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VOLUME 42 JOURNAL OF APPLIED METEOROLOGY DECEMBER 2003 Terminal Doppler Weather Radar Observation of Atmospheric Flow over Complex Terrain during Tropical Cyclone Passages C. M. SHUN,S.Y.LAU, AND O. S. M. LEE Hong Kong Observatory, Hong Kong, China (Manuscript received 11 December 2002, in ®nal form 25 May 2003) 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 ¯ow patterns in the lower atmosphere. The TDWR Doppler velocity datasets reveal features not previously observed with conventional instruments. These include shear lines, reverse ¯ow, small-scale vortices, streaks of low-speed ¯ow set against a high-speed background, as well as gap-related downslope high-speed ¯ow. HovmoÈller 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 ments and numerical simulations was conducted in 1994 The Hong Kong International Airport (HKIA) came (Neilley et al. 1995) prior to the airport opening. This into operation in July 1998. Immediately south of HKIA study identi®ed two sources of TIWT downwind of Lan- is the mountainous Lantau Island. Figure 1 shows the tau: (i) moderate turbulence associated with gravity location of the airport and the complex terrain of Lantau wave dynamics for critical-level ¯ow under stably strat- Island. Lantau is oriented east-northeast±west-south- i®ed conditions and (ii) severe mechanical turbulence west, with a width of about 5 km and length of about associated with deep uniform ¯ow. It also concluded 20 km. In the middle, several peaks, namely, Nei Lak that terrain-induced wind shear was considerably less Shan (NLS; 751 m MSL), Lantau Peak (LP; 934 m intense and less frequent as compared with terrain-in- MSL), Sunset Peak (869 m MSL), and Lin Fa Shan duced turbulence, with peak wind shear events only (766 m MSL), form a U-shaped ridge. Saddlelike cols marginally signi®cant and occurring during very sig- as low as 340±460 m MSL separate these peaks, the ni®cant turbulence episodes. As part of this study, Clark most prominent one being the Tung Chung Gap (340 et al. (1997) used a high-resolution numerical model to m MSL) between Lantau Peak and Sunset Peak. To simulate a case of severe turbulence reported by a King support airport operations, a Terminal Doppler Weather Air research aircraft downwind of Lantau during the Radar (TDWR) system was installed in 1997 for de- passage of Tropical Storm Russ on 7 June 1994. Clark tecting microburst and wind shear associated with thun- et al. (1997) concluded that mechanical effects rather derstorms and other weather events. Weather sensors for than gravity wave dynamics dominated the ¯ow dis- wind shear and turbulence alerting also include ground- tortion and generation of turbulence downwind of Lan- based anemometers installed in and around HKIA, and tau under deep uniform ¯ow. Aircraft pilot reports re- their locations are indicated in Fig. 1 to facilitate the ceived since the opening of HKIA, however, indicate discussion below. An overview of the wind shear and that, apart from terrain-induced turbulence, signi®cant turbulence-alerting facilities and new developments to wind shear also occurred downwind of Lantau in both improve the alerting techniques is given in Shun (2003). stably strati®ed conditions and deep uniform ¯ow. To better understand the occurrence of terrain-in- This paper presents interesting but complex three- duced wind shear and turbulence (TIWT) generated by dimensional atmospheric ¯ow and terrain-induced wind air¯ow over Lantau, a study based on ®eld measure- shear phenomena observed by the TDWR downwind of Lantau during passages of two tropical cyclones, one Corresponding author address: C. M. Shun, Hong Kong Obser- in 1997 and one in 1999. Wind patterns not previously vatory, 134A Nathan Road, Hong Kong, China. observed by conventional instruments under similar E-mail: [email protected] conditions, especially their spatial and temporal varia- q 2003 American Meteorological Society 1697 Unauthenticated | Downloaded 09/29/21 07:25 PM UTC 1698 JOURNAL OF APPLIED METEOROLOGY VOLUME 42 The TDWR is a 250-kW peak power C-band system with a high-performance antenna with half-power beam- width of 0.558. It is essentially the same system imple- mented by the U.S. Federal Aviation Administration (FAA) (Michelson et al. 1990) at more than 40 airports in the United States. The radar has a highly stable kly- stron-based ampli®er that enables clutter suppression of up to 55 dB. Although the TDWR was designed to detect microbursts and wind shear associated with thunder- storms, its radar data enables detection of wind shear brought about by other mechanisms under rainy con- ditions. The TDWR routinely makes azimuthal scans at different elevation angles in accordance with the scan strategy in operation. The radar base data (re¯ectivity, Doppler radial velocity, spectrum width, and signal-to- noise ratio) have range gate spacing of 150 m and an azimuth interval of 18. The lowest elevation angle em- ployed for automatic wind shear detection is 0.68. Dur- ing normal operation, the ``monitor mode'' essentially makes continuous 3608 azimuthal scans at different el- evation angles. When the TDWR detects either wind FIG. 1. Map of HKIA and its surrounding areas. Terrain contours shear or signi®cant precipitation area, it will automat- are given in 100-m intervals. Locations of the TDWR and ground- based anemometers are indicated. The tick marks along the extended ically switch to the ``hazardous weather mode.'' In this axes of the runways give the speci®c distance (1, 2, and 3 n mi) from mode, the radar makes sector azimuthal scans con®ned the runway ends. to the airport approach and departure areas only. This enables the wind shear detection scan to be repeated at intervals of less than 1 min. For operational reasons, no tions, will be highlighted. Section 2 provides back- RHI scans are made in these modes. ground information on the use and interpretation of the Because the TDWR has to operate in a high-clutter TDWR data shown in the subsequent sections. Sections environment with both stationary and moving targets, 3 and 4 present TDWR Doppler velocity observations sophisticated clutter-removal algorithms are used to pro- captured during the passage of Typhoon Victor in 1997 vide quality-controlled radar base data (``conditioned and Typhoon Maggie in 1999. Although both tropical data'') for automatic wind shear detection. These al- cyclones brought strong southerly ¯ow to the Lantau gorithms include the use of (i) 55-dB frequency domain area, different wind patterns were observed on these ®lters that provide high-pass band ®ltering of targets two occasions. As will be discussed, these differences with small or zero radial velocity; (ii) four different were due to the highly three-dimensional complex ter- Clutter Residue Editing Maps (CREMs) for removal of rain of Lantau projecting different cross sections toward residual stationary clutter at the four lowest elevation the different ¯ow directions, namely, southwesterly ver- angles (0.68, 1.08, 2.48, and 6.08); (iii) clutter polygons sus southerly. These two occasions, therefore, warrant to eliminate sidelobe contamination at 0.68 and 1.08 for separate discussions. Section 5 summarizes our current the area over and south of Lantau Island, over which understanding of the observed phenomena, suggests ar- the main radar beam is blocked by the Lantau terrain; eas to be explored further to improve this understanding, and (iv) point target ®lters that remove spikelike anom- and provides conclusions to the paper. alies in radar re¯ectivity caused by moving objects, such as aircraft, birds, automobiles, and marine vessels. The system is also provided with velocity dealiasing [using 2. TDWR in Hong Kong dual scans with different pulse repetition frequencies The TDWR is strategically located at Tai Lam Chung (PRFs)], signal-to-noise thresholding, and range obscu- (Fig. 1), about 12 km northeast of HKIA, to enable a ration±editing algorithms to ensure that the Doppler ra- clear view of the airport runways, and the approach and dial velocity data are free from contamination. After departure areas. Because its line-of-sight toward the air- passing through these quality-control algorithms, the port is practically aligned with the orientation of the conditioned data are then processed by several weather runways, the Doppler radial velocity thus observed pro- detection and warning algorithms for automatic gen- vides a good estimate of the head or tail wind component eration of wind shear alerts and graphical products for and, hence, the possible wind shear experienced by land- users. The radar data presented in this paper are all ing or departing aircraft. To avoid beam blockage by conditioned data. nearby ships, the radar antenna of the TDWR was in- For presentation of TDWR data in this paper, the stalled at about 60 m MSL. conditioned data from azimuthal scans in polar coor- Unauthenticated | Downloaded 09/29/21 07:25 PM UTC DECEMBER 2003 SHUN ET AL. 1699 FIG. 2. Surface pressure chart at 1500 UTC 2 Aug 1997 with the track of Victor overlaid. dinates were mapped onto a display in Cartesian co- ordinates with grid resolution of 100 m 3 100 m.