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471 Alerting of terrain-induced windshear using wind data measured over the mountains Gerrit Kuhlmann and Thomas Hauf University of Hannover, Hannover, Germany P.W. Chan * Hong Kong Observatory, Hong Kong, China 1. INTRODUCTION 10-m high anemometers are installed on somev alleys and hilltops of Lantau Island, such as Pak Kung Au The Hong Kong International Airport (HKIA) is (PKA), Tai Fung Au (TFA), Ngong Ping (NGP) and situated in an area of complex terrain. To its south is Sham Wat hilltop (SW1). The locations of these the mountainous Lantau Island with peaks rising to stations could be found in Figure 1. With pilot about 1000 m above mean sea level with v alleys as windshear reports collected during the operation of low as 400 m in between. It is surrounded by seas in these weather stations, a systematic analysis is the other three directions. The complex topography carried out in this paper to f ind out any benefits of results in low-level windshear encountered by the using the wind data f rom these anemometers in the landing/departing aircraft at HKIA. In aviation issuance of windshear alerts and warnings, by meteorology, low-level windshear refers to sustained comparing their performance with that based on the change of headwind of 15 knots or more at a height of conventional NLS crosswind rule. Three quantities 1600 f eet or below. The major type of windshear at deriv ed from the anemometer readings are HKIA is terrain-induced airflow disturbance when considered, namely, crosswind, gust f actor and winds f rom east, southeast through southwest blow standard deviation of the wind speed. ov er Lantau Island. It accounts for about 70% of the pilot reports of low-level windshear. The next For windshear alertingservices at an airport, the common type is sea breeze, which accounts f or about pilots would be warned through windshear warnings 20% of the pilot reports. The remaining 10% of the that are broadcast in the Automatic Terminal windshear reports is caused by gust f ront, microburst Information Service (ATIS) and windshear alerts that and low-level jet. It should however be noted that are relayed to the pilots by air traffic controllers v ia pilots tend to avoid penetrating thunderstorms, and so radio communication. The ATIS warnings last at the actual percentage of occurrence of least half an hour and are issued by aviation weather thunderstorm-induced windshear should be higher. f orecasters after assessing the likelihood of the occurrence of low-level windshear by considering the Since the majority of the low-level windshear at mesoscale to microscale meteorological patterns. HKIA occurs in clear air condition, two Doppler LIght On the other hand, windshear alerts are issued Detection And Ranging (LIDAR)systems are operated automatically by machine-based algorithms, e.g. in by the Hong Kong Observatory (HKO) for the alerting the Windshear and Turbulence Warning System of windshear. Some background materials of (WTWS) at HKIA. They are updated moref requently low-lev el windshear at HKIA and its alerting by using than windshear warnings, for instance, an update rate LIDAR data could be f ound in Shun and Chan (2007). of every 1 minute in WTWS. The f requent updating Bef ore LIDARs were introduced to HKIA, windshear is necessary because of the transient and sporadic alerting had been achieved by making reference to nature of low-level windshear, such as those anemometer readings inside and around HKIA. associated with terrain-induced airflow disturbances. Such anemometer-based windshear rules are still useful nowadays at times of the maintenance of the 2. DATA SOURCES LIDARs and underthe weather conditions in which the data coverage of the LIDAR is limited, e.g. in rain and The anemometers considered in the present in low cloud-base situations in spring-time. paper are shown in Figure 1. Their heights above mean sea level are given in Table 1. The Among the anemometer-based windshear rules, anemometers on the ground measure the wind speed the crosswind rule, namely, the crosswind component and direction by cup-star and wind v ane respectively. of Nei Lak Shan (NLS, location in Figure 1) with For the weather buoys, propeller-ty pe wind sensors respect to the runway orientation exceeding a certain are used. As a start, 1-minute mean winds from threshold f or the issuance of windshear these anemometer stations are considered in the warnings/alerts, had been developed and adoptedf or paper. For the gust, the maximum v alue of the HKIAfor a long time (since about 2002). A number of running 3-second mean over every 1minute interval is new anemometers have since been set up around taken. HKIA in the recent y ears. For instance, five weather buoys have been deployed over the seas to the east The pilot reports of low-level windshear and and west of HKIA in order to measure the winds turbulence have been used as “sky truth” of the directly under thef light paths. They have a height of occurrence of windshear. Only those reports of about 8 m above sea level. Moreover, standard windshear events occurring within 3 nautical miles from the runway ends are considered. As astart, this paper f ocuses on windshear reports over the * Corresponding author address: P.W. Chan, Hong most-used arrival runway corridor in the spring-time, Kong Observatory, 134A Nathan Road, Hong Kong namely, arriving at the north runway of HKIA from the email: [email protected] west (07LA, location in Figure 1). The longer (in the region of 30 – 40%) in orderto achieve a meteorological reason for each windshear report has hit rate of about 0.75. Though the runway been analyzed by examining all the available weather anemometers are directly located under the flight data. Based on the results of this analysis, only the paths of the aircraft, their readings may only be windshear reports arising f rom terrain-disrupted representative of the conditions along the flight paths. airf low are studied in the present paper. A total of On the other hand, the effect of Lantau terrain on the 519 reports over 07LA are examined here for the occurrence of airflow disturbances to be encountered spring (February to April) in the y ears 2004 – 2007. by the aircraft over 07LA may be better captured by Each windshear alert (when the quantity derived from the anemometers higher up on the mountains or even the anemometer reading meets certain criteria) is the v alleys of Lantau Island. taken to last 1 minute in the calculation of total alert duration for a certain anemometer-based windshear 4. GUST FACTOR rule. It hits a pilot windshear report when there is a windshear alert within 5 minutes backward in time Gust f actor is taken as the ratio between the from the moment of windshear occurrence, 3-second gust within a minute and the mean wind considering the possible time elapsed between the speed of the corresponding minute. Similar to Figure issuance of the alert and the report by the pilot. 2, the frequency distributions of windshear occurrence and all data f or the various gust f actor thresholds of 3. CROSSWIND some anemometers inside and around HKIA are shown in Figure 4. It could be seen that, for most The conventional crosswind rule is considered, anemometers, the frequency distributions do not show namely, the 1-minute meancrosswind component with up having separate peaks except SW (Sham Wat, respect to the runway orientation exceeds a certain Figure 1). The resulting plot of hit rate v ersus alert threshold. As a first step, the frequency distributions duration is given in Figure 5. As expected, it turns of all data and during the occurrence of out that the gust-f actor rule based on SW has the best terrain-induced windshear over 07LA are plotted as a perf ormance. In fact, it works even better than the f unction of crosswind component of the various crosswind rules. For an alert duration of about 17%, anemometer stations. The results are shown in the hit rate reaches about 0.82. The present study is Figure 2. Moments of windshear are based on the based on the pilot reports of a f ew years only and reporting times of low-level windshear by the pilots. more data would be necessary to compare the perf ormance of crosswind-based and It could be seen f rom Figure 2 that, f or certain gust-f actor-based rules. anemometers, the frequency distributions of windshear occurrence and non-occurrence appear to 5. STAND ARD DEVIATION hav e distinct peaks and smaller overlapping areas. Such stations include NLS, YTS (Yi Tung Shan, The standard deviation of the wind at an another hilltop station on Lantau Island, Figure 1), anemometer station is calculated based on the 15 PKA, TFA and SW1. The crosswind threshold is numbers of 1-minute mean wind speed in the last 15 v aried to see how well the crosswind data could be minutes ending at the current minute. The f requency used for providing windshear alerts. The distributions of windshear occurrence and all data at perf ormance of the crosswind rules f or different diff erent values of the standard deviation are shown in anemometers is studied by examining the plot of hit Figure 6. It could be seen that separate peaks are rate v ersus alert duration (i.e. the percentage of the discernible for TFA, SW, SW1 and WB1 (location in total alert duration over the total period of times that Figure 1). Even f or the runway anemometers such data are available f rom the anemometer under as R2W and R2C, separate peaks with not-so-large consideration).

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