## 3 Results and Discussion of Two Reported Windshear Events
### Windshear Event of 17 February 2024  
The synoptic pattern in the evening of 17 February 2024 (Figure 9a) is quite similar to that on 12 February 2024, namely, a broad, high-pressure area bringing easterly winds to Hong Kong. The observations near the airport at around 18H on that day is shown in Figure 9b. Note that, possibly arising from the mountain wake, the weather buoy at the west of the airport showed north-northwesterly wind, though the wind speed was light. The windshear event occurred at around 10 UTC (18H local time). The microwave radiometer data at King's Park (the radiosonde station in Hong Kong) are shown in Figure 10a. Slight temperature inversion shows up between the 925 and 975 hPa levels. However, there was no low-level jet in this case from the Sham Shui Po wind profiler data around that time (which is located about 1 km to the north of King's Park, Figure 10b). The terrain-induced windshear in this event, if any, is believed to have arisen from mechanical turbulence arising from moderate southeasterly winds impeding on the Lantau terrain under a slight capping inversion, such that there was disturbed flow in the lee side further down the airport arena without significant vertical propagation. The LIDAR observations are shown in Figure 11. From the PPI Doppler velocity plots, for both three-degree and six-degree elevation angles, reverse flow (in green) appears to the west of the prevailing southeasterly flow (Figure 11a). It is found to extend quite far north to affect the 07 LA runway corridor. Because of the sharp boundary between the reverse flow and the prevailing southeasterly wind, windshear alerts were triggered at about 1 nautical mile away from the western end of the north runway (Figure 11a). In this event, there were two pilot reports of windshear over 07 LA, one with a headwind gain of 10 knots at a height of 200 feet at 09:53 UTC, and another with a headwind gain of 15 knots at a  
Figure 8: The simulated horizontal wind profile (**a**), vertical wind profile (**b**), and virtual temperature (**c**) at the Sha Lo Wan wind profiler/RASS. height of 300 feet at 10:05 UTC. Both were captured by GLYGA, and they were consistent with LIDAR observations. The LIDAR EDR map is shown in Figure 11b. Again, moderate turbulence (in green) extended from the mountains of Lantau Island towards the 1 nautical mile region of 07 LA. Figure 9: Synoptic weather pattern (**a**) and observations near HKIA (**b**) for the 17 February 2024 windshear case. All the figures are north–south oriented. Figure 10: The temperature (black) and dew point (red) profiles from the microwave radiometer at King’s Park at 10 UTC (**a**), and the wind profile from the Shan Shui Po wind profiler in HKT (**b**). The Sha Lo Wan wind profiler/RASS data are shown in Figure 12. At around 10 UTC (18H), slightly stronger winds were observed below 500 m above mean sea level (Figure 12a), with strong upward motion (in red, about 4-5 m/s, Figure 12b). The virtual temperature was mostly constant up to that height and dropped gradually at higher altitudes (Figure 12c). Figure 11: The LIDAR windshear panel, with the PPI velocity image and headwind profiles (**a**) and EDR image (**b**) for the windshear case of 17 February 2024. All the figures are north–south oriented. The model was initialized at 06 UTC, 17 February 2024. The simulated three-degree LIDAR picture is shown in Figure 13a. The simulation captures very well the mountain wake extending from Nei Lak Shan (location in Figure 1b), so that reverse flow appears in the prevailing southeastly flow. The wake shows up more clearly in the two-dimensional  
Figure 12: The time–height cross section of wind (**a**) and vertical velocity (**b**) for the 17 February 2024 windshear case; (**c**) shows selected virtual temperature profiles. wind field at a height of about 100 m above sea level (Figure 13b). As such, Nei Lak Shan may have a wake extending downstream up to 4 to 5 km to cause terrain-induced windshear over 07 LA.