## 3 Results and Discussion of Two Reported Windshear Events  
### Windshear Event of 12 February 2024  
The surface isobaric chart at 12 UTC on that day (which is 20H in Hong Kong time, HKT = UTC + 8 h) is shown in Figure 1a. The chart was produced by the duty forecaster through analyzing the available surface observations. There was a broad area of high pressure to the south of Japan. An easterly ridge established itself over the southeasterly coast of China, bringing moderate easterly winds to Hong Kong near the surface. The observations from the weather stations around HKIA are shown in Figure 1b. East to southeasterly winds prevailed over the airport area. However, at higher altitudes, e.g., the mountaintops of Lantau Island, the winds reached a strong force (colored blue in Figure 1b), with a wind speed of 25 to 30 knots. Figure 1: _Cont._  
The radiosonde ascent closer to the time (13 UTC) is shown in Figure 2. From the temperature profile (Figure 2a), no significant temperature inversions are shown in the atmospheric boundary layer. In the wind-speed profile (Figure 2b), there is a jet of around 10 m/s between 600 to 1000 m above mean sea level, which is consistent with the observation of stronger winds over the mountaintops (Figure 1b), which are around 1000 m above mean sea level. The airflow disturbances downstream of Lantau Island are mostly believed to arise from mechanical turbulence as a result of the easterly jet impeding on the Lantau terrain. LIDAR observations are shown in Figure 3. Figure 3a shows the standard panel of the glide path scan windshear alert generation algorithm (GLYGA), with the three-degree and six-degree plan position indicator (PPI) scans of Doppler velocities from the central and northern LIDAR at HKIA shown in the middle. The panels on the far right and far left are the headwind profiles for the various runway corridors. The principles of GLYGA can be found in [10]. Significant windshear has been captured for runway corridor 07 LA, with the alert shown (+16 knots at one nautical mile final) and highlighted in blue in the headwind profile. There were two windshear reports from pilots for 07 LA during this event, namely, a headwind gain of 15 knots at a height of 300 feet at 13:55 UTC and a headwind gain of 15 knots at a height of 100 to 300 feet at 14:04 UTC. They were both captured by GLYGA, and the GLYGA alerts were consistent with the pilot reports of windshear. Figure 3b shows the eddy dissipation rate (EDR) plot from the three-degree PPI of the central LIDAR around that time. The EDR map was generated according to the method described in [16]. Moderate turbulence (around 0.3, colored green in Figure 3b) shows up between the runway end and 1 nautical mile away for 07 LA. Figure 1: Surface isobaric chart for the 12 Feb 2024 windshear case (**a**) and the surface observations from nearby HKIA (**b**), with winds on hilltops enclosed by the ellipse in orange. The landing runways 07 LA, 07 CA, and 07 RA are shown in (**b**). The grid in (**b**) has a size of 0.02 degrees, i.e., around 2 kilometers. For all maps used in this communication, the top of map is the north, as indicated in (**a**). All the figures are north-south oriented. The observations from the Sha Lo Wan wind profiler/radio acoustic sounding system (RASS) (location in Figure 1b) are shown in Figure 4. At around 14 UTC (22H local time), there was a jet of strong force below around 600 m above mean sea level (colored blue in Figure 4a), with downward velocity of around 3-4 m/s in that region (Figure 4b), and a virtual temperature profile (Figure 4c) generally following the saturated adiabatic lapse rate (broken straight lines in Figure 4c). The virtual temperature was generally constant at higher altitudes. Figure 3: _Cont_. Figure 2: Radiosonde ascent. The upper plot (**a**) is temperature and dew point, and (**b**) is wind speed and direction. The model run was initialized at 12 UTC, 12 February 2024.