Wednesday, 25 January 2012
Observation and Numerical Simulation of An Event of Vortex/Wave Shedding From a Mountain near the Hong Kong International Airport
In the evening of 11 April 2011, vortex/wave shedding from a mountain to the south of the Hong Kong International Airport (HKIA) occurred following the establishment of fresh to strong easterly airflow in Hong Kong. This paper documents the observations by the various meteorological instruments in the event and discusses the possibility of forecasting the vortex/wave shedding using a high resolution numerical weather prediction (NWP) model. The vortex/wave shedding was well observed by the Light Detection And Ranging (LIDAR) system at HKIA. The vortex/wave appeared as an area of reversed flow against the background east to southeasterly airflow in the airport area. It was detached from the mountain and moved to the northwest with the background southeasterly flow, with gradual weakening in terms of decreasing spatial extent of the area of reversed flow. The vortex/wave brought about significant windshear to both arrival flight paths to the west of HKIA, as evidenced from the headwind profile data measured by the LIDARs and the aircraft data. It had rather large vertical extent, reaching the top of the boundary layer, but did not extend downward to the surface, as shown from the vertical scans of the LIDAR and the surface wind observations. The shedding event is successfully captured by high-resolution NWP model with a spatial resolution of 50 m. The model output could forecast the shedding at 6 to 7 hours ahead of the event, indicating the possible occurrence of terrain-induced windshear in the form of vortex/wave shedding from a mountain. As a result, the numerical simulation could be useful in giving an early alert to the aviation weather forecaster about the significant windshear event, which could be hazardous to the landing aircraft.