## 1 Introduction  
Situated along the southern coast of China, Hong Kong is very often affected by tropical cyclones every summer. On average, there are about four to seven tropical cyclones occurring within 500 km of Hong Kong each year. Hong Kong is also a highly urbanized city. There are many bridges over waters, as well as many open sections of railways. The Hong Kong Observatory (HKO) is responsible for providing weather support for the operation of the bridges and some of the railways, especially in tropical cyclone situations. There are wind speed limits designed for the operation of the bridges and the open sections of the railways. The observatory may be asked to give a trend for the future wind speed given the current wind situation, i.e., to predict whether there will be an increasing trend or decreasing trend in the wind speed, or whether the wind speed will remain roughly the same, in order for the bridge operator to consider closing the bridge. The timely closing of the bridge is of the utmost importance as wind-induced accidents, including the overturning of vehicles, have been reported worldwide [1; 2; 3]. For a recent summary of the literature on vehicle safety assessments along lengthy bridges exposed to high winds, readers can refer to [4]. There have been enquiries about the technical feasibility of forecasting the exceedance of wind speed limits of a bridge or the open section of a railway when Hong Kong is affected by tropical cyclones--for example, with an advance assessment of several hours ahead. The exact forecast of the timing of the exceedance of/failure to reach the wind speed limit is rather challenging, but there could be requests for such services. In recent years, with the increasing computational power at an affordable cost, it has been possible to perform high-resolution numerical weather prediction (NWP) modeling coupled with computational fluid dynamics (CFD) modeling at a high spatial scale, e.g., in the order of meters, along bridges or open sections of the railway in order to study the technical feasibility of supporting bridge/road/railway operation with such an approach. The feasibility of using NWP-CFD coupled models to predict urban winds due to tropical cyclones has also been demonstrated recently [5; 6; 7; 8; 9; 10; 11; 12]. The NWP models are necessary to provide realistic boundary conditions to drive the high-resolution CFD simulations, which can resolve the complex flow patterns around buildings and other structures in urban areas. For example, [PERSON] and [PERSON] conducted a CFD simulation with the k-epsilon turbulence model and boundary conditions from the mesoscale model MM5 to investigate the pressure forces acting on a steel storage building during Hurricane Katrina [5]. They found that the simulated pressure exceeded the damage threshold determined by structural engineers and thus explained the observed damage. [PERSON] et al. studied the wind gusts in an urban district of Osaka City, Japan during Typhoon Jebi [8] by merging a mesoscale weather research forecast (WRF) model and large-eddy simulation (LES) with the Smagorinsky turbulent model [13]. The maximum simulated wind gust was 60-70 ms\({}^{-1}\) and was comparable to the measured values. [PERSON] et al. used a WRF-CFD coupled model to study the wind speeds and pressure fields surrounding a tall building located in a densely built area of Hong Kong during Typhoon Kammuri [11]. The simulated wind-induced vibration of the tall building was also in good agreement with the measurement data. The bridge under consideration in this study, Cross Bay Link, opened in 2022. It is the first marine viaduct in Hong Kong that contains a carriageway, cycle track, and footway.