond resolution. Together with the fast and the slow electric field records, the narrowband radiation field at 3 and 30MHz signals were also measured simultaneously. Out of a total of 107 flashes, 30 flashes had two strokes, 7 had three strokes and 3 flashes had four strokes. The arithmetic and geometric means of the interstroke intervals were found to be 116 and 70 ms, respectively. The arithmetic and geometric mean ratio between the peak electric field of the Subsequent Return Stroke (SRS) and the first Return Stroke (RS) were 0.48 and 0.36, respectively. Of the 40 positive multiple-stroke ground flashes, 5% have at least one SRS with field peak higher than the first RS. The percentage of SRS with field peaks greater than the first RS was 6%. In our best of our knowledge, this is the first time a large sample of positive return strokes in Sweden was analysed. It was found to be statistically more significant than the previous studies.

Electric Field Signatures in Wideband, 3 MHz and 30 MHz of Negative Ground Flashes Pertinent to Swedish Thunderstorms

In this study, the electric field signatures of negative ground flashes pertinent to the Swedish thunderstorms were recorded simultaneously during the summer of 2014 using wide (up to 100 MHz) and narrow (at 3 MHz and 30 MHz as central frequencies) bandwidth antenna systems. The electric field signatures were recorded for a time duration of 250 ms. In the analysis, the whole flash was considered and a total of 98 flashes were chosen where electric field signatures of all wideband, 3 MHz and 30 MHz signals were present. It is observed that preliminary breakdown pulses are stronger radiators at 3 and 30 MHz compared to the return strokes. A comparison of our results with those of the previous studies obtained from different geographical regions clearly shows that the strength of preliminary breakdown pulses is higher in the temperate region (Sweden for instance) and is a function of latitude.

Unique Stroke-Multiplicity of Positive Lightning in Malaysia

This paper presents the unique characteristics of positive lightning generated vertical electric field waveforms recorded in Malaysia. The focus of the study was the waveforms due to positive lightning and their number of return strokes in a flash. A total of 172 lightning ground flash records were analysed, out of which 57 flashes contained positive lightning with a number of them having high numbers of subsequent return strokes which is somewhat unusual with observations elsewhere. The field measurements were done by using a parallel plate antenna coupled with a recording system. In contrary to the majority of positive lightning return strokes that has been recorded previously, where the average multiplicity is one, this study showed an average multiplicity of 3.86 which is almost four times higher than that is usually observed.

Contribution to Lightning Parameters Study Based on Some American Tropical Regions Observations

This paper presents a recent characterization review of three cloud-to-ground lightning parameters [lightning peak current, ground flash density, and keraunic level (KL)] used in engineering applications based on available data from studies conducted in some tropical countries of Central and South America (Brazil, Colombia, Costa Rica, and Venezuela), considering seasonal and spatial variation aspects. There are some differences between the behaviors of the analyzed parameters in tropical areas in comparison with those published in the literature for temperate ones. The most significant conclusion is that historical data suggest that the highest ground flash density are located between latitude of 8° and 10° north. This value is much higher than the highest values reported for countries located in temperate regions. It is seen from the KL analysis that this parameter varies depending on the latitude with values up to 200 thunderstorm days (TDs).

Characteristics of negative lightning leaders to ground observed by TVLS

The Thunder and VHF lightning Locating System (termed TVLS) is established and utilized to observe leader behaviors of negative cloud to ground (CG) flashes. This system takes advantages of VHF broadband interferometer and thunder imaging technique, which could provide the temporal and quasi-3D spatial evolution of lightning discharges. In conjunction with synchronized electric field changes (E-changes) and electric field derivatives (dE/dt) records, 10 leaders from two CG flashes are presented and analyzed. Based on the characteristic evolution of leader velocities, E-changes, dE/dt waveforms and VHF intervals, three stepped leaders, five dart leaders and two dart-stepped leaders are identified. The stepped leaders behave impulsive while approaching ground, with average speed (1.3∼3.9)×105m/s. All normal dart leaders presented here exhibit irregular (or termed “chaotic”) fluctuations in E-change and dE/dt waveforms, with the similar speeds ((1.0∼2.9)×107m/s) and durations ((300∼700)μs) of the “chaotic” leaders observed by other investigators. The irregular fluctuations would be weak if the channels keep conductive until the leader enters the less conductive branches, coinciding with VHF radiations in time sequence. The dart-stepped leader could be divided into the dart stage and the stepped stage by a transition region, which usually lies around the branch junctions of previous active channel. The dart stage resembles the normal dart leader, and the stepped stage usually associates with regular pulse trains in E-change and dE/dt waveforms.

Analyzing a Heavy Rainfall Process Based on Dual-Doppler Radar and Lightning Data

In order tofurther understand the application of Doppler radar data in mesoscale weather system as well as thedynamical structureof precipitationsystem,a large-scaledand more accurate inversion resultis aimed to get,basing on the new ly-proposed fuzzy logical algorithm of cloud classification and the two-step variational method which is applied to dual-Doppler radar. The data come from Dual-Doppler Radar' s(S-band) observation in Hefei and Fuyang,Anhui province. A heavy rainfall processoccurred in Anhui Province on 7 ~ 10 June in 2010 is analyzed by utilizing vertical accumulative total liquid water content(VIL),rainfalland distribution of lightning. The results show that this persistentrainfall is associated with the wind fieldsw hichcantains a convergence in low er layers and a divergence in higher layers,a cyclonic wind rotation and a wind shear line. This rainfall process which is based on stratiformis show n to be a hybridcumuluscloudprecipitation. Besides,the stratiform lightning is few during this rainfall and mainly focuses on the negative ground flashes. The heavy rain occurred20 minutes or so after lightning. The value of VIL in this heavy rainfall is small,which is less than 3 kg ·m-2.There are strong relationships among cloudclassification,the area of rain,VIL and Radar's echo.

Spatial and temporal variations of thunderstorm activities over Sri Lanka

Spatial and temporal variation of frequencies of thunderstorms over Sri Lanka using thunder day data is presented. A thunder day is simply a calendar day in which thunder is heard at least once at a given location. Two sets of data were collected and analyzed: annual totals for 10 climatological stations for a period of 50 years and monthly totals for 20 climatological stations for a period of 20 years. The average annual thunder days over Sri Lanka was found to be 76. Among the climatological stations considered, a high number of annual thunder days was recorded in Ratnapura (150 days/year), followed by Colombo (108 days/year) and Bandarawela (106 days/year). It appears that there are no widespread long-term increasing or decreasing trends in thunderstorm frequencies. However, Colombo, the capital of Sri Lanka which has over two million people shows an increasing trend of 0.8 thunder days per year. Although there is a high variability between stations reporting the number of thunder days, the overall pattern within a year is clear. Thunderstorm frequencies are high during two periods: March–May and September–November, which coincide with the first inter-monsoon and second inter-monsoon periods. Compared to the dry zone, the wet zone, especially the southwestern region, has high thunderstorm activity. There is a clear spatial difference in thunderstorm activities during the southwest and northeast monsoon seasons. During both these seasons, enhanced thunderstorm activities are reported on the leeward side of the mountain range. A slight reduction in the thunderstorm activities was found in the high elevation areas of the hill country compared to the surrounding areas. A lightning ground flash density map derived using annual thunder days is also presented.

Lightning Distribution Characteristics of Beijing-Shanghai High-speed Railway Corridor

In order to optimize the lightning protection design and to ensure the safe, stable operation of Beijing-Shanghai high-speed railway(B-S railway), we plotted a map of the average annual ground flash density of the areas along B-S railway by cloud-to-ground flash data of Hebei, Beijing, Tianjin, Anhui, Jiangsu, Shanghai and Shandong during 2005 to 2011, which were obtained by lightning location systems. The ground flash density gradually increased from north to south, and it remained less than 4.7 times/(km2?a) in North China. Applying the grid method to the line corridor, we meshed B-S railway into 126 statistical sections, which were labeled as NO.1~126, respectively, from north to south, and each one covered a 10 km×10 km area. According to the statistics of ground flash density in each section, the most vulnerable sections to lightning were located: the sections in the southwestern and the south central of Jiangsu Province were finally confirmed as the key sections of lightning protection along the whole B-S railway. Meanwhile, we defined other three statistical sections, namely, the North China, the Shandong, and the East China sections. The lightning current amplitude probability distributions of these three sections were counted and their distribution curves were fitted into fitting expressions.The results indicate that the lightning current amplitude probability distributes in lognormal patterns.

A Method for Retrieving the Ground Flash Fraction and Flash Type from Satellite Lightning Mapper Observations

AbstractAn analytic perturbation method is introduced for retrieving the lightning ground flash fraction in a set of N lightning flashes observed by a satellite lightning mapper. The value of N must be large, typically in the thousands, and the satellite lightning optical observations consist of the maximum group area (MGA) produced by each flash. Moreover, the method subsequently determines the flash type (ground or cloud) of each of the N flashes. Performance tests of the method were conducted using simulated observations that were based on Optical Transient Detector (OTD) and Lightning Imaging Sensor (LIS) data. It is found that the mean ground flash fraction retrieval errors are below 0.04 across the full range 0–1 under the nominal conditions defined. In general, it is demonstrated that the retrieval errors depend on many factors (i.e., the number N of satellite observations, the magnitude of random and systematic instrument measurement errors, the ground flash fraction itself, and the number of samples used to form certain climate distributions employed in the method). The fraction of flashes accurately flash typed by the method averaged better than 78%. Overall, the accuracy of ground flash fraction and flash-typing retrievals degrade as the simulated population ground and cloud flash MGA distributions become more identical. Finally, because the analytic perturbation method was found to be quite robust (i.e., it performed well for several arbitrary underlying MGA distributions), it is not restricted to the lightning problem studied here but can be applied to any inverse problem having a similar problem statement.

A study of the 21 March 2012 tornadic quasi linear convective system in Catalonia

This study presents a description of a quasi linear convective system that took place in Catalonia (NE Spain) on 21 March 2012 producing heavy rainfall, moderate lightning activity and a weak tornado in the village of Ivars d'Urgell around 19 UTC after local sunset. A post-event survey indicated EF0 and EF1 damage in houses of the village – roofs and ceilings, broken windows, fences and walls and trees knocked down – along a track approximately 4km long and about 20m wide. Doppler radar observations show that the parent thunderstorm that spawned the tornado was one of a series that developed along a convective line that moved from S to N, initiating convective activity in terms of precipitation and lightning in the Mediterranean Sea and moving inland in S Catalonia (Tarragona and Salou coastal areas, producing local flash floods). Convective activity remained several hours with series of thunderstorms developing along the same paths. The synoptic situation was dominated by a high pressure ridge extending from northern Africa to central Europe, with a closed maximum sea level pressure area around 1036hPa over northern France, southern Germany and Austria. On the other hand a relative low pressure area seen on 850hPa and upper levels was present over the Iberian Peninsula, favouring a southern maritime flow from the Mediterranean between the forward part of the low pressure area and the high pressure system which blocked the advance of the low to the east. In the study we examine both the synoptic environment and storm scale observations with Doppler radar and total lightning data (cloud to ground and intracloud flashes) that lead to this cool-season severe convective event which is remarkable given the fact that, unlike in this case, most reported tornadoes in the region occur during the warm season (with peaks in August and September) and during daylight hours (6 to 18 UTC).

Ground to Cloud Lightning Flash Currents and Electric Fields: Interaction with Aircraft and Production of Ionosphere Sprites

This paper presents for the first time a case for the importance of ground to cloud (upward leader) lightning flash parameters for safety testing of direct aircraft-lightning interaction and protection of wind turbines, as well as the importance of radiated electric fields for indirect lightning-aircraft interaction and generation of electric discharges called sprites and halos in the ionosphere. By using an electric circuit model of the transverse magnetic waves along the return stroke channel, electric currents at ground level as well as cloud level are determined for both the cloud to ground lightning flash and the ground to cloud lightning flash. We show that when an aircraft triggers lightning, the electric currents will be much more severe in current magnitude, rate of rise of currents, and frequency spectrum than otherwise and are more severe than the parameters observed for the usual and well monitored (and measured) cloud to ground (downward leader) flashes. The rate of rise of currents and the frequency spectrum of the ground to cloud lightning flash are also given here. The electric fields radiated by the lightning flashes that would appear in the ionosphere are presented for both the earth flash and the ground to cloud flash.

Lightning-initiator type of narrow bipolar events and their subsequent pulse trains

Previous observations show that some narrow bipolar events (NBEs) can initiate intracloud discharges, but the role of NBE as lightning initiation is still unclear. During the summer of 2013, 827 NBEs were detected with a 3-D LF lightning location system in Osaka, Japan. Out of 638 positive NBEs, 103 occurred as the initial events of lightning flashes. These initiator-type NBEs, called “INBEs” in this paper, are always followed by positive pulse trains whose locations show upward propagations probably from the main negative charge region to the upper positive charge region. Most of INBEs develop into intracloud flashes. Only two INBEs develop into positive ground flashes and five INBEs develop into negative ground flashes. Pulse widths and peak amplitudes of electric field change waveforms of INBEs are almost the same as those of normal NBEs. A major difference is that INBEs have much lower discharge heights. Most of INBEs are lower than 10 km while normal NBEs are mainly higher than 10 km. Characteristics of positive pulse trains following INBEs are closely related with discharge heights of INBEs. Higher INBEs are usually followed by weaker, fewer, and less frequent positive pulses with slower upward propagations. As the height increases to above 10 km, NBEs are usually no longer followed by such positive pulses.

Analysis on Lightning Striking Selectivity and Lightning Protection

According to the national standard (GB-50057-1994) of lightning protection design, we can calculate the number of lightning ground flash density around them. However, this technique would have some defects. In order to improve the predicting accuracy of the lightning striking of tall buildings, there are three methods, i.e. a broken line method, the method of Faraday cage, and ball method. The electrical geometry model of rolling ball method as the theoretical basis for calculating is easy. In order to analyze lightning striking selectivity of buildings, the shielding between buildings were discussed based on the electric geometric model (ball method). It is different lightning probability for different monomer buildings, building groups, different distance, different geographical environment, and climate environment. And some protection measurements were proposed to improve the lightning shielding performances.

Analyzing the transmission structures of long continuing current processes from negative ground flashes on the Qinghai‐Tibetan Plateau

AbstractThis paper presents a new overlap and progressive method using either a one‐point charge model or a point dipole model for studying the electric field change data of lightning and tests the suitability of the proposed method by analyzing the slow electric field change of the long continuing current data from two negative ground flashes. The current measurements of the ground flashes are from the Composite Observing Experiment for Lightning project carried out in the Datong region of Qinghai Province, China, in 2009, obtained from field observations synchronized through high‐precision GPS clocks at seven observation stations. Analysis shows that the time‐averaged current can reach 800 A, and the current intensity relates to the height of the top of the return stroke before the continuing current process. The two current processes studied in this work neutralized a large amount of negative charge, 39.5C and 60.8C, respectively. The neutralized charges from the negative charge layer are typically at 2.5–4.7 km above ground. The intracloud transition of the positive leader is a complex process, and the horizontal propagation of the positive leader plays an important role in current duration as well as horizontally influencing the center of the electrical dipole moment with an obvious change of up to 3.0 km. The proposed analysis method is very useful compared to previous methods in terms of determining infinitesimal changes in long continuing currents. This work's new analysis method will help increase understanding of the fine physical processes of long continuing current processes.

Probabilistic evaluation of failure risk of transmission line surge arresters caused by lightning flash

Transmission line surge arresters (TLAs) are vital elements for protection of power networks against the lightning surges. The proper selection of TLAs leads to a reduction of the lightning-related outages as well as the costs imposed to the power utility, because of the undelivered energy and damage to the equipment. The selection of the optimal arrester depends on how well its stresses can be estimated. Although a ground flash very frequently consists of multiple strokes, it is usual to consider only a single lightning stroke to calculate the lightning-related failures. A probabilistic method is presented for risk calculation that considers multiple strokes of each lightning flash to estimate the annual outage rate of TLAs. In the presented method, once the equivalent wave of the flash is computed, the probability of arrester failure is estimated by means of two indices, including total charge and maximum energy stress by the flash, separately. The energy stress caused by the flash is calculated using a few electromagnetic transient program (EMTP) simulations. Then, the obtained results from EMTP simulation are utilised to estimate the probability of failure. The accurate evaluation of failure probability of arresters permits the power utility to select the optimal arresters, considering a desirable average life.

Nordic Lightning Information System: Thunderstorm climate of Northern Europe for the period 2002–2011

A 10-year statistics (2002–2011) of the Nordic Lightning Information System (NORDLIS) are presented. NORDLIS is a joined lightning location network between Norway, Sweden, Finland, and Estonia, comprising in 2011 of 32 lightning location sensors. Our data set contains a total of 4,121,649 cloud-to-ground (CG) flashes. We show the regional and temporal distribution of lightning in Northern Europe during the study period. Our results indicate that the average annual ground flash density values are greatest in Southern Sweden, Baltic countries and Western Finland. The average number of thunderstorm days is largest in the Baltic countries and Southwestern Sweden, and the annual number of ground flashes has varied during the study period from 250,000 to 620,000. The largest observed daily number of ground flashes is 51,500, and the largest daily ground flash density is about 5CGskm−2; this has occurred in southern Sweden in July 2003. The average daily number of ground flashes peaks in mid-July–early-August. Cold season (October–April) thunderstorms occur frequently over the North Sea west of Norway and in the west coast of Denmark. Our results also show that an intense thunderstorm may occur practically anywhere in the Northern Europe except for certain maritime and mountain areas.

Comparative Study on Substation Shielding Due to Direct Lightning Strokes

Malaysia is one of the many countries that experience high lightning related activities. In fact, according to Malaysian Meteorological Department the Ground Flash Density (GFD) in Malaysia, it is rated at the fifth place in the ten most lightning cities of the world, with a high keraunic level, which is 240 thunderstorm days per year, and in other words, a Ground Flash Density (GFD) of 48.3 flashes per square kilometer per year. In the power systems, high keraunic level would contribute to high possibility of power interruptions such as disruption, degradation, damage and downtime. These outages would ultimately lead to revenue losses and reduction of network reliability. These lightning related interruptions may be in terms of direct lightning strikes to the lines or to the equipments in the substation. By the use of Mat Lab GUI (Graphic Users Interface), this study presents a simple computer program which uses the electro-geometric model (EGM) for the designing of substation shielding systems. The EGM uses the concept where the protection zone of a lightning system lies within the radius where the upward channel initiates and propagates through the air terminal to meet the downward leader. This interception point is called “the point of discrimination” and is where the downward leader decides its final jump. The distance at which the last jump occurs is known as the striking distance. With the use of the striking distance and the mathematical equations developed by Young, Brown-Whitehead, IEEE-1992 (IEEE T&D Committee Equations) and IEEE-1995 (IEEE Substations Committee Equations). This project aimed to investigate, understand and analyse the substation protection by means of masts and shield wires. The analysis is extended to account for lightning protection provided by single mast to two masts as well as from single shield wire to double shielding wires. The outcomes of these four equations will be compared.

Negative cloud-to-ground lightning flashes in Malaysia

The characteristics of the negative cloud-to-ground lightning flashes in Malaysia are studied by analyzing the electric fields generated by the whole flash in nanosecond resolution. A total of 405 strokes obtained from 100 successive negative cloud-to-ground lightning flashes were analyzed, which were recorded from seven convective thunderstorms during the southwest monsoon period, i.e. from April to June 2009. It was found that the total number of interstroke intervals has an arithmetic mean value of 86ms, a geometric mean value of 67ms and does not depend on the return stroke order. Of the 100 negative ground flashes, 38 flashes (38%) have at least one subsequent return-stroke (SRS) whose electric field peak was greater than that of the first return-stroke (RS). Furthermore, 58 (19%) out of 305 SRS have electric field peak larger than those of the first RS. The arithmetic and geometric mean ratio between the peak electric field of the SRS and the peak electric field of the first RS are 0.7 and 0.6, respectively. The percentage of single-stroke flashes was 16% while the mean number of strokes per flash and maximum number of stroke per flash were 4 and 14, respectively.