雷暴中双极性窄脉冲事件的位置与辐射强度

双极性窄脉冲事件（NBE）是一类特殊的大气放电现象，能产生强甚低频/低频（VLF/LF）和甚高频（VHF）辐射。为了探索NBE发生的气象环境和放电特性，选出重庆双频段闪电定位网络在一次雷暴过程中观测到的608次正极性NBE（简称正NBE）和82次负极性NBE（简称负NBE），对比发生位置和辐射强度。结果表明:正NBE主要分布于7~15 km高度处，归一化到距离辐射源100 km处的VLF/LF电场变化峰值的平均值为13.4 V·m-1，平均VHF辐射功率为73.5 kW。负NBE主要发生在两个高度范围，72例负NBE分布于16~20 km高度，它们倾向于发生在30~35 dBZ回波顶高大于18 km的对流云顶及附近，其平均归一化VLF/LF电场变化峰值为42.7 V·m-1，平均VHF辐射功率为76.9 kW。10例负NBE分布于4~8 km高度，全部发生于对流核内部。其平均归一化的VLF/LF电场变化峰值为2.7 V·m-1，平均VHF辐射功率为18.2 kW。从统计结果看，在VLF/LF频段，上部负NBE的辐射强度普遍强于正NBE和下部负NBE；在VHF频段，上部负NBE的辐射强度与正NBE基本相当，大于下部负NBE；下部负NBE在两个频段的辐射通常弱于正NBE。

Locations and Radiation Strength of Narrow Bipolar Pulses in a Thunderstorm

Narrow bipolar pulses (NBE) are special flashes in thunderstorms which are different from regular in-cloud discharges and cloud-to-ground discharges. They can produce intense radiation in both VLF/LF and VHF bands. To explore the meteorological environment and discharge characteristics of NBE, locations and radiation strength of 608 positive NBE and 82 negative NBE detected in a thunderstorm day are analyzed using the dual band 3D lightning locating system in Chongqing. Results show that positive NBE occur at the altitude of 7-15 km, with the average altitude of 10.0 km. According to the radar reflectivity of positive NBE, they can be divided into three groups. 49 positive NBE, which occur in the thunderstorm cores (reflectivity), are categorized as Group Ⅰ. 350 NBE occurring in regions outside cores with the reflectivity higher than 5 dBZ are categorized as Group Ⅱ. The rest 209 positive NBE are Group Ⅲ. The radiation strength of these three groups are in descending order on both bands. The mean value of all positive NBE VLF/LF electric field change peaks normalized to 100 km is 13.4 V·m-1. The mean value of their VHF radiant powers is 73.5 kW. Negative NBE are generally produced in two regions in the thunderstorm. Among 82 negative NBE, 72 of them occur at the altitude of 16-20 km, and the average altitude is 18.0 km. They occur on or beside 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-1. The mean value of the VHF radiant powers is 76.9 kW. 10 negative NBE occur at the altitude of 4-10 km, whose average altitude is 6.0 km. They all occur in thunderstorm cores. The mean value of VLF/LF electric field change peaks normalized to 100 km is 2.7 V·m-1. The mean value of VHF radiant powers is 18.2 kW. According to statistical results, the radiation strength of the upper negative NBE is mostly stronger than those of positive NBE and the lower negative NBE on VLF/LF band. In VHF band, values are similar, both of which are stronger than the lower negative NBE. The radiation strength of the lower negative NBE is weaker than that of positive NBE in both bands.