Ground Flash Density Research Articles

Spatial distribution and frequency of lightning activity and lightning flash density maps for Australia

The spatial distribution and frequency of lightning activity in Australia have been analyzed using lightning data obtained by ground‐based lightning detection instruments denoted CIGRE‐500 and CGR3 and by NASA satellite‐based instruments denoted OTD and LIS. The geographical distribution of lightning incidence is described by a map of total lightning flash density, Nt (i.e., cloud‐to‐ground and intracloud flashes). A high level of lightning activity, Nt > 10 km−2yr−1, is observed in the northern parts of Australia, and a decrease in total flash density occurs southward to Nt < 5 km−2yr−1 in the central and southern parts of Australia. The peak lightning occurrence is in the northwestern part of the Australian continent with Nt values up to about 35 km−2yr−1 centered around 16°S 126°E. A reduction in Nt by a factor of about 10 for a change in latitude from 10°S to 40°S was found, which is in agreement with the earlier studies. The data from all the sources were used to estimate the cloud flash‐to‐ground flash ratio, Z, which at the studied localities was found to be in a range of values from 0.75 to 7.7. We concluded that for the range of latitude over Australia the most representative value of Z is about 2 ± 30%, and it is relatively independent of latitude. We used this to develop a map of average annual lightning ground flash density, Ng, the first for Australia. Ng varies from over 6 km−2yr−1 in the northern parts of Australia to about 1 km−2yr−1 and below in the southern parts.

Study of the Winter Lightning Ground Flash Density Investigated by the Lightning Location Systems

A network of lightning location systems covering the entire Japanese Archipelago has been operational since the late 1980s. However, the operational records appear to be distorted relative to winter lightning flash densities. The maximum flash density registered was 0.3 flashes/km/sup 2/ per one winter season, a mere one-tenth of the corresponding figure for severe summer lightning anywhere along the archipelago. On the other hand, the fault frequencies of overhead transmission lines constructed in the severe winter lightning districts are comparable with those of severe summer lightning districts. The author submits to resolve the apparent disparity by introducing two analytical parameters, namely 1) the propagation velocity of the return strokes of the winter lightning, and 2) the seasonal difference in the altitude of the lightning charge in the cloud. Consequently, the author proposes a certain seasonal modification of the threshold level applicable to the operation of lightning location systems.

Geographical and seasonal characteristics of the relationship between lightning ground flash density and rainfall within the continent of Australia

Ground-based observations of annual rainfall and lightning incidence collected over periods ranging from 9 to 22 years at 23 stations around the continent of Australia were used to compute values of ‘rain yield’, defined as the mass of rain produced per lightning ground flash (units: kg fl − 1) over a given area of ground. The rain yield was found to vary considerably with geographical location, season and climatic conditions. Of the 23 stations, 5 were mid-continental and these showed a mean rain yield of 2.64 × 10 8 kg fl − 1 in contrast to the coastal and near-coastal stations that showed a corresponding mean value of 9.91 × 10 8 kg fl − 1. The difference was statistically significant at the confidence level of 95%. When the stations were classified according to seasonal climate zones, the winter and winter-dominant rainfall stations showed a rain yield of 1.28 × 10 9 kg fl − 1 while the summer and summer-dominant rainfall stations showed a significantly lower value of 5.44 × 10 8 kg fl − 1. Again the difference was statistically significant at the 95% confidence level. Every one of the 23 stations showed mean winter rain yields that were significantly higher than the summer values. These differences are attributed to surface heating which controls such parameters as cloud base height and convective available potential energy in the atmosphere. In terms of the behaviour of the rain yield with geographical, seasonal and climatic conditions, the Australian observations are in good agreement with studies in other parts of the world.

Dissecting the anomaly: A closer look at the documented urban enhancement in summer season ground flash densities in and around the Houston area

The Houston “lightning anomaly” was examined using nine years of cloud‐to‐ground (CG) lightning data (1995–2003). We find that the local Houston CG lightning anomaly, is a persistent feature, but statistically non‐unique along the Gulf Coast. The results also suggest that although the Houston area sees an increased frequency of lightning producing storms, storms occurring further inland appear to produce more lightning on an event basis. Although hypotheses invoking anthropogenic influences have been offered to explain the Houston flash density anomaly, it seems equally plausible that mesoscale influences along the coastline may also be important.

Lightning Fatalities on the South African Highveld

A review of the Southern Africa medical literature shows a paucity of published data regarding lightning fatalities. The South African Highveld has a lightning ground flash density of 6 to 9 flashes/km/year, with a high incidence of thunderstorm days per year (some 40-70). The Highveld has a largely urban population, many of whom have low socioeconomic status and poor education, housing, and other infrastructures and hence (possibly) are at greater exposure risk. Thirty-eight victims of lightning-related death were identified from the records of the 6 large medicolegal mortuaries on the South African Highveld, serving a population of approximately 7 million, for the period 1997 to 2000. Analysis of the records revealed that 95% of all victims were black, 79% were male, and the average age was 36 years. Lightning strikes occurred from September through to April (normal summer rainfall period), and the most strikes took place in the late afternoon (3:00 pm to 6:00 pm). All except 1 case occurred outdoors. In the autopsy reports, mention was made of singeing of hair in 68% of cases, and mention of damage to clothing was made in 26% of cases. Cutaneous thermal injuries were noted in 34 of the 38 cases, with apparent electrothermal injuries of the feet noted in 4 cases. Fifty-two percent of victims sustained some form of associated blunt-force injury (including abrasions, contusions, etc). Specific keraunopathologic injuries were described in only 2 of the cases. Twenty-one cases had some form of internal organ injury. This study serves to illustrate the relatively high incidence of lightning strikes in the region and calls for a more systematic and detailed investigative protocol in lightning-related deaths.

Lightning Data Observed With Lightning Location System in Guang-Dong Province, China

This paper presents an investigation into cloud-to-ground lightning activity data collected by a lightning detection system in the Guang-Dong Province, the southernmost in China. Annual lightning days are acquired and ground flash density maps are constructed from the database, which contains the information of more than one million flashes. The lightning days are compared with annual thunderstorm days collected by the Guang-Dong Meteorological Agency, and a weak correlation is noted. The correlation between ground flash density and thunderstorm day is studied, and an empirical formula is presented. The relationship between transmission line faults and ground flash density is discussed in this paper too. It is noted that the correlation with the faults caused by lightning is relatively high. The correlation coefficient does not significantly vary with the threshold of discharge current set for constructing the map of revised ground flash density.

Evaluation of the LLS efficiency effects on the ground flash density, using the Italian lightning detection system SIRF

The Italian Lightning Detection System started at the end of 1994 and has been recording lightning data since then. The efficiency of the whole network is evaluated with the usual projection software. Considering this projection as the real efficiency is possible to weight the calculated ground flash density, getting a much more significant estimation of this fundamental parameter. At the end of 1999 four sensors were added to the network to increase the detection efficiency over the Southern part of the Italian territory and over the main islands of Sardinia and Sicily. An extrapolation of the network upgrade was performed on the previous years of data using the same efficiency correction method. We developed a dedicated software, which takes into account the real behavior of each sensor and calculates the efficiency of the network with this information. The efficiency obtained can be used to correct the ground density map, avoiding holes of efficiency due to temporary sensor problems. After the corrections significant changes in the ground flash density map can be seen, showing some areas of high density that were missed in the usual uncorrected mapping of data. This suggests that influence of Lightning Location System efficiency has to be carefully taken into account in ground flash density calculations.

Lightning risk assessment using fuzzy logic

A lightning risk assessment necessarily implies a set of uncertainties related to lightning strike probability, presence of a damage channel (type of coupling) or presence of a victim (equipment, human beings, structure itself, etc.). In order to quantify and qualify these uncertainties a risk assessment methodology using fuzzy logic is proposed. Fuzzy logic seems to be a powerful tool capable of combining linguistic and numeric variables in order to estimate the subjectiveness involved in risk analysis and determine whether a risk level is acceptable or not. The methodology proposed in this paper is a contribution to the analysis of risk of damage due to lightning, particularly applied in Tropical Regions, where parameters such as ground flash density—GFD, and lightning current peak value—LCPV present a special behavior and determine the frequency and Intensity of lightning phenomenon.

Verification of lightning strike incidence as a Poisson process

Recordings of lightning strikes to a wide range of practical structures are used to investigate the statistical nature of lightning strike incidence. Using the scatter of the lightning incidence onto a given attractive area, comparisons are made between the theoretically estimated and the observed strike incidence. It is shown that the strike frequency data are well described by a Poisson distribution for the ground flash density and a log-normal distribution for lightning current. The uncertainties in the estimation of ground flash density are determined, taking into account factors such as year-to-year variability and the observation time span.

Evaluation of the Guang Dong lightning location system with transmission line fault data

The Guang Dong lightning location system (GDLLS) was put into operation in 1997. Its performance, including detection efficiency and location accuracy, was evaluated with the data of the recorded lightning faults on 110, 220 and 500 kV transmission lines in 1997–1999. Lightning parameters, such as thunderstorm days, ground-flash density and peak lightning current were estimated with the lightning data collected by GDLLS. It is found that the detection efficiency is as high as 86%, and the median lightning accuracy is about 1.3 km. The correlation between the number of detected lightning flashes and recorded transmission line faults is weak, but can be improved if more appropriate analysis methods or data are employed.

A diagnostic analysis of the Kennedy Space Center LDAR network: 2. Cross‐sensor studies

Range dependencies in total (intracloud and cloud to ground) lightning observed by the Kennedy Space Center Lightning Detection and Ranging (LDAR) network are established through cross comparison with other lightning sensors. Using total lightning observed from space by the Lightning Imaging Sensor (LIS), LDAR flash detection efficiency is shown to remain above 90% out to 90–100 km range, and to be below 25% at 200 km range. LDAR VHF source location error distributions are also determined as a function of range and are found to be asymmetric with standard deviation increasing roughly as r2. Range normalization schemes for total VHF source density are tested and shown to yield significant improvements in correlation with National Lightning Detection Network (NLDN) ground flash density at hourly, daily, monthly, and climatological timescales (up to 50% over uncorrected source densities using an exponential‐in‐range correction factor with 40–50 km e‐folding scale).

Cloud-to-Ground Lightning throughout the Lifetime of a Severe Storm System in Oklahoma

Cloud-to-ground (CG) lightning data are examined relative to digitized radar data for a storm system that occurred in Oklahoma on 26 May 1985. This system evolved through three stages: 1) two lines of cells, one near the dryline and the other 60 km ahead of it; 2) a supercell storm; and 3) a mesoscale convective system (MCS). The behavior of lightning in each stage was different. Initially no ground flashes were observed in either line until reflectivity increased to $46 dBZ and vertically integrated liquid (VIL) increased to $10 kg m22; then ground flash rates remained ,1.2 min21 for .1 h. Most ground flashes in the line of storms near the dryline were negative (18 2CG, 3 1CG), while most in the leading line were positive (11 1CG, 3 2CG), a pattern of polarity opposite to what usually has been observed. Approximately 3 h after radar detected the first storm, ground flash rates increased to .5 min21 and remained so for 6 h. A mesocyclone formed approximately 30 min after flash rates exceeded 5 min 21, and a few positive ground flashes occurred near it. Ground flash rates increased briefly to .20 min21 as the mesocyclone dissipated and then remained .10 min21 as a squall line formed along the outflow boundary from the dissipating supercell and produced a stratiform region. Most ground flashes in this MCS occurred in the convective line and had negative polarity. The few ground flashes in the stratiform region tended to be positive (42 1CG, 32 2CG during 3 h). Durin g1ho f theMCS, ground flash rates decreased and then increased again simultaneously in both the convective and stratiform regions, a previously undocumented behavior. It is possible that this was caused by updrafts in both the convective line and stratiform region changing at roughly the same time. It is also possible that most ground flashes in the stratiform region originated near the convective line, and so were influenced by the line. Overall trends in ground flash density, flash relative frequency, reflectivity, VIL, and severe hail reports appeared similar as the storm system evolved.

Diurnal variation of lightning characteristics around Java Island

The diurnal variation of lightning activity on and around Java Island, Indonesia, is examined by using the observation results from a magnetic direction finder network. The magnetic direction finder network has four direction finders on Java Island and has been operating since 1994. Lightning is active during the rainy season, usually from October to May. The highest ground flash density on Java Island was 16 flashes per km2 per year. The diurnal variation of the lightning activity over land shows its peak at around 1500 LT. There is a small peak in the early morning around 0300–0500 LT over the sea, several hours prior to the reported peak times of the cold cloudiness, an indicator of deep convection observed from meteorological satellites. Existence of close sea‐land interaction related to the activity of deep convection in this area is suggested from the diurnal variations in the lightning activity. The diurnal variation is also found in the peak currents of negative first return strokes over land, medians from 25 kA around noon to 30 kA around midnight. The lightning activity over the ocean does not show such diurnal variation and delivers currents of higher amplitudes with a median of 36 kA.

Lightning Ground Flash Density and Thunderstorm Duration in the Continental United States: 1989–96

Abstract The mean annual flash density, thunderstorm duration, and flash rates were calculated using 121.7 million cloud-to-ground lightning flashes in the continental United States for the period 1989–96. Florida had flash densities over 11 flashes km−2 yr−1, while the Midwest, Oklahoma, Texas, and the Gulf Coast had densities greater than 7 flashes km−2 yr−1. There was a relative minimum in flash density (three flashes km−2 yr−1) in the Appalachian Mountains and Missouri. Thunderstorm duration values exceeded 120 h yr−1 in Florida and 105 h yr−1 in New Mexico, Arizona, and the Gulf Coast. The maximum annual flash rates exceeded 45 flashes h−1 in the Midwest, along the Florida coasts, and along the mid-Atlantic coast with the minimum flash rates, 15 flashes h−1, over the Appalachian and Rocky Mountains. The relationship between thunderstorm duration and flash density is Flash_Density = 0.024(Flash_Hours)1.29 producing expected flash densities that are within 30% of the measured densities for over 70% of ...

Convective Structure of Hurricanes as Revealed by Lightning Locations

Cloud-to-ground lightning flash locations were examined for nine Atlantic basin hurricanes using data from the National Lightning Detection Network. A common radial distribution in ground flash density was evident: a weak maximum in the eyewall region, a clear minimum 80–100 km outside the eyewall, and a strong maximum in the vicinity of outer rainbands (210–290-km radius). These results are consistent with the authors’ previous study of Hurricane Andrew. None of the storms showed this characteristic radial structure during prehurricane stages. The results support the division of precipitation in the hurricane into three distinct regimes. The eyewall is a unique phenomenon but shares some attributes with deep, weakly electrified oceanic monsoonal convection. The region outside the eyewall and under the central dense overcast has characteristics of the trailing stratiform region of mesoscale convective systems, including a relatively high fraction of positive polarity flashes. The outer bands, with mean maximum flash density at the 250-km radius, contain the vast majority of ground flashes in the storms. Eyewall lightning, defined as that within 40 km of the center, was examined for four moderate-to-strong hurricanes. Such lightning occurred episodically during hurricane stage, with 93% of hourly intervals containing no detected flashes. Eyewall lightning outbreaks over water always occurred at the beginning of or during times of intensification, but often were indicative of the imminent end of deepening. It is proposed that the existence of such inner core lightning might reveal the presence of an eyewall cycle. For the one storm with available aircraft reconnaissance data, eyewall cycles were reliably identified by the occurrence of inner core lightning, and inner core lightning appeared only during such cycles. Suggestions are made as to how eyewall flashes in existing hurricanes might be used to help predict hurricane intensity change.

Spatial and temporal distribution of lightning activity around Java

Cloud‐to‐ground lightning flashes on and around the island of Java, Indonesia, in the tropical maritime continent region, were observed continuously by a magnetic direction‐finder network for lightning location from December 1994 to January 1996. The annual ground flash density, estimated by correcting the raw data by using the detection efficiency of the network, ranged from less than 2 to about 16 flashes/y/km2 on the island. The average annual ground flash density over the entire island was 3.2 flashes/y/km2, as opposed to 0.24 flashes/y/km2 on the Indian Ocean about 100 km south of the island. The majority of the flashes on the land occurred during November to April in the rainy season. In the dry season, lightning was active only in the western part of the island. The amount of monthly precipitation over the island was related to the monthly number of ground flashes, but the ratio of the precipitation to the ground flash density differed from season to season, from 4×108 kg/flash in the rainy season to 1.3×109 kg/flash for other months. The ratio observed in the rainy season is equal to that for the break period in the rainy season observed at Darwin, Australia, in the same maritime continent region. The annual ratio for the entire island was 6×108 kg/flash. The diurnal variation of the lightning activity averaged over the island in the set season showed a single peak in the afternoon with the peak time of about 1530 LT, similar to those reported for land stations in the tropics. The diurnal variation over the ocean had a peak in the early morning, showing the same characteristic observed in the western Pacific. The diurnal variation of the lightning activity on the sea of offshore regions around Java showed an almost out‐of‐phase pattern to the variation on the land, indicating the influence of the land‐sea effect.

A five-year study of lightning activity using a CGR3 flash counter in Gaborone, Botswana

We have used a CGR3 lightning flash counter to gather lightning type and occurrence data over five complete years in Gaborone, Botswana. The results show that the mean ground flash density is 4.6 km−2 yr−1. Of these, 4.9% of ground flashes lower a net positive charge. The overall ratio of intracloud flashes to ground flashes is 1.9, the value being slightly higher during the nights than the daytime. A typical lightning season lasts from September to May, the activity reaching a peak between November and January. There is virtually no lightning at all during the dry winter months of June, July and August. The number of flashes per storm shows a marked decrease as the season progresses. In the diurnal cycle, the peak lightning activity occurs at around 19 h, which is somewhat late when compared with most other locations. However, together with observations in the Central United States, it seems to suggest that the peak activity occurs later in the day at mid-continental locations.

Lighting Ground Flash Density in the Contiguous United States: 1992–95

Abstract Cloud-to-ground lightning data for the years 1992–95 have been analyzed for geographical distribution of total flashes, positive flashes, and the percentage of flashes that lower positive charge to ground. In the contiguous United States the measured total cloud-to-ground lightning flash counts were 16.3 million (1992), 24.2 million (in both 1993 and 1994), and 22.3 million in 1995. The maximum flash densities occurred in Florida in 1992 (9–11 flashes per square kilometer) and in the Midwest in 1993 (11–13 flashes per square kilometer), coinciding with the storms and floods that dominated the summer of 1993 in the Midwest. In 1994, the area of maximum flash density was again in Florida (11–13 flashes per square kilometer). In 1995, the flash density maxima (9–11 km−2) were in southern Louisiana and near the Kentucky–Illinois border. Positive flash densities had maxima in the Midwest in all four years with values of 0.4 (1992), 1.0 (1993), 0.7 (1994), and 1.8 flashes per square kilometer (1995). T...

A Climatology, Synoptic Assessment, and Thermodynamic Evaluation for Cloud-to-Ground Lightning in Georgia: A study for the 1996 Summer Olympics

A lightning climatology within 50 km of nine outdoor venue locations for the 1996 Summer Olympics has been produced. Spatial and temporal patterns were analyzed for July and August from 1986 through 1993. Unusually active and inactive lightning days were isolated, and thermodynamic variables examined. At the inland sites, no pattern was found in the spatial distribution of cloud-to-ground lightning; that is, the lightning locations were random. At the one coastal site, Savannah, an inland maximum in ground flash density was observed. Although there was great day-to-day variability, there was a diurnal progression of lightning with a broad minimum from 0600 to 1400 UTC and a sharp maximum near 2200 UTC. Composite synoptic charts were produced for eight selected active days and eight selected inactive days. At the 500-hPa level the composite dewpoint depression in central Georgia was approximately 8°C less on active days than on inactive days. At the 850-hPa level the vector-averaged wind fields on active days revealed weakly anticyclonic southwesterly flow throughout Georgia. On inactive days, the vector-averaged winds exhibited a large anticyclone centered in northern Georgia. Some correlation was found between cloud-to-ground lightning activity and several of the thermodynamic variables. The most highly correlated was a form of convective available potential energy with a correlation coefficient of 0.70. The Showalter stability index and K index had correlation coefficients of 0.60 and 0.56, respectively. Logistic regression equations were developed to forecast active and inactive lightning days from thermodynamic variables and persistence. Days of unusually low lightning activity were more accurately identified through logistic regression than days of unusually high lightning activity. To aid in forecasting lightning days, the historical probability of active or inactive lightning days is provided as a function of the logistic model output.

Observations of High Ground Flash Densities of Positive Lightning in Summertime Thunderstorms

Abstract Observations of summertime thunderstorms indicate that positive polarity cloud-to-ground lightning activity can occur with rates as high as 67 flashes in 5 min and spatial densities up to 0.60 flashes per square kilometer per hour. All ground flashes in a storm may be positive for substantial periods. Using data from a nationwide network of magnetic direction finders, 24 storms with high ground flash densities of positive lightning were found on 11 days in June and July 1989 in the Great Plains of the United States. The periods of high-density positive lightning persisted an average of 4 h, longer than the lifetime of a typical single thunderstorm cell. In most cases, they occurred at or near the beginning of the storms' cloud-to-ground lightning activity. Supporting data suggest that the production of high rate and high percentage of positive ground flashes may be associated with exceptionally tall storms that exhibit a stage of early, rapid increase in radar echo-top height and product large hail.