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* Corresponding Author Address: P.W. Chan, Hong Kong Observatory

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* Corresponding Author Address: P.W. Chan, Hong Kong Observatory 10.4 Latest developments of windshear alerting services at the Hong Kong International Airport P.W. Chan *, C.M. Shun and M.L. Kuo Hong Kong Observatory, Hong Kong, China 1. INTRODUCTION (GFS) of the Hong Kong Government has also been upgraded recently to collect wind data at a high Low-lev el windshear and turbulence could be frequency (up to 20 Hz) in order to provide the sky hazardous to the aircraft. At the Hong Kong truth data of windshear and turbulence within the International Airport (HKIA), windshear andturbulence planetary boundary layer. alerts are issued if they are expected to occur below a height of 1600 f eet or within 3 nautical miles away This paper summarizes the above-mentioned from the respective runway end. Windshear alerting dev elopments in the last year or so at HKIA f or services at HKIA are provided by the Hong Kong improving windshear and turbulence alerting services. Observatory (HKO). A number of algorithms have Some preliminary results of these studies would be been developed to give automatic windshear alerts, giv en. such as the anemometer-based alerts and the LIDAR Windshear Alerting System (LIWAS). With all such 2. DUAL LIWAS eff orts, the hit rate of pilot windshear reports has got to about 90% in the recent y ears with the alert duration The LIWAS algorithm has been used per hit on a decreasing trend. operationally for the arrival runway corridors of HKIA since 2005. The technical details of the algorithm Despite the generally satisfactory performance could be f ound in Shun and Chan (2008). With a of the windshear alerting services, enhancement single LIDAR working at the air traffic control complex works are still on-going in a number of aspects. On of HKIA, the algorithm did not perform so well for the the windshear alerting side, LIWAS algorithm has only most used departure runway corridor of HKIA in the been developed f or arrival runway corridors and the spring-time windshear season (namely, departing corresponding algorithm for departure runway from the south runway of HKIA to the east, 07RD) corridors needs to be established. In particular, the because the laser beam has a rather large angle length of the windshear ramp appears to be a critical (more than 30 degrees) with respect to the runway f actor in setting up the alerting criterion. The existing orientation around the rotation point of the aircraft. windshear algorithms are mainly based on wind data collected by the LIDARs and the surface-based Dual LIWAS has been established at HKIAsince anemometers. The use of other remote-sensing early 2008, namely, one LIDAR serving a specific instruments, such as the spectral width data of the runway of the airport. A schematic diagram of dual Terminal Doppler Weather Radar (TDWR) in rain and LIWAS is shown in Figure 1. Under this the thermodynamic profiles provided by a configuration, the laser beam of each LIDAR is better ground-based microwave radiometer, would need to aligned with the respective runway. Data are also be explored. Moreover, there is increasing concern updated moref requently, namely, headwind profilef or on the effect of buildings on the low-level airflow and a particular runway corridor is available every 1 – 2 the building-associated turbulence should be minutes. With the improvement of data coverage, measured as well. LIWAS algorithm is optimized f or 07RD by making ref erence to the pilot windshear reports. An intensive Another major issue with the development of observation period of 07RD windshear occurrence windshear/turbulence alerting services is the was conducted in liaison with the Hong Kong air traffic establishment of the “sky truth” dataset based on control (ATC) and local airlines in the spring of 2009. which the alerting algorithms are established. At the moment, the pilot reports of low-level windshear and The length of windshear ramp detected by the turbulence form the most important source of the “sky LIDAR could be a critical factor in considering the truth”. However, they are prone to subjective issuance of windshear alerts (see the discussions in judgement of the pilots and more objective dataset Shun and Chan (2008)). In the development of would be useful in determining the quantitative LIWAS algorithm for 07RD, two cut-off lengths of relationship between the magnitude of windshear ramps have been examined, namely, the windshear/turbulence and the meteorological data conventional value of 4 km in the existing LIWAS measured at the time of the event. Tothis end, HKO algorithm (4 km is based on the maximum length used obtains the Quick Access Recorder (QAR) data from in the alerting of microburst by the TDWR) and a the local airlines andconducts collaborative study with shorter length of 3 km. The impact of different ramp the National Aerospace Laboratory (NLR) in the lengths on hit rate, f alse alarm rate and alert duration Netherlands to develop the software f or calculating are considered. The study results of LIWAS f or the windshear and turbulence quantities out of the flight spring and early summer of 2009 (January to May) are data. The meteorological measuring system of a summarized in Tables 1 and 2. For the departure f ixed-wing aircraft of the Government Flying Service runway corridors (07RD, and 25LD v iz. departingfrom the south runway of HKIA to the west), the use of a short ramp length (3 km) does not change the hit rate * Corresponding author address: P.W. Chan, Hong v ery much, but could result in much shorter alert Kong Observatory, 134A Nathan Road, Hong Kong duration and smaller false alarm rate. The latter is email: [email protected] particularly evident for 25LD runway corridor over west) and 07RD. A temperature inversion of about 1 which ATC has helped to collect a significant number degree Celsius could be seen between 250 and 650 of null windshear reports f or algorithm development m above ground. The cross-mountain airflow at that purpose. The performance of LIWAS f or the arrival time may have resulted in the warming of an elevated runway corridors in thestudy period is given in Table 2. lay er of the air within the boundary layer. The In general, LIWAS performs well with a hit rate of 3.2-degree conical scan of the LIDAR in the early about 80% and a false alarm rate of 34% or less. morning of 20 January 2009 is shown in Figure 3c. The hit rate f or 25RA (arriving at the north runway of An east to southeasterly jet is discernible over the HKIA from the east) is rather low in the study period airport and a mountain wake (lighter winds coloured in because of the extended f ailure of the north runway brown to grey) could be analyzed to the west of HKIA. LIDAR (note: the hit rate of 07LA is not very much This windshear event is a ty pical case of aff ected because the windshear mostly occurred terrain-disrupted airflow in spring-time. The bey ond 1 nm, which was well covered by the south combined use of LIDAR and microwave radiometer runway LIDAR). data could give the aviation weatherf orecasters more confidence in the issuance of manual windshear It is noted that, though with improved alignment warnings. between the laser beam of the LIDAR and the runway orientation, the hit rate of 07RD windshear is about 4. SHORT-RANGE LIDAR 2/3 only. The missed cases have been studied whenev er QAR data are available. Some examples Apart from the mountains, buildings may also of missed cases are shown in Figure 2. There are a result in disturbances in the low-lev el winds to be number of reasons for the misses, namely, (a) the encountered by the landing/departing aircraft. A headwind change measured by the LIDAR is slightly study of the potential effects of buildings on the less than that encountered by the aircraft (see Figure low-lev el winds was conducted inthe summer of 2009 2(a)), probably because of the transient and sporadic by setting up a short-range LIDAR on the rooftop of an nature of terrain-induced airflow disturbances, (b) the exhibition centre (Figure 4a). This LIDAR has 20 measurement range of the LIDAR is limited due to the range gates with a selectable gate size between 30 low cloud base (see Figure 2(b)), and (c) both aircraft and 75 m. As such, it can measurethe radialvelocity and LIDAR do not show significant windshear of the wind up to a maximum range of 600 to 1500 m. whereas the pilot perceives the occurrence of It is more suitable than the existing LIDARs in the significant windshear with subjectivity (see Figure measurement of building-associated turbulence 2(c)). For the three reasons, point (b) is more related because of the small gate size (the existing LIDARs to the instrumentation limitation that may not be hav e a gate size of 105 m) and more frequent data improved with the use of the LIDAR technology alone. update (every 20 seconds v ersus 1 – 2 minutes of the The use of more sophisticated technologies, such as existing LIDARs). The scanning area of this cloud radar that measures the winds within clouds, short-range LIDAR with respect to the 25RAf light path may need to be explored in order to f ully capture the is shown in Figure 4b. This LIDAR makes horizontal headwind profile along the flight path in humid and scan only within a sector size of about 40 degrees in cloudy conditions. azimuth, and the scan plane is just below the arriving aircraft over 25RA.
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