## 1 Introduction  
Situated in an area of complex terrain, the Hong Kong International Airport (HKIA) may be affected by terrain-induced airflow disturbances at times. The disturbed airflow can result in rapid and significant changes in wind speed and direction over a short distance. When such changes result in a sudden shift in headwind experienced by aircraft, especially close to the ground during takeoff or landing, it may pose a safety threat to the aircraft. Windshear refers to a change of headwind/tailwind by 15 knots or above because of changes in wind speed and/or wind direction, resulting in a change in the lift of the aircraft. Low-level windshear refers to the windshear that occurs below 1600 feet, in accordance with the definition of the International Civil Aviation Organization (ICAO). It is particularly hazardous to the operation of the aircraft because the plane is already close to the ground, leaving little room for maneuvering. There are many causes of windshear, e.g., synoptic weather patterns [1], complex terrain [2, 3], frontal systems, thunderstorms, sea breeze, etc. A summary of windshear can be found in [4], in terms of detection and prediction. For detecting windshear, there are numerous examples of the combined use of weather radar (microwave radar) and LIDAR (laser radar), such as in HKIA and other airports(e.g., [3, 5, 6]). Windshear is an issue for quite a number of airports around world, with examples including [7, 8, 9].  
There have been quite a number of studies on the terrain-induced windshear and turbulence at HKIA, such as [10, 11], including both the detection of wind changes using light detection and ranging (LIDAR) systems and numerical forecasting of the occurrence of windshear and turbulence. The previous studies mainly focused on the two-runway configuration of the airport.  
The third runway of HKIA was officially commissioned on 25 November 2022. It is situated at a distance of around 4 km from the foot of the mountain ranges of Lantau Island to its south, which has peaks rising up to about 1 km above mean sea level. Landing on this new runway from the west is designated as 07 LA. With further distance from the mountains of Lantau, it is suspected that the terrain-induced airflow disturbances may not extend far enough away to affect this new runway. However, in the year after the commissioning of this runway, the air traffic industry in Hong Kong was still very much affected by the post-COVID-19 situation, so aircraft operation was far below that back in 2018. As a result, the windshear occurrence based on pilot reports in 2023 might not be indicative of the actual conditions of windshear and turbulence for the third runway. In early 2024, the air traffic in Hong Kong had gone back to around 70% of the pre-COVID-19 era, and the windshear reports from the spring-time peak windshear season in 2024 might be considered to be more representative of the impact of the Lantau terrain, if any, on wind changes over the third runway. Two events of pilot-reported windshear are examined through observational analysis as well as numerical simulations to examine whether the terrain effect from Lantau could still result in windshear over the third runway.