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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]

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]

Dispersion modelling is a key component of modern emergency responses to catastrophic atmospheric releases. However, periodic algorithmic advances are needed to effectively use new datasets acquired with modern remote sensing instruments. This work demonstrates that coherent Doppler lidar can be used to provide valuable new inputs for dispersion models. While related research seeks to retrieve other required inputs for dispersion modelling systems, for example velocity vectors from radial velocities, this paper assembles and contextualizes analytical and algorithmic approaches for an improved understanding of dispersion characteristics in specific atmospheric scenarios using Doppler lidar data. Longitudinal (along-wind), lateral (cross-wind), and vertical dispersion parameters are calculated and used to estimate eddy diffusivities based on Gaussian curve fitting and first-order closure. Empirical relations based on similarity theory are used to verify these estimates, and reasonable agreement is found between the two approaches. Several improvements are also suggested for the lidar scanning techniques to facilitate retrieval of dispersion parameters. Copyright © 2010 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

Wind information in urban areas is essential for many applications related to air pollution, urban climate and planning, safety of drone‐related operations, and assessment of urban wind energy potential. These applications require accurate wind forecasts, and obtaining this information in an urban environment is challenging as the morphology of a city varies from street to street, altering the wind flow. Remote sensing techniques such as Doppler lidars (light detection and ranging) provide a unique opportunity for wind forecast verification as they can provide both the vertical profile of the horizontal wind and the spatial variation in the horizontal domain at high resolution. In this study, the performance of numerical weather prediction (NWP) models, analysis systems, and large‐eddy simulation (LES) models have been analysed by comparing the modelled winds against Doppler lidar observations under various atmospheric conditions and from season to season, in the coastal environment of Helsinki, Finland. The long‐term mean vertical profile of the modelled horizontal wind shows good agreement with observations; the NWP model and the analysis systems selected here exhibit different strengths and weaknesses depending on the atmospheric conditions but no significant diurnal variation in performance. However, both the model and analysis systems show differences in their spatially‐averaged bias when investigating different wind directions. LES verification shows that these models can potentially provide winds down to street level, given pre‐computed scenarios of atmospheric conditions. For Helsinki, the observed winds are stronger during winter than summer, and, on average, higher wind speeds were observed at the urban site than the sub‐urban site. [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]

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]

Roughness length ( Z0) is traditionally used as the primary measure of the aerodynamic roughness of a surface, but is notoriously difficult to estimate. This study takes a new approach to the estimation of Z0, using high resolution LIDAR data coupled with spatial processing techniques to provide estimates of effective roughness length ( Z0eff) based upon the prevailing wind direction and the height of the surface elements (e.g. buildings, trees) within a defined area of upwind fetch. The range of roughness values obtained using this new technique is consistent with published values obtained from detailed boundary layer experiments, and is shown to distinguish between a variety of landuse categories, ranging from high density urban areas to rural farmland. Indeed, comparisons in Z0eff values between different landuse classes using a detailed land-cover dataset have revealed significant differences in surface roughness between landuse classes, giving confidence not only in the technique itself but also to the validity of the land-cover dataset used in the study. Copyright © 2011 Royal Meteorological Society [ABSTRACT FROM AUTHOR]