Altitude and polarity transitions of corona activity in thundercloud tops reaching the lower stratosphere

Abstract Thunderclouds occasionally penetrate into the stratosphere, forcing the exchange of air masses between the troposphere and the stratosphere. Observations from space have shown that blue corona discharges are often generated in such thundercloud tops and are the optical manifestation of so-called Narrow Bipolar Events (NBEs) observed in radio signals. They come with two polarities, the positive that brings net negative charge aloft, and the negative that brings negative charge towards the earth. While their production appears to depend on convection, the cause and nature of such discharges are not well known. Here we report the observations by a lightning detection array of unusual amounts of NBEs during a tropical storm on the coastline of China. NBEs of negative polarity are predominantly observed at the top of the cloud reaching the tropopause and into the stratosphere, and positive NBEs are primarily at lower altitudes. We find that either positive or negative polarities dominate at a time, shifting between the two with a ~30 min period. The period is close to the typical time of development of thunderstorm cells, suggesting that the polarity depends on the phase of the storm cell. Furthermore, we find that the lightning jump of negative NBEs is associated with the occurrence of above-anvil cirrus plumes (AACPs) of ice crystals and water vapor in the lower stratosphere resulting from the strong updraft. We propose that variations in updrafts induce changes in the altitude and charge concentrations of the cloud layers, which lead to the polarity transition. Our results have implications for studies of the chemical perturbations of greenhouse gas concentrations at the tropopause by corona discharges at the tropopause.