A study on lightning discharge precursor by taking advantages of simple structure and observation easiness of Tibetan thunderstorms

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We have performed a lightning observation campaign in Lhasa city, Tibet plateau region of China in the summers of 2005 and 2006. We found1. Almost all Tibetan lightning occurred from late after noon to mid night. Thunderstorms differ greatly in lightning activity with some thunderstorms producing 20 flashes within one minute and some only a few even during their whole life period.2. Many of the thunderstorms exhibit positive dominant electric fields at the ground and tend to produce smaller ratio of cloud-to-ground (CG) discharges. Among the CG discharges recorded, nearly 90% of them are negative ones. In general summer thunderstorms, the favorite condition for triggering a CG discharge may be the presence of the opposite polarity of pocket charge beneath a main charge region. For Tibeuan thunderstorms, a negative pocket charge may not exist just beneath the main positive charge which produced the dominant positive electric fields. This may account for the small number of CG discharges.3. The lightning discharges are found to be initiated mainly at two heights : one is roughly 2 km and the other is 4 km above the ground. These results strongly suggest that the initial dominant leader of an intracloud discharge could be either a negative leader or a positive leader depending on its initial height. The large microsecond scale pulses usually observed at the initial stage of an intracloud discharge appeared to be generated when a negative leader is propagating within a positive charge region.4. The CG ratio differs much during different stages of thunderstorms which suggest that favorite electrical charge structures for producing CG exist.