LIDAR Observations of Wind Shear Induced by Mountain Lee Waves

Shun et al (2003a) documented weather radar observations of periodic shedding of vortices, velocity streaks and gap-related descent of high-speed flow caused by the complex terrain of Lantau Island to the south of the Hong Kong International Airport (HKIA) during the passage of tropical cyclones. Shun et al (2003b) reported LIDAR (Light Detection And Ranging) observations of suspected cases of hydraulic jump and complex wind flow in clear-air wind shear conditions. In this paper we present recent Doppler LIDAR observations of standing lee waves induced by the Lantau terrain which resulted in wind shear to a departing aircraft flying through it. In particular, both the Doppler radial velocity (Figure 1) and backscatter (Figure 2) returns at 4.5 degrees elevation reveal an interesting zig-zag pattern associated with the lee waves. The wind flows revealed by the LIDAR at the lower elevation angles of 1.0 degree (Figure 3) and 0.0 degree (Figure 4) are however rather different due to the complexity of the Lantau terrain.

Hong Kong implemented a LIDAR at HKIA in mid-2002 to enhance the detection and warning of wind shear under clear-air conditions. Making use of azimuthal scans at three different elevation angles, the three-dimensional structure of the complex flow resulting in the zig-zag pattern will be analysed and diagnosed in this paper. We will also compare this case with a previous one on 17 March 2000 in which a similar zig-zag pattern was depicted by the weather radar. On that day, numerous wind shear reports with magnitudes of up to 35 kt (18 m/s) headwind change were received from aircraft. We will try to relate the observed phenomena with the presence of low-level inversion and characteristics of the vertical wind profiles. Aircraft data from commercial flights for these cases, where available, will be analyzed.

Reference:

C.M. Shun, S.Y. Lau and S.M. Lee, 2003a, "Terminal Doppler Weather Radar Observation of Atmospheric Flow over Complex Terrain during Tropical Cyclone Passages", J. Appl. Meteor., 42, 1697-1710.

C.M. Shun, C.M. Cheng and O. Lee, 2003b, "LIDAR Observations of Terrain-induced Flow and its Application in Airport Wind Shear Monitoring", International Conference on Alpine Meteorology (ICAM) and Mesoscale Alpine Programme (MAP) Meeting, Brig, Switzerland, 19-23 May 2003.