Analysis of Low-Level Windshear Cases at the Hong Kong International Airport Under Prevailing Easterly Wind Situations

Abstract

During spring, low-level windshear occurs at the Hong Kong International Airport (HKIA) under various easterly wind conditions associated with low-level jets, terrain and sea breezes. This paper analyses in detail several cases of low-level windshear by considering pilot reports, aircraft data and measurements from Doppler Light Detection and Ranging (LIDAR) systems. The LIDARs are found to provide crucial data for capturing windshear features related to terrain, low-level jet and synoptic forcing. Their glide-path scan data are highly useful for constructing the headwind profiles, which are used to generate windshear alerts. The LIDAR-based headwind profiles are consistent with pilot reports and aircraft data for a variety of windshear cases. Through case studies, this paper also demonstrates the subjective nature of pilot windshear reports, and the transient and sporadic nature of terrain-induced windshear. In a significant windshear case with 16 pilot reports, the capability of a high-resolution numerical weather prediction (NWP) model to provide windshear alerts is evaluated. The model is found to be skilful in capturing the pilot reports through relative operating characteristics (ROC) curves, though the results should be interpreted with the limited sample size in mind. This paper shows the usefulness of LIDAR in providing detection-based windshear alerts and the potential capability of a high-resolution NWP model in providing forecast windshear alerts with a lead time of more than 24 h.