High resolution numerical modelling of windshear episodes at the Hong Kong International Airport

Terrain-induced airflow disturbances and sea breeze are two main causes of windshear at HKIA. While numerical modelling facilitates understanding and has the potential of forecasting the occurrence of windshear, its application at HKIA meets the challenge of resolving the rugged mountainous terrain and complicated land-sea distribution in the airport area.

This paper uses a numerical model (Regional Atmospheric Modelling System [RAMS] version 4.4) to study the typical windshear episodes at HKIA. It is nested from the Regional Spectral Model (RSM) of the Observatory and performs simulation down to a horizontal resolution of 200 m for the airport area. The main features of the windshear episodes are found to show up in the model results, such as mountain wake of Lantau Island (an island with peaks rising to almost 1000 m to the south of HKIA) and movement of the sea breeze front west of the airport.

Windshear may be encountered by aircraft flying through the wake of the mountains on Lantau Island in southeasterly wind situations in spring-time. Idealized simulations are carried out with RAMS to study how the mountain wake is related to the strength of the low-level jet as well as the magnitude and height of the boundary layer temperature inversion. A regime diagram of mountain wake occurrence is constructed with the Froude number and non-dimensional mountain height as the controlling parameters. This kind of diagrams is useful for the aviation weather forecasters to assess the chance of the occurrence of terrain-induced disturbances over the runway corridors.