Warm season convective variability in snow mountain range and heavy rains in Taipei

Convective activity around metropolitan Taipei City is greatly affected by the Snow Mountain Range (SMR), especially in the warm season with weak synoptic conditions. Pronounced convective activity is frequently observed around the mountain range and causes severe flash floods in the basin of Taipei. The formation mechanisms of a daytime and a nighttime extremely heavy rain-flash flood event are investigated in this study. The results show the daytime heavy rain event was associated with severe air-mass afternoon thunderstorms with a complex structure. Storm initiation was due to insolation-produced sea breeze and upslope flows. The short-duration torrential heavy rain occurred during the merger of convective cells. Radar analysis identifies that cell merging was associated with enhanced low-level convergence induced by the sea breeze and prior storm-produced outflows. On the contrary, the heavy rain at night highlighted the importance of locally enhanced baroclinicity due to intrusion of cold air. Elevated convection was triggered by the lifting of the SW warmer air above the shallow cold surface to form stagnant convection in the basin. Convective cells that formed over the coastal region along the baroclinic zone propagated eastward and merged with the convective cells in the basin. The intensified convection produced heavy rain. Signatures of polarimetric radar parameters are also described to examine the possible differences in the characteristics of bulk cloud microphysics between the daytime and the nighttime heavy rain cases. ? 2021 by World Scientific Publishing Co. Pte. Ltd.