Locations And Radiation Characteristics Of Narrow Bipolar Events In A Thunderstorm Based On The Observation Data Of Dual Bands

Narrow Bipolar Events(NBE)are special flashes in the thunderstorms that are different from regular in-cloud discharges and cloud-to-ground discharges.They can produce intense radiation on both VLF/LF and VHF bands.To explore the meteorological environment and discharge characteristics of NBE,we analyze the locations and radiation strength of 608 positive NBE and 82 negative NBE detected in a thunderstorm day by the dual bands 3D lightning locating system in Chongqing.The results show that positive NBE occur at around 7~15 km altitude between main negative charge layer and upper positive charge layer,of which the average altitude is 10.0 km.According to the radar reflectivities of positive NBE,they can be divided into three groups.49 positive NBE,which occur in the thunderstorm cores(reflectivies≥30 dBz),are GroupⅠ.350 are GroupⅡ.They occur on the regions outside the cores and meanwhile the relectivities are higher than 5 dBz.The rest 209 positive NBE are GroupⅢ.The radiation strength of these three groups are in descending order on both bands.There is no correlation between the isolation of positive NBE and its position in radar echo,but the positive NBE that initiate other lightning all occur below 10.5 km altitude.The mean value of all the positive NBE’ VLF/LF electric field change peaks normalized to 100 km is 13.4 V/m.The mean value of their VHF radiant powers is 73.5 kW.According to the mean value,the positive NBE in the convection core region are slightly stronger than the positive NBE outside the core region on dual bands.Negative NBE are generally produced on two regions in the thunderstorm.Among the 82 negative NBE,72 of them occur between upper positive charge layer and negative screening charge at the cloud top.They are at 16~20 km altitude,of which the average altitude is 18.0 km.They occur on or beside the tops of thunderstorms with 30~35 dBz echo heights higher than 18 km.The mean value of their VLF/LF electric field change peaks normalized to 100 km is 42.7 V/m.The mean value of the VHF radiant powers is 76.9 kW.10 negative NBE occur at 4~10 km altitude between main negative charge layer and lower positive charge layer,whose average altitude is 6.0 km.They all occur in the thunderstorm cores.The mean value of the VLF/LF electric field change peaks normalized to 100 km is 2.7 V/m.The mean value of the VHF radiant powers is 18.2 kW.According to the statistical results,the radiation strength of the upper negative NBE is mostly stronger than the positive NBE’ and the lower negative NBE’ on VLF/LF band.On VHF band the values are similar,both of which are stronger than the lower negative NBE’.The radiation strength of the lower negative NBE is weaker than positive NBE’ on both bands.The discharge processes of two positive NBE are modeled by the MTLE model.The length of each discharge channel is 422 m and 328 m.The breakdown velocity of each current is-44 m/μs and-40 m/μs.The peak current of each discharge channel is-40.3 kA and-55.5 kA.The charge moment of each process is 27.0 C? m and-40 C?m.