Spatiotemporal and Structural Differences of Two Types of <scp>MCSs</scp> and Their Circulation Environments Before and After Monsoon Onset Over South China

ABSTRACT This study investigates the spatiotemporal characteristics, structural features and environmental conditions of mesoscale convective systems (MCSs) over the coastal region of South China from April to June during 2013–2017, using satellite–gauge merged precipitation, cloud‐to‐ground (CG) lightning data, radar observations and reanalysis data. All analysed MCSs generate rainfall and are classified into two categories based on lightning activity: rainfall‐only MCSs (without observed CG lightning) and lightning‐producing MCSs (rainfall with CG lightning). Their spatial structures, rainfall and lightning behaviours and associated circulation patterns are compared, with emphasis on variations before and after monsoon onset. Results show that lightning‐producing MCSs are generally larger in both horizontal and vertical extent than rainfall‐only MCSs, especially after monsoon onset, when both types become more vertically developed. Before monsoon onset, lightning‐producing MCSs are less frequent but produce stronger rainfall. After monsoon onset, their frequency increases while rainfall intensity weakens. In contrast, rainfall‐only MCSs show increases in both frequency and rainfall intensity after monsoon onset. The formation of lightning hotspots is influenced by both the frequency of lightning‐producing MCS occurrences and their internal lightning intensity, with the latter playing a more dominant role. Circulation types favourable for MCS development differ significantly. Lightning‐producing MCSs tend to occur along the western flank of the subtropical high under southwesterly low‐level winds, with convective instability peaking in the afternoon. Rainfall‐only MCSs are more common within the subtropical high under southeasterly flows, where nocturnal convergence and orographic lifting promote their development. These circulation differences are shaped by seasonal variations in moisture, thermodynamics and topography. This study provides a comprehensive understanding of MCS‐related rainfall and lightning, offering insights for forecasting and model evaluation.