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In this paper, to realize a better adaptive method for the lightning electric field signal denoising, we firstly compared the decomposition results of three methods called the EMD (empirical mode decomposition), the CEEMDAN (complete ensemble empirical mode decomposition with adaptive noise), and the EWT (empirical wavelet transform) by artificial signals, respectively, and found that the EWT was better than the other two methods. Then, a MEWT (modified empirical wavelet transform) method based on the EWT was presented for processing the natural lightning signals data. By using our MEWT method, we processed three types of electric field signal data with different frequency bands radiated by the lightning step leader, the cloud pulse and the return stroke, respectively, and the VLF (very low frequency) lightning signals propagating different distances from 500 km to 3500 km, by using the data of the fast electric field change sensors from Nanjing Lightning Location Network (NLLN) in 2018 and the data of the fast electric field change sensors and the VLF electric antennas from the NUIST Wide-range Lightning Location System (NWLLS) in 2021. The results showed that our presented MEWT method could adaptively process different lightning signal data with different frequencies from the step leader, the cloud pulse, and the return stroke; for the lightning VLF signal data from 500 km to 3500 km, the MEWT also achieved a better noise reduction effect. After denoising the signal by using our MEWT, the detection ability of the fast electric field change sensor was improved, and more weak lightning signals could be identified. [ABSTRACT FROM AUTHOR]

The effects of real terrain on the lightning magnetic fields and location accuracy are studied by using two-dimensional finite difference time-domain method. Two estimation methods of wave arrival time (using the time corresponding to the peak field and 10% threshold of the peak field) are applied to the arrival time delay, and then using the ToA technique with three approaches of arrival time differences to evaluate the location accuracy. It is found that the effect of real terrain on the lightning magnetic fields is due to the enhancement of reflection wave and propagation attenuation of terrain topography. The arrival time of peak value method is delayed from 0.15 to 2.3 μs, while that of 10% threshold method varied from −0.11 to 0.92 μs. The lightning location accuracy using the 10% threshold method is least, about tens of meters. Furthermore, considering the specific frequency bandwidth of detection sensors, three-order 10- to 500 kHz Butterworth filter was applied. After filtering, the effect of real terrain on the lightning magnetic fields becomes more obvious. The 10% threshold method is still the best way for lightning location, and the frequency bandwidth of different sensors on the effect of the location accuracy cannot be ignored. [ABSTRACT FROM AUTHOR]

In this article, the effect of a mountain located on the propagation path of lightning-radiated electromagnetic fields was systematically studied by using the two-dimensional finite-difference time-domain method (2-D FDTD), considering the influence of the return stroke (RS) type, the ground conductivity, the mountain height and the width at the base, its position, and the observation distance. The field peak reduction and the wave arrival time delay, compared with those for the perfectly conducting flat ground, are analyzed in detail. The results show that, with an increase in the mountain height, the field peak reduces and the wave arrival time delay increases. The wave arrival time is insensitive to the change of mountain widths. The terrain envelope method can be used to estimate the additional time delay introduced by the mountain. For a mountain lower than 1 km or so, the additional attenuation effect of the mountain on the field peak can be ignored for most practical purposes. Furthermore, the propagation process of the lightning-radiated electromagnetic wave over the mountain was studied by analyzing the spatiotemporal evolution characteristics of the field. The analysis shows that the electromagnetic wave propagates to the back of the mountain through the diffraction process at the mountaintop. [ABSTRACT FROM AUTHOR]

This paper analyzes the effect of lightning strikes to an oblique cone-shaped mountain on the lightning-induced voltages on the power distribution line. The lightning-generated electromagnetic fields are calculated by using a three-dimensional finite-difference time-domain method in Cartesian coordinates, and the lightning-induced voltages are computed by means of Agrawal coupling model. The simulation results are presented for an oblique cone-shaped mountain with different inclining angles, and for different relative positions of the distribution line placed over nonflat lossy terrain. It is found that, when lightning strikes the top of an oblique cone-shaped mountain, the lightning-induced voltages are closely related with the mountain inclining angles where the distribution line is located above as well as the other mountain inclining angles; in general, the peak values of induced voltages increase with increasing inclining angles. For example, for the constant angle β = 45° (the inclining angle where the line is located above), the induced voltages at the close end of line for α = 26.6°, 45°, and 63.4° (the inclining angle opposite to angle β) is increased by 24%, 60%, and 106% as compared with that for flat ground, respectively. [ABSTRACT FROM AUTHOR]

This paper evaluates the lightning-induced voltages of two typical multiconductor lines configurations with or without shielding wires over lossy ground for striking the cone-shaped mountain top, by means of a cylindrical two-dimensional finite-difference time-domain method and Agrawal coupling model. It is found that the lightning-induced voltages both for the vertical and the horizontal configurations can be enhanced obviously for striking to the cone-shaped mountain, compared with the case of flat terrain, especially for the lower finite ground conductivity. The larger mountain inclining angle results in the higher peak values of lightning-induced voltages, which can be explained by the enhancement of lightning electromagnetic fields due to wave reflection of the transition between mountain terrain and flat ground. Moreover, the shielding wires can effectively reduce the voltages induced at both ends of the phase lines. [ABSTRACT FROM AUTHOR]

This paper presents the computed lightning-radiated vertical electric field and azimuthal magnetic field at the ground level at observing distances ranging from 0.3 to 50 km from the lightning channel for striking the cone-shaped mountain top, by using the two-dimensional finite-difference time-domain method in cylindrical coordinate systems. It is found that the mountain height and the inclination angle have a significant effect on the lightning-radiated electromagnetic field both for the close and far observing distances. For the lightning-radiated vertical electric field, the enhancement of the far field beyond several kilometers (mainly radiation field component) is due to the reflection process in the transition between the mountain terrain and the flat ground, but the reduction of close field near the mountain terrain (mainly quasi-electrostatic field component) is due to the shadowing effect, but not due to such reflection. For the lightning-radiated azimuthal magnetic field, the field values for different mountain terrains, including the mountain height and inclination angle, increase both for close and far observing distances. [ABSTRACT FROM AUTHOR]

In this paper, we have employed the 3-D finite-difference time-domain technique to analyze the influence of the vertically stratified conducting ground (mixed path) on the lightning-induced voltages of the overhead line at 60 and 200 m from the lightning strike point by using decomposing method. In order to analyze the effect range caused by mixed path, we assumed that the mixed path has much different conductivities with 100:1 ratio. When the conductivity near the strike point (0.1 S/m) is larger than that near the overhead line section (0.001 S/m), the induced voltage clearly increases with the increase of the closest distance from the center of the line to the interface between the two media (dl). However, for strike point with a less conducting earth (0.001 S/m) than the overhead line section (0.1 S/m), when dl is less than 30 m, the influence of the stratification should be considered; when dl is larger than 30 m, it can just be regarded as a homogeneous soil with the conductivity of 0.1 S/m equal to that near the overhead line. For the smaller ratio of conductivities of the mixed path, its effect on the induced voltages decreases. [ABSTRACT FROM AUTHOR]

In this paper we have studied the propagation effect of frequency-dependent soil (FDS) on the far vertical electric fields radiated by subsequent lightning strike to tall objects with heights from 50 to 300 m. It is found that the field propagation attenuation along FDS is obviously less than that case where the parameters are assumed to be constant (low-frequency conductivity at 100 Hz, LFC), and with the decrease of LFC, the corresponding field attenuation increases. When LFC is equal to or larger than 0.01 S/m, the effect of FDS can be ignored. However, when LFC is 0.0001 S/m, the field peak for FDS may reach as much as 2.5 times of that earth with LFC for strike to a 300-m-tall object. [ABSTRACT FROM PUBLISHER]

In this paper we have validated and revised the general expressions for far-field-current factors presented by Bermudez et al. (2005, 2007) for lightning strike to tall objects on perfectly and finitely conducting ground, respectively. For the perfectly conducting ground, it is found that when the risetime of lightning return stroke current (RT) is larger than 5 h / c ( h is the height of tall object and c is the light speed), the overestimation caused by the traditional method predicting the lightning current peak is within 10% beyond a distance of 20 km from the lightning channel, and with the decrease of observed horizontal distance d , the error will increase due to the effect of induction field component. For example, when d is 10 km, the overestimation is about 20% for strike to a 300-m-tall object. For the finite conductivity ranging from 0.01 S/m to 0.001 S/m, when the lightning strikes the 300-m-tall object, the lightning current peak predicted from the measured magnetic field peak according to the traditional method is overestimated ranging from about +5% to +20% (positive means overestimation while negative means underestimation) and the derivation value is more within a distance of 20 km because of induction field component; When the lightning strikes the 50-m-tall object, the predicted current peak according to the traditional method has an error ranging from about +5% to −15%. [ABSTRACT FROM AUTHOR]

Based on the two-dimensional finite-difference time-domain (2-D FDTD) method, we have validated the time-domain method presented by Barbosa (B-P formula) for calculating the lightning-radiated horizontal electric field at the earth surface within distances from 30 to 1000 m from the lightning channel, considering the horizontally stratified earth. The results show that, except for distances in the vicinity of the lightning channel (e.g., 30 m), the B-P approximate formula can approximately predict the field peak value from 50 to 1000 m with a satisfactory accuracy within errors of 10% and its accuracy for subsequent return strokes seems to be more or less the same as that for first return strokes. The rise-time part of the field wave predicted by the B-P approximate formula is similar to that predicted by the 2-D FDTD, however, the tail of the wave predicted by the B-P approximation may be shorter than that by the 2-D FDTD (e.g., 30 and 50 m). [ABSTRACT FROM AUTHOR]

The impulse charge moment change (iCMC) is an important electrical property of cloud-to-ground (CG) lightning. In this paper, a new method of measuring the iCMC at distances of several hundred kilometers is proposed. The method is based on the vertical electric field below 1 kHz measured by the widely used fast electric field antenna with low frequency/very low frequency (LF/VLF) band. The impulse response of Earth-ionosphere waveguide (EIWG) is modeled using a finite difference time domain (FDTD) method considering an anisotropic ionosphere. By comparing the observed waveform with the simulated impulse response, the lightning discharge is classified into the impulsive discharge and the non-impulsive discharge. For the impulsive discharge, its iCMC is obtained directly by comparing the measured ELF waveform to the modeled impulse response at the same distance. For the non-impulsive discharge, its current moment waveform is assumed to be a sum of two Heidler's functions, and the genetic algorithm is used to search the unknown parameters in the functions. The good agreement between the measured ELF waveform and the simulated waveform implies that the extracted current moments are reasonable. This method can be used to continuously monitor the lightning iCMC in a given time and space. [ABSTRACT FROM AUTHOR]

The equivalent propagation method adopts a variable propagation velocity in lightning location, minimizing the location error caused by various factors in the long-range lightning location network. To verify the feasibility of this method, we establish a long-range lightning location network in China. A new method is used to extract the ground wave peak points of the lightning sferics and is combined with the equivalent propagation velocity method for lightning location. By comparing with the lightning data detected by the lightning locating system called advanced direction and time-of-arrival detecting (ADTD) that has been widely used for tens of years in China, the feasibility of this method is initially verified. Additionally, it is found that the relative detection efficiency of our long-range lightning location network can reach 53%, the average location error is 9.17 km, and the detection range can reach more than 3000 km. The equivalent propagation method can improve the average location accuracy by ~1.16 km, compared with the assumed light speed of lightning-radiated sferic from the lightning stroke point to the observation station. The 50th percentile of the equal propagation velocity is 0.998c, which may be used in the long-range lightning location networks. [ABSTRACT FROM AUTHOR]

In this paper we have analyzed the effect of frequency dependent soil (FDS) on the lightning-induced voltages caused by lightning subsequent return stroke for strike to tall objects ranging from 100 m to 300 m. It is found that the effect of FDS on the induced voltages peak can be approximately ignored when the low frequency conductivity (LFC) is equal to or larger than 0.01 S/m, and with the decrease of LFC, the effect of FDS on the lightning induced voltages is more obvious. Compared with the constant LFC, the induced voltage peak becomes less for FDS. For example, for a constant LFC of 0.001 S/m, the ratio of the induced voltages peak value for FDS to that for LFC is 83.2% at the line center and 66.8% at the line end for strike to 300-m-tall object, respectively. By using the decomposition method, we divide the lightning induced voltages into two components named by the incident induced waves ( V i ) related with the vertical field and scattered induced waves ( V s ) related with horizontal field, and it is found that FDS results into a less initial peak of tangential horizontal field along the overhead line and further results into a less induced voltage. Also, compared FDS with LFC, the FDS reduces the disparity of lightning induced voltages caused by different tall objects. For example, for the constant LFC, the induced voltage peak for strike to 300-m-tall object is 1.69 times larger than that for strike to 50-m-tall object. However, for the case of FDS, the corresponding ratio is about 1.2. [ABSTRACT FROM AUTHOR]