Mountain waves near Hong Kong International Airport: observations and high‐resolution model analysis

Hong Kong International Airport (HKIA) is located in an area with complex terrain (Figure 1). To its south is the mountainous Lantau Island with mountain peaks rising to approximately 1000m above mean sea level (amsl), with valleys as low as 400m in between. To the north-northeast is Castle Peak with a height of approximately 300m amsl. The mountains can produce airflow disturbances that may be hazardous to aircraft landing at or departing from the airport. Mountain waves have been observed in the airport region following the introduction of Doppler Light Detection and Ranging (LiDAR) systems see (Shun and Chan (2008) for technical specifications). Shun et al. (2004) provide the first account of a mountain wave observation in this region. With the introduction of highresolution numerical weather prediction (NWP) models, mountain waves at HKIA have been studied in detail through case studies; an example can be found in Chan and Hon (2016a). Through comparison with LiDAR observations, it has been observed that the 200m resolution NWP model reproduces the Doppler velocity signature of mountain waves very well. The mountain waves studied so far are mostly to the east of the airport island, downwind of Yi Tung Shan which is a minor peak of the Lantau Island, approximately 750m amsl (Figure 1). Based on LiDAR observations accumulated over the years, it can be inferred that there are many more mountain wave events occurring around HKIA that would be interesting to observe and worthy of more detailed studies. Furthermore, compared to well-studied mountain ranges in other parts of the world (for example, the Rocky Mountains (Guarino et al., 2018) or the Alps (Jiang and Doyle, 2004)), the Lantau Island is only a few times smaller in terms of altitude but much smaller in terms of horizontal dimensions. Microscale flow phenomena generated by the Lantau mountains with generally steeper slopes can be expected to pose additional challenges to the spatio-temporal resolution of remote-sensing observations and numerical models. This paper documents recent observations of mountain waves over previously less studied areas of HKIA: namely, the western and central parts of the airport island (respectively downwind of Nei Lak Shan, a minor peak approximately 750m high, and Pak Kung Au, a gap; locations in Figure 1), and the area west of Castle Peak. Upper-air soundings are examined to investigate if the observed waves are consistent with classical mountain wave theory by computing the Scorer parameter (Scorer, 1949). The Scorer parameter is especially useful to detect conditions necessary for the existence of trapped lee waves; however, this does not confirm the presence of actual trapped wave modes, for which the resonant condition must also be fulfilled. For the case of a hydraulic jump, the numerically simulated feature is consistent with the previous comprehensive study by Vosper (2004). The performance of a sub-kilometre resolution NWP model in simulating the mountain waves is also studied. The mountain waves are studied in detail based on the results of the numerical simulations, for example, using vertical transects across the waves to visualise the structures in velocity distributions. It is expected that the paper can serve as a useful documentation of mountain waves for similar studies in other mountainous areas, as well as a demonstration of the capability of fine-resolution NWP models in predicting such microscale phenomena under real-time settings.