Simple numerical modelling on Urban Heat Island in Hong Kong

Urbanization, including land use changes, building developments, emission of heat and human activities, has a significant impact to our environment and would also contributions to urban heat island (UHI) effect. UHI effect means the cooling rates in the urban space is slower than rural space. Provision of greenery, water feature, ventilation and high albedo material could reduce UHI effect and improve the thermal comfort of the outdoor environment. However, the lack of quantitative understanding about the UHI effect has prevented us systematically implement mitgrtion strategy during urban planning in Hong Kong. Traditional computational fluid dynamics (CFD) approach is often used to study the microclimate of a small-scale area. The large domain size of the UHI modelling would induce problem of high computational power, long simulation time and complex setting. Therefore, traditional CFD approach is rarely used to study the UHI phenomenon. This research has developed a simplified CFD approach to quantify the UHI effect. The new approach used volume averaging approach on the Navies-Stoke equation and developed sets of sub-models for simplification. The new approach is able to simulate the thermal effect through radiation, conduction, convection, evaporation and etc, to re-create the thermal flow effect within the urban space under coarse grid model. Hong Kong local weather data has been analysed to understand the impact of the urban characteristic to the thermal environment. Based on the analysis, Ta Kwu Ling is selected for the reference rural weather station for calculating UHI Intensity. A database has been created using GIS data, which stored the local urban characteristic of Hong Kong, including the greenery area, road area, construction area, building area, sea area, average building and terrain height. The new approach has reduced the input requirement, modelling complexly, simulation time and computational power for simulating UHI effect in an urban space. The Tsim Sha Tsui Case study has shown the new approach can reduce the number of required grid cells 4000 times and reduce the required CPU time from 50,000 hours (CPU × 1) to 0.1 hour (CPU × 1) as compared with the traditional CFD approach. Thus, the new UHI modelling approach is more practical to be applied during the urban planning process as compared to the traditional. This thesis lays a foundation to predict the outdoor thermal environment and the UHI impact and to guide a sustainable urban design and planning through a performance-based approach