Charge structure of an isolated thunderstorm on the Tibetan Plateau and the formation of bolt-from-the-blue lightning

<p indent="0mm">The Qinghai-Tibetan Plateau is the highest in the world, and it is located in an area jointly influenced by the Indian monsoon and westerly winds. With its unique thermal and dynamic effects and complex topographical features, thunderstorms and convective activities occur frequently in the summer over most of the Plateau. The thunderstorm days in the central and eastern Plateau are similar to that of the southeastern coastal region with the most thunderstorm processes and lightning activities in China. However, the life cycle of thunderstorms over the Plateau is usually shorter, and their convective intensity is weaker, compared with those over prominently lower altitude regions. Thunderstorms over the Plateau not only show unique convective structures, but their electrical characteristics also differ significantly from those of thunderstorms over plain areas. Due to the difficulty of observation in the Plateau regions and the limitations in lightning detection technologies, it remains a challenge to understand the intercloud charge structure and lightning initiation of the thunderstorms over the main body of the Plateau region. By utilizing very high frequency (VHF) broadband interferometer lightning mapping technique, weather radar, and radio sounding, a thunderstorm process that produced two rare “bold-from-the-blue” flashes was observed in the Lhasa area located in the central Plateau. The mechanism of the “bold-from-the-blue” flashes was revealed through accurate mapping of the discharge processes in high spatial and temporal resolution, and the corresponding charge structure characteristics inside the thunderstorm were also retrieved. At the initial stage of the thunderstorm, it exhibited a negative dipole charge structure with an upper negative and bottom positive pattern, which is completely different from the evolution of charge structure in the thunderstorms over prominently lower altitude regions. When the thunderstorm developed into its mature stage, it exhibited a tripolar charge structure stacked with positive, negative, and positive charge regions from the top to bottom of the thunderstorm. These regions corresponded to environmental temperature zones below –30°C, from –30°C to –15°C, and above –10°C, respectively. The “bold-from-the-blue” flashes were associated with the upper dipole. The discharge originally initiated between the central negative and the upper positive charge region, with the positive leader developing downward to the lower negative charge region and the negative leader upward to the upper positive charge region. When the upward negative leader reached the upper positive charge region, it propagated horizontally and got out from the body of the small storm cell. The discharge ultimately became the “bold-from-the-blue” discharge process, spreading away from the cloud body into the clear sky area and striking the ground. The grounding points of the two “bold-from-the-blue” flashes were 3.6 and <sc>3.8 km</sc> from the precipitation edge of the storm, respectively. The horizontal scale of the thunderstorm cell was relatively small with positive charge regions in both the upper and lower parts of the cloud. The unbalanced upper dipole in the smaller storm body, where the upper positive charge region was weaker than the lower negative charge region, was the main reason for triggering the “bolt-from-the-blue” flashes. This study not only sheds light on the mechanism of the rare “bold-from-the-blue” discharge process, but also provides direct observational evidence on the diversity of intercloud charge structure and lightning activity characteristics inside the thunderstorm over high-altitude regions.