||In this thesis, multi-scale modelling is used to study the mesoscale and microscale meteorology (MMM) in complex terrain and building geometry. The first part of this thesis couples a mesoscale model (MM5) and a microscale model (CALMET) to provide more detailed and realistic wind field simulations over Guangdong (GD), Pearl River Delta (PRD) and Hong Kong (HK), well known as regions with complex terrain. After model validation, the MM5/CALMET system is utilized to generate wind maps over GD, PRD and HK with resolutions of 3 km, 250 m and 100 m respectively. The wind maps are utilized to analyze year-to-year variations (2004-2006) of wind availability over southern China and investigate the wind energy potential over GD, PRD and HK. The second part of the thesis couples the MM5/CALMET system with a multi-layer, non-steady-state puff dispersion model, “CALPUFF”. This is used to identify the SO2 contribution of PRD and HK power plants, road vehicles and marine vessels to the air quality of HK in summer and winter. In this study, the observation data at 11 general Environmental Protection Department (EPD) stations and simulation results are analyzed. In-zone and out-zone stations are defined to examine the SO2 contribution of emission sources to different parts of HK. Beside large-scale pollution problems, this thesis also investigates the finer scale (microscale) pollutions, as the “wall effect” in the urban area is a concern in HK. In this study, computational fluid dynamics (CFD) is utilized to achieve a deeper understanding of the wind speed reduction at the pedestrian level (2 m above ground) due to differing height of frontal high-rise buildings. The retention time of pollutants emitted from road vehicles is also investigated to quantify the influence of pollutant dispersion in a street canyon environment.