Showing papers on "Lightning published in 2006"

Detection efficiency of the VLF World-Wide Lightning Location Network (WWLLN): initial case study

Abstract: . An experimental Very Low Frequency (VLF) World-Wide Lightning Location Network (WWLLN) has been developed through collaborations with research institutions across the world, providing global real-time locations of lightning discharges. As of April 2006, the network included 25 stations providing coverage for much of the Earth. In this paper we examine the detection efficiency of the WWLLN by comparing the locations from this network with lightning location data purchased from a commercial lightning location network operating in New Zealand. Our analysis confirms that WWLLN favours high peak current return stroke lightning discharges, and that discharges with larger currents are observed by more stations across the global network. We then construct a first principles detection efficiency model to describe the WWLLN, combining calibration information for each station with theoretical modelling to describe the expected amplitudes of the VLF sferics observed by the network. This detection efficiency model allows the prediction of the global variation in WWLLN lightning detection, and an estimate of the minimum CG return stroke peak current required to trigger the network. There are strong spatial variations across the globe, primarily due to station density and sensitivity. The WWLLN is currently best suited to study the occurrence and impacts of high peak-current lightning. For example, in 2005 about 12% of the global elve-producing lightning will have been located by the network. Since the lightning-EMP which produce elves has a high mean rate (210 per minute) it has the potential to significantly influence the ionosphere on regional scales.

Performance Assessment of the World Wide Lightning Location Network (WWLLN), Using the Los Alamos Sferic Array (LASA) as Ground Truth

Abstract: The World Wide Lighting Location Network (WWLLN) locates lightning globally, using sparsely distributed very low frequency (VLF) detection stations. Due to WWLLN’s detection at VLF (in this case ∼10 kHz), the lightning signals from strong strokes can propagate up to ∼104 km to WWLLN sensors and still be suitable for triggering a station. A systematic evaluation of the performance of WWLLN is undertaken, using a higher-frequency (0–500 kHz) detection array [the Los Alamos Sferic Array (LASA)] as a ground truth during an entire thunderstorm season in a geographically confined case study in Florida. It is found that (a) WWLLN stroke-detection efficiency rises sharply to several percent as the estimated lightning current amplitude surpasses ∼30 kA; (b) WWLLN spatial accuracy is around 15 km, good enough to resolve convective-storm cells within a larger storm complex; (c) WWLLN is able to detect intracloud and cloud-to-ground discharges with comparable efficiency, as long as the current is comparable;...

Numerically Simulated Electrification and Lightning of the 29 June 2000 STEPS Supercell Storm

Abstract: A three-dimensional dynamic cloud model incorporating airflow dynamics, microphysics, and thunderstorm electrification mechanisms is used to simulate the first 3 h of the 29 June 2000 supercell from the Severe Thunderstorm Electrification and Precipitation Study (STEPS). The 29 June storm produced large flash rates, predominately positive cloud-to-ground lightning, large hail, and an F1 tornado. Four different simulations of the storm are made, each one using a different noninductive (NI) charging parameterization. The charge structure, and thus lightning polarity, of the simulated storm is sensitive to the treatment of cloud water dependence in the different NI charging schemes. The results from the simulations are compared with observations from STEPS, including balloon-borne electric field meter soundings and flash locations from the Lightning Mapping Array. For two of the parameterizations, the observed “inverted” tripolar charge structure is well approximated by the model. The polarity of th...

Voltages induced on an overhead wire by lightning strikes to a nearby tall grounded object

Abstract: The aim of this study was to identify conditions under which the presence of tall strike object can serve to increase or decrease lightning-induced voltages on a nearby overhead wire. We examined the ratios of magnitudes of lightning-induced voltages on the overhead wire for the cases of strikes to a tall object and to flat ground as a function of distance from the lightning channel d, current reflection coefficients at the top of the strike object rhotop and at the bottom of the strike object rhobot, the current reflection coefficient at the channel base (in the case of strikes to flat ground) rhogr, and the return stroke speed v. Lightning-induced voltages were computed using the finite-difference time-domain (FDTD) method. The transmission line (TL) model was used to find the distribution of current along the lightning channel and the strike object. The ratio of magnitudes of lightning-induced voltages for tall-object and flat-ground cases increases with increasing d (ranging from 40--200 m), decreasing rhobot(<1), decreasing rho top (<0, except for the case of rhobot=0), and decreasing v (

The Chisholm firestorm: observed microstructure, precipitation and lightning activity of a pyro-cumulonimbus

Abstract: . A fire storm that occurred on 28 May 2001 and devastated the town of Chisholm, ~150 km north of Edmonton, Alberta, induced a violent fire-invigorated cumulonimbus cloud. This pyro-cumulonimbus (pyro-Cb) had overshooting tops of 2.5–3 km above the tropopause, and injected massive amounts of smoke into the lower stratosphere. Fortunately, this event occurred under good coverage of radar, rain gauge, lightning and satellite measurements, which allowed in-depth documentation of the event, and gave us an opportunity to study the cloud top morphology and microstructure, precipitation and cloud electrification of the pyro-Cb. The combination of heat and smoke created a cloud with extremely small drops, which ascended rapidly in violent updrafts. There appeared to be little freezing up to the homogeneous freezing isotherm level of −38°C. A cloud with such small and short-lived highly supercooled drops is incapable of producing precipitation except for few large graupel and hail, which produced the observed radar echoes and charged the cloud with positive lightning. The small cloud drops froze homogeneously to equally small ice particles, for which there is no mechanism to aggregate into precipitation particles, and which hence remain in the anvil. The lack of significant precipitation implies that only a small fraction of the smoke is scavenged, so that most of it is exhausted through the anvil to the upper troposphere and lower stratosphere. Comparisons with other cases suggest that a pyro-Cb does not have to be as violent as the Chisholm case for precipitation to be strongly suppressed. However, this level of convective vigor is necessary to create the overshooting updraft that injects the smoke into the lower stratosphere.

The Art and Science of Lightning Protection

Abstract: Preface 1. What is lightning? 2. Lightning damage 3. General methods for lightning protection: Faraday cages, topological shields and more practical approaches: cone of protection and rolling sphere methods 4. Structure protection: air terminals and down conductors 5. Structure protection: grounding 6. Surge protection for electronics in low-voltage electrical systems 7. Humans and animals 8. Lightning warning 9. Airships, airplanes, and launch vehicles 10. Ships and boats 11. Trees 12. Overhead and underground power and communication lines 13. Lightning elimination 14. So, what do we know and what don't we know about lightning protection? References Index.

Total Lightning Observations with the New and Improved Los Alamos Sferic Array (LASA)

Abstract: Since 1998, Los Alamos National Laboratory (LANL) has deployed an array of fast electric field change sensors in New Mexico and Florida in support of LANL’s satellite lightning observations. In April 2004, all the sensors were significantly upgraded and improved, and a new array was deployed in north-central Florida. This paper describes the operations of the new array and reports the first 12 months of lightning observations. The new array is about 10 times more sensitive than the previous one and can capture millions of discharge events during a stormy day in Florida. In this paper, the array’s lightning location accuracy, minimum detectable peak current, and ratio of intracloud-to-cloud-to-ground flashes are analyzed. Some case studies that illustrate the storm evolution, lightning classification, and radar comparisons are presented. A new three-dimensional capability of the array is demonstrated.

Lightning flashes conducive to the production and escape of gamma radiation to space

Abstract: [1] Gamma radiation observed in space has been associated with lightning flashes in thunderstorms. These special flashes do not appear to be the large and energetic positive ground flashes that also produce sprites. Considerations of gamma ray attenuation in air indicate that such flashes may not produce gamma radiation at sufficient altitude to enable their escape to space. High-altitude intracloud lightning, most prevalent in the tropics where the tropopause is also high, may be a necessary source.

Small ice crystals and the climatology of lightning

Abstract: [1] Vigorous debate still surrounds the cloud electrification process and unexplained regional variations in lightning activity. Here, we show that climatological maxima in lightning activity are associated with small effective diameter D-e of ice crystals near cumulonimbus cloud tops. This relationship, unlike lightning's more well-known relationship with cloud top height, is consistent over land and ocean. Since multiple studies indicate that D-e is reduced by atmospheric aerosol, this relationship strengthens previous suggestions of a role for aerosols as well as dynamics in electrification. Moreover, the angular distribution of backscattered radiance shows that modest (similar to 10%) D-e decreases reflect large (similar to 2x) increases in the number of small (< similar to 30 mu m) particles N, a finding supported by cloud model simulations. Both relationships provide an important new test of cloud microphysics and/or electrification models. (Less)

Electrification of volcanic plumes

Abstract: Volcanic lightning, perhaps the most spectacular consequence of the electrification of volcanic plumes, has been implicated in the origin of life on Earth, and may also exist in other planetary atmospheres. Recent years have seen volcanic lightning detection used as part of a portfolio of developing techniques to monitor volcanic eruptions. Remote sensing measurement techniques have been used to monitor volcanic lightning, but surface observations of the atmospheric electric Potential Gradient (PG) and the charge carried on volcanic ash also show that many volcanic plumes, whilst not sufficiently electrified to produce lightning, have detectable electrification exceeding that of their surrounding environment. Electrification has only been observed associated with ash-rich explosive plumes, but there is little evidence that the composition of the ash is critical to its occurrence. Different conceptual theories for charge generation and separation in volcanic plumes have been developed to explain the disparate observations obtained, but the ash fragmentation mechanism appears to be a key parameter. It is unclear which mechanisms or combinations of electrification mechanisms dominate in different circumstances. Electrostatic forces play an important role in modulating the dry fall-out of ash from a volcanic plume. Beyond the local electrification of plumes, the higher stratospheric particle concentrations following a large explosive eruption may affect the global atmospheric electrical circuit. It is possible that this might present another, if minor, way by which large volcanic eruptions affect global climate. The direct hazard of volcanic lightning to communities is generally low compared to other aspects of volcanic activity.

Terrestrial gamma ray flashes and lightning discharges

Abstract: [1] Analysis of ELF/VLF broadband data from Palmer Station, Antarctica indicates that 76% Terrestrial Gamma-ray Flashes (TGFs) detected on the RHESSI spacecraft occur in association with lightning-generated radio atmospherics arriving from near the footprint of RHESSI and within a few ms of the TGF. The remaining TGFs are not associated with any radio atmospheric, thus by implication CG lightning. The peak currents of TGF-associated lightning discharges are often among the most intense from a given storm, with the degree of this association apparently varying between oceanic and land regions. The time-integrated ELF energy of the associated sferics (and thus the lightning charge moment) exhibit much less tendency to be large. Statistical analysis of the spread in arrival time suggests a ∼2 ms variance due to factors other than geometry and measurement error.

Electrification and Lightning in an Idealized Boundary-Crossing Supercell Simulation of 2 June 1995*

Abstract: A nonhydrostatic cloud model with electrification and lightning processes was utilized to investigate how simulated supercell thunderstorms respond when they move into environments favorable for storm intensification. One model simulation was initialized with an idealized horizontally varying environment, characteristic of that observed across an outflow boundary in the west Texas Panhandle on 2 June 1995 with larger convective available potential energy (CAPE) and wind shear on the boundary’s cool side. That simulation was compared with a control simulation initialized without the boundary. The simulated rightmoving supercell rapidly increased in updraft strength and volume, low-level rotation, radar reflectivity, and 40-dBZ echo-top height as it crossed the boundary, whereas the supercell that did not cross the boundary failed to intensify. For the same kinematic and microphysical evolution and the same inductive charging parameterization, four noninductive (NI) charging parameterizations were tested. In all four cases, there was a general tendency for the charge regions to be lofted higher within the updraft after crossing the boundary. Once the precipitation regions between the main storm and a secondary storm started merging farther on the cool side of the boundary, a gradual deepening and strengthening of the lowest charge regions occurred with relatively large increases in hail and graupel volume, charging rates, charge volume, charge density, and intracloud and cloud-to-ground (CG) flash rates. The negative charge present on graupel within the downdraft appeared to have a common origin via strong NI charging within the midlevel updraft in all four NI cases. Positive channels were more consistent in coming closer to the ground with time compared to negative channels within this graupel and hail-filled downdraft (four of four cases). Those NI schemes that also set up a positive dipole (three of four cases) or inverted tripole (two of four cases) above the downdraft had downward-propagating positive channels that reached ground as positive CG (CG) flashes. The best overall performance relative to the 2 June 1995 CG lightning observations occurred within one of the rime-accretion-rate-based schemes and the Gardiner scheme as parameterized by Ziegler.

Seasonal and diurnal variation of lightning activity over southern Africa and correlation with European whistler observations

Abstract: . Lightning Imaging Sensor (LIS) data have been analysed to ascertain the statistical pattern of lightning occurrence over southern Africa. The diurnal and seasonal variations are mapped in detail. The highest flash rates (107.2 km-2 y-1) occur close to the equator but maxima are also found over Madagascar (32.1 km-2 y-1) and South Africa (26.4 km-2 y-1). A feature of the statistics is a relatively steady contribution from over the ocean off the east coast of South Africa that appears to be associated with the Agulhas current. Lightning statistics are of intrinsic meteorological interest but they also relate to the occurrence of whistlers in the conjugate region. Whistler observations are made at Tihany, Hungary. Statistics reveal that the period of most frequent whistler occurrence does not correspond to the maximum in lightning activity in the conjugate region but is strongly influenced by ionospheric illumination and other factors. The whistler/flash ratio, R, shows remarkable variations during the year and has a peak that is narrowly confined to February and March.

Can Lightning Observations be Used as an Indicator of Upper-Tropospheric Water Vapor Variability?

Abstract: Lightning activity in thunderstorms is closely related to the intensity of vertical updrafts indeep convective clouds that also transport large amounts of moisture into the upper troposphere. Small changes in the amount of upper-tropospheric water vapor (UTWV) can have major implications for the Earth's climate. New evidence is presented showing a strong connection between the daily variability of tropical lightning activity and daily uppertropospheric water vapor concentrations from the NCEP-NCAR reanalysis. Our results over the African continent show that the NCEP upper-tropospheric water vapor peaks one day after intense lightning activity in the Tropics. Given the many caveats related to the NCEP UTWV product over Africa, these results need to be interpreted with caution. However, since global lightning activity can be monitored from a few ground stations around the world via the Schumann resonances, we suggest the possible use of continuous lightning observations for studying the daily varia...

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

Abstract: [1] 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.

Lightning protection system for a wind turbine blade

Abstract: The invention relates to a lightning protection system for a wind turbine blade, where the blade is a shell body made of a composite material and comprises a root area and a tip end. The lightning protection system comprises at least one lightning receptor arranged freely accessible in or on the shell unit surface on or in the immediate vicinity of the tip of the blade, and a lightning conductor made of electrically conductive material extending within the shell body along substantially the entire longitudinal direction of the blade. The lightning receptor and the lightning conductor are electrically connected by means of a connection area. The lightning conductor in its entire longitudinal direction as well as the connection area between the lightning conductor and the lightning receptor are electrically insulated in order to prevent lightning striking through the surface of the blade.

Generation of neutrons in giant upward atmospheric discharges

Abstract: It has been shown that the efficiency of the reactions of the fusion of deuterons in the atmosphere is very low; therefore, nuclear fusion cannot be responsible for the generation of neutrons by lightning. The generation of neutrons in thunderstorm fields is attributed to photonuclear reactions in giant upward atmospheric discharges over thunderclouds.

Double-Exponential Expression of Lightning Current Waveforms

Abstract: A lightning waveform is usually defined by the rise time tf , pulse length tp and peak value I. In order to represent lightning current waveforms in an analytic manner, the double-exponential formula is employed to fit with the lightning current waveforms. This paper describes the relationship between the waveform parameters tf, tp, I and the formula constants alpha, beta and A. An effective algorithm is proposed to calculate alpha, beta and A. The algorithm gives a reasonable expression for alpha, beta and A from the data points of lightning current waveforms. Numerical examples are derived from the algorithm for a few types of lightning current waveforms that are commonly used in lightning protection design. A better agreement appears between calculated and standard waveforms

Fine‐resolution simulation of the channel structures and propagation features of intracloud lightning

Abstract: [1] Intracloud lightning discharges are simulated in 12.5 m (fine) and 250 m (coarse) resolution for 2-dimensional domain by using an improved Stochastic Lightning Model. Simulation results indicate that the bi-level branched channel structure, horizontal extending ranges and maximum vertical electric field changes obtained from fine resolution lightning model are in better agreement with previously observed data than those from coarser model. The IC flash channels from fine lightning model have the fractal feature with fractal dimension of 1.43, and propagation tendency is dependent on the potential distribution. In addition, fine resolution lightning modeling shows that after IC flash initiation at the boundary between positive and negative potential zones, potential wells attract the leaders of opposite polarity into the central area and prevent their outward expansion. Leaders can propagate throughout regions of net charge of the opposite polarity, but they avoid the isolated charge areas of the same polarity.

A Statistical Procedure to Forecast Warm Season Lightning over Portions of the Florida Peninsula

Abstract: Sixteen years of cloud-to-ground lightning data from the National Lightning Detection Network and morning radiosonde-derived parameters are used to develop a statistical scheme to provide improved forecast guidance for warm season afternoon and evening lightning for 11 areas of the Florida peninsula serviced by Florida Power and Light Corporation (FPL). Logistic regression techniques are used to develop equations predicting whether at least one flash will occur during the noon–midnight period in each area, as well as the amount of lightning that can be expected during this same period, conditional on at least one flash occurring. For the amount of lightning, the best results are achieved by creating four quartile categories of flash count based on climatology, and then using three logistic equations and a decision tree approach to determine the most likely quartile. A combination of forward stepwise screening and cross validation are used to select the best combination of predictors that are most...

Relationship between cloud-to-ground lightning and precipitation ice mass: A radar study over Houston

Abstract: [1] Using seven summer-seasons (1997–2003) of NEXRAD data (over 46,000 volumes), coincident climatologies of cloud-to-ground (CG) lightning flash densities and radar-derived, column integrated precipitation ice mass (IM) were developed, extending global studies of IM and lightning to more regional and cell scales around Houston, TX. Results indicate that local maximums in CG lightning were indeed accompanied by peaks in IM. Extending previous global findings to cell-scales, we establish a link between a storm's ability to generate enhanced concentrations of mixed-phase IM, and its ability to generate lightning. Relative to the documented CG lightning “anomaly” over Houston, these results imply that unique aspects of the Houston urban area must first generate an anomaly in convective intensity and precipitation ice, thereby generating an anomaly in lightning; causal hypotheses must be capable of explaining either increased frequency and/or intensity of convection, and then relating these to the enhancement of IM and lightning production.

New findings about the influence of smoke from fires on the cloud-to-ground lightning characteristics in the Amazon region

Abstract: [1] In the last decade many authors have found evidence on the influence of smoke from fires on the cloud-to-ground (CG) lightning characteristics, both in the lifetime scale of individual storms and on a climatological scale. In general, the observations indicate that the peak current and percentage of positive flashes increase in thunderstorms injected with smoke, while the negative peak current decreases. This article investigates the validity of these findings in thunderstorms injected with large concentrations of smoke from fires in the Amazon region, comparing CG lightning characteristics in the dry (polluted) and wet (clean) seasons, as well as using a new methodology, based on results of a numerical simulation model of atmospheric transport of tracers coupled to a biomass burning emission model. The results confirm the previous findings and, in addition, give a first estimation of the sensitivity of these parameters to changes in the smoke concentration.

Benjamin Franklin and Lightning Rods

Abstract: Franklin’s work on electricity and lightning earned him worldwide fame and respect—ideal assets for brokering aid from France during the American Revolution.

Estimation of input energy in rocket‐triggered lightning

Abstract: [1] Electric fields in the immediate vicinity (within 0.1 to 1.6 m) of the triggered-lightning channel were measured with Pockels sensors at the International Center for Lightning Research and Testing at Camp Blanding, Florida. These fields and the associated currents measured at the base of a 2-m strike object were used to compute the input power and energy, each per unit channel length and as a function of time, associated with return strokes in rocket-triggered lightning. In doing so, we assumed that the vertical component of the electric field at horizontal distances of 0.1 to 1.6 m from the lightning attachment point is not much different from the longitudinal electric field inside the channel (Borovsky, 1995). The estimated mean input energy over the first 50 μs or so is between 103 and 104 J/m, consistent with predictions of gas dynamic models, but one to two orders of magnitude smaller than Krider et al.'s (1968) estimate for a natural-lightning first stroke, based on the conversion of measured optical energy to total energy using energy ratios observed in laboratory long-spark experiments. The mean channel radius and resistance per unit channel length at the instance of peak power are estimated to be 0.32 cm and 7.5 Ω/m, respectively.

Magnetic fields and loop Voltages inside reduced- and full-scale structures produced by direct lightning strikes

Abstract: This paper presents a numerical electromagnetic analysis of magnetic fields and loop voltages inside reduced- and full-scale lightning protection systems (LPSs) "structures" resulting from direct lightning strikes. The method of moments is employed to model the whole structure in three dimensions except the lightning channel. The lightning channel is simulated by the well-known transmission-line model (TL model), where the influence of the lightning-channel generated electric and magnetic fields are taken into account. Three distinct LPSs were modeled, namely, reduced-scale model with return conductors (RSRC), reduced-scale model with lightning channel (RSLC), and full-scale model with lightning channel (FS). The computed results of magnetic fields and magnetic-field derivatives were verified versus some experimental results for the RSRC model. In addition, the scale factor for all the measured quantities were also checked as functions of the geometrical scale factor for the positive and the negative first stroke currents. The lightning shielding performance with and without bonding was investigated for three distinct lightning stroke types, namely, the negative first, the negative subsequent, and the positive strokes. The voltages and currents generated in loops located inside the struck FS LPS were computed with and without bonding and grounding resistance and for different lightning current waveforms, locations and inclination of the lightning channel, and return stroke velocity.