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ABSTRACT
A Terminal Doppler Weather Radar (TDWR) started operation in Hong Kong, China, in 1997 for monitoring wind shear associated with thunderstorms affecting the Hong Kong International Airport. The airport was built on land reclaimed from the sea and lies to the immediate north of the mountainous Lantau Island, which has hills rising to nearly 1000 m. Since 1997, the airport experienced a number of tropical cyclone passages, some bringing strong southerly winds across these hills. Under these conditions the TDWR captured interesting but complex How patterns in the lower atmosphere. The TDWR Doppler velocity datasets reveal features not previously observed with conventional instruments. These include shear lines, reverse flow, small-scale vortices, streaks of low-speed flow set against a high-speed background, as well as gap-related downslope high-speed flow. Hovmoller diagrams constructed from the Doppler velocity data bring out in considerable detail periodic shedding of vortices and transient wind patterns in the wake of the hills.
1. Introduction
The Hong Kong International Airport (HKIA) came into operation in July 1998. Immediately south of HKIA is the mountainous Lantau Island. Figure 1 shows the location of the airport and the complex terrain of Lantau Island. Lantau is oriented east-northeast-west-southwest, with a width of about 5 km and length of about 20 km. In the middle, several peaks, namely, Nei Lak Shan (NLS; 751 m MSL), Lantau Peak (LP; 934 m MSL), Sunset Peak (869 m MSL), and Lin Fa Shan (766 m MSL), form a U-shaped ridge. Saddlelike cols as low as 340-460 m MSL separate these peaks, the most prominent one being the Tung Chung Gap (340 m MSL) between Lantau Peak and Sunset Peak. To support airport operations, a Terminal Doppler Weather Radar (TDWR) system was installed in 1997 for detecting microburst and wind shear associated with thunderstorms and other weather events. Weather sensors for wind shear and turbulence alerting also include ground-based anemometers installed in and around HKIA, and their locations are indicated in Fig. 1 to facilitate the discussion below. An overview of the wind shear and turbulence-alerting facilities and new developments to improve the alerting techniques is given in Shun (2003).
To better understand the occurrence of terrain-induced wind shear and turbulence (TIWT) generated by airflow over Lantau, a study based...