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Enhanced Positive Cloud-to-Ground Lightning in Thunderstorms Ingesting Smoke from Fires
Abstract
Smoke from forest fires in southern Mexico was advected into the U.S. southern plains from April to June 1998. Cloud-to-ground
lightning (CG) flash data from the National Lightning Detection Network matched against satellite-mapped aerosol plumes imply
that thunderstorms forming in smoke-contaminated air masses generated large amounts of lightning with positive polarity (+CGs).
During 2 months, nearly half a million flashes in the southern plains exhibited +CG percentages that were triple the climatological
norm. The peak currents in these +CGs were double the expected value. These thunderstorms also produced abnormally high numbers
of mesospheric optical sprites.
... A number of studies show that dust could affect the development of thunderstorm clouds as cloud condensation nuclei (CCN), ice nuclei (IN), or giant cloud condensation nuclei (GCCN) (DeMott et al., 2003;Levin, 2005;Rosenfeld et al., 2008;Van den Heever et al., 2006), and thus influence lightning activity (Khain et al., 2008;Lyons et al., 1998;Orville et al., 2001). With abundant liquid water content, CCN increases the cloud droplet concentrations, but slows down the collision speed and thus decreases the cloud droplet size (Twomey, 1977;Twomey et al., 1984). ...
... Using a 2D mixed-phase cloud model with spectral microphysics, Khain et al. (2008) found that aerosols dramatically increase the mass content of supercooled water and ice, as well as vertical velocities, which allows collisions and charge separation within a single cloud zone. Many studies have shown that aerosols can significantly enhance lightning activity (Altaratz et al., 2014;Coquillat et al., 2022;Orville et al., 2001;Qie et al., 2021a;Qie et al., 2020;Sun et al., 2021) and affect the positive cloud-to-ground (+CG) lightning ratio (Farias et al., 2009;Kar et al., 2009;Lyons et al., 1998;Murray et al., 2000). Research by Shukla et al. (2022) showed that the dust aerosols provided sufficient CCN and IN, which was conducive to charge separation in the cloud and was one of the reasons for the disastrous lightning activity in India on April 16, 2019. ...
... Atmospheric pollutants in super cooled cloud droplets would cause graupel particles to be charged negatively in a warmer temperature region, and a wider negative charge region is conducive to the generation of negative CG lightning, thus reducing the percentage of positive CG lightning. On the other hand, the effect of smoke aerosol from fires on the percentage of positive CG lightning is opposite to that of urban aerosols (Lyons, 1998;Murray et al., 2000;Lang and Rutledge, 2006). The smoke aerosol hypothesis considers that smoke aerosol would narrow the droplet spectrum, and the narrow droplet spectrum leads to the graupel particles charging positively between −10°C and −20°C in the thunderstorm, which would lead to a wider distribution of lower positive charge region, which is conducive to the generation of positive CG lightning. ...
... The influence of aerosol on positive lightning may be covered by the strong total CG lightning and the complex relationship between aerosol and convection systems. Previous studies (Lyons, 1998;Murray et al., 2000;Steiger et al., 2002;Naccarato et al., 2003;Lang and Rutledge, 2006) have found that urban anthropogenic aerosols and smoke from forest fires have significant effects on positive lightning, and it is believed that aerosols affect positive lightning by affecting the electrification process of cloud particles. In our study, we tend to consider that aerosols affect charge structure and thus positive lightning by influencing convective intensity. ...
This study discussed the influence of aerosols on the relative frequency of positive cloud-to-ground (CG) lightning and its dependence on thermodynamic and cloud-related factors in Sichuan during the warm season from 2005 to 2017. The relative frequency of positive CG lightning is defined as the proportion of positive CG lightning flashes to total CG lightning flashes. Although the total CG lightning density in the Western Sichuan Plateau is significantly lower than that in the Sichuan Basin, the relative frequency of positive CG lightning is higher than that in the basin. Convective available potential energy (CAPE) and vertical wind shear in the low-to-mid level of the troposphere (SHEAR-5 km) are the controlling factors of positive CG lightning frequency. A small CAPE and a large SHEAR-5 km represent weak convection, which is more conducive to the generation of positive CG lightning. The upper main positive charge region in a thriving thunderstorm is higher from the ground, which is not conducive to the transport of positive charge to the ground, so it is not conducive to the generation of positive CG lightning. In the basin, the relationship between aerosols and positive lightning is not significant, which may be due to the strong total CG lightning and the low proportion of positive CG lightning. In the plateau, both sulfate aerosol and black carbon (BC) aerosol have a significant inhibition effect on the positive CG lightning relative frequency. Sulfate aerosol stimulates the ice-phase process through a microphysical effect and promotes the development of convection. The distribution of the main positive charge center is higher, which is not conducive to the transport of positive charge to the ground and the generation of positive CG lightning. The significant heating effect of BC aerosol on the lower troposphere makes the convective development more vigorous and is not conducive to the occurrence of positive CG lightning.
... More small droplets in the warm cloud leads to droplet growth suppression due to their similar fall speeds and increased competition for available water vapor. For a given updraft, these small droplets are more easily carried aloft, leading to a large cloud liquid water content in the mixed-phase layer, positive charging of graupel and anomalous storms (Albrecht et al., 2011;Fernandes et al., 2006;Lyons et al., 1998). ...
... Verifying this result with a larger and largely different set of storms during RELAMPAGO-CACTI using reanalysis data of aerosol mass concentration led to a similar behavior. This suggests that CCN may not be an important factor for development of anomalous storms in Cordoba, contrary to other study findings (Albrecht et al., 2011;Lyons et al., 1998). Preliminary analysis of the low-level flow indicates that some normal events with larger CCN concentration might have been influenced by southward flow from the polluted Cordoba metro area. ...
In this study we explored the environmental conditions hypothesized to induce a dominant charge structure in thunderstorms in the province of Cordoba, Argentina, during the RELAMPAGO‐CACTI (Remote sensing of Electrification, Lightning, And Mesoscale/microscale Processes with Adaptive Ground Observations‐Clouds, Aerosols, Complex Terrain Interactions) field campaigns. Hypothesized environmental conditions are thought to be related to small warm cloud residence time and warm rain growth suppression, which lead to high cloud liquid water contents in the mixed‐phase zone, contributing to positive charging of graupel and anomalous charge structure storms. Data from radiosondes, a cloud condensation nuclei (CCN) ground‐based instrument and reanalysis were used to characterize the proximity inflow air of storms with anomalous and normal charge structures. Consistent with the initial hypothesis, anomalous storms had small warm cloud depth caused by dry low‐level humidity and low 0°C height. Anomalous storms were associated with lower CCN concentrations than normal storms, an opposite result to the initial expectation. High CAPE is not an important condition for the development of anomalous storms in Argentina, as no clear pattern could be found among the different parameters calculated for updraft proxy that would be consistent with the initial hypothesis.
... While it is unclear how aerosol impacts differ in terrestrial versus maritime environments, we expect findings from other regions and modeling studies to have some applicability to the MC. These include a host of aerosol-induced micro-and macro-physical changes in clouds (e.g., Tao et al., 2012;Dey et al. 2011), such as delays in warm rain formation (Berg et al., 2008) and congestus and overall storm invigoration (e.g., Lyons et al., 1998;Wang et al., 2009;Storer et al., 2014). Modeling studies are largely consistent in warm phase cloud processes, but less so as ice nucleation begins to take hold (Khain et al., 2005;van den Heever et al., 2006;Saleeby et al., 2010;Cotton et al., 2012;Fan et al., 2013;Grant and van den Heever, 2015;Sheffield et al., 2015;Mace and Abernathy, 2016;Gryspeerdt et al., 2017). ...
The NASA Cloud, Aerosol, and Monsoon Processes Philippines Experiment (CAMP ² Ex) employed the NASA P-3, Stratton Park Engineering Company (SPEC) Learjet 35, and a host of satellites and surface sensors to characterize the coupling of aerosol processes, cloud physics, and atmospheric radiation within the Maritime Continent’s complex southwest monsoonal environment. Conducted in the late summer of 2019 from Luzon Philippines in conjunction with the Office of Naval Research Propagation of Intraseasonal Tropical OscillatioNs (PISTON) experiment with its R/V Sally Ride stationed in the North Western Tropical Pacific, CAMP ² Ex documented diverse biomass burning, industrial and natural aerosol populations and their interactions with small to congestus convection. The 2019 season exhibited El Nino and associated drought, high biomass burning emissions, and an early monsoon transition allowing for observation of pristine to massively polluted environments as they advected through intricate diurnal mesoscale and radiative environments into the monsoonal trough. CAMP ² Ex’s preliminary results indicate 1) increasing aerosol loadings tend to invigorate congestus convection in height and increase liquid water paths; 2) lidar, polarimetry, and geostationary Advanced Himawari Imager remote sensing sensors have skill in quantifying diverse aerosol and cloud properties and their interaction; and 3) high resolution remote sensing technologies are able to greatly improve our ability to evaluate the radiation budget in complex cloud systems. Through the development of innovative informatics technologies, CAMP ² Ex provides a benchmark dataset of an environment of extremes for the study of aerosol, cloud and radiation processes as well as a crucible for the design of future observing systems.
... Many studies have shown that the increased aerosol concentration not only increases the lightning flash rate but also reverses the polarity of charge generation processes inside thunderclouds. Lyons et al. (1998), Murray et al. (2000), Fernandes et al. (2006), and Rosenfeld et al. (2007) have observed higher rates of positive cloud-to-ground flashes with high peak currents in the presence of smoke from forest fires. Pawar et al. (2017) have shown that high aerosol concentration with adequate ice-nuclei non-inductive charging mechanism can produce a strong and widespread positive charge region in the lower portion of the cloud. ...
Six cases of dust storms that occurred over the northwestern and northern parts of India have been studied here to understand the effect of dust on the lightning characteristics of these storms. Satellite pictures show high dust content on all six storm days, and ground station data show that visibility was reduced to less than 500 m. Lightning data observed by the lightning detection network indicate that these six cases of convective dust storms produced more than 30 percent of positive CG lightning in the total CG lighting, which is considered to be very high compared to ordinary thunderstorms. Further, analysis indicates that the current carried by positive lightning is much higher than negative lightning. In some cases, the lightning flash rate reached more than 200 flashes per min (fpm). Observation shows that all these thunderstorms were accompanied by dust, and hence, incursion of the abundance of dust particles into the cloud was likely to be very high. Many earlier observations have suggested that aerosols can affect thunderstorms’ microphysical (such as vertical distribution of the hydrometers) and electrical characteristics. With the abundance of dust particles in the cloud and increased positive CG discharges, it has been proposed that increased dust particles can modify the vertical distribution of ice particles inside a thundercloud and affect the lightning flash rate as well as polarity.
... The relationship between lightning and LIW has been widely investigated by several studies (e.g., [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]). It is currently accepted that the process involved in the formation of LIW is composed of three phases, i.e., ignition (fire triggering), survival (smoldering), and arrival (flaming combustion) [4]. ...
Lightning is the main precursor of wildfires in Arizona, New Mexico, and Florida during
the fire season. Forecasting the occurrence of Lightning-Ignited Wildfires (LIW) is an essential tool to reduce their impacts on the environment and society. Long-Continuing-Current (LCC) lightning is proposed to be the main precursor of LIW. The long-lasting continuing current phase of LCC lightning is that which is more likely to ignite vegetation. We investigated the meteorological conditions and vegetation type associated with LIW and LCC lightning flashes in Arizona, New Mexico, and Florida.
We analyzed LIW between 2009 and 2013 and LCC lightning between 1998 and 2014 and combined lightning and meteorological data from a reanalysis data set. According to our results, LIW tend to occur during dry thunderstorms with a high surface temperature and a high temperature gradient between the 700 hPa and the 450 hPa vertical levels for high-based clouds. In turn, we obtained a high lightning-ignition efficiency in coniferous forests, such as the ponderosa pine in Arizona and New Mexico and the slash pine in Florida. We found that the meteorological conditions that favor fire ignition and spread are more significant in Florida than in Arizona and New Mexico, while the meteorological conditions that favor the occurrence of LIW in Arizona and New Mexico are closely related with the meteorological conditions that favor high lightning activity. In turn, our results indicate high atmospheric instability during the occurrence of LIW. Our findings suggest that LCC(>18 ms) lightning tends to occur in thunderstorms with high relative humidity and ice content in the clouds, and with low temperature in the entire troposphere. Additionally, a weak updraft in the lower troposphere and a strong one in the upper troposphere favor the occurrence of LCC (>18 ms) lightning. We found that the meteorological conditions that favor the occurrence of LCC (>18 ms) lightning are not necessarily the preferential meteorological conditions for LIW.
... g., Schultz et al. (2008); Van The linkage between lightning and forest fires has been widely investigated by several authors in different regions using data provided by aircraft campaigns, satellite observations and lightning-detection networks [e. g., Lyons et al. (1998) ;Anderson 35 (2002); Stocks et al. (2002); Wotton and Martell (2005); Hall and Brown (2006); Fernandes et al. (2006); Kochtubajda et al. (2006); Lang and Rutledge (2006); Rosenfeld et al. (2007); Hall (2007); Altaratz et al. (2010); Dowdy and Mills (2012); Nauslar et al. (2013); Lang et al. (2014); Veraverbeke et al. (2017)]. In Canada, LIW account for 80% (Anderson, 2002) of total area burned, while in the western states of the US, the total area burned by LIW is about 60% (Nauslar et al., 2013). ...
Lightning is the major cause of natural ignition of wildfires worldwide and produces the largest wildfires in some regions. Lightning strokes produce about 5% of forest fires in the Mediterranean basin and are one of the most important precursors of the largest forest fires during the summer. Lightning-ignited wildfires produce significant emissions of aerosols, black carbon and trace gases, such as CO, SO2, CH4 and O3, affecting air quality. Characterization of the meteorological and 5 cloud conditions of lightning-ignited wildfires in the Mediterranean basin can serve to improve fire forecasting models and to upgrade the implementation of fire emissions in atmospheric models.
The association between aerosol and lightning has been investigated with long-term decadal data (2005-2014) for lightning, aerosol optical depth (AOD), relative humidity, and effective cloud droplet size. To understand the complex relationship between aerosol and lightning, two different regions with different climatic and weather conditions, a humid region R1 (22°-29° N, 89°-92° E) and an arid region R2 (23°-28° N, 70°-76° E) of northern India, were chosen for the study domain. The results show that lightning activity was observed to occur more over the humid region R1, i.e., 1141 days (1/3 of total days), than over the arid region R2, i.e., 740 days (1/5 of total days). Also, over the humid region R1, the highest lightning flash density was recorded as nearly 4.6 × 10-4 flashes/km2/day observed for 18 days (1.5%); on the contrary, over the arid region R2, the maximum lightning flash density was observed to be 2.5 × 10-4 flashes/km2/day and occurred for about 22 days (2.9%). The analysis shows that a nonlinear relationship exists between aerosol and lightning with a highly associated influence of relative humidity. A very significant positive and negative co-relation that varies with relative humidity has been observed between AOD and lightning for both humid and arid regions. This shows relative humidity is the key factor in determining the increase or decrease of lightning activity. This study also shows that the larger the cloud droplet size, the higher the relative humidity and vice versa. This study emphasizes that aerosol concentration in the atmosphere influences cloud microphysics by modulating the size of cloud droplets and thereby regulating the lightning frequency. The atmospheric humidity is the driving factor in deciding the positive or negative co-relationship between aerosol and lightning.
Supplementary information:
The online version contains supplementary material available at 10.1007/s00024-022-02981-6.
Lightning occurrence at higher latitudes in northwestern Europe is by far less frequent than mainland continental and the Mediterranean during most of the year. Yet, as recent studies suggest, this region harbors a large fraction of the most energetic lightning flashes on Earth, commonly referred to as superbolts. In this study, we examine the time/locations of intense cloud-to-ground (CG) strokes (>200 kA in absolute value), provided by Météorage for the 10.5-year period (from Jan 2010 to Jul 2020), to present a high-resolution map of their distribution, pointing out relevant discrepancies observed between -CG and + CG, respectively. We additionally investigate the potential of superbolts to result in short-lived optical phenomena above thunderstorms, collectively known as transient luminous events (TLEs). Observations in the region indicate that isolated superbolts with substantial charge moment change can produce sprites during low active marginal winter thunderstorms, in the absence of concurrent IC/CG activity several minutes before and after the event. An example is described when 3 sprites were captured in a similar context during the night of 7th/8th February 2016. We suggest that: i) convergence and aerosols advection from sea surface and busy shipping lanes may favour deep convection and cloud electrification on the English Channel with respect to surrounding areas. Inherent differences in cloud charge structure of sea based storms could lead to faster negative leader vertical velocity than those for storms over land, on average, and hence in larger peak currents, determining the winter peak of negative superbolts in the area; ii) areas occupied by the most populated superbolt clusters can be used to conduct future research in the region, aimed at better characterising microphysical properties of superbolts and their potential in generating TLEs.
A clear association between large peak current cloud-to-ground lightning flashes of positive polarity and sprites and elves in the stratosphere and mesosphere has been previously demonstrated. This paper reports on the first climatology of large peak current cloud-to-ground (LPCCG) lightning flashes compiled from the U.S. National Lightning Detection Network. Analysis of almost 60 million CG flashes from 14 summer months (1991–95) reveals distinct geographic differences in the distribution of positive and negative polarity LPCCGs, arbitrarily defined as flashes with peak currents 75 kA. Large peak current positive CGs (LPCCGs) are concentrated in the High Plains and upper Midwest, the region in which a large majority of optical sprite and elves observations have been obtained. By contrast, large peak current negative CGs (LPCCGs) preferentially occur over the coastal waters of the Gulf of Mexico and the southeastern United States. A total of 1.46 million LPCCGs were found, of which only 13.7% were CGs. Almost 70% of the LPCCGs, however, occurred in the central United States (30–50N, 88–110W). The percentage of all LPCCGs that were positive approached 30% in the central United States compared to 4.5% for the remainder of the country. A CG is 3.1 times more likely to exceed 75 kA than is a CG flash on a national basis. Yet in terms of absolute numbers for all ranges of peak current 75 kA, negative CGs are clearly dominant. For peak currents 75 and 200 kA, negative CGs outnumbered positive CGs by ratios of 6.4 and 4.1, respectively. In the central United States, however, during evening hours the number of LPCCGs almost reaches parity with LPCCGs. Average stroke multiplicity also exhibited regional differences. Over a half million negative CGs and over 1000 positive CGs were found with multiplicity 10.
It is generally accepted that atmospheric pollutants can be transported over distances described as synoptic in scale. However, the redistribution and removal of the various pollutant species may be significantly influenced, often in a highly episodic way, by phenomena which are inherently mesoscale in nature.
Transient luminous events (sprites, blue jets, elves) above large mesoscale convective systems (MCSs) over the U.S. High Plains have been routinely monitored from the Yucca Ridge Field Station near Fort Collins, Colorado using ground-based low-light video systems. We analyzed 36 sprites above the Nebraska MCS of August 6, 1994. The results lend further support to the hypothesis that sprites are almost uniquely associated with positive cloud-to-ground (+CG) lightning flashes. Sprite-associated +CGs also averaged substantially larger peak currents than the remaining +CG population (81 kA versus 30 kA in this storm system). There is some evidence that sprite-associated +CGs also have higher stroke multiplicity. This study yields no evidence of sprites associated with negative CG events. In the central United States an additional requirement appears to be that the parent MCS has a contiguous radar reflectivity area exceeding 20-25,000 km 2. The majority of the sprites occur above the large stratiform precipitation region and not the high-reflectivity convective core of the MCS. Triangulation of a limited number of paired images (from September 7, 1994) suggests that the sprite is generally centered within 50 km of the parent +CG. Assuming the +CG provides the range, single-image photogrammetric analyses provide estimates of the maximum vertical extent of the sprites. For this storm the sprite tops averaged 77 km with a maximum of 88 km. The bases averaged 50 km but with a few sprite tendrils extending as low as 31 km.
A prescribed fire ignited in western Ontario on August 10, 1989 burned from 1400 to 1700 EDT, generating a plume cloud structure including a portion resembling an anvil. From 1500-1630, 21 ground flashes lowering positive charge from the cloud as well as four negative flashes near the beginning of the time period that were not recognized as ground flashes. These four flashes out of a total of 37 flashes exceeding 90 V/m were the only flashes perceived as lowering negative charge; all the others lowered positive charge. The median current of the positive flashes to ground was 6 kA. Simple charge distribution models indicate that the plume and associated cloud had a monopolar charge distribution with positive charge at altitude and that the fire was most probably the source of this charge. A charge generation rate of 2 nC/sq m/s by the fire could generate enough charge to satisfy the flash discharge rate and amount using the simplest model.
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). The annual mean percentage of flashes that lowered positive charge to ground was between 4% and 5% for the three years, 1992-94, but increased to 9.3% in 1995. The monthly values of the percentage of positive flashes ranged from 3% (August 1992) to 25% (December 1993). The positive flash maxima in the Midwest appear to be near the geographical areas in which cloud-ionosphere discharges (sprites) have been reported.
Further laboratory studies have been made of the charge transferred when ice crystals collide and then separate from an ice target. Previous results indicated increased charge transfer when cloud droplets were present. The new data show that the size distribution of the droplets is important. When the cloud droplets are too small to collide with the growing target, it charges negatively; for the same liquid water content but with larger droplets that hit the target, it charges positively. The results suggest that the relative growth rate and the surface states of both the interacting bodies may be important in controlling the sign of the charge transfer. It is suggested that these results may explain the observation of negative charge centers in thunderstorms at temperatures warmer than -20°C.
On 20/21 August 1993, deep convective storms occurred across much of Arizona, except for the southwestern quarter of the state. Several storms were quite severe, producing downbursts and extensive wind damage in the greater Phoenix area during the late afternoon and evening. The most severe convective storms occurred from 0000 to 0230 UTC 21 August and were noteworthy in that, except for the first reported severe thunderstorm, there was almost no cloud-to-ground (CG) lightning observed during their life cycles. Other intense storms on this day, particularly early storms to the south of Phoenix and those occurring over mountainous terrain to the north and east of Phoenix, were prolific producers of CG lightning. Radar data for an 8-h period (2000 UTC 20 August-0400 UTC 21 August) indicated that 88 convective cells having maximum reflectivities greater than 55 dBZ and persisting longer than 25 min occurred within a 200-km range of Phoenix. Of these cells, 30 were identified as `low-lightning' storms, that is, cells having three or fewer detected CG strikes during their entire radar-detected life cycle. The region within which the low-lightning storms were occurring spread to the north and east during the analysis period.Examination of the reflectivity structure of the storms using operational Doppler radar data from Phoenix, and of the supportive environment using upper-air sounding data taken at Luke Air Force Base just northwest of Phoenix, revealed no apparent physical reasons for the distinct difference in observed cloud-to-ground lightning character between the storms in and to the west of the immediate Phoenix area versus those to the north, east, and south. However, the radar data do reveal that several extensive clouds of chaff initiated over flight-restricted military ranges to the southwest of Phoenix. The prevailing flow advected the chaff clouds to the north and east. Convective storms that occurred in the area likely affected by the dispersing chaff clouds were characterized by little or no CG lightning.Field studies in the 1970s demonstrated that chaff injected into building thunderstorms markedly decreased CG lightning strikes. There are no data available regarding either the in-cloud lightning character of storms on this day or the technical specifications of the chaff being used in military aircraft anti-electronic warfare systems. However, it is hypothesized that this case of severe, but low-lightning, convective storms resulted from inadvertent lightning suppression over south-central Arizona due to an extended period of numerous chaff releases over the military ranges.
In 4 of the 15 storms, ground flash activity was dominated by positive cloud-to-ground lightning throughout most of the life of the storm. In 11 storms, the dominant polarity of ground flashes switched from positive to negative sometime during the mature stage of the storm. In all cases observed by Doppler radar, storms dominated by positive flashes had at least some rotation, and in most cases they were low-precipitation or classic supercell storms. In all cases for which hail verification efforts were vigorous, large hail was reported during the period when positive ground flashes dominated. In the 11 storms that were tornadic, tornadoes occurred either during or after the period when positive ground flashes dominated. It is inferred that the dominant polarity of lightning is strongly influenced by mesoscale properties of the atmosphere, possibly through systematic effects on other storm properties related to severe weather. -from Authors
No abstract available.
The U.S. National Lightning Detection Network TM (NLDN) has provided light- ning data coveting the continental United States since 1989. Using information gathered from more than 100 sensors, the NLDN provides both real-time and historical lightning data to the electric utility industry, the National Weather Service, and other government and commercial users. It is also the primary source of lightning data for use in research and climatological studies in the United States. In this paper we discuss the design, implementation, and data from the time-of-arrival/magnetic direction finder (TOA/MDF) network following a recent system-wide upgrade. The location accuracy (the maximum dimension of a confidence region around the stroke location) has been improved by a fac- tor of 4 to 8 since 1991, resulting in a median accuracy of 500 m. The expected flash detection efficiency ranges from 80% to 90% for those events with peak currents above 5 kA, varying slightly by region. Subsequent strokes and strokes with peak currents less than 5 kA can now be detected and located; however, the detection efficiency for these events is not quantified in this study because their peak current distribution is not well known.