## 3 Aircraft Observation
### First Encounter--Surface and Radar Observations and Windshear Alerts  
At around 21:35 Hong Kong time (=UTC + 8 h) on 30 April 2024, an aircraft approached the north runway of HKIA from the east for landing. The runway the aircraft intended to use was runway 25 right approach, denoted as R25R. Surface observations at that time as shown in Figure 2a show that the runway was under prevailing north-to-northwesterly winds at that time, indicating northerly crosswind on the runway. In particular, for the anemometer near the touchdown of the north runway from the east (anemometer R3E), the wind was very gusty and the maximum gust reached 21.5 m/s as seen from Figure 2b. The aircraft attempted to land but failed to do so, so that it conducted a missed approach and flew to the west. Figure 2: _Cont._  
From the radarscope, intense echoes with reflectivity in excess of 53 dBZ were found to sweep across HKIA from the west to the east as shown in Figure 3a,b). At the time instance when the aircraft was on its final approach, the intense echoes were affecting the eastern end of the north runway with significant divergence as captured by the TDWR which can be seen in Figure 4a,b), with a difference in radial winds of about 15 m/s over a distance of around 5 km (radial winds towards the TDWR minus those away from the TDWR). The S-band weather radar at Tate's Cairn showed low and even negative \(Z_{\text{DR}}\) values near the north runway of HKIA in the lower atmosphere around 1 km in height as shown in Figure 5a,b). This suggested the occurrence of downward atmospheric motion [8] in the form of downdrafts and affected the north runway. The SLW PAWR also provides useful information about the EDR. At a height of about 1 km, the EDR was found to reach around 0.45 to 0.5 m\({}^{2/3}\)s\({}^{-1}\) in 13:35 to 13:36 UTC which can be seen in Figure 6a,b), i.e., severe turbulence could be expected in the airport area. Figure 3: _Cont._  
Figure 2: (**a**) Wind observations at 13:36 UTC on 30 April 2024 when north-to-northwesterly winds prevailed over the north runway of Hong Kong International Airport (The yellow ellipse shows the anemometers over the north runway with winds picking up from the northwest at around 21:35 Hong Kong time on 30 April 2024). (**b**) Time series of 3 s gusts recorded by the R3E anemometer station from around 13:20 to 13:50 UTC on 30 April 2024 with maximum gusts reaching 21.5 m/s between 13:31 UTC and 13:32 UTC. Figure 4: _Cont._  
Figure 3: Radar reflectivity from (**a**) a 0.1\({}^{\circ}\) PPI scan from the S-band weather radar at Tate’s Cairn at 13:36 UTC on 30 April 2024 and (**b**) a 0.9\({}^{\circ}\) PPI scan from the X-band SLW PAWR at 13:14 UTC on 30 April 2024 (the blue triangle and cross indicate the locations of the mesocyclone and microburst detected by the SLW PAWR, respectively). Both radars showed reflectivity in excess of 53 dBZ associated with the line of intense echoes sweeping across HKIA from the west to the east on that night. Figure 4: The (**a**) radial velocity and (**b**) reflectivity plots over HKIA by the BP TDWR at 13:36 UTC on 30 April 2024. Figure 5: _Cont_. Figure 5: (**a**) Differential reflectivity (\(Z_{\text{DR}}\)) based on a 0.1\({}^{\circ}\) PPI scan from the S-band weather radar at Tate’s Cair at 13:36 UTC on 30 April 2024 (The blue ellipse shows low and even negative \(Z_{\text{DR}}\) values in the lower atmosphere) and (**b**) a cross-section of \(Z_{\text{DR}}\) along the A–B line in panel (**a**). Low or even negative \(Z_{\text{DR}}\) values were observed near the downdraft. Figure 6: _Cont._  
From the time series perspective, the change in 3 s gusts recorded by the R3E anemometer at the north runway associated with the occurrence of microbursts is shown in Figure 2b. In addition, the vertical wind speed measured by HKO's wind profiler at SLW shows a downward velocity of \(-\)5.14 m/s, \(-\)7.63 m/s and \(-\)8.26 m/s at around 150 m height at 13:10 UTC, 13:50 UTC and 14:00 UTC, respectively which can be seen in Figure 7a.