Urban Modulation of Cloud-to-Ground Lightning Activity in a Megacity Revealed by Multi-Source Observations

Urbanization can modify the near-surface environment for thunderstorm development, but how megacity surfaces affect the spatial distribution of cloud-to-ground (CG) lightning within urban areas remains unclear. Taking Beijing as a representative megacity, this study uses multi-source observations, including CG lightning flashes, automatic weather station (AWS) observations, radar data, reanalysis, urban land-use classification data, and DEM data, to examine the spatiotemporal characteristics of CG lightning and their relationships with the urban thermal–dynamic environment, synoptic background, and thunderstorm evolution. The results show that 304 thunderstorms passed over or affected the Beijing built-up area, producing 6.96 × 104 CG flashes within the analysis domain. CG lightning exhibited clear interannual variability and reached its diurnal peak from late afternoon to early evening. However, high-density CG flash centers did not persistently occur over the urban core, but were more frequently located near the built-up edge and adjacent transition zones. Urban heat island intensity (UHII) partly corresponded to the diurnal variation in CG lightning, but its interannual and spatial relationships with CG activity were weak. Further analysis suggests that, under a weak background, higher pre-storm UHII and lower wind speed (WS) may help to maintain local convergence and upward motion, which may partly contribute to increased CG lightning within the urban core. In contrast, under the strong synoptic background, bifurcated thunderstorms were associated with lower pre-storm UHII, higher pre-storm WS, and CG lightning concentrated near the urban edge. These findings improve our understanding of how megacity surfaces may modulate thunderstorm-related CG lightning activity and provide observational context for spatially differentiated lightning monitoring within urban areas.