Microphysical and Kinematical Characteristics of Merged and Isolated Convective Cells over the Complex Terrain of the Taipei Basin

This study investigates the microphysical and kinematic characteristics in extreme afternoon thunderstorm (ATS) rainfall during Taiwan-Area Heavy rain Observation and Prediction Experiment (TAHOPE)/Prediction of Rainfall Extremes Campaign In the Pacific (PRECIP) IOP 2 on 31 May 2022. The high-quality S-Pol radar observations and the multi-Doppler winds provided valuable information about the organization of convection over complex terrain. There were two episodes of severe ATS in this event. Episode 1 (1200-1400 LST) featured multiple cell merger (MCM) favored by terrain-induced circulation. The updrafts merged in tandem with MCM while ZDR columns became “ZDR crown” which is defined as a ZDR column with a 5-km-height width > 8 km. Afterward, maximum vertical velocity increased dramatically to ~20 m/s and graupel reached up to 12 km MSL. In contrast, episode 2 (1500-1700 LST) exhibited isolated cells with weak updrafts (< 10 m/s) and more snow aloft. Evolution of ZDR column and updraft metrics revealed that in episode 1, the maximum height of the ZDR column was highest (7.0 km) at the timing of MCM and preceded the highest updraft speed by more than 10 min. Meanwhile, the maximum width of the ZDR column increased twofold to 8 km. The updraft velocity and area followed a similar evolution, seemingly related to the ZDR column width, highlighting that ZDR column width is a promising forecasting parameter on thunderstorm evolution. Furthermore, the “ZDR crown” signature may be a manifestation of the updraft merger process, suggesting less dilution of updraft buoyancy due to entrainment.