Thunder Events Research Articles

Tracking Lightning Through 3D Thunder Source Location With Distributed Acoustic Sensing

AbstractInvestigating lightning is of key significance in understanding the lightning mechanism and mitigating lightning hazards. We reported an experiment of investigating lightning through three‐dimensional (3D) thunder locating using a Distributed Acoustic Sensing (DAS) array in Hefei, China. In this experiment, we recast a 7.7 km long urban telecom optical fiber cable as 3,850 sensors using the DAS technique. From dense DAS recording, we manually identified 101 thunder events during six positive cloud‐to‐ground (CG) lightning flashes within 20 min. The DAS recorded thunder signals are dominated by direct acoustic waves rather than air‐ground coupled surface waves. The thunder events were then located using the arrival times of thunder signals. The locations and amplitudes of thunder events are generally consistent with those from the conventional lightning detection data set and broadband magnetic field. There is likely a correlation between the maximum strength of thunder events and the highest peak current for individual CG flashes. Moreover, the comparison with weather radar observations indicates that lightning usually originated from areas of high reflectivity (e.g., ≥50dBz) with diffusely distributed (from ground surface to ∼5 km altitude) thunder events and extended in a narrow altitude range of 3–5 km to areas with low radar reflectivity.

Inversion of acoustic thunder source spectral model from thunder-induced seismic waves in megacity

SUMMARYThunder-induced seismic waves recorded at dense seismic stations in Seoul, South Korea are analysed for inversion of thunder source spectra. Thunder-induced seismic waves from four local thunder events are analysed. A theory is introduced for the inversion of acoustic source spectra from thunder-induced seismic waves. In the course of source-spectral inversion, the propagation and acoustic-to-seismic coupling effects are counted. The thunder-induced seismic signals were well identified at distances of <∼20 km. Direct acoustic-to-seismic coupled seismic waves present apparent phase velocities of sound speed in atmosphere (340 m s−1). Thunder-induced seismic waves are dominant at high frequencies (>20 Hz). Vertical peak ground accelerations of thunder-induced seismic waves in local regions (0.024–0.110 m s−2 at distances of 2.4–3.7 km) are equivalent to the ground motion levels induced by a moderate-size (∼M5) earthquake at regional distances. The thunder-induced acoustic waves in the atmosphere are obtained by removing the acoustic-to-seismic coupling effect and site-response effect from the observed thunder-induced seismic waves. The quality factors for acoustic wave attenuation in the atmosphere are determined. Urban landscapes and atmospheric effects cause strong acoustic attenuation over atmospheric absorption. Acoustic thunder source spectra are determined by stacking the inverted acoustic spectra at all stations. The peak frequencies of acoustic thunder source spectra are around 34–36 Hz, suggesting the acoustic energy per unit length in lightning strikes to be ∼4 × 106 J m−1. Local seismic records are applicable for the investigation of thunder and lightning properties.

Seismic event detection in urban Singapore using a nodal array and frequency domain array detector: earthquakes, blasts and thunderquakes

SUMMARYDetection of seismic events at or below the noise level is enabled by the use of dense arrays of receivers and corresponding advances in data analysis methods. It is not only important to detect tectonic events, but also events from man-made, non-earthquake sources and events that originate from coupling between the solid Earth and the atmosphere. In urban environments with high ambient noise levels the effectiveness of event detection methods is unclear, particularly when deployment restrictions result in an irregular receiver array geometry. Here, we deploy a dense nodal array for 1 month in the highly populated city state of Singapore. We develop a new detection method based on image processing that we call spectrogram stacking, which detects anomalous, coherent spectral energy across the array. It simultaneously detects multiple classes of signal with differing spectral content and aids event classification, so it is particularly useful for signal exploration when signal characteristics are unknown. Our approach detects more local events compared to the traditional short-term average over long-term average and waveform similarity methods, while all methods detect similar numbers of teleseismic and regional earthquakes. Local events are principally man-made non-earthquake sources, with several events from the same location exhibiting repeating waveforms. The closest earthquake occurs in peninsular Malaysia, in an area where no earthquakes have previously been detected. We also detect ground motion over a wide frequency range from discrete thunder events that show complex coupling between acoustic and elastic wavefield propagation. We suggest that care should be taken deciphering local high-frequency tectonic events in areas prone to thunder storms.

Statistical analysis of storm electrical discharges reconstituted from a lightning mapping system, a lightning location system, and an acoustic array

AbstractIn the framework of the European Hydrological Cycle in the Mediterranean Experiment project, a field campaign devoted to the study of electrical activity during storms took place in the south of France in 2012. An acoustic station composed of four microphones and four microbarometers was deployed within the coverage of a Lightning Mapping Array network. On the 26 October 2012, a thunderstorm passed just over the acoustic station. Fifty‐six natural thunder events, due to cloud‐to‐ground and intracloud flashes, were recorded. This paper studies the acoustic reconstruction, in the low frequency range from 1 to 40 Hz, of the recorded flashes and their comparison with detections from electromagnetic networks. Concurrent detections from the European Cooperation for Lightning Detection lightning location system were also used. Some case studies show clearly that acoustic signal from thunder comes from the return stroke but also from the horizontal discharges which occur inside the clouds. The huge amount of observation data leads to a statistical analysis of lightning discharges acoustically recorded. Especially, the distributions of altitudes of reconstructed acoustic detections are explored in detail. The impact of the distance to the source on these distributions is established. The capacity of the acoustic method to describe precisely the lower part of nearby cloud‐to‐ground discharges, where the Lightning Mapping Array network is not effective, is also highlighted.

Thunderstorms caused by southern cyclones in Estonia

The relationships between the frequency and duration of thunderstorms, lightning and southern cyclones over Estonia are presented for the period 1950-2010. A total of 545 southern cyclones and 2106 thunderstorm days were detected, whereas 11.3% of the observed thunder days were associated with southern cyclones. At the same time, 29.2% of all southern cyclones were accompanied by thunderstorms. In the thunder season, however, this percentage was much higher, reaching up to 80% in summer months. The number of thunder days was largest when the centres of southern cyclones passed a measuring station at a distance less than 500 km. The number of cloud-to-ground lightning strikes related to southern cyclones was larger than that of any other thunder events. The results of our study demonstrate that the intensity of thunderstorms related to southern cyclones is higher than that of other thunderstorms. Correlation analysis revealed statistically significant relationships between the frequency of thunder days related to southern cyclones and the frequency of southern cyclones, also between the frequency of thunder days related to southern cyclones and days of other thunder events.

Observational Study of Pre-monsoon Thunder Events over Delhi and Guwahati Region

Thunder events are common during pre-monsoon season (March -May). In this paper thunder events of Delhi and Guwahati for the month of May and April have been analysed with Global Navigation Satellite System (GNSS) derived precipitable water vapour and Moderate Resolution Imaging Spectroradiometer (MODIS) derived Aerosol Optical Depth (AOD) at 550 nm data. It is observed that GNSS derived precipitable water vapour have the potential to nowcast or characterize the mesoscale event. In all cases the AOD and Integrated Precepitable Water Vapour (IPWV) increases before the thunder event which accompanied with duststorm and consequently increase the aerosol loading in the atmosphere. The rate of increase of IPWV (d/dt(IPWVdt)) is observed approximately 3-4 mm /hour during the convective initiation and relative increase of AOD is 150-300 %. The various thermodynamic parameters of radiosounding to understand better the thunder activity are also highlighted in the paper.

Observational study of pre-monsoon thunder events over Delhi and Guwahati region

Observational study of pre-monsoon thunder events over Delhi and Guwahati region

Thunder Events in China: 1980-2008

Using data collected at 517 weather stations in contiguous China over the period 1980-2008, characteristics of thunder events have been investigated. These characteristics include geographical dist...

Infrasound from thunder: A natural seismic source

A small array consisting of five three‐component short‐period surface seismometers, a three‐component borehole seismometer, and five infrasound microphones was built to investigate thunder‐induced ground motions. Data from two thunder events with similar N‐wave waveforms but different horizontal slownesses are chosen as examples of data collected by the array. These impulsive acoustic waves excited P and S reverberations in the near surface that depend on both the incident wave horizontal slowness and the velocity structure in the upper 30 meters at the site. Although the depth of the borehole is relatively shallow compared to a seismic wave wavelength, velocity amplitude in the radial component decays as much as 63 percent with depth but vertical component amplitudes are unaffected consistent with air‐coupled Rayleigh wave excitation. Naturally occurring thunder appears to be a useful seismic source to empirically determine site resonance characteristics for hazards assessments.

Lake-Effect Rain Events

Abstract Seven years of autumnal (September–November) precipitation data are examined to determine the characteristics of lake-effect precipitation downwind of Lake Erie. Atmospheric conditions for each lake-effect event are compiled and the mean atmospheric environment for rain events is constructed and compared to conditions for lake-effect snow events. It is found that lake-effect precipitation occurs approximately one out of every five days with a diurnal peak in precipitation intensity during the afternoon and evening. The greatest number of lake-effect days occurs in October followed by November and then September. Comparison of these results to regional precipitation climatologies strongly suggests that the season of lake enhanced precipitation begins in late summer. Precipitation is predominantly rain throughout September and October and snow after the first week of November. A transition period of both rain and snow occurs in early November. Analysis of thunder events for the 7-yr period show a l...

An Assessment of Thunderstorm Probability Forecasting Skill

Abstract An analysis of the 11-year (1983–1993) skill of subjective 24-h thunderstorm probability forecasts made by staff and students at the State University of New York at Albany is presented. Skill is measured relative to the long-term National Weather Service climatology of thunder heard at 10 U.S. cities using the well-known Brier verification score. The 11-year average skill was 21.8% (standard deviation of 3.9%), and the annual increase in skill was a disappointing 0.1%. The sample correlation coefficient between the frequency of thunder events and the year-to-year variability of forecast skill was 0.53 for all cities combined.

Relationships between Thunderstorms and Cloud-to-Ground Lightning in the United States

Abstract Data from networks of lightning sensors operated during 1986–89 were employed to perform climatic assessments of cloud-to-ground (CG) flashes, and of the relationship between CG flashes and thunder events, as reported at 62 first-order stations. The average annual CG flash pattern resembles the nation's thunderstorm pattern, with lightning being most frequent at points in the southeast United States (>18 000 flashes in Florida) and least frequent at West Cost stations (< 100 flashes). Flashes in the intermontane area are relatively less than thunderstorms, and greater than 50% of all thunder events there are due to intracloud lightning. An anomalous high in CG frequencies exists from Virginia to New York. Cluster analysis identified ten regions of similar storm activity with several reflecting localized influence (Great Lakes, Florida, the Piedmont, and Arizona). Thunder events provide poor estimates of CG lightning incidence and durations. Cloud-to-ground flash data reveal that 20% (far West) an...

Relations of Thunderstorms and Cloud-to-Ground Lightning Frequencies

Temporal and spatial relationships between thunderstorms (events) and flashes were investigated using data for 1983–85 for 25 first-order stations (10 in the West and 15 along the East Coast). Thunder events were compared with flashes within three ranges: 5 km, 10 km, and 20 km, around each station. Cluster analysis revealed six geographic regions: Florida, Southeast (South Carolina, Georgia), Mid-Atlantic (Virginia, Maryland Pennsylvania), Northeast (New York and New England), Rocky Mountains, and an intermontane area. Periods of multiple flashes not within thunder events and within 10 km of a point (most realistic for audibility), revealed that 10% to 20% (depending upon region) of all thunderstorms were missed. Also, 13% (Rockies) to 44% (Mid-Atlantic) of all thunderstorms have recorded durations too short (missed flashes before their reported start), and the average underestimated durations were from 55% (Northeast Mid-Atlantic) to 26% (Rockies). Flashes isolated in time and space, due to locational errors, represent 1% of all flashes in the east and 3% to 5% in the west where the data are poorer. Errors in flush data appear minimal but the errors in thunder events are sizable. Thunder events and flash frequencies related well based on major features in their average areal patterns and their between-year changes at stations. Correlations of fishes with events varied; their annual point frequencies had coefficients of +0.83 (east) and +0.67 (west). Durations of events and Bash frequencies were poorly correlated with skewed distributions (often large flash frequencies in a few storms). The percent of all recorded flashes (within 10 km) in thunder events varied from 28% to 44% at western stations and from 13% to 20% at eastern stations. Thunder events with ≥ 1 flash varied widely, from 71 % of all events at Washington, D.C. to 30% at Boston. Major east-west differences existed in the frequency of thunder events with flashes, reflecting poorer audibility of thunder in the west. Part of the difference is due to flash recording problems in the west, leaving flash frequencies that are underestimates of the true values. latitudinal distributions were marked with north-to-south increases in thunder events and their durations, frequency of flashes, and number of flashes not in events. More missed flashes in the south suggested that atmospheric conditions in northerly U.S. latitudes enhance audibility. With a 20-km sampling radius, between 6 (Northeast) and 23 (Southeast) thunder events are not recorded yearly, but these averages drop to 1 (Northeast) and 4 (Southeast) based on flashes within 5 km. The data on thunderstorms is generally poor from two perspectives: 1) the recorded data miss sizable numbers of storm events, and 2) when recorded, 30% to 50% often underestimate durations based on nearby lightning activity.

Thunder events and cloud‐to‐ground lightning frequencies

Data on thunder days, thunder events (discrete periods of nearly continuous thunder), and cloud‐to‐ground lightning flashes for portions of the United States were compared to discern how well the frequencies of thunder days, events, and lightning flashes related. Across the United States, thunder day frequencies related well to events with correlation coefficients of +0.97 to +0.84, being lowest in areas where multiple storm periods in a day occur most frequently. The number or duration of thunder events does not relate well to lightning flash frequencies; annual thunder events and durations explain less than 30% of the variance in lightning flashes. This poor relationship is caused by four factors: more than 50% of all thunder events had no cloud‐to‐ground lightning; many flashes (22–26% in the east and 31–40% in the west) occurred at times not in thunder periods, the frequency of lightning in thunder periods was extremely skewed, with 50% of all flashes occurring in 10% of the periods; and locational errors inherent in the detection system affect the preciseness of the relationships. The poor event‐lightning relationships indicate that historical values of thunder activity cannot be used to reconstruct historical lightning frequencies. The results also raise serious questions about the accuracy of the thunder event and thunder day records; clearly, they underestimate the actual storm activity at a weather station.

Climatography of Thunder Events in the Conterminous United States. Part II: Spatial Aspects

Abstract The average temporal and spatial distributions of thunder events (periods of discrete thunder activity heard at a point) in the conterminous United States were found to be generally similar to those of thunder days. Annual averages of thunder events peak along the Gulf Coast (>100) and are also quite high in the central United States (Kansas, Missouri, Illinois with >75 events), and in the southwest (Arizona with 60 events). Thunder events are least along the west coast (<20) and in the northeast (<30). Multiple events per day are greatest in the Midwest (Illinois, Iowa) averaging 1.7 events per summer day, and are also high in the southwest (Arizona) with 1.5 events. This causes these two maxima in thunder event activity to be more pronounced than those found on the pattern of average thunder days. The average patterns for the thunder event frequencies, multiple events per day, and durations reveal that convective activity is weakest and shortlived along the west coast and in the northeast. The ...

Climatography of Thunder Events in the Conterminous United States. Part I: Temporal Aspects

Climatography of Thunder Events in the Conterminous United States. Part I: Temporal Aspects

Effect of atmospheric absorption on the acoustic power spectrum of thunder

Recordings of six thunder events have been analyzed in 1/3-octave bands over the frequency range 50–500 Hz to determine the relative acoustic power spectrum. The received power spectra have been corrected for propagation loss due to viscosity, thermal conductivity, and molecular relaxation. The correction, which is strongly dependent upon atmospheric conditions that were not measured, yields spectra so nearly identical to suggest that the six thunder events have identical spectra (within 5 dB) in the frequency range considered. Subject Classification: 28.20; 20.35.

Acoustical analysis of thunder signatures

Seven thunder signatures resulting from cloud-to-ground lightning strokes have been analyzed in third-octave bands spanning the frequency range from 25 to 500 Hz. Atmospheric absorption was found to contribute significantly to the change in the signature from source to receiver. Although atmospheric absorption is not large, it is frequency dependent, and hence can not be ignored when attempting to reconstruct the source spectrum from measurements taken several thousand feet away. A program has been developed which converts received sound pressure level (SPL) to initial power level (IPL) for given frequency bands. No attempt was made to determine an absolute measure of the acoustic energy released during each event. These initial results indicate that the spectra of different thunder events are sufficiently consistent to allow thunder to be used as a source for studying low-frequency absorption and atmospheric effects which lead to absorption.

On the power spectrum and mechanism of thunder

Forty thunder events from intracloud and cloud-to-ground lightning were analyzed by analog and digital power spectrum methods. Thunder exhibits a low Q(0.5 to 2) spectrum with peak power observed at frequencies from <4 to 125 Hz. Significant differences are noted between thunder from intracloud and cloud-to-ground lightning. Intracloud thunder spectrums show a mean peak value of power at 28 Hz with a mean total acoustic energy of 1.9×1013 ergs. Cloud-to-ground spectrums show a mean peak value at 50 Hz with a mean total acoustic energy of 6.3×1013 ergs. The mean total acoustic efficiency for discharges to ground is calculated to be 0.18%. The thunder power spectrum is time varying. The mechanism of thunder production by a thermally driven expanding channel as described by A. A. Few appears to account for the dominant frequencies observed from many cloud-to-ground flashes, but cannot explain the high-energy low-frequency peaks of some cloud-to-ground and most intracloud discharges. The electrostatic mechanism of C. T. R. Wilson and of S. A. Colgate is proposed to explain the large energy peaks at frequencies less than 10 Hz.