A Modeling Study of the Severe Afternoon Thunderstorm Event at Taipei on 14 June 2015: The Roles of Sea Breeze, Microphysics, and Terrain

Abstract

On 14 June 2015, a severe afternoon thunderstorm event developed within the Taipei Basin, which produced intense rainfall (with a rainfall rate of 131 mm h⁻¹) and urban-scale flooding. Cloud-resolving simulations using the Weather Research and Forecasting (WRF) model were performed to capture reasonably well the onset of the sea breeze and the development and evolution of this afternoon thunderstorm system. The WRF model had four nested grids (with the finest grid size of 0.5 km) in the horizontal direction and 55 layers in the vertical direction to explicitly resolve the deep convection over complex terrain.

It was found that convection was initiated both by the sea breeze at foothill and by the upslope wind at the mountain peak. Convective available potential energy (CAPE) was increased from 800 to 3200 J kg⁻¹ with abundant moisture transport by the sea breeze from 08 to 12 LST, fueling large thermodynamic instability for the development of the afternoon thunderstorm. Strong convergence between the sea breeze and cold-air outflow triggered further development of intense convection, resulting in heavy rainfall over Taipei City.

Microphysics sensitivity experiments showed that evaporative cooling played a major role in the propagation of cold-air outflow and the production of heavy rainfall within the basin plain (terrain height < 100 m), whereas melting cooling played a minor role. The terrain-removal experiment indicated that the local topography of Mount Datun at the coastal region may produce the channel effect through the Danshui River Valley, intensify sea-breeze circulation, and transport more moisture. This terrain-induced modification of sea breeze circulation made its dynamic and thermodynamic characteristics more favorable for convection development, resulting in a stronger afternoon thunderstorm system with heavy rainfall within Taipei City.