image: A lightning flash stroked at the Central Radio and Television tower on 7 July 2017, photographed by Kuirun Zhong.
Severe thunderstorms often produce lightning, heavy precipitation, hails, and wind gusts, and cause significant economic losses and casualties in the region they passed through. So, their prediction is one of the primary concerns of not only weather forecasters but also local government and public. However, the accurate forecasting on severe thunderstorm is still a great challenge because of its complicated mechanism of formation and enhancement.
Beijing metropolitan region is one of the most developed areas in China and is also influenced greatly by severe thunderstorms. The complex terrain, dynamic and thermal characteristics of Beijing and its surrounding areas, as well as the multi-process and multi-scale interactions inside the thundercloud, make the development and propagation of the lightning weather system in Beijing very complicated.
"Efforts to understand the effects of complex thermal dynamics and microphysical characteristics on lightning, five consecutive years of collaborative observation experiments on severe convective thunderstorms were conducted in Beijing", said the fist author Xiushu Qie from the Institute of Atmospheric Physics, Chinese Academy of Sciences. "The primary detection equipment contains a 3D lightning location system - Beijing Lightning Network (BLNET), two X-band dual linear polarization Doppler radars and four laser raindrop spectrometers, etc. The data from the mesoscale meteorological observation network in Beijing operated by China Meteorological Administration was also utilized."
The researchers stated: "The lightning location data from the BLNET has the advantages of the 3D location of both intracloud and cloud-to-ground lightning with high-precision and being free from the terrain influences, providing new insights into the structure of the severe thunderstorms, especially the small-scale storm cells."
The researchers found that squall lines and multi-cell thunderstorms are the two main types of lightning disaster weather systems affecting Beijing. Lightning weather systems often generate and develop in the western and northern mountainous areas. The heat island effect and boundary layer characteristics of Beijing urban areas play important roles in the propagation and enhancement of thunderstorms passing over. The highest lightning density regions are in the east of Changping, the middle and east of Shunyi District, and the central city of Beijing. The lightning frequency of very strong thunderstorms can produce hundreds of lightning flashes per minute in the central Beijing.
Analyzing the lightning in thundercloud, the authors found that lightning mostly occurs in areas of strong convection and transition areas about 10 km away from the edge of strong convection, where the strong updraft or wind shift distributed. The growth and collision of ice-phase particles not only enhances the lightning process in the cloud, but also facilitates the generation of hailstones. The jump increase in lightning frequency can early warn the occurrence of hailfall and short-term heavy precipitation. The early warning time can reach nearly half an hour.
"By establishing the relationship between lightning frequency and the model parameters, such as water vapor mixing ratio, ice phase particle content, updraft velocity and radar reflection factor, lightning data can be assimilated into numerical weather prediction models at the cloud-resolved scale", the authors said. "Lightning data assimilation can significantly improve the forecast of severe convection and heavy precipitation".