## 1 Introduction  
Afternoon thunderstorms (ATs), which feature short-duration intense rainfall, typically develop in summer when the sun heats the air near the ground [1, 2]. Compared with mesoscale convective systems such as squall lines, cold fronts, and tropical cyclones, ATs have smaller scales of space and time [3]. Taiwan, with a total land area of 36,000 km\({}^{2}\), is an island with a warm and humid climate and features complex terrain changes (right panel of Figure 1). The north-south oriented mountain range in the middle of Taiwan island has an average height of 2 km and contains 200 mountains over 3000 m high, with a maximum height of approximately 4 km [4]. Therefore, convective storms in Taiwan tend to develop when the wind flows smoothly up the windward side of the mountains during hot weather. Ref. [5] used data from conventional weather stations and automatic weather stations (AWSs) to analyze the diurnal variation of rainfall in different seasons and found that the most pronounced afternoon convection was over the western mountain slopes of Taiwan under the unstable atmosphere in summer. Similar diurnal sea-land breeze signatures of precipitation have also been found along the west coast of Taiwan [5, 6, 7]. In summer, approximately 75% of the rainfall in the Taipei basin was demonstrated to be produced by ATs triggered by the interaction between the wind blowing from the sea and the mountains to the south of the basin [8, 9]. The sea breeze transports abundant moisture from the ocean to the Taipei basin and increases the convective available potential energy from 800 to 3200 J kg\({}^{-1}\), which favors convection development and induces heavy rainfall [10]. As indicated, studies of ATs have mainly focused on northern Taiwan (subtropical climate); less research attention has been paid to southern Taiwan and the tropical climate. During summer months, with weak synoptic forcing in Taiwan, most of the convective processes associated with thunderstorms occur in the afternoon. Short-duration intense rainfall can result in flooding, mudslides, and aircraft accidents [11; 12; 13]. Such intense rainfall can reduce horizontal visibility, causing dangerous situations for the take-off or landing of aircraft or rendering take-offs and landings impossible. Triggered by local circulations such as the sea-land breeze and mountain-valley flow [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16] in the summer in Taiwan, ATs occur frequently and tend to significantly influence to the take-off or landing of aircraft. Predicting the characteristics of ATs and other small-scale weather phenomena is challenging as such phenomena are triggered by prevailing winds, especially at the airport, which only covers a few kilometers from the airport periphery. An AT event typically affects a given locality for approximately an hour during its passage overhead, and the entire lifetime of the event can be several hours. Therefore, along a pathway of hundreds of kilometers, an AT event can cause a substantial threat to flight safety [17; 18]. The Weather and Research Forecast (WRF) model, a mesoscale numerical weather prediction system that uses regional data assimilation can improve the prediction of rainfall [19; 20; 21]. The purpose of the WRF data assimilation (WRFDA) system is to obtain  
Figure 1: The locations of 134 AWSs (green dots) used in this study. Variables of Pingtung meteorological station located at the Pingtung Airport (red star), and three AWSs of Cengwen (station 1, red dot), Yuemei (station 2, red dot), and Yanpu (station 3, red dot) were used to describe the background conditions of the event day. accurate initial conditions for numerical models and determine the best reliable atmospheric state using observations and short-range forecasts. Conventional atmospheric sounding devices such as the radiosonde, which measures the vertical profile of the atmosphere, is limited by low temporal resolution and high cost.