Investigation On The Negative Narrow Bipolar Events Based On The Multi-station Observations

Narrow Bipolar Events(NBE)are characterized with extremely strong HF/VHF radiation,strong and short duration(10-30 μs)VLF/LF bipolar pulses radiation and possible initiating stage of the intracloud discharge,which has attracted the attention from researchers.The negative NBE,which occurs near the cloud top of thunderstorms,is closely related to the strong convective activity of thunderstorms.It can be used as an indicator for monitoring the global strong convection and severe weather,and is of great significance for understanding the energy coupling between the stratosphere and the middle layer.Therefore,it has received great attention from the academic community.In this dissertation,the multi-station GPS time synchronization lightning VLF/LF locating system,named Jianghuai Area Sferic Array,is used to compare and analyze the phenomenological characteristics of positive and negative NBE.The optical data of space-based ISUAL simultaneous observations are also used to explore the spatio-temporal relationship between negative NBE and Blue jets,and analyzes the meteorological characteristics of thunderstorms producing outbreak negative NBE.The main conclusions are summarized as follows:(1)Statistical analysis of the phenomenological and discharge parameters of positive and negative NBE in the Jianghuai area.The results show that the negative NBE tends to occur at a height of 13-20 km with an average height of 16 km.The positive NBE occurs at 7-20 km with an average height of 9.7 km.About 4%of the positive NBE occurs at the height of the negative NBE.The electric field peak statistics show that the average value of the electric field peak normalized to 100 km for the negative NBE is 12.3 V/m,which is significantly larger than the 8.7 V/m of the positive NBE.The statistical results of waveform characteristics show that the time characteristic of negative NBE is significantly smaller than that of positive NBE.The statistical results of the discharge parameters show that the average peak current moment of the negative NBE is 36.0 kA-km,which is larger than the 29.6 kA-km of the positive NBE.However,the charge moment(0.25 C-km)of the negative NBE and the positive NBE(0.27 C-km)are not much different.(2)The first observations of the negative NBE associated with Blue jet.Based on the thunderstorm data of two negative NBE recorded by Jianghuai Area Sferic Array,and compared with the space-based ISUAL synchronous optical data,it is found that the blue jet event is always associated with the negative NBE.On the time scale,the negative NBE occurs within 1ms of the blue jet event.On the spatial scale,the horizontal position between blue jet and the associated negative NBE is within the error range,and the associated negative NBE height is at the thunderstorm cloud top position.This synchronous observation confirms the negative is the initial stage of the blue jet,while the blue jet event is the optical representation of the negative NBE out of the cloud.(3)The outbreak of negative NBE is always accompanied with convective surge in the thunderstorms.By analyzing the maternal thunderstorms of three explosive nucleus NBE at different latitudes,it is found that the convective activity of the mother thunderstorms with explosive negative NBE is relatively strong.The maximum height of thunderstorm clouds is higher than the tropopause height.From the radar profile,the generation of negative NBE is always accompanied by the convective surge phenomenon of thunderstorms,and has a good consistency with the maximum height change of 15 dBZ,indicating that the sudden strong updraft rise is critical to the initiation of negative NBE.The negative NBE is more concentrated in the vicinity of the strong echo region(greater than 40 dBZ),while the positive NBE mainly occurs in the peripheral position of the strong convection nucleus(20-30 dBZ),indicating that the negative NBE has a good indication for tracking strong thunderstorms.(4)Retrieving the diurnal variation characteristics of the ionosphere D layer based on NBEs.We introduce a method retrieving the virtual reflection height of the lower ionosphere by analyzing the skywaves of one distinct type of lightning discharges called Narrow Bipolar Events(NBEs).The method is implemented toJASA for systemic monitoring of ionospheric D-layer.From the recording of two thunderstorms in 2012 of JASA,the variations of ionospheric D layer reflection height during sunrise and sunset time had been analyzed.The preliminary results show that:the height of D-layer falls linearly in a rate of 5.9km/h during sunrise,while rises linearly in a rate of 8.6km/h during sunset;During daytime,the short-time fluctuation of D-layer height is linearly correlated to the flux density of solar radiations;The diurnal variation pattern of D-layer shows a significant asymmetry.The results during the day and sunrise-sunset time suggest that electron production in the lower ionosphere is dominated by the effect of solar radiation accelerating the neutral atmospheric molecules.In combination with the technique to detect the lower ionosphere with time domain waveforms of lightning return strokes,it is potential to monitor the lower ionosphere in real-time by setting up a local network that is capable of recording lightning radiation field waveforms in a continuous manner.The research work in this paper would enrich our understanding of the phenomenological characteristics of negative NBE,reveal the close relationship between negative NBE and blue discharges near the cloud top of thunderstorm,and expound the meteorological characteristics of thunderstorm producing outbreak negative NBE,which could advance our knowledge of the initiation mechanism of negative NBE.