A method for revising the inversion error of lightning intensity considering the influence of surface propagation

Abstract

The present invention relates to a revision method for lightning intensity inversion error considering the influence of surface propagation, the method comprising: gridding and dividing the research area; using the global digital elevation model and three-dimensional height data, adopting the two-dimensional time-domain finite difference method, establishing a lightning electromagnetic transmission model under a real environment, calculating the lightning electromagnetic field propagation law from each grid point to each detection station, and obtaining the attenuation factor A; establishing a database of the attenuation factor A; according to the grid point where the lightning location is given by the lightning location system, finding the attenuation factor A from the lightning location to different detection stations; revising the relationship between the electromagnetic field and the current peak value under the existing ideal situation; then inverting, averaging the inversion results of different detection stations, and giving the mean and variation range of the inverted lightning intensity. The present invention effectively improves the inversion accuracy of lightning intensity, and provides technical support for business needs such as disaster accident identification and evaluation.

Description

Technical Field

[0001] The invention belongs to the technical field of lightning disaster monitoring and early warning, and in particular relates to a method for revising lightning intensity inversion errors taking into account the influence of surface propagation. Background Art

[0002] The electromagnetic pulse (LEMP) generated by lightning discharge can propagate in the cavity formed by the earth and the ionosphere. When studying the problem of electromagnetic wave propagation on a non-global scale, this space is usually called the Earth-ionosphere waveguide (EIWG). In the Earth-ionosphere waveguide, LEMP can propagate along the surface of the earth in the form of ground waves, or it can be reflected back and forth between the ground and the lower ionosphere and propagate over a long distance in the form of sky waves. The lightning electromagnetic waves observed within tens to hundreds of kilometers from the discharge channel are mainly ground waves, and the sky waves have a small amplitude and a significant time interval between them and the ground waves. The limited conductivity of the ground, the undulating surface, and the curvature of the earth are the main factors affecting the propagation of ground waves in the LF frequency band of lightning. In the range of hundreds to thousands of kilometers, the sky waves are enhanced and overlap with the ground waves, and the parameters of the lower ionosphere become the main factors affecting the far field of lightning. For the ELF-band electromagnetic waves generated by lightning discharge, it is impossible to distinguish between ground wave and sky wave components when they propagate in the Earth-ionosphere waveguide. Therefore, it can be considered that the ELF-band electromagnetic waves propagate in the Earth-ionosphere waveguide in the form of guided electromagnetic waves.

[0003] In recent years, the research on ground waves generated by lightning ground lightning return strokes has mainly focused on two aspects: the study of lightning electromagnetic radiation under complex lightning conditions, mainly including the impact of lightning strikes on high towers (or buildings) and mountains; the impact of complex propagation paths on the propagation of lightning ground waves, mainly including the impact of uneven soil conductivity distribution and soil dispersion effect on ground wave propagation, the impact of rough surface on ground wave propagation, and the impact of irregular and complex terrain on ground wave propagation.

[0004] The reason why rough surface (including natural terrain and urban buildings) has an impact on the electromagnetic field of ground lightning return stroke is that the folds and undulations of the ground surface increase the surface impedance, which correspondingly increases the impact on the propagation of the return electromagnetic field. Feinberg first proposed that the irregular undulations of the earth's surface may increase the surface impedance, but the article did not mention the specific calculation method. Senior assumed that the surface undulations were periodic and obtained a set of equations for calculating equivalent impedance, but the calculation method was extremely complicated. Barrick used the Leontovich boundary condition and Rayleigh-Rice electromagnetic scattering theory to solve the equivalent surface impedance of vertically polarized waves on rough surfaces. Hu et al. pointed out that for ground lightning return electromagnetic waves, the impedance of vertically polarized waves is approximately the same as the impedance of horizontally polarized waves. In 1994, Ming and Cooray directly used the Newmann-Pierson empirical model of the wave height density spectrum to analyze and calculate the impact of wave undulations on the propagation of return electromagnetic fields. The results show that the undulating sea waves cause the electromagnetic field above 10MHz to decay quickly within a range of tens of kilometers; if the sea surface is flat, the electric field change rate decays by 10% after propagating 50km; if it is undulating, the decay exceeds 30%. Compared with the situation on the ocean, the conductivity of land is much lower than that of seawater. The continuous undulations of land mountains may have a greater impact on the propagation of ground lightning return electromagnetic fields, but there are very few studies. There are only studies on isolated mountains and isolated buildings. For example, Zhang Mingxia et al. and Zhang et al. used the moment method and two-dimensional FDTD to study the influence of isolated mountains on lightning radiation magnetic fields. The results show that the higher the mountain and the steeper the mountain, the greater the impact on the magnetic field.

[0005] Soto et al. used the FDTD algorithm to study the impact of lightning on the conical mountain on the close-range electric and magnetic fields on the mountain surface and on the flat ground near the foot of the mountain. The results showed that the vertical and horizontal electric fields on the mountain would be enhanced when lightning struck the conical mountain. When the flat ground at the foot of the mountain was considered, the reflection process of the electromagnetic wave could be seen in the simulated field waveform. Subsequently, Soto et al. further used the "two-step" algorithm based on the Agrawal coupling model to analyze the impact of lightning on the mountain on the coupling voltage. Paknahad et al. used the finite element method to analyze the impact of lightning on the conical mountain on the lightning electromagnetic field and coupling voltage above and below the ground.

[0006] Azadifar et al. used northeastern Switzerland The lightning current data measured on the tower evaluated the detection performance of the European Cooperation for Lightning Detection (EUCLID). Their research results showed that the peak lightning current given by the European Lightning Location Network was about 1.8 times the peak lightning current measured on the high tower. Subsequently, Li et al. added the irregular terrain on the real ground wave propagation path to the two-dimensional time-domain finite difference algorithm, and used the measured lightning current waveform on the Santis tower and the measured synchronous electric field waveform data at 15km to study the impact of the real surface on the lightning electromagnetic field. The results show that considering The lightning vertical electric field waveform simulated after considering the real terrain around the mountain is consistent with the measured waveform, and the far-field peak is greatly enhanced compared with the case of lightning striking the ground, which is consistent with the results given by Azadifar et al.

[0007] In short, when the lightning electromagnetic field propagates along the ground surface, due to the limited conductivity of the soil and the influence of factors such as the mountain, the lightning electromagnetic field gradually attenuates. Therefore, when using the long-distance lightning electromagnetic field to invert the lightning current intensity, the attenuation caused by surface propagation must be considered. However, in actual work, since the attenuation factor is not easy to obtain, it is usually assumed that the ground surface is a smooth ideal conductor surface, which makes the inverted lightning current intensity have a relatively large deviation. Therefore, it is very necessary to develop a set of algorithms that can test and revise the lightning current intensity in real time, which has very important practical application value for lightning disaster accident investigation and disaster warning. Because the identification of lightning disaster accidents first needs to understand the intensity of lightning. The greater the lightning current intensity, the more obvious the degree of the disaster. For example, the reason why the disaster caused by this type of lightning is relatively large is that its lightning current intensity is large, usually up to about several hundred kA, and the corresponding energy and radiation power are also large. It is more significant in the fields of lightning tripping disasters, lightning fires, and petrochemical safety of power line towers.

[0008] How to overcome the shortcomings of existing technologies is an urgent problem to be solved in the field of lightning disaster monitoring and early warning technology. Summary of the invention

[0009] The purpose of the present invention is to solve the deficiencies of the prior art and to provide a method for revising the lightning intensity inversion error taking into account the influence of surface propagation.

[0010] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0011] A method for revising the inversion error of lightning intensity considering the influence of surface propagation includes the following steps:

[0012] Step (1), gridding the study area into 0.01°×0.01° grids;

[0013] Step (2), using the global digital elevation model and three-dimensional height data, adopting the two-dimensional time-domain finite difference method, establish a lightning electromagnetic transmission model under a real environment, calculate the lightning electromagnetic field propagation law from each grid point to each detection station, that is, obtain the ratio of the lightning electromagnetic field peak value at different distances and different directions in the real environment to that in the ideal situation, and obtain the attenuation factor A; attenuation factor A = real value / ideal value;

[0014] Step (3), establishing a database of the attenuation factor A when each 0.01°×0.01° grid point arrives at different measuring stations;

[0015] Step (4), according to the grid point where the lightning location is given by the lightning location system, search from the database to find out the attenuation factor A from the lightning location (the grid point is the lightning location) to different detection stations (Note: the attenuation factor A from the lightning location to different detection stations is different);

[0016] Step (5), according to the result of step (4), revising the relationship between the electromagnetic field and the current peak value under the existing ideal condition;

[0017] Step (6), inversion is performed according to the revised content of step (5), the inversion results of different detection stations are averaged, and the average and variation range of the inverted lightning intensity are given.

[0018] Further, preferably, the three-dimensional height data is Google Earth three-dimensional height data.

[0019] Further, preferably, step (5) is specifically as follows: Under ideal surface conditions, the inversion formula of lightning current intensity is:

[0020]

[0021]

[0022] Among them, I peak is the peak value of lightning current, d is the observation distance, v is the return stroke velocity, c is the speed of light, μ is the magnetic permeability, E0 peak is the peak value of the electric field strength under ideal conditions, B0 peak is the peak value of magnetic induction intensity under ideal conditions; ε0 is the vacuum permittivity;

[0023] According to the two-dimensional finite-difference time-domain method, a lightning electromagnetic transmission model in a real environment is established, and the lightning electric field propagation attenuation database of the area of ​​concern is calculated. The calculation formula under the above ideal situation is revised as follows:

[0024]

[0025]

[0026] Among them, E peak is the peak value of the electric field strength in real conditions, B0 peak is the peak value of magnetic induction intensity under actual conditions; A is the attenuation factor obtained in step (2).

[0027] Considering that the existing lightning location data deviation is about 1 km, the present invention grids the study area and divides it into 0.01°×0.01° grids.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] By using the technical means of the present invention, the intensity of lightning current can be obtained more accurately through telemetry, which is very important for in-depth understanding of the lightning discharge process and its disaster-causing mechanism. Because the identification of lightning-caused disasters first requires the intensity of lightning to be understood. The greater the lightning current intensity, the more obvious the degree of the disaster. For example, the reason why the disaster caused by this type of lightning is relatively large is that its lightning current intensity is large, usually up to several hundred kA, and the corresponding energy and radiation power are also large. It is more significant in the fields of lightning tripping disasters on power line towers, lightning fires, petrochemical safety, etc.

[0030] Therefore, the present invention effectively improves the inversion accuracy of lightning intensity, which is of relatively important value for the prevention of multi-type meteorological disasters, because meteorological disaster prevention and mitigation is an important area that all walks of life in the country must pay attention to. The present invention provides technical support for business needs such as disaster accident identification and evaluation, monitoring and early warning. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a complex topographic map of Yunnan. S1-S8 represent the locations of eight detection stations, and RS1-RS5 represent lightning occurrence points in different directions.

[0032] Figure 2 The vertical profile of the terrain from the lightning point to different measuring stations (terrain elevation fluctuation);

[0033] Figure 3 2D FDTD calculation method considering complex terrain;

[0034] Figure 4 The electromagnetic parameters of the dielectric conformal grid are equivalent;

[0035] Figure 5 Schematic diagram of the effect of real terrain on lightning electric field propagation (real terrain and ideal flat terrain);

[0036] Figure 6This is a graph showing the improvement rate of FDTD calculation efficiency after using the mobile calculation domain technology as the window width (Lw) and the simulation domain width (L) change. DETAILED DESCRIPTION

[0037] The present invention is further described in detail below in conjunction with embodiments.

[0038] Those skilled in the art will appreciate that the following examples are only used to illustrate the present invention and should not be considered to limit the scope of the present invention. If no specific techniques or conditions are specified in the examples, the techniques or conditions described in the literature in the art or the product specifications are used. If the manufacturer of the materials or equipment used is not specified, they are all conventional products that can be purchased.

[0039] Step 1: Use the Global Digital Elevation Model Version 2 (GDEM V2) and Google Earth 3D height data and use conformal meshing to solve complex boundary problems.

[0040] The real topography of the mountains is extremely complex, so the present invention uses conformal grid technology for numerical calculation. Figure 4 The figure shows the equivalent schematic diagram of electromagnetic parameters in the dielectric conformal grid. Assume that the electromagnetic parameters of mountain medium 1 and air medium 2 are ε1, σ1, μ1, σm1 and ε2, σ2, μ2, σm2 respectively. Letters A and B represent horizontal electric field nodes, C and D represent vertical electric field nodes, and F represents horizontal magnetic field node. When the conical mountain is a finite conductivity medium, for the conformal grid on the boundary, it is necessary to reintroduce equivalent dielectric parameters at the corresponding electromagnetic field nodes.

[0041] Step 2: Establishment of lightning electromagnetic field calculation model

[0042] like Figure 3 As shown in FIG. 1 , when calculating the electromagnetic field of lightning return stroke, the present invention adopts the modified transmission line model with exponential current decay with height (MTLE), assuming that the return stroke current amplitude decays exponentially with the channel height, and the attenuation factor λ is 2000 m; the return stroke velocity v is 1.5×10 8 The base current waveform at the bottom of the channel adopts a double Heidler function model.

[0043] The FDTD model in two-dimensional cylindrical coordinates has a total computational domain height of 11 km, a spatial grid size of Δr = Δz = 10 m in the horizontal and vertical directions, and a time step of Δt of 16.67 ns, which meets the current stability condition. The upper and right boundaries of the simulation domain use CPML absorbing boundaries. The soil thickness below the surface is 500 m, the ground conductivity and soil conductivity σ are both 0.001 S / m, and the relative dielectric constant εr is 10.

[0044] The present invention uses a 2D FDTD algorithm to calculate the lightning electromagnetic field in a real environment, which requires parallel calculation to change the calculation time. The present invention mainly adopts a method of moving the calculation domain, which can greatly shorten the calculation time.

[0045] Figure 6 The figure shows the improvement rate of FDTD calculation efficiency after using the mobile calculation domain technology as the window width (Lw) and the simulation domain width (L) change. The window width is the grid size in the FDTD algorithm, and the simulation width is the size of the entire calculation domain. As can be seen from the figure, as the width of the simulation domain increases, the improvement rate of FDTD simulation efficiency after using the mobile calculation domain technology gradually increases. In other words, the larger the spatial scale of the FDTD simulation domain, the greater the improvement of the simulation efficiency of the mobile calculation domain on FDTD. For example, when the width of the moving window is 6km, when the spatial scale of the simulation is greater than 150km, the FDTD simulation time after using the mobile calculation domain is only 1 / 5 of that when the mobile calculation domain is not used.

[0046] The third step is to calculate the lightning electric field propagation attenuation law in the area of ​​concern: calculate the electromagnetic field attenuation factor A at different distances and different propagation paths. The specific steps are as follows:

[0047] 1) Establish a database of the attenuation factor A when each 0.01°×0.01° grid point reaches different measuring stations;

[0048] 2) According to the grid point where the lightning location is given by the lightning location system, search from the database to find out the attenuation factor A from the lightning location (the grid point is the lightning location) to different detection stations (Note: the attenuation factor A from the lightning location to different detection stations is different);

[0049] 3) Revise the existing ideal relationship between the electromagnetic field and the current peak;

[0050] 4) The inversion results of different detection stations are averaged to give the mean and variation range of the inverted lightning intensity.

[0051] The method of the present invention is used to Figure 1The complex terrain of Yunnan area shown in the figure is studied, which includes 8 detection stations S1-S8 and 5 lightning occurrence points RS1-RS5. The vertical profile of the terrain (terrain elevation fluctuation) from the 5 lightning strike points to different stations is shown in the figure. Figure 2 As shown. Figure 5 Schematic diagram of the impact of real terrain on the propagation of lightning electric field (real terrain and ideal flat terrain).

[0052] The above shows and describes the basic principles, main features and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for revising the inversion error of lightning intensity considering the influence of surface propagation, characterized in that: The steps include: Step (1) grid the study area according to 0.01 ° ×0.01 ° Grid points are used for division; Step (2), using the global digital elevation model and three-dimensional height data, adopting the two-dimensional time-domain finite difference method, establish a lightning electromagnetic transmission model in a real environment, calculate the lightning electromagnetic field propagation law from each grid point to each detection station, that is, obtain the ratio of the lightning electromagnetic field peak value at different distances and different directions in the real environment to that in the ideal situation, and obtain the attenuation factor A; Step (3), establish each 0.01 ° ×0.01 ° Database of attenuation factors A when grid points arrive at different stations; Step (4), searching from the database according to the grid point where the lightning location is given by the lightning location system to find out the attenuation factor A from the lightning location to different detection stations; Step (5), according to the result of step (4), revising the relationship between the electromagnetic field and the current peak value under the existing ideal condition; Step (6) performs inversion according to the revised content of step (5), averages the inversion results of different detection stations, and gives the average and variation range of the inverted lightning intensity.

2. The method for revising the inversion error of lightning intensity considering the influence of surface propagation according to claim 1 is characterized in that: The 3D height data is Google Earth 3D height data.

3. The method for revising the lightning intensity inversion error considering the influence of surface propagation according to claim 1 is characterized in that: Step (5) is as follows: Under ideal surface conditions, the inversion formula for lightning current intensity is: （1）； （2）； in, is the peak value of lightning current, d is the observation distance, v is the return velocity, c is the speed of light, μ0 is the magnetic permeability, E0 peak is the peak value of the electric field strength under ideal conditions, B0 peak is the peak value of magnetic induction intensity under ideal conditions; is the vacuum permittivity; According to the two-dimensional finite-difference time-domain method, a lightning electromagnetic transmission model in a real environment is established, and the lightning electric field propagation attenuation database of the area of ​​concern is calculated. The calculation formula under the above ideal situation is revised as follows: （3）； （4）； Among them, E peak is the peak value of the electric field strength in real conditions, B peak is the peak value of magnetic induction intensity under real conditions; A is the attenuation factor obtained in step (2).

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