Observational Analysis of Convective Evolution During a Cloudburst-Type Extreme Precipitation Event in Shanghai Under Weak Vertical Wind Shear

Under climate warming, frequent short-duration extreme precipitation events in coastal megacities exacerbate urban waterlogging, whereas the associated convective mechanisms over complex underlying surfaces remain poorly understood. On 21 July 2023, an extreme short-duration rainfall event (14:00–19:00 LST, peak intensity 127.3 mm h−1) struck Shanghai under weak vertical wind shear (VWS) conditions that cannot be fully explained by classic storm dynamics. Based on multi-source observations, this study shows that the middle and lower troposphere was controlled by warm, moist southwesterly flows, with highly favorable thermodynamic conditions (CAPE ~3300 J kg−1, CIN near zero) that only required weak local lifting to trigger convection. Both 0–1 km and 0–6 km VWS were below 7 m s−1, maintaining stable, upright updrafts that favored high precipitation efficiency. The formation and maintenance of the quasi-linear convective system and the resultant extreme precipitation depended critically on the southerly sea breeze, local mesoscale convergence, and cold pool feedback. Convergence induced by the complex underlying surface (urban friction, high-rise building blocking) played important roles in initiating convective cells, while the interaction between cold pool outflows and the sea breeze from the East China Sea and Hangzhou Bay sustained the system, which evolved into a unique “fish-shaped” rainstorm. Driven by dominant convective propagation toward unstable inland areas, the system moved west–southwestward across the coastal zone into central urban Shanghai. This mechanism differs from both the cold pool–VWS balance under strong shear and the urban convective relay propagation mode under weak VWS documented in previous studies. These findings provide new observational insights into the formation and maintenance of weak-shear, short-duration extreme rainfall in coastal megacities, and carry important implications for identifying convectively prone zones, optimizing spatial development patterns, and improving climate-resilient land management and urban planning practices.