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 NBEproducing 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.
