An assessment indicator for air ventilation and pollutant dispersion potential in an urban canopy with complex natural terrain and significant wind variations

In an urban planning context, an assessment indicator for evaluating a city's dispersion potential is beneficial, especially if the city has a complex natural terrain and significant wind variations. A study was conducted to implement an urban canopy drag indicator, taking site wind variation into account by involving both wind speed and direction in the calculations. Hong Kong (HK) was taken as an example due to its complicated natural topography and wind characteristics. A spatial distribution of an urban canopy drag over HK was determined based on wind data from 2004. The urban canopy drag values in three highly urbanized areas in HK, including Kowloon West, Kowloon East and Hong Kong Island North, were obtained and are discussed in detail. A fluid particle tracking program was developed and applied to identify the major wind paths in Kowloon West, with an area of approximately 5.5 × 6 km as an example. We analyzed the diurnal variation in the dispersion times and the major wind paths in the region during both summer and winter. Our results estimated that the horizontal dispersion times of Kowloon West during both winter and summer were approximately 20 min. By combining the wind paths from both seasons, we identified several major wind paths and critical ventilation areas in Kowloon West. This paper demonstrates the potential use of an urban canopy drag indicator for assessing air ventilation and pollutant dispersion in a city planning context.