Urban Enhancement of Organized Convective Systems and Extreme Rainfall Along the South China Coast Under Distinct Synoptic Patterns

Abstract Rapid urbanization significantly modulates extreme rainfall over the Guangdong‐Hong Kong‐Macao Greater Bay Area (GBA) along the South China coast. Based on 10 years of surface rainfall observations and radar mosaics, this study identifies the urban‐mountain interface in the northeastern GBA as the primary hotspot for extreme short‐term heavy rainfall events (76 cases) and tracks the associated organized convective systems (OCSs). This urban hotspot is governed by two dominant synoptic patterns: the southwesterly monsoon pattern (P1) and the low‐level vortex pattern (P2). While both primarily involve propagating and local OCSs, the local type predominates (more than 55%). These OCSs exhibit distinct organizational characteristics. Compared to P1, local OCSs in P2 appear as smaller, slower, and more locally confined features. Notably, the urban modulation is synoptic‐pattern‐dependent, yet it consistently enhances local OCSs as they pass the urban‐mountain interface. Local OCSs in P1 display a coherent single‐peak expansion, resulting from the temporal alignment between the intensification of inbound convection and the development of urban‐initiated cells. Conversely, those in P2 exhibit a double‐peak structure, reflecting a separation between the rapid intensification of incoming convection near the urban boundary and the later incubation of new cells within the city. Despite these evolutionary divergences, the peak rainfall consistently occurs approximately 3 hr after urban entry or initiation, indicating a pronounced urban amplification of rainfall.