Lagrangian characterization of terrain induced turbulence based on LIDAR observations: flow characteristics and airplane approaches at Hong Kong International Airport (HKIA)

The set of tools developed in Tang et al. (2009) is used to further study the evolution of airflow structures near HKIA. Lagrangian Coherent Structures (LCS) from three synoptic flow cases between April 2008 and February 2009 are extracted in consecutive LIDAR scans. Among these flow structures, structure generation and shedding from velocity anomalies aloft mountain peaks and ridges have been identified. The alternating streaks of forward-time and backward-time LCS highlighters indicate regions of counter rotating vortices downwind of the terrain nearby HKIA. Also, relatively large scale structures (persistent ridges of updraft) seem to be associated with the larger mountain gaps on Lantau Island. These dynamical pictures of the airflow structures around the airport could provide useful conceptual model of the terrain-induced windshear phenomena for better understanding by aviation users and weather forecasters.

To further understand the interaction of the airflow structures with the aircraft, wind data obtained from landing aircraft is analyzed and compared with the LCS extracted from LIDAR data. It turns out that there is good correlation between the locations of the LCS indicating the most unstable air motion and the “jittering” motion of the aircraft along a 3-degree glide path. The potential applications of LCS identification will be discussed.