A team of researchers from the University of Science and Technology of China (USTC), led by Professors LEI Jiuhou, ZHU Baoyou, and Associate Professor LIU Feifan, has made significant strides in understanding the mechanisms behind corona discharges at thunderstorm cloud tops, a phenomenon that plays a critical role in the Earth's atmospheric chemistry. Their findings, published in the prestigious journal Nature Communications on August 26, introduce a new conceptual model that could reshape our understanding of these high-altitude electrical discharges.

Corona discharges, often manifesting as blue flashes near the tops of thunderstorms that penetrate into the stratosphere, are a key element in the transfer of energy and materials from the troposphere to the higher layers of the atmosphere. These discharges, particularly narrow bipolar events (NBEs), can influence the concentrations of greenhouse gases such as nitrogen oxides and ozone in the stratosphere, thereby impacting the Earth's radiation balance.

Schematic diagram of large-scale low ionospheric disturbance induced by cloud top discharge (Image by USTC).

Traditionally, it was believed that cloud-top discharges were caused by imbalances in cloud charge distribution triggered by conventional lightning. However, due to the observational challenges posed by cloud cover and Rayleigh scattering, the exact initiation mechanisms of these events have remained elusive, attracting significant interest from the scientific community.

Utilizing an advanced ground-based lightning detection array, the research team observed NBEs during a typhoon on the Chinese coastline, uncovering a robust polarity competition between different NBE types at the cloud top. The findings indicate that positive NBEs occur predominantly during the convective uplift phase at the overshooting top of the cloud, whereas negative NBEs prevail during the convective downdraft phase, typically associated with cirrus plumes in the lower stratosphere. This observation led to the development of a new model suggesting that the intensity of convection modulates the altitude of charged layers within the cloud, which in turn governs the occurrence of cloud-top discharges.

A new conceptual model of cloud top discharge induced mechanism dominated by convective degree (Image by USTC).

These findings clarify the mechanisms of cloud-top discharges and their effects on stratospheric chemistry, setting the stage for more detailed studies on the broader role of thunderstorms in atmospheric processes.

Paper link: https://www.nature.com/articles/s41467-024-51705-y

(Written by CHEN Yehong, Edited by WU Yuyang, USTC News Center)