Urban morphology modulates thunderstorm process and associatied cloud-to-ground lightning activity over Beijing metropolitan region

Abstract. The effect of urban barriers may have a significant impact on the patterns of thunderstorm processes and lightning activity, but there is still a lack of comprehensive mechanical explanations. The observational analysis carried out in this study found that cloud-to-ground (CG) lighting activity tends to cluster around the outer boundaries of the mega cities, while, on the opposite, CG gathers within the small-sized city. When a squall line originating from a type of mesoscale convective system (MCS) known as '0713' passed through the built-up area, the barrier effect of the rough underlying surface contributed to the separation of the cold pool. This led to weakening of vertical airflow and breaking of the convergence line, ultimately triggering the bifurcation of the thunderstorm. Simulation results complement these observations. When buildings outside the 5 Ring Road (RR) are replaced with bare soil, the separation of the cold pool is minimized. Furthermore, the density of the buildings also influenced the strength of the barrier effect. Therefore, the specific urban morphologies were identified as a critical factor in modulating cloud-to-ground (CG) lightning activity and the organization process of thunderstorms. This study offers a fundamental foundation and technical support for predicting and assessing urban cloud-to-ground (CG) lightning risks. It holds significant implications for understanding excess urban warming, its prediction and assessment, and the resulting thermal risk, influenced by factors such as ventilation, sea breezes, and the geophysical environment in coastal cities.