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This paper analyses over 10,000 quality‐controlled pilot reports of low‐level wind shear ('wind shear') at the Hong Kong International Airport (HKIA; ICAO code: VHHH) between 2001 and 2019. HKIA is well known for its susceptibility to wind shear, which is a potential hazard for aircraft during landing and take‐off. Wind shear at HKIA exhibits strong seasonality with double peaks in the middle of the spring and summer months. There is a strong diurnal cycle in report numbers, peaking towards the afternoon, as modulated by air traffic at HKIA. On average, there are 115 days per year with wind shear reported, with possible power law distribution in the number of days with different number of reports. By comparing the background wind distribution during those minutes with pilot reports against the climatological distribution, wind shear is observed to be, on average, favoured under higher wind speeds (≥6 m s−1) and reduced under lower speeds with shifted frequency peak and heavier tail, although statistical behaviour can differ significantly across individual runway corridors. Preferred background wind directions for wind shear occurrence highlight the role of terrain influence upstream of HKIA. While positive shear (65.6%) is more often reported than negative shear (34.4%), both types of wind shear events appear to follow an exponential distribution in shear magnitude, with similar rates of decay. This is by far the largest statistical study of low‐level wind shear at an airport. Results can serve as an important local reference in addition to being a useful example for airports around the world. [ABSTRACT FROM AUTHOR]

A case of vortex/wave shedding at a mountain near Hong Kong International Airport (HKIA) in a tropical cyclone situation is studied in this paper. Compared with similar studies at HKIA in the past, this paper contains novel results by considering both actual observations and numerical simulation of the event, and by studying the shedding in a neutral boundary layer instead of a stably stratified atmosphere in the spring time. The vortex/wave shedding is clearly observed from the radial velocity imagery of Terminal Doppler Weather Radar (TDWR). The shedding period is about 7 min, which is consistent with previous results in the literature. The vortex/wave adjacent to the background strong southeasterly flow also leads to significant wind shear over the arrival runway corridor to the west of the north runway of HKIA. It may also be associated with the significant headwind changes encountered by an arriving aircraft that had to conduct missed approach due to the rapidly fluctuating winds. The shedding event is simulated by using the Regional Atmospheric Modelling System (RAMS) version 4.4 with a horizontal resolution down to 50 m. The model is found to capture the vortex/wave shedding successfully. The simulated shedding period is similar to actual observations, though the associated area of reverse flow is much smaller. As such, with faster computers, it may be possible to forecast the occurrence of significant wind shear associated with vortex/wave shedding in an area of complex terrain and provide earlier alert to the aviation weather forecasters. Copyright © 2012 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

A weak waterspout was observed at the Hong Kong International Airport on the morning of June 8, 2020. This paper documents the meteorological observations of this weak system, which was well captured by surface wind sensors and a Doppler weather radar, and this study also investigated the capability of a high-resolution numerical weather prediction model to simulate the system in a near real-time configuration. Despite the short-lived nature of the vortex, the model effectively reproduced the occurrence of a weak vortex around the time the system was observed, although with positional errors of the order of a few kilometres. The simulated vertical cross-sections of the reflectivity and Doppler velocity were consistent with the actual radar observations and reflected the slanting nature of the micro-vortex. These results may serve as a useful reference for the study of weak waterspouts worldwide. [ABSTRACT FROM AUTHOR]

A significant wind shear event occurred in the early morning of 27 December 2009 at the Hong Kong International Airport (HKIA), leading to the diversion of three aircraft to the airport of Shenzhen (about 39 km to the north-northwest of HKIA). This paper documents the meteorological observations and predictions for the event. The significant wind shear arose from disruption of the prevailing easterly to southeasterly flow by the complex terrain to the south of the airport. It was well captured by the Doppler Light Detection and Ranging (LIDAR) systems at HKIA and, as such, the aircraft were given sufficient warning about the occurrence of significant wind shear. The headwind changes along the glide paths as measured by the LIDAR were consistent with the aircraft measurements. The observations of the remote-sensing instruments during the event, including the LIDAR, are described in the paper. Moreover, this paper discusses the possibility of including wind shear forecast information for flight planning purpose for this particular case. The forecast wind field near HKIA based on a high-resolution numerical weather prediction (NWP) model is described. For the present wind shear event, it appears that the occurrence of terrain-disrupted airflow near HKIA may be forecast about 6 h ahead. The timing of significant wind shear is, however, not reproduced well in the NWP forecast. Copyright © 2011 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

Small-scale, fast-evolving airflow disturbances, possibly due to upstream building clusters, are known to affect aircraft landing at corridor 25RA of Hong Kong International Airport (HKIA), China, under southerly or southwesterly winds. The paper presents a new algorithm, based on the concept of coherent integrated differential velocity (CIDV), for the automatic alerting of such hectometric (scale of hundreds of metres) turbulence features using short-range LIDAR (SRL) scans at 30 m radial resolution and an observation frequency of around every 20 s by Hong Kong Observatory (HKO). The CIDV identifies the cumulative effect of coherent patches of radial velocity changes occurring along the landing flight path by comparing two consecutive subminute SRL scans. Verification was conducted between June and September 2017 against a selection of relevant pilot reports, with a positive skill level observed over the whole range of the CIDV thresholds considered. When simulating a realistic setting where the SRL would issue alerts in conjunction with HKO's operational Windshear and Turbulence Warning System (WTWS), the CIDV algorithm would bring a gain in hit rate between 5% and 25%, with a corresponding increase in alert duration between 0.4% and 20%. The chosen CIDV threshold of 0.4 m·s-1 nautical mile (nm) can be physically interpreted as an average radial velocity change of 4.0 m·s-1 sustained over a distance of < 200 m (or between 2 and 3 s of flight), which agrees with reported values of crosswind changes considered significant for a landing aircraft and also pilots' perception of the characteristics of such events. [ABSTRACT FROM AUTHOR]

ABSTRACT Vortex/wave shedding is sometimes observed in the Hong Kong International Airport ( HKIA) region by such sophisticated instruments as Doppler LIght Detection And Ranging ( LIDAR) systems. During the past three years several vortex/wave shedding cases have been documented, often to the west of HKIA, downwind of the mountains which can block and disturb the prevailing airflow from the southeast. These shedding movements might trigger turbulence and wind shear, threatening aircraft landing from the west. This paper documents, for the first time, that such shedding can also occur at the runway corridor to the east of the airport's south runway, caused by mountains on the eastern side of Lantau Island. It calculates the shedding period and Froude number of the events, comparing these data with results reported in the literature. It also discusses, for the first time, the use of a computational fluid dynamics ( CFD) model to simulate these events, based on homogeneous initialization only and using the upper air ascent data closest in time to the events. The simulations were shown to successfully reproduce the main features of the shedding, pointing to the possibility of applying this kind of simulation method in real-time forecasts to provide early warning services for aircraft taking off from HKIA. [ABSTRACT FROM AUTHOR]

ABSTRACT The performance and aviation application of minisodars at Hong Kong International Airport are discussed in this paper. There are two minisodars at the airport and they have been in use for more than 2 years. The data availability at various measurement heights shows that the minisodars in general provide reasonably good coverage for aviation applications. The minisodar's wind data compare well with other measurements, including the anemometer data and LIDAR (light detection and ranging) data. The applications of minisodar data are then described. They are found to provide useful indications of low-level windshear, mechanical turbulence associated with complex terrain, and downburst associated with a thunderstorm. Through these case studies, the minisodars are found to provide good supplementary data to other remote sensing meteorological instruments, such as LIDAR and radar, in the monitoring of low-level wind changes that may be encountered by the aircraft. [ABSTRACT FROM AUTHOR]

At about 0155 UTC, 22 February 2009, an aircraft departing from the south runway of the Hong Kong International Airport to the east experienced significant wind shear of headwind loss. This may contribute towards the tail strike of the aircraft. This paper documents the meteorological observations in this tail strike event. The case occurs in a background atmosphere with a stable boundary layer and fresh east to southeasterly winds near the surface. The surface anemometer readings do not indicate the occurrence of significant headwind drop over the south runway. The glide-path scan data of a Doppler Light Detection and Ranging (LIDAR) system over the runway corridor concerned also do not indicate significant changes of the headwind due to blind zone of the LIDAR and geometrical constraint. However, the wind data measured on board the aircraft show that the tail strike may be due to a wind change from headwind of 19 knots to a tailwind of 5 knots over the western and middle parts of the south runway when the aircraft was on rotation. The headwind drop appears to be due to a jet of more easterly component near the surface occurring over the western part of the south runway. This drop could be captured if the LIDAR's headwind profiles over different runway corridors could be combined, and the present case suggests that, for wind shear alerting purposes, it may be necessary to try out combinations of headwind profiles over different parts of the same runway. The possibility of forecasting the significant wind shear in this case is also studied using a numerical weather prediction (NWP) model. The model results show that it may not be possible to forecast the event by using the direct model output alone, but would need to consider both the simulated wind direction as well as the forecast gust near the surface. Here the gust is forecast based on a physical approach as applied to the NWP model output. Copyright © 2012 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

ABSTRACT Two aircraft had to conduct missed approaches in the overnight period of 8-9 September 2010 due to intense thunderstorm activity at the Hong Kong International Airport (HKIA). The missed approach event is studied in detail in this paper using the available flight data and other meteorological measurements. It appeared that the missed approaches were attributable to strong tailwind associated with the downdraft of the thunderstorm. Timely wind shear/microburst alerts had been provided to the pilots in this case based on the measurements from a Terminal Doppler Weather Radar (TDWR) and ground-based anemometer measurements at HKIA. The nowcasting and forecasting aspects for such an event are also discussed. The Aviation Thunderstorm Nowcasting System (ATNS) developed by the Hong Kong Observatory (HKO) is found to provide adequate nowcasting of thunderstorm activity for this particular case and the aviation weather forecasters could be alerted of such intense convective weather at the airport more than half an hour ahead. On the forecasting side, the mesoscale meteorological model used by HKO may provide an indication of the strong tailwind associated with the thunderstorm at 6-7 h ahead, though the exact timing of the tailwind at the airport may be delayed by a couple of hours compared with the actual situation. Using the model the downdraft of the thunderstorm in this event is examined in detail. [ABSTRACT FROM AUTHOR]

In the evening of 11 April 2011, vortex/wave shedding from a mountain to the south of the Hong Kong International Airport (HKIA) appeared to occur following the establishment of fresh to strong easterly airflow in Hong Kong. This paper documents the observations by the various meteorological instruments of the event and discusses the possibility of forecasting the vortex/wave shedding using a high resolution numerical weather prediction (NWP) model. The vortex/wave shedding was well observed by the Light Detection and Ranging (LIDAR) system at HKIA. The vortex/wave appeared as an area of reversed flow against the background east to southeasterly airflow in the airport area. It was detached from the mountain and moved to the northwest with the background southeasterly flow, with a gradual decrease of the spatial extent in the area of reversed flow. The vortex/wave brought about significant wind shear to both arrival flight paths to the west of HKIA, as evidenced from the headwind profile data measured by the LIDARs and the aircraft data. It had rather large vertical extent, reaching the top of the boundary layer, but did not extend downward to the surface, as shown from the vertical scans of the LIDAR and the surface wind observations. The shedding event is successfully captured by high-resolution NWP model with a spatial resolution of 50 m. The model output has a lead time of 6-7 h ahead of the event, indicating the possible occurrence of terrain-induced wind shear in the form of vortex/wave shedding from a mountain. As a result, the numerical simulation could be useful in giving an early alert to the aviation weather forecaster about the significant wind shear event, which could be hazardous to the landing aircraft. Copyright © 2011 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

In the morning of 5 March 2010, an aircraft experienced tail strike when it departed from the south runway of the Hong Kong International Airport to the west. This paper documents the meteorological conditions in the tail strike event, and discusses the alerting and the forecasting aspects of the event. Based on the analysis of the aircraft manufacturer, the tail strike is believed to arise from significant wind shear (headwind loss) experienced by the aircraft. By analysing the surface and the LIDAR observations, it appears that the significant wind shear is related to disruption of the background southwesterly flow by the complex terrain near the airport. From the surface observations, there is a microscale cyclone at the western part of the south runway. The radial velocity image of the LIDAR also depicts an area of reverse flow in that region against the background southerly flow. The alerting of the wind shear based on the two wind shear alerting algorithms developed by the Hong Kong Observatory is discussed, and it turns out that the anemometer-based wind shear algorithm could successfully capture this wind shear occurring close to the ground. On the other hand, due to geometrical constraint, the LIDAR does not pick up the headwind loss experienced by the aircraft. The forecasting of this wind shear event is considered by performing high resolution (down to a horizontal resolution of 50 m) using a mesoscale numerical weather prediction model. The simulated headwind profile at the event time is generally consistent with the aircraft data, though the headwind loss in the simulation is smaller than the reality. Together with the forecast wind gusts, it may be possible to alert the aviation weather forecaster at an earlier time about the occurrence of terrain-induced wind shear over the south runway in this particular event. Copyright © 2011 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

A case of severe low-level windshear at the Hong Kong International Airport was investigated in this study using large eddy simulation (LES) with a spatial resolution of 40 m. The computer simulation results were compared with the actual observations at the airport, including Doppler light detection and ranging (LIDAR) systems and surface automatic weather stations. The LES was found to be capable of reproducing the tiny vortices associated with the terrain-disrupted airflow. Some interesting features of surface temperature and vertical velocity were also observed in the computer simulation. The simulated and observed meteorological parameters had satisfactory comparison results, although the micro-scale variability of such parameters still could not be captured by the high-resolution LES. Finally, the computer-simulated headwind profiles appeared to have skills in capturing the low-level windshear reports from the pilots. The headwind refers to the component of the wind along the direction of the flight path to be encountered by the arrival/departing aircraft. The study was novel in the use of the rather large domain for LES simulation and in the application to low-level windshear alerting. [ABSTRACT FROM AUTHOR]

Low-level windshear poses serious hazards to the flight safety of civil aircraft. The objective of the study was to assess the risk of windshear occurrence within 1 hr at runway corridors at Hong Kong International Airport (HKIA). Windshear report records from HKIA and weather data around HKIA were obtained from Hong Kong Observatory (HKO) between January 2016 and December 2018. The rare-event logistic regression was employed to evaluate windshear risk considering the imbalanced nature of the data. Several explanatory variables were found to be significantly correlated with the risk of the hourly occurrence of the windshear. Results indicated that at HKIA, March, April, July and August are associated with a higher likelihood of windshear events, while November suffered a significantly lower windshear risk. Besides, winds from the east, southeast, south and southwest are more likely to cause windshear events at HKIA. However, the study found that it was hard to predict windshear occurrence precisely only with the hourly weather variables due to the transient and sporadic nature of windshear. Therefore, more refined data are needed to improve the prediction performance of regression models for windshear occurrence. The present study is the first to analyse the windshear occurrence at airports using the rare-event logistic regression method. Findings from the study can provide some references and valuable information for related researchers to understand low-level windshear. [ABSTRACT FROM AUTHOR]

Located at the southeastern coast of Guangdong Province of China, Hong Kong is susceptible to the effects of severe convective weather and tropical cyclones. The Hong Kong International Airport (HKIA) is susceptible to windshear and microbursts caused by convective weather. Due to the complex terrain around the HKIA, windshear caused by strong winds of tropical cyclones across complex terrain can be as strong as microbursts generated by thunderstorms. A Terminal Doppler Weather Radar (TDWR) has been used to detect microbursts and windshear in real time to safeguard aviation safety. The TDWR at Brothers Point, Tuen Mun, Hong Kong, was optimized to detect windshear and microbursts in a highly cluttered environment. Stationary and moving clutters were suppressed and radar velocity data were dealiased by the hybrid multiple pulse repetition interval method during radar system optimization. A case on October 4, 2015 showed that the TDWR performed satisfactorily with a probability of detection and a false alarm ratio of 91% and 1% respectively in detecting microbursts by comparing microburst features identified by the TDWR and human truth, which is a method to identify areas of interest by analysts. [ABSTRACT FROM AUTHOR]

With the aim of developing a forecasting system for the prediction of adverse meteorological conditions in the vicinity of complex terrain for aircraft landing at Hong Kong International Airport, the linearized, quasi‐empirical FLOWSTAR model has been configured to represent a series of eight landing events (2015–2017); half of these events reported significant wind shear and, for two, the landing attempt was aborted. Meteorological data used as input to the model have been derived from Hong Kong Observatory radiosonde measurements; these are the most reliable available despite being recorded at a location distant from the airport and at one or more hours before or after landing events. In general, the model predicts stronger wind shear for cases where adverse conditions have been reported. Analysis of the magnitude of wind shear generated as a result of wind veering and backing helps to quantify the importance of nonlinear flow features such as trapped lee waves. The model is able to represent strong downslope flows and recirculating flow regions (rotors) generated by the mountainous region located upwind of the airport. Comparisons of model predictions against LIDAR traces indicate that the model replicates wind shear for 50% of cases where it was recorded, although the location and magnitude of the wind shear usually differs between the modelled and measured datasets; the lack of agreement is likely to be because the input meteorological conditions are not wholly representative of the upstream conditions and the LIDAR trace does not exactly coincide with the aircraft trajectory. When airports are located in hilly terrain, near‐ground flow features such as wind shear and turbulence periodically cause disruption to aircraft movements. The FLOWSTAR model has been configured to represent favourable and unfavourable landings at Hong Kong International Airport; the model predicts high wind shear at some locations along the flight path (black dotted line). Quantitative analyses of wind shear generated as a result of wind veering and backing demonstrate the importance of flow features such as trapped lee waves. [ABSTRACT FROM AUTHOR]

ABSTRACT Hong Kong International Airport ( HKIA) is situated in an area of complex terrain. Mountains near the airport can lead to terrain-disrupted airflow disturbances, which may bring about hazardous weather phenomena for aviation, such as low-level windshear and turbulence. Timely forecasting of such airflow disturbances would be useful in providing early warnings to the pilots. The Hong Kong Observatory has been running a super high resolution numerical weather prediction model suite, the aviation model ( AVM), since late 2013, with spatial resolution up to 200 m immediately around HKIA. After testing and tuning over the past year, the performance of the AVM in forecasting terrain-disrupted airflow at the airport area is investigated in this study. Two typical cases of such complex airflow are considered, Foehn wind and mountain wave. The AVM is found to capture successfully some salient features of the airflow as observed around HKIA through comparison with light detection and ranging (LIDAR) and weather buoy observations. The model also helps in shedding new light on the airflow from the vertical cross sections of the winds across the mountains. Limitations of the AVM are also discussed. [ABSTRACT FROM AUTHOR]

ABSTRACT Rapid fluctuations of wind shear hazard factor (or F-factor) calculated from the headwind profile measured by Doppler Light Detection and Ranging ( LIDAR) may make it difficult for use in wind shear alerting at an airport such as Hong Kong International Airport ( HKIA). Preprocessing by flight simulator or post-processing by averaging over a certain length are attempted in this study to smooth the F-factor and enhance its skills on wind shear alerting. The F-factor so obtained is used for alerting of wind shear (headwind change exceeding 15 knots) for departing flights. Through case studies and statistical studies, the LIDAR-based F-factors are found to have good correlation with the F-factor determined from flight data (the Quick Access Recorder, or QAR, data from the commercial jets). Using the relative operating characteristics ( ROC) curves, the LIDAR-based F-factor obtained by smoothing over a window size of 300-500 m is found to have good skills in wind shear alerting, and the performance is comparable with, or even better than, the LIDAR-based F-factor using the flight simulator approach. By choosing an optimal alerting threshold specific for each runway corridor, the F-factor successfully alerts 86% of pilot wind shear reports with the percentage of time on alert of 12%, which is found to be satisfactory for real-time use. The performance of the F-factor on stronger wind shear (headwind change of 25 knots or more) is also studied. [ABSTRACT FROM AUTHOR]

ABSTRACT A theoretical and computational study is presented of the wind flow over the mountain ridges to the south of Hong Kong International Airport given the upwind profiles of 'mean' velocity and temperature. A sensitivity study shows how large variations in wind speed, wind shear and wind direction occur on the approach path when the upwind flow from the southeast is stably stratified above the boundary layer with a significant inversion no more than a few hundred metres above the ridge (height about 450 m). The Froude number is close to unity. The effects of vertical wind shear across the inversion layer reduce the speed-up, and directional wind shear changes the surface flow direction. The observed horizontal length scales along the approach are about equal to the projected widths of the predominant features of the ridges and valleys. For an extreme wind event there was found to be satisfactory agreement between the wind speed measured by a landing aircraft and the predictions of the fast quasi-linear analytically based computer model FLOWSTAR. This suggests that near real time application of the model and near real time monitoring with remote and in situ instruments can be used to predict these extreme flow events and hence give warnings about them. Conclusions are drawn about how such predictive modelling could be developed. [ABSTRACT FROM AUTHOR]