<title>Terminal Doppler Weather Radar (TDWR) observation of atmospheric flow over complex terrain during tropical cyclone passages</title>

Abstract

To facilitate warning of low-level wind shear associated with convective storms, a Terminal Doppler Weather Radar (TDWR) was installed about 12 km to the northeast of the Hong Kong International Airport (HKIA). The HKIA is located just off the northern shore of an island known as Lantau. The HKIA lies on the lee side of the complex terrain of Lantau when winds come from the east through southwest.With the commissioning of the TDWR in 1997, interesting high-resolution radar data were collected in strong southerly flows during tropical cyclone passages. These data sets reveal the complex low-level atmospheric flow in the vicinity of the HKIA, including streaks of low-speed flow, reverse flows, small-scale vortices and high-speed gap flows . Animation sequences of the radar images suggest existence of von Kármán vortex streets and vortex shedding in the wake regions. These phenomena could induce strong shear regions which led to significant low-level wind shear for landing/departing aircraft. Analysis of on-board flight data for a wind shear event experienced by a landing aircraft in strong southeasterly flow revealed that terrain-induced features with horizontal scale less than 1 km brought significant air speed changes to the aircraft over a short duration of time.

Observation and Numerical Simulation of Terrain-Induced Windshear at the Hong Kong International Airport in a Planetary Boundary Layer without Temperature Inversions

“…The MBA area is oriented north-northeast to south-southwest, just downstream of Siu Ho Wan, and alerts (headwind loss of 40 knots) are in force for the runway corridors to the east of the southern runway of HKIA. This MBA is similar to the one documented in Shun and Lau [2].…”

“…However, besides thunderstorms, terrain-disrupted airflow can also bring about microburst, called the terrain-induced microburst, if the headwind loss is found to reach 30 knots or more and in rain, for example, in tropical cyclone or southwest monsoon situation. One case of terrain-induced microburst has been documented in Shun and Lau [2], which discusses terrain-induced microburst in neutral atmospheric condition as associated with the strong winds of a tropical cyclone. In the present paper, this kind of microburst in southwest monsoon is considered, but in terms of prevailing wind flow, atmospheric boundary layer structure, and the orientation/location of terrain-induced microburst, the two cases are rather similar.…”

Windshear detection by Terminal Doppler Weather Radar during Tropical Cyclone Mujigae in 2015

“…It was a C‐band single‐polarization radar with an antenna 7.6 m in diameter. Its beam width was around 0.55° and the range resolution was 150 m (Shun, ; Shun and Lau, ). The TDWR dealiased data by automatically selecting two pulse repetition frequencies in each volume scan which lasted for 5 min.…”

“…The TDWR dealiased data by automatically selecting two pulse repetition frequencies in each volume scan which lasted for 5 min. Dealiased velocity fields were used to update a “wind field model” of the system (Shun and Lau, ). The system could detect velocity from −80 to +80 m/s.…”

“…Various studies showed that the TDWR at TLC observed terrain‐induced features over the HKIA during tropical cyclone events. Shun and Lau () showed that the TDWR observed an area with significant windshear of about 60 m/s which was related to terrain‐induced velocity streaks and reverse flows over a runway of the HKIA under the influence of southwesterly winds associated with Tropical Cyclone Victor in 1997. In another event during Tropical Cyclone Maggie in 1999, air speed loss reaching 13 m/s experienced by a flight was consistent with terrain‐induced vortices and high‐speed gap flows observed by the TDWR (Shun and Lau, ).…”