Recently the world’s population demographics crossed an important threshold: more people now live in urban than in rural areas (UN 2006)

Recently the world’s population demographics crossed an important threshold: more people now live in urban than in rural areas (UN 2006). Projections show that the urbanization continues at a significant rate of ~ 1.5% per year, presenting difficult challenges of providing basic needs such as clean air, water and security for urban dwellers while managing available resources efficiently. In this context, planners need to take into account the symbiotic workings of several urban systems -for example, environmental, ecological, transportation, energy, material, social and political -and their complex interplay. This paper concerns one of the relevant aspects, namely, the meteorology and turbulence in urban airsheds, which has important implications in air quality, energy usage and human comfort. Noting that more than ~ 70% of the world topography is in areas replete with mountains, valleys and escarpments, of particular interest will be the transport and dispersion of contaminants in urban areas located in complex terrain, which have profound implications in determining the quality of air and pathways of contaminant releases (and hence emergency response). Although it is generally sufficient to make air quality calculations on scales on the order of neighborhood scales (~ a few hundreds of meters to km scales), calculations of contaminant dispersion in downtown areas (e.g. Central Business Districts CBDs) for emergency response need to be on a much smaller scale, paying attention to the details of the building clusters and roadway canyons within. Therefore, a successful modeling system ought to cascade from synoptic (~ 1000 km) to personnel (1 m) scales, incorporating the effects of terrain, vegetation and building canopies (Figure 1). One of the ways of parsing such continuum is to develop a nested modeling system, starting from a large scale model (say NCEP eta model) to a personal scale model. In the modeling system to be discussed, the CFD code developed by Baik et al. (2003) is used for small scales with the intermediate scales calculated using a mesoscale model (MM5).