## 2 Synoptic Background and Meteorological Equipment  
Synoptically, southern China was under the influence of a trough of low pressure at the surface, as shown in the isobaric chart in Figure 1a. In the inland area, there was cooler continental air associated with the northeast monsoon. At the same time, southwesterly winds prevailed along the coast in association with an area of low pressure over the western part of southern China. The confluence of cooler continental air and more humid and warmer air from the south led to the occurrence of heavy rain over the south China coast. In Hong Kong, heavy rain and thunderstorms first developed to the west and then travelled eastwards, following the mid-level westerly flow to affect HKIA and then the urban areas of the city. Severe gusts were associated with the thunderstorms. Downdrafts and downbursts are also rather common in such situations.  
Figure 1: _Cont._  
In order to observe the gusty winds, the Observatory operates a network of around 20 anemometers (10 m above ground or sea level for anemometers installed on weather buoys) inside and around HKIA. But for efficient monitoring and alerting of downbursts that could be hazardous to aircraft, a network of microwave weather radars has also been set up in Hong Kong as shown in Figure 1b. In particular, the C-band TDWR located about 20 km to the northeast of the airport is used to provide microburst alerts for aircraft. The details of the microburst alerting algorithm could be found in [5,6]. The algorithm is mainly based on convergence/divergence features of the radial velocity from the TDWR. At the same time, an X-band phased array weather radar (PAWR) is operated at Sha Lo Wan (SLW), which can provide 1 min rapid updates of radar imagery with a high range resolution of 30 m for observing the wind streaks coming out from Lantau Island, a mountainous island to the south of the airport. The technical characteristics of the SLW PAWR can be found in [7,8]. The spectral width of the SLW PAWR could also be used to calculate the EDR (details of the algorithm could be found in [9]). With the provision of a high spatiotemporal resolution for radar imagery and having an antenna height of around 34 m above sea  
Figure 1: **(a)** The surface isobaric chart at 12 UTC on 30 April 2024. **(b)** The locations of the phased array weather radar (PAWR) at Sha Lo Wan (SLW); S-band weather radars at Tate’s Cairn (TC), Tai Mo Shand (TMS) and Qiu Yu Tan (QYT); and C-band terminal Doppler weather radar (TDWR) at Brothers Point (BP).  
level, the SLW PAWR is particularly useful for monitoring high-impact weather such as tornadoes, waterspouts and hail [10; 11; 12] as well as low-level winds and precipitation [8]. For an overview of the rain cells in Hong Kong, an S-band long-range surveillance radar with a maximum detection range up to around 512 km has been set up on a hilltop called Tate's Caim at a height of about 588 m above the mean sea level, and it is around 30 km to the east of HKIA. Apart from radar reflectivity and Doppler velocity, its spectral width could be used to provide territory-wide EDR map as well for aviation applications. During inclement weather, another HKO S-band weather radar situated at Tai Mo Shan, the highest peak in Hong Kong that is about 968 m above the mean sea level and around 25 km to the east-northeast of HKIA, is very often switched to rapid scan operation, which can provide a 0.0\({}^{\circ}\) PPI scan at every minute to help monitor the evolution of weather systems.