Cold Pool–Shear Interaction and Convective Cell Merging in Afternoon Thunderstorms over Southwestern Taiwan: A Case Study of 6–7 August 2022

Afternoon thunderstorms (ATs) frequently occur over southwestern Taiwan during the warm season and pose significant challenges to aviation operations owing to their rapid development, intense rainfall, strong winds, and abrupt reductions in visibility. Despite their operational importance, the physical mechanisms governing the propagation and longevity of ATs in the Chiayi region remain poorly documented in the international literature. This study examines two contrasting AT events that occurred near Chiayi Airport on 6 and 7 August 2022, representing a short-lived and a long-lived convective system, respectively, under similar weak synoptic conditions. A comprehensive observational analysis is conducted using dual-polarization radar data, three-dimensional wind field retrievals, surface meteorological observations, and ERA5 reanalysis data. The evolution of convective structure, cold-pool dynamics, vertical wind shear, mid-level moisture, and convective cell interactions is analyzed to identify the mechanisms responsible for the differing storm behaviors. On 6 August, convection initiated in association with terrain-induced local circulations but rapidly weakened before reaching Chiayi Airport. Radar-derived vertical structures reveal a shallow and nearly upright updraft accompanied by strong low-level outflow. The cold-pool propagation speed exceeded the magnitude of the environmental vertical wind shear, allowing the cold pool to undercut the updraft and suppress sustained convective regeneration, consistent with an unfavorable cold-pool–shear balance within the Rotunno–Klemp–Weisman (RKW) framework. Limited mid-level moisture and weak convective cell merging further contributed to the short-lived nature of the system. In contrast, the 7 August event was characterized by a more favorable, though non-ideal, vertical wind structure, enhanced mid-tropospheric moisture, and persistent convective cell merging. The cold-pool propagation speed was comparable to the environmental shear, indicating a near-balanced RKW state. Successive merging of convective cells broadened the updraft region, enhanced system-scale low-level convergence along the gust front, suppressed mid-level dry-air intrusion, and promoted continuous new cell formation. As a result, the convective system evolved into a deep, organized, and long-lived thunderstorm that propagated northeastward and directly impacted Chiayi Airport. The comparison demonstrates that while a favorable cold-pool–shear balance is a necessary condition for maintaining organized convection, it is not sufficient to explain thunderstorm longevity in isolation. Convective cell merging, modulated by mid-level moisture, acts as a critical amplifying mechanism that stabilizes and prolongs the cold-pool–shear interaction under weak synoptic forcing. These findings provide new insights into the dynamics of afternoon thunderstorms over complex coastal–mountain environments and have important implications for short-term convective forecasting and aviation weather operations in southwestern Taiwan.