Spatial and Temporal Characteristics of Mesoscale Convective Systems in China's Three Major Urban Agglomerations and the Urban Impact on Them

Mesoscale convective systems(MCSs) can generate severe disasters, including extreme precipitation, hail, flooding, thunderstorms, and strong winds, and are significantly influenced by weather circulation and local geographical conditions. For MCSs occurring in urban areas, urban forcing plays a crucial role in regulating their activity, though the associated impacts are highly complex. Utilizing 15 years of high-resolution radar network products from the China Meteorological Administration, this study identifies and tracks MCSs across three major urban agglomerations, analyzing their spatiotemporal distribution characteristics. It is observed that MCSs in the Pearl River Delta predominantly occur from April to June, in the Yangtze River Delta from May to July, and in the Beijing-Tianjin-Hebei region from June to August. In the Yangtze River Delta and Beijing-Tianjin-Hebei regions, MCSs are more frequently initiated at night (20:00–08:00 Beijing Time), whereas in the Pearl River Delta, they are more commonly initiated during the day (08:00–20:00 Beijing Time). Moreover, the spatial distribution patterns and movement directions of MCSs in these three major urban agglomerations exhibit distinct variations and differences between the cold half-year (October–March) and warm half-year (April–September), as well as between daytime and nighttime. To assess urban impacts, MCSs traversing urban areas were further selected to analyze the spatiotemporal distribution characteristics of their trajectory points and changes before and after crossing cities. It is evident that trajectory points within Pearl River Delta cities exhibit higher numbers, larger areas, and greater intensity during daytime. In contrast, trajectory points within Yangtze River Delta cities show higher numbers but smaller areas and lower intensity during daytime. Trajectory points within Beijing-Tianjin-Hebei cities demonstrate higher numbers, smaller areas, and lower intensity during nighttime. Additionally, significant changes in trajectory point characteristics were observed before and after urban crossing. For instance, trajectory points in the Pearl River Delta and Beijing-Tianjin-Hebei regions expanded in area upon entering cities, whereas those in the Yangtze River Delta contracted. Finally, correlation coefficients were used to identify relationships between various environmental and urbanization factors and the characteristics of MCSs traversing urban clusters.