## 6 Influence of Urban Dynamic Effect on Thunderstorm Processes and Lightning Activity  
In addition to the urban air pollution and thermal effect, scholars have also recognized the driving force of the urban dynamic effect on lightning activity [8; 27; 95]. Compared to natural underlying surfaces, the roughness of urban surfaces is significant. Urban buildings have a strong drag effect on the horizontal flow field, and the wind field converges, rises, and flows around in the windward direction of the city [96]. [PERSON] et al. [24] posited that the friction of the urban underlying surface increases the momentum flux in the urban near-surface layer, leading to enhanced atmospheric convergence, which contributes to the development of thunderstorm systems. However, compared to the urban thermal effect, the vertical motion intensity when urban airflow \"climbs\" is minimal [97], which is insufficient to trigger convective activities. As shown in Figure 8, the urban barrier effect refers to the phenomenon where the amount of lightning or precipitation in the center of the city is significantly lower than that in the periphery of the city. This effect can lead to the bifurcation of thunderstorms, which move around cities [27; 98], with the peak area of the lightning and rainfall generated by the thunderstorm system concentrated in the periphery of cities [28; 30]. [PERSON] et al. [99] analyzed 91 thunderstorm systems in Indianapolis from the summer of 2000-2009 and found that thunderstorms with large horizontal scales split over the city and merge downstream of the urban area. [PERSON] et al. [100] posited that the increase in the buildings in the urban area of Beijing may break the squall line into convective cells and promote the bifurcation of rainfall clouds, but the barrier effect of the urban canopy is less than its thermal effect on strong convection. [PERSON] et al. [6] conducted a systematic review of the research progress on the impact of the urban barrier effect on strong convection processes, concluding that current research on the barrier effect is not yet comprehensive. Currently, scholars have explored the impact of the urban barrier effect on thunderstorms from a climatological perspective [29; 30; 101]. During weak UHI periods, summer thunderstorms with strong synoptic backgrounds bifurcate and detour through the city [30], and the flash density shows a low value trend in the city center [28]. The synoptic backgrounds of thunderstorms in Beijing are classified into weak synoptic backgrounds and strong synoptic backgrounds (Figure 9). After statistical analysis, it was found that over 60% of CG flashes in built-up areas were produced by the strong weather system, which was also an important cause of the occurrence of the barrier effect [102]. So, a strong synoptic background may be an important condition for the barrier effect on the spatial patterns of a long series of CG lightning in the built-up areas in Beijing. Additionally, some scholars believe that when the wind speed near the surface exceeds 4-6 m/s, the urban barrier effect is significant [29]. However, some scholars have observed the opposite phenomenon [26], where thunderstorm systems under weak synoptic conditions also bifurcate at the junctions of urban and rural areas. Therefore, the synoptic hypothesis proposed by previous studies cannot fully explain the formation mechanism of the barrier effect. The characteristics of urban morphology include the size of the city and the spatial configuration of urban building groups. The city size is considered to be an important factor affecting thunderstorm activities. [PERSON] et al. [34] used radar echo data to study the impact of four cities in the Central Plains of the United States on thunderstorm frequency and intensity and found that larger cities may increase the incidence and intensity of downwind thunderstorms in such cities. Additionally, the spatial configuration of urban agglomerations also has a tendency to alter thunderstorm activities.