基于不间断闪电波形采集的一次皖北雷暴负地闪特征观测

Some Properties of Negative Cloud-to-Ground Flashes over North Anhui Based on Continuous Electromagnetic Field Observations

利用具有不间断信号记录能力系统记录的闪电波形资料,借助人工识别判定各次地闪的回击过程,研究了2012年7月9日发生在安徽淮北地区一次夏季雷暴过程的连续负地闪活动特征。该次雷暴3141次负地闪中,单回击地闪占15.5%,最大回击次数为18次,每次地闪平均回击次数为4.2次;在整个雷暴期间,单回击地闪比例和每次地闪平均回击次数存在显著的大小变化。负地闪过程平均持续时间为363.7 ms,相邻回击间隔时间的几何平均为75.3 ms,雷暴成熟期的回击间隔明显小于其它阶段。总体上后继回击强度远弱于首次回击,10031例后继回击与首次回击峰值强度之比几何平均为0.49,但是强于首次回击的后继回击过程并不罕见,34.8%的多回击地闪至少有一次后继回击强度大于首次回击,全部后继回击中有19.7%的强度超过首次回击。后继回击强度和回击间隔呈明显的随回击序号的系统性变化特征。约有38.6%的负地闪伴随可辨的双极性脉冲活动,该脉冲活动与首次回击之间的时间间隔几何平均为23.2 ms。

By analyzing wideband electromagnetic field waveform records obtained by the system capable of recording electric and magnetic field of distant lightning discharges in a continuous manner,overall 3141 negative cloud-to-ground flashes were identified through manual selection for a thunderstorm occurred in Anhui,Huaibei Area on 9 July 2012,among which 15. 5% contained only single stroke. The maximum number of strokes per flash was 18 and the average number of strokes per flash was 4. 2. The percentage of single-stroke flash and the mean number of strokes per flash showed significant variations during the storm life cycle. The average duration of the negative flashes was 363. 7 ms. 10031 inter-stroke intervals gave a geometric mean value of 75. 3 ms. It seemed that CG flashes during active stage had slightly shorter intervals than other stages. The amplitude ratio of subsequent stroke to the first stroke presented a geometric mean of 0. 49 for 10031 subsequent strokes,confirming that subsequent strokes were generally much weaker than first return stroke. However,the cases of amplitude of subsequent strokes greater than which of first return strokes is not rare. Our data indicated that about 34. 8% of multiple-stroke flashes contained at least one subsequent stroke whose amplitude is greater than that of the first stroke and 19. 7% of all subsequent strokes whose amplitude is greater than first return strokes. Both the relative amplitude and the preceding time interval of subsequent strokes showed a significant dependence on the stroke order. About 38. 6% of negative cloud-to-ground flashes were observed to associate with discernible bipolar pulse activities of preliminary breakdown,with a geometric mean value of 2. 3 ms preceding the first strokes.