Why the least nocturnal convection initiation yields the most upscale convective growth over South China

Upscale convective growth (UCG) is the key process forming mesoscale convective systems that dominate warm-season heavy rainfall. A recent decade of South China events shows coastal UCG is misaligned with convection initiation (CI): it peaks at night, when CI is weakest. Nocturnal UCG reflects higher organizational efficiency, not more frequent triggering, despite daytime peak instability and CI. Both contrasts arise from the same boundary-layer wind cycle. By day, opposing sea-breeze and offshore oceanic departure winds yield a weak, narrow coastal convergence band, and northward storm motion carries systems away from it. At night, onshore oceanic departure winds converge with the land breeze, forming a stronger, broader lifting corridor, while the same southerly enhancement reorients the low-level wind maximum, making storm motion zonal and band-aligned. Enhanced coastal lifting and along-band propagation favor nocturnal UCG. Climate-sensitive land-sea thermal contrast and phase-locked oceanic wind anomalies could reshape coastal heavy-rainfall hazards beyond South China.