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ABSTRACT
The charge distributions in a thundercloud play an important role in the initiation and propagation of lightning discharges. To further understand the effects of charge distributions on lightning discharge, the authors conducted a very high-frequency (VHF) lightning observation campaign during the 2006/07 monsoon in Darwin, Australia, using a VHF broadband digital interferometer (DITF). A C-band polarimetric weather radar to estimate the precipitation profiles such as hydrometeor classification was operated by the Bureau of Meteorology (BOM) Research Centre. Cloud-to-ground (CG) and intracloud (IC) flashes were initiated from the outer and the inner parts of the upper side of the graupel regions, respectively. In the cases of CG flashes, the negative leaders travel first about 10 km horizontally through positive charge regions and then begin to bend toward the ground when they reach the edge of the positive charge regions where there is no graupel region underneath. In contrast, in the cases of the IC flashes the negatively charged graupel regions block the downward developments of negative leaders. It is noted that positive charge regions could facilitate the extension of the horizontal negative leader. These results may suggest that lightning flash types are closely dependent on their initiation locations and the surrounding charge distributions. The experimental results are consistent with other previous observation results and charge model simulations.
1. Introduction
Cloud-to-ground (CG) flashes with lightning channels horizontally longer than 10 km before the return stroke (RS) have been reported in the literature (e.g., Fuquay 1982). These types of flashes have preceding long intracloud (IC) development (e.g., Nag and Rakov 2009). They have been observed in both summer (Rust 1986; Rison et al. 1999) and winter thunderstorm seasons (Takeuti et al. 1978). Previous studies indicate that propagation characteristics of lightning channels, especially their direction of propagation, have a relationship with the local charge structure (Williams et al. 1989; Mazur et al. 1995; Mazur and Ruhnke 1998; Wiens et al. 2005).
A lightning flash is thought to be initiated by a bidirectional leader that develops into either a cloud-to-ground or an intracloud flash from the regions of maximum electric field (E field) (e.g., Mazur 1989). Mazur and Ruhnke (1998) show with numerical solutions of a cloud charge model that the lightning current continues to flow both when...