Baoyou Zhu

Numerical study on relationship between lightning types and distribution of space charge and electric potential

The favorable factors that affect the initiation of intracloud (IC) lightning and cloud-to-ground (CG) flashes are investigated by using a fine resolution two-dimensional lightning model. Simulation results indicate that potential at initiation point is a key to decide whether downward leader reaches ground. The absolute values of initiation potential of CG flashes are greater than 30 MV, while the absolute values of initiation potential of IC lightning are basically less than 30 MV. Since potential field is determined by space charge distributions, polarities and types of lightning discharges are also dependent on relative locations and magnitudes of positive and negative charge zones near initiation points. In general, dipoles (positive charge above negative) initiate normal IC lightning and positive CG flashes and inverted dipoles are associated with inverted IC lightning and negative CG flashes. The magnitude of upper charge near initiation point is generally larger than that of lower charge when CG flashes occur, and the two are comparable when IC lightning occur. For CG flashes, the magnitude of lower charge near initiation points should be enough for initiation breakdown, but not so strong that the lower potential well prevents propagation to ground. In addition, CG flashes are initiated within various special charge distributions, only if the reference potentials at initiation point are far away from 0 MV.

Observations of narrow bipolar events during two thunderstorms in Northeast China

Here we report characteristics of more than 100 cases of Narrow Bipolar Events (NBEs) occurred during two thunderstorms on the basis of our analysis of the dataset collected by a GPS based three-station time of arrival (TOA) lightning location system deployed in Northeast China during the summer of 2010. All of the NBEs were of positive polarity and the amplitude of VLF/LF initial pulse of NBE was comparable to that of return strokes. The waveform characteristics of 166 NBEs that occurred within 150 km were consistent with the results of other reports. The average value of initial peak width of the bipolar pulse is 7.8±1.5 μs, full width at half maximum is 4.6±1.0 μs, and the ratio of initial peak amplitude to overshoot peak is 2.1±0.6. The inferred source height of NBEs occurred in two thunderstorms averaged 9.6 and 7.4 km above mean sea level, whereas the corresponding virtual ionosphere heights at two different times were 89 and 78 km, respectively. In NBE-producing thunderstorms, NBEs tended to burst during the active stage of normal lightning; nevertheless, no determinate quantitative relations could be established between them. In Storm 0711, NBEs were inclined to cluster at the proximity of particular convective cores with high radar reflectivity, and NBEs tended to move consistently at the front area of those NBE-producing convective cores during the evolution of thunderstorm.

An improved ray theory and transfer matrix method‐based model for lightning electromagnetic pulses propagating in Earth‐ionosphere waveguide and its applications

An improved ray theory and transfer matrix method-based model for a lightning electromagnetic pulse (LEMP) propagating in Earth-ionosphere waveguide (EIWG) is proposed and tested. The model involves the presentation of a lightning source, parameterization of the lower ionosphere, derivation of a transfer function representing all effects of EIWG on LEMP sky wave, and determination of attenuation mode of the LEMP ground wave. The lightning source is simplified as an electric point dipole standing on Earth surface with finite conductance. The transfer function for the sky wave is derived based on ray theory and transfer matrix method. The attenuation mode for the ground wave is solved from Focks diffraction equations. The model is then applied to several lightning sferics observed in central China during day and night times within 1000 km. The results show that the model can precisely predict the time domain sky wave for all these observed lightning sferics. Both simulations and observations show that the lightning sferics in nighttime has a more complicated waveform than in daytime. Particularly, when a LEMP propagates from east to west (Φ = 270°) and in nighttime, its sky wave tends to be a double-peak waveform (dispersed sky wave) rather than a single peak one. Such a dispersed sky wave in nighttime may be attributed to the magneto-ionic splitting phenomenon in the lower ionosphere. The model provides us an efficient way for retrieving the electron density profile of the lower ionosphere and hence to monitor its spatial and temporal variations via lightning sferics.

Simultaneous observations of electric field changes, wideband magnetic field pulses, and VHF emissions associated with K processes in lightning discharges

We studied simultaneous electric field changes, microsecond-scale (VLF/LF) magnetic field pulses, and VHF emissions associated with K processes in 37 cloud and 54 cloud-to-ground lightning flashes ...

Study of earth-ionosphere waveguide effect on lightning pulse with Ray Theory

In this paper we present an half analytic electromagnetic wave model for the study of a lightning-produced VLF/LF pulse propagating in the earth-ionosphere waveguide (EIWG). The property of the lightning pulse propagating in the ionospheric D region is retrieved in frequency domain by using the Transfer Matrix Method coupled with presumed electron density and electron-collision rate profiles. With the retrieved reflection coefficients, a Ray Theory based transfer function from the lightning source to the receiver is derived. The propagation attenuation through spherical ground with presumed conductance is also calculated. The model is applied to the data from a lightning detection network in central China. The simulation results match perfectly with the ground waves and sferics observed by the network, indicating that the model is an efficient tool to investigate the interaction of the lightning pulse with the ionospheric D region and the earths magnetic field.