Progress in local scale flow and dispersion modelling

This review paper provides an overview of current understanding of local scale flows and dispersion with attention to the urban canopy layer and related spatial and temporal scales. The presence of buildings and topographic features are responsible for a vast number of processes ranging from simple drag and friction effects, wakes, corner vortices, flow separation and reattachment to differential heating leading to local thermal circulation. In highlighting key processes at various spatial-temporal scales, it will be shown lesson learnt from recent laboratory and field experiments. Progress made in understanding physical mechanisms occurring in streets, between groups of buildings and above, has inspired the advance of new conceptual models suitable for operational applications and development of sub-grid parameterizations within "urbanized" mesoscale weather prediction models. Among recent developed conceptual framework the one of city breathability is an example of how integrated knowledge (from physics-based understanding to computational fluid dynamics) can capture salient aspects of ventilation and dispersion in cities. After reviewing the relevant processes, the role of buildings, urban morphology and thermal characteristics are examined in view of delineating future developments and challenges.