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Abstract A tornado/waterspout associated with a supercell thunderstorm was observed in the adjacent waters of southern China on the evening of 18 April 2019. The case was documented using surface weather observations and Doppler radar data. A weather station near the tornado/waterspout, recorded wind gusts corresponding to hurricane intensity for a few seconds, never observed before in the region. The Doppler velocity associated with the tornado/waterspout was analysed and the vertical profile of the velocity could be very useful for wind engineering applications in the region. Dual‐polarization observations also were analysed and exhibited some similar signatures documented in supercell tornadoes in other parts of the world. Predictability of the tornado/waterspout was studied using a numerical weather prediction model. It showed that the location and timing of the waterspout/tornado could be roughly reproduced, and the model might be used to give earlier alerting to the vessels operating in the south China coastal waters.

Abstract 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 Situated just north of the mountainous Lantau Island, Hong Kong International Airport (HKIA) can experience terrain‐induced airflow disturbances under certain meteorological conditions, such as during a tropical cyclone. For approaching aircraft, such disturbances can lead to wind shear, jeopardizing flight safety. In this numerical study, large‐eddy simulations of turbulent airflow over Lantau Island are performed under neutrally stratified conditions with a view to understanding how terrain‐induced disturbances affect the headwind and crosswind experienced by aircraft landing at HKIA. Analysis of 1 min output data shows that, consistent with previous studies, high‐speed transient flow disturbances can emerge from gaps in the mountains of Lantau Island, producing spatiotemporal variations in wind speed and direction along the glide paths. An attempt is made to quantify the relative likelihood of wind shear by analysing how frequently wind‐speed fluctuations due to terrain interaction exceed predefined thresholds within a prescribed time window. The results show that wind shear is most likely to occur (1) on the eastern approach to the south runway (25L) in a southerly background wind; and (2) on the western approach to the south runway (07R) in a southeasterly background wind. These findings are consistent with the present operations at HKIA, where the north runway, which has a lower likelihood of wind shear, is used as the predominant landing runway. The simulations also show that the proposed third runway, which is currently under construction, will see an even lower likelihood of wind shear because it is the farthest of the three runways from Lantau Island. The study provides new insight into the unsteady flow behaviour at an airport surrounded by complex terrain, contributing to aviation safety.

Terrain-induced windshear at Hong Kong International Airport (HKIA) could be hazardous to the landing and departing aircraft. Such windshear occurring in a planetary boundary layer without temperature inversions is studied in this paper by using the data from the Terminal Doppler Weather Radar and Light Detection and Ranging systems. A high resolution numerical model, called aviation model (AVM), is also employed to find out its capability to forecast the occurrence of such windshear. The model is found to have skills in capturing the terrain-induced windshear, including the terrain-induced microburst due to the mountains of Lantau Island. Moreover, the windshear detection algorithm as applied to the AVM output, called AVM-GLYGA, is able to give advance alert for the occurrence of low-level windshear. The model also offers new dataset, such as vertical velocity and vertical cross sections across the windshear feature, to study the terrain-induced windshear phenomena with new insights. The AVM is found to have good skills in depicting the terrain-disrupted airflow at the airport area, and more comprehensive study would be conducted to study the skills of AVM-GLYGA as compared with pilot windshear report as sky truth.