Cloud-to-ground Flash Density Research Articles

Observation and Simulation of Lightning Strikes in an Offshore Wind Turbine Cluster

AbstractBased on the detection data of the Guangdong‐Hong Kong‐Macau lightning location system from 2013 to 2022, this paper analyzes the characteristics of lightning strikes in an offshore wind turbine cluster (WTC) in Guangdong. This is the first time to quantify the lightning strikes difference between the inner and outer wind turbines (WTs) and discuss the impact mechanisms of WTCs on the characteristics of lightning strikes by the simulation results of electric potential distribution. The observation results show that the stroke density, cloud‐to‐ground (CG) flash density, and average peak current within the affected area increase by 10.1%, 11%, and 5.1%, respectively, but the lightning multiplicity changes little. The inner and outer WTs show different characteristics of lightning strikes. The stroke density, lightning multiplicity, and average peak current in the area of outer WTs increase by 21%, 4.1%, and 6.8%, respectively, while in the area of inner WTs decrease by 20.2%, 19%, and 3.9%, respectively. The CG flash density in the area of outer WTs increases by 16.1% but changes little in the area of inner WTs. The simulation results show there is a significant difference in the electric potential distribution between the inner and outer WTs: the distortion effect of the outer WTs on spatial electric potential is stronger than that of the inner WTs at a greater height, but the difference is not obvious at a lower height. Such a difference in electric potential distribution leads to different characteristics of lightning strikes between the inner and outer WTs.

Spatiotemporal patterns of long series of cloud-to-ground lightning in Beijing and its cause

Previous studies on the climatic characteristics of lightning activity in Beijing are sparse due to the lack of long-term, continuous and stable lightning location data. In this paper, the long series of lightning location data during 2009–2018 were applied to reveal the spatiotemporal distribution of cloud-to-ground (CG) lightning in Beijing. The spatial characteristics were low west and high east, and the temporal characteristics were frequent in summer. In the complex terrain areas of northeast Beijing, there was a positive relationship between CG density and altitude, with the correlation coefficient higher than 0.8. The northeastern trumpet-shaped terrain had the forced lifting effect on thunderstorms, forming lightning-intensive areas of 500 km2. In addition, urban heat island (UHI) effect provided the necessary thermal conditions for the development of thunderstorms, forming lightning-intensive areas in the built-up areas, with the highest value of CG flash density exceeding 6 fl/km2. Note that the lightning-sparse areas in the urban center exhibited a distinct barrier effect on the spatial patterns of summertime CG lightning in the built-up areas, which was mainly modulated by the type of synoptic background.

Enhancement of Cloud-to-Ground Lightning Activity Caused by the Urban Effect: A Case Study in the Beijing Metropolitan Area

To investigate the possible impact of urban development on lightning activity, an eight-year (2010–2017) cloud-to-ground (CG) lightning dataset provided by the National-Wide Lightning Detection Network in China was analyzed to characterize the CG lightning activity in the metropolitan area of Beijing. There is a high CG flash density area over the downtown of Beijing, but different from previous studies, the downwind area of Beijing is not significantly enhanced. Compared with the upwind area, the CG flash density in the downtown area was enhanced by about 50%. Negative CG flashes mainly occurred in the downtown and industrial area, while positive CG flashes were distributed evenly. The percentage of positive CG flashes with Ipeak ≥ 75 kA is more than six times that of the corresponding negative CG flashes in the Beijing area. The enhancement of lightning activity varies with season and time. About 98% of CG flashes occurred from May to September, and the peak of CG diurnal variation is from 1900 to 2100 local time. Based on the analysis of thunderstorm types in Beijing, it is considered that the abnormal lightning activity is mainly responsible for an enhancement of the discharge number in frontal systems rather than the increase of the number of local thunderstorms. In addition, there is a non-linear relationship between pollutant concentrations and CG flash number, which indicates that there are other critical factors affecting the production of lightning.

The European lightning location system EUCLID – Part 2: Observations

Abstract. Cloud-to-ground (CG) lightning data from the European Cooperation for Lightning Detection (EUCLID) network over the period 2006–2014 are explored. Mean CG flash densities vary over the European continent, with the highest density of about 6 km−2 yr−1 found at the intersection of the borders between Austria, Italy and Slovenia. The majority of lightning activity takes place between May and September, accounting for 85 % of the total observed CG activity. Furthermore, the thunderstorm season reaches its highest activity in July, while the diurnal cycle peaks around 15:00 UTC. A difference between CG flashes over land and sea becomes apparent when looking at the peak current estimates. It is found that flashes with higher peak currents occur in greater proportion over sea than over land.

A 10-Year Study on the Characteristics of Thunderstorms in Belgium Based on Cloud-to-Ground Lightning Data

Abstract Temporal and spatial distributions of cloud-to-ground (CG) lightning in Belgium are analyzed. Based on data from the European Cooperation for Lightning Detection (EUCLID) network, spanning a period of 10 years between 2004 and 2013, mean CG flash densities vary between 0.3 km−2 yr−1 in the west up to 2.4 km−2 yr−1 toward the east of Belgium, with an average flash density of 0.7 km−2 yr−1. The same behavior is found in terms of thunderstorm days and hours, where in the east most of the activity is observed, with a drop-off toward the coast. The majority of lightning activity takes place in the summer months between May and August, accounting for nearly 90% of the total activity. Furthermore, the thunderstorm season reaches its highest activity in July in terms of CG detections, while the diurnal cycle peaks between 1500 and 1600 UTC. A correlation is found between the estimated peak currents and altitude, with on average higher absolute peak currents at lower elevations and vice versa. In addition, a cell tracking algorithm is applied to the data to monitor the behavior of the individual cells. It is found that the lightning cells travel at an average speed of about 25 km h−1, with a preferred northeasterly direction of movement. At last, CG flash rates are strongly related to the cell area.

The characteristics of lightning risk and zoning in Beijing simulated by a risk assessment model

Abstract. In this study, the cloud-to-ground (CG) lightning flash/stroke density was derived from the lightning location finder (LLF) data recorded between 2007 and 2011. The vulnerability of land surfaces was then assessed from the classification of the study areas into buildings, outdoor areas under the building canopy and open-field areas, which makes it convenient to deduce the location factor and confirm the protective capability. Subsequently, the potential number of dangerous lightning events at a location could be estimated from the product of the CG stroke density and the location's vulnerability. Although the human beings and all their material properties are identically exposed to lightning, the lightning casualty risk and property loss risk was assessed respectively due to their vulnerability discrepancy. Our analysis of the CG flash density in Beijing revealed that the valley of JuMaHe to the southwest, the ChangPing–ShunYi zone downwind of the Beijing metropolis, and the mountainous PingGu–MiYun zone near the coast are the most active lightning areas, with densities greater than 1.5 flashes km−2 year−1. Moreover, the mountainous northeastern, northern, and northwestern rural areas are relatively more vulnerable to lightning because the high-elevation terrain attracts lightning and there is little protection. In contrast, lightning incidents by induced lightning are most likely to occur in densely populated urban areas, and the property damage caused by lightning here is more extensive than that in suburban and rural areas. However, casualty incidents caused by direct lightning strokes seldom occur in urban areas. On the other hand, the simulation based on the lightning risk assessment model (LRAM) demonstrates that the casualty risk is higher in rural areas, whereas the property loss risk is higher in urban areas, and this conclusion is also supported by the historical casualty and damage reports.

Seasonal, Regional, and Storm-Scale Variability of Cloud-to-Ground Lightning Characteristics in Florida

Abstract 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.

Climatology of lightning in the Czech Republic

The Czech Hydrometeorological Institute (CHMI) has utilized lightning data from the Central European Lightning Detection Network (CELDN) since 1999. The CELDN primarily focuses on the detection of cloud-to-ground (CG) lightning but intra-cloud (IC) lightning detection is also available. Lightning detection is used by the CHMI forecasters as an additional source to radar and satellite data for nowcasting of severe storms. Lightning data are also quantitatively used in automatic nowcasting applications. The quality of lightning data can be evaluated using their climatological characteristics. Climatological characteristics are also useful for defining decision thresholds that are valuable for human forecasters as well as for automatic nowcasting applications. The seven-year period from 2002 to 2008, which had relatively even-quality lightning data, was used to calculate the spatial and temporal distributions of lightning. The monthly number of CG strokes varies depending on the season. The highest number of CG strokes occurs during summer, with more than 20 days of at least five detected CG strokes on the Czech Republic territory in June and July. The least number of CG stokes occurs in winter, with less than three days per month having at least five detected CG stokes. The mean diurnal distribution of CG strokes peaks between 1500 and 1600 UTC and reaches a minimum between 0500 and 0800 UTC. The average spatial distribution of CG strokes shows sharp local maxima corresponding with the locations of the TV broadcast towers. The average spatial distribution of CG flash density, calculated on a 20 × 20 km grid, shows the maximum (3.23 flashes km − 2 year − 1 ) in the western part of Czech Republic and the minimum (0.92 flashes km − 2 year − 1 ) in the south-southeast of the Czech Republic. In addition, lightning characteristics related to the identified convective cells, such as distribution of the lightning stroke rates or relation to the radar derived by Vertically Integrated Liquid, are presented.

A decade of cloud‐to‐ground lightning in Canada: 1999–2008. Part 1: Flash density and occurrence

Flash density and occurrence features for more than 23.5 million cloud‐to‐ground (CG) lightning flashes detected by the Canadian Lightning Detection Network (CLDN) from 1999 to 2008 are analyzed on 20 × 20 km equal area squares over Canada. This study was done to update an analysis performed in 2002 with just three years of data. Flashes were detected throughout the year, and distinct geographic differences in flash density and lightning occurrence were observed. The shape and locations of large scale patterns of lightning occurrence remained almost the same, although some details were different. Flash density maxima occurred at the same locations as found previously: the Swan Hills and Foothills of Alberta, southeastern Saskatchewan, southwestern Manitoba and southwestern Ontario. A region of greater lightning occurrence but relatively low flash density south of Nova Scotia occurred at the same location as reported previously. New areas of higher flash density occurred along the US border with northwestern Ontario and southern Quebec. These appear to be northward extensions of higher flash density seen in the previous study. The greatest average CG flash density was 2.8 flash km−2 y−1 in southwestern Ontario, where the greatest single‐year flash density (10.3 flash km−2 y−1) also occurred. Prominent flash density minima occurred east of the Continental Divide in Alberta and over the Niagara Escarpment in southern Ontario. Lightning activity is seen to be highly influenced by the length of the season, proximity to cold water bodies and elevation. The diurnal heating and cooling cycle exerted the main control over lightning occurrence over most land areas; however, storm translation and transient dynamic features complicated the time pattern of lightning production. A large portion of the southern Prairie Provinces experienced more than 50% of flashes between 22:30 and 10:30 local solar time. The duration of lightning over a 20 × 20 km square at most locations in Canada is 5–10 h y−1, although the duration exceeded 15 h y−1 over extreme southwestern Ontario. Lightning occurred on 15–30 days each year, on average, over most of the interior of the country. The greatest number of days with lightning in a single year was 47 in the Alberta foothills and 50 in southwestern Ontario. Beginning and ending dates of the lightning season show that the season length decreases from north to south; however, there are considerable east‐west differences between regions. The season is nearly year‐round in the Pacific coastal region, southern Nova Scotia, southern Newfoundland and offshore.

Cell mergers and their impact on cloud-to-ground lightning over the Houston area

A previous hypothesis advanced from observational studies such as METROMEX suggests that the intensity, frequency, and organization of cumulus convection may be impacted by the forcing of enhanced merger activity downstream of urban zones. A resulting corollary is that cities may exert an indirect anthropogenic “forcing” of parameters related to convection and associated phenomena such as lightning and precipitation. This paper investigates the urban-merger hypothesis by examining the role of convective cell mergers on the existence and persistence of the Houston lightning “anomaly”, a local maximum in cloud-to-ground (CG) lightning activity documented to exist over and east of Houston. Using eight summer seasons of peak columnar radar reflectivity, CG lightning data and a cell-tracking algorithm, a two-dimensional cell merger climatology is created for portions of Eastern Texas and Louisiana. Results from the tracking and analysis of over 3.8 million cells indicate that merger-driven enhancements in convection induce a positive response (O 46%) in ground flash densities throughout the domain, with areas of enhanced lightning typically being co-located with areas of enhanced merger activity. However, while mergers over the Houston area (relative to elsewhere in the domain) do result in more vigorous convective cells that produce larger CG flash densities, we find that CG lightning contributions due to mergers are distributed similarly throughout the domain. Hence while we demonstrate that cell mergers do greatly impact the production of lightning, the urban cell merger hypothesis does not uniquely explain the presence of a local lightning maximum near and downstream of Houston.

Statistical characteristics of ground lightning flashes over the Korean peninsula using Cloud-to-Ground lightning data from 2004–2008

Lightning is one of most serious threats to successful launch operations. The objective of this study was to investigate the characteristics of Cloud-to-Ground (CG) lightning flashes in the South Korean peninsula from statistical information, to support space launch operations. The CG flash density, number of flashes, flash polarity, flash hours, median peak current and the interrelationship between flash hours and flash density were calculated using lightning observation data for 2004–2008 from the Korea Meteorology Administration (KMA). During this period 2,314,034 CG strokes and 1,152,475 CG flashes over the South Korean peninsula were detected by the Korean Meteorological Administration Lightning Detection Network (KLDN). Lightning occurred intensively during June, July and August. The mean percentage of positive flashes was 8.72%. The median peak current of positive flashes showed seasonal variations, while the median peak current of negative flashes did not. The maximum and minimum of median peak current for positive flashes was 76.2 kA in January and 17.6 kA in August respectively. Linear regression analysis was conducted to assess the relationship between flash hours and density. The correlation coefficient was derived by 0.5680.

Improvements in the detection efficiency model for the Brazilian lightning detection network (BrasilDAT)

The detection efficiency (DE) is the most important performance gauge of a lightning detection network (LDN). Moreover, the main motivation for evaluating the DE of a LDN is to separate the geographical variations of the CG lightning parameters from the variations regarding the network performance. A review of previous relative DE techniques and simple methods to correct the cloud-to-ground (CG) lightning flash density maps is presented. In addition, recent improvements in the flash DE model for the Brazilian lightning detection network (BrasilDAT) are discussed. The DE estimated values are based on the sensor individual DE probability functions, which are derived from a large amount of CG stroke data provided by the network considering different distances from the sensor and specific peak current ranges. The new approach provides better results when compared with the previous developments, since the calculation of the sensor DE probability functions neglects the lightning data provided by the minimum number of reporting sensors. Hence it is possible to minimize the unrealistic enhancement of the DE closer to the network boundaries (“border effect”) without affecting significantly the performance inside the network. The main result is a more realistic correction of the CG flash density maps, particularly at the outermost network areas, leading to an improvement in the model sensitivity.

Total flash density and the intracloud/cloud‐to‐ground lightning ratio over the Iberian Peninsula

Analysis of the total, intracloud and cloud‐to‐ground lightning flash density and the intracloud (IC) to cloud‐to‐ground (CG) ratio (Z = IC/CG) over the Iberian Peninsula (IP) has been performed using 5 years of combined data from the Spanish lightning detection network and the NASA Optical Transient Detector (OTD). The spatial distributions of total flash density and CG flash density show significant differences, especially on the northwest of the IP, the southern island of the Balearic Islands, and the southern/southeastern Mediterranean coastline. The spatial average of the annual value of Z is 3.48. A correlation was found between Z and latitude (correlation coefficient: −0.9). A relationship between Z and the total flash density was also observed (Z ∼ f0.5; correlation coefficient: 0.48). The spatial pattern of Z is reminiscent of that of the percentage of positive CG‐flashes, and high Z values seem to occur where the percentage of positive flashes is high.

Mapping the cloud-to-ground lightning occurrence in Fiji

Using the Time Of Group Arrival (TOGA) of sferics two year (Jan2003-Dec2004) annual average of about 3,200 and 1,200 cloud-to-ground (CG) flashes were recorded in the two larger islands, Viti Levu and Vanua Levu, Fiji. A two year average CG flash density showed maxima of 0.80 and 0.35 flashes/km2 per year in two islands. The diurnal variation showed peak activity between 14:00 hrs- 16:00 hrs LT and the seasonal variation showed enhanced lightning incidences during November-April. Data from Lightning Imaging Sensor (LIS) aboard the NASA Tropical Rainfall Measuring Mission (TRMM) satellite are also presented for the same period at the two locations. Lightning in a 10 km �10 km area was compared with total rain accumulation at a site midway in the area which shows good correlation.

Cloud-to-ground flash patterns for Atlanta, Georgia (USA) from 1992 to 2003

We analyzed the patterns of cloud-to-ground (CG) lightning flashes around Atlanta, Georgia (USA), a region that has undergone an intense conversion from natural to anthropogenic land uses. For the 12 yr period from 1992 to 2003, annual average CG flash densities of 6 to 8 flashes km -2 emerged around Atlanta. These values are 50 to 75% higher than in the surrounding rural areas, and comparable to flash densities along the Atlantic coast of Georgia. High flash densities extended over a large swath of Atlanta, and into Gwinnett County, a heavily suburbanized, rapidly growing county to the northeast. Urban flash production peaked during the summer (May through June) and exhibited more night and early morning activity (18:00 to 06:00 h) than in surrounding rural areas. Atlanta's higher flash densities do not result from isolated flash production over the city; rather they develop when the large scale atmospheric setting favors widespread lightning throughout the region. Maps of flash counts by interval classes also revealed where flash density maxima emerge in different county regions around the city. A large area of reduced positive polarity flashes developed along the arc of At- lanta's loop highway, Interstate 285. This area also trended south along the corridor of Interstate High- way 75 into central Georgia. This pattern suggests that automobiles may be a source of particulate matter, which is hypothesized to reduce the percentage of positive flashes.

Where Is the Real Cloud-to-Ground Lightning Maximum in North America?

Abstract A local maximum in cloud-to-ground (CG) lightning flash density over northwestern Mexico as measured by the U.S. National Lightning Detection Network is examined in detail. Corrections are derived for the relative detection efficiency of the network in this region that is outside the perimeter of the sensors. The need to adjust the parameters of the signal normalization model used for the network is also documented. New propagation model parameters are employed to derive relative detection efficiency corrections for northwestern Mexico and these are then used to show that the actual CG flash density over the region significantly exceeds that observed in Florida, the area with the highest flash density within the perimeter of the network.

Remote sensing of cloud‐to‐ground lightning location using the TOGA of sferics

AbstractUsing the Time Of Group Arrival (TOGA) of sferics about 3700 cloud‐to‐ground (CG) flashes were recorded in one year (2003) in the largest island, Viti Levu in Fiji. A maximum CG flash density of 0.49 flashes/km2per year was recorded in two locations in the island. The seasonal variation showed enhanced lightning incidences during November–April and the diurnal variation showed peak activity from 14:00–16:00 h local time. Five‐year data of the Lightning Imaging Sensor (LIS) aboard the NASA Tropical Rainfall Measuring Mission (TRMM) satellite are also presented, which show good correlation of seasonal and diurnal variations with those obtained from TOGA measurements. Copyright © 2005 Royal Meteorological Society

Cloud‐to‐ground lightning enhancement over Southern Louisiana

Fourteen years (1989–2002) of cloud‐to‐ground (CG) lightning data show a significant enhancement of lightning associated with Lake Charles and Baton Rouge, Louisiana. A peak density value of 7 flashes km−2 yr−1 exists on the western side of the Lake Charles urban area. A comparison of the Louisiana CG flash density distribution with the locations of PM10 (particulate matter less than 10 μm in diameter) sources strongly suggests that pollution plays a key role in lightning enhancement. Urban and sea breeze effects can be neglected. The values of median peak negative current show a sharp difference between land and the Gulf of Mexico; inland values are near 24 kA, while over the Gulf waters immediately offshore are over 30 kA. This observation, along with a relative minimum of negative peak current from the mouth of the Mississippi River southeastward seems to support the hypothesis that the underlying surface characteristics influence the calculated negative current distribution.

Cloud-to-Ground Lightning in Linear Mesoscale Convective Systems

Recently, three distinct archetypes for midlatitude linear mesoscale convective systems (MCSs) have been identified: those comprising convective lines with trailing stratiform precipitation (TS), leading stratiform precipitation (LS), and parallel stratiform precipitation (PS). While cloud-to-ground (CG) lightning in TS MCSs has received a great deal of study in recent years, linear MCSs exhibiting leading or parallel stratiform precipitation have received comparatively little attention. This paper documents the arrangement and frequency of CG lightning for four LS and four PS MCSs that occurred during May 1996 and May 1997. On average, the LS cases bore some similarity to previously studied TS MCSs; they produced predominantly positive CG lightning (1CG) early in their lifetimes (concomitant with convective precipitation) and late in their lifetimes (concomitant with stratiform precipitation). In contrast with previously published results for MCSs in general, the LS MCSs in the present study overall had relatively low negative CG (2CG) flash densities, yielding higher percentages of 1CGs than other MCS archetypes. While the PS systems had higher average flash densities than the LS systems, they did not exhibit prolonged 1CG modes early in their lifetimes, and they produced a lower overall fraction of 1CGs. Notably, the presence of line-parallel flow may render a somewhat unique electrical character in PS systems. In particular, the PS MCSs in this study exhibited enhanced CG lightning in the vicinity of decaying convective cells that were embedded within their stratiform precipitation regions.

The CG lightning activity of a storm causing a flash‐flood

On August 7, 1996, a flood of the Rio Gallego, in Aragon (Spanish central Pyrénées) wiped out the Biescas' campground, killing more than 80 persons. From the observations of two meteorological radars, this thunderstorm exhibited some of the radar characteristics of a supercell. It spread almost exactly over the Rio Gallego catchment area, up‐stream of Biescas, and it remained above this area for about two hours, without any advection. The rainfall reached 100 mm h−1 and the total cumulative amount locally reached 200 mm. The cloud‐to‐ground (CG) flash density averaged over 5 × 5 km² areas, was exceptionally high, around 2 km−2, and the peak flash rate averaged over 5‐minute periods reached 11.6 min−1 within the cell area. The evolution of the CG flash rate and the radar reflectivity were closely correlated. The flash rate reached high values before the arrival of the precipitation at the ground. We observe also a very good agreement between the location of the intense rain at the ground and the high CG flash density. The rainfall water volume per flash was 3×104 m³, a value in agreement with previous work. We propose that quantitative measurements of the lightning activity may provide valuable information for flash‐flood nowcasting and short‐term forecasting in mountainous areas.