Abstract
We have been designing and developing Broadband Observation network for Lightning and Thunderstorm (BOLT) which locates radiation sources in 3D associated with lightning discharges. The BOLT consists of four or more LF receivers which detect electromagnetic (EM) waves in a wide frequency range from 5kHz to 500kHz. We have been operating the BOLT in Kansai, Japan, since Oct. 2012, for locating preliminary breakdowns (PB) followed by lightning discharges. We define PB from its pulse width and interpulse duration. In addition, we classified PB pulses into +PB and -PB, depending on initial polarity of PB pulses in the atmospheric electricity sign convention. In this paper, we identified 342 +PB pulses and 139 .PB pulses. We found that most of the -PB pulses occurred about 2.5 km higher than +PB. From a comparison of radar reflectivity factor it is found that most of the +PB and .PB pulses, respectively, occurred in the lower part or below the high reflectivity regions, and in the upper part or above the high reflectivity region. These results indicate that the difference of the polarity of PB pulses is determined by the polarity of charge region around locations of PB pulses.
