Operational LIDAR-based system for automatic windshear alerting at the Hong Kong International Airport

The Hong Kong Observatory has developed the first operational system in the world to automatically alert windshear at an airport based on the Doppler LIDAR.  The system makes use of the LIDAR scans along the glides paths of HKIA to construct the headwind profiles to be encountered by the aircraft and detects the ramps of significant wind change along the glide paths.  The headwind profile vividly illustrates the sporadic and transient nature of terrain-induced airflow disturbances (an example shown in Figure 1 below), which is the main cause of windshear at HKIA.  Moreover, headwind loss and gain could occur over the same runway corridor (Figure 2).  The algorithm is specially developed for alerting this kind of windshear.

This paper gives an overview of the windshear detection algorithm and the quality control measures on the LIDAR velocity data.  The algorithm's performance in the two peak windshear seasons in Hong Kong (namely, spring-time and summer to early autumn) is also presented.  It is found to capture 70% or more of the pilot reports of significant windshear encounter over the commonly used runway corridors.  Moreover, integration of the LIDAR-based alerts into the Observatory's Windshear and Turbulence Warning System (WTWS) is discussed.

Figure 1: The headwind profiles along the runway corridor 25LD (i.e. departure from the south runway of HKIA towards the west) as indicated by the LIDAR wind measurements at 14:41 and 14:43 UTC, 30 August 2004.  Strong southerly winds prevailed over HKIA on that day and the air was disturbed as it climbed over the hills on Lantau Island (an island south of HKIA with peaks rising to 1000 m).  As shown by the LIDAR, there was a headwind loss followed by a headwind gain at 14:41 UTC over the region highlighted in green in the figure, but the headwind sequence was reversed 2 minutes later.  This shows the transient nature of terrain-induced windshear.  An aircraft departing at 25LD runway corridor at 14:41 UTC on that day reported encounter of significant windshear.

Figure 2: An example of the display of the LIDAR-based windshear detection algorithm.  The left panel shows the 1-degree PPI scan of the LIDAR at 9:06 UTC, 14 February 2005, giving an overview of the wind pattern over the western part of HKIA (red/yellow refers to the wind blowing away from the LIDAR whereas blue/green means towards the LIDAR).  The headwind profile measured by the LIDAR over the approach corridor to the north runway of HKIA (i.e. corridor 07LA) is given on the right panel.  It shows that, for terrain-induced airflow disturbances, both headwind gain and loss could co-exist over the same runway corridor.  The abrupt wind changes are successfully captured by the windshear detection algorithm (highlighted in red).  At that time, an aircraft over 07LA conducted missed approach due to windshear.