## 3 Results  
### Overview of the Event Day  
When synoptic forcing is weak, most of the convection is triggered by diurnal heating during summer months in Taiwan [52]. On 5 August 2018, a weak synoptic-scale forcing environment was dominant in southern Taiwan. As ATs on the event day had a major impact during 0500-1100 UTC with a maximum precipitation intensity during 0600-0800 UTC, the following discussion mainly focused on the period of 0600-0800 UTC. After sunrise, a sea breeze began to form in the coastal areas of southern Taiwan due to the increasing difference between the air temperature over land and over sea. As indicated by surface wind observations at 0600 UTC (1400 LST; Figure 4a), the sea breeze formation in southern Taiwan increased the surface wind speed as the prevailing winds and the sea breeze were from the northwest, and the observed peak wind speed in the southern coastal regions of Taiwan was 8-12 kt. However, the wind speed decreased as the wind moved inland. The convective activity around the mountains in southern Taiwan was enhanced after the air from the sea breeze was forced to move upward along the mountains. High composite radar reflectivity greater than 45 dBZ (Figure 4d) signifying intense precipitation that was closely associated with thunderstorms, was largely observed in the mountain regions of Chiavi, Kaohsiung and Pingtung. Due to the effect of convection cells with high composite radar reflectivity of 45 dBZ, intense precipitation occurred, which reduced the horizontal visibility of the Pingtung meteorological station located at Pingtung Airport (Figure 1) from 10 km to 8 km (Figure 5a). Surface wind observations at 0700 UTC (1500 LST) revealed that the convective activity increased sharply on the land surface (Figure 4b), and the convection cells with high composite radar reflectivity (>45 dBZ) moved west toward the coast of southern Taiwan. As the onshore wind was strengthened due to the sea breeze effect, the convergence zone occurred at a smaller scale. Convergence lines (Figure 4e) formed rows of showers or thunderstorms, including over Chiavi, Tainan, Kaohsiung and Pingtung, and maximum surface wind speeds (Figure 4b) were approximately 18-22 kt over the area in the northeast of Pingtung Airport. Even though no obvious rainfall variations occurred in Pingtung Airport, horizontal visibility at the Pingtung weather station (star sign in Figure 1) gradually decreased from 8 km to 7 km (Figure 5a). Figure 4: Surface wind observations (kt; vector) at **(a)** 0600, **(b)** 0700, and **(c)** 0800 UTC and composite radar reflectivity (dBZ; shaded) at **(d)** 0600, **(e)** 0700, and **(f)** 0800 UTC on 5 August 2018 from 134 AWSs. The location of Pingtung Airport (Pingtung meteorological station) was depicted as red star in **(a–c)** and white star in **(d–f)**, respectively. The blue lines in **(a–c)** indicated the convergence line of ATs. Wind speeds more (less) than 8 kt were colored in red (black). Surface wind observations at 0800 UTC (1600 LST) revealed the convergence line (Figure 4c) along the coast area. The composite radar reflectivity (Figure 4f) of Pingtung indicated that the row of thunderstorm had passed over Pingtung Airport. Under the influence of thunderstorm, horizontal visibility at the Pingtung meteorological station decreased from 7 to 5 km (Figure 5a). To reduce the risk of aviation accidents, pilots in Pingtung Airport are prohibited from flying under visual flight rules and are required to obey the instrument flight rules when horizontal visibility is less than or equal to 5 km. Therefore, although the observed maximum hourly rainfall was only approximately 2.5 mm (Figure 5a) during 0700-0800 UTC, the reduced horizontal visibility of 5 km exerted a great impact on aviation operations. As the thunderstorm moved offshore and began to weaken, the horizontal visibility at the Pingtung meteorological station during 0900-1200 UTC (1700-2000 LST) increased from 6 to 9 km (Figure 5a).