On Nocturnal Extreme Rainfall Associated With Low‐Level Rotation in the Coastal Urban Agglomeration of South China: A Case Study

Abstract Influenced by monsoonal winds and complex topography, the megacity cluster of the Guangdong‐Hong Kong‐Macao Greater Bay Area (GBA) serves as a regional rainfall center. Using 6 years of rain gauge data, this study identified the northeastern GBA as a hotspot for extreme hourly rainfall, especially during nighttime. Using high‐resolution assimilation data from a four‐dimensional variational Doppler radar system, we investigated storm‐scale dynamics and microphysics of a nocturnal event with a peak hourly accumulation of 167.8 mm. Results showed that the urban heat island amplified local thermal contrasts as the cold pool approached, leading to strong northeasterly outflows exceeding 14 m/s at 150 m. Consequently, despite the predominant east‐southeastward movement driven by upper‐level west‐northwesterly winds, the intense convection shifted southwest toward the urban area and intensified significantly. The maximum hourly rainfall was related to the quasi‐stationary mesoscale boundary that triggered repeated convection. Before the most intense 5‐min rainfall of 34 mm, low‐level vertical velocity increased significantly, driven mainly by buoyancy‐induced and nonlinear dynamical accelerations. However, the nonlinear acceleration played a key role in maintaining strong vertical velocity at lower levels. The nonlinear acceleration was associated with low‐level meso‐ γ ‐scale rotation and vertical vorticity peaking below 2.5 km. In the southern region, vorticity was mainly attributed to stretching from strong convergence along the cold pool outflow boundary, while the tilting effect dominated the vorticity to the west, related to intense 0‐3‐km vertical wind shear. Microphysically, extreme rain rates were primarily driven by active warm‐rain processes and a higher concentration of medium‐sized raindrops resembling maritime convection.