Estimating the Effect of Transit Oriented Developments on Wind Speed and Turbulence

A transit oriented development (TOD) is typically a compact community of homes, offices, and shops built within walking distance of a transit station, such as a light rail or a bus station. TODs can change the dispersive ability of the atmosphere relative to that in open terrain. We are conducting a field study of dispersion within TODs in which we measure particle concentrations and meteorology within different built environments within Los Angeles, and simultaneously measure meteorology at a location located upwind of the urban area. Our goal is to develop a model that allows us to understand the effect of the built environment parameters such as the building height, plan area fraction, and frontal area fraction, on the dispersion within a TOD. In this paper we use the observations obtained in our field study to evaluate several alternative models that explain how the urban area modifies the turbulence and wind speed that is measured at the upwind location. One model uses a two-step approach. An internal boundary layer model is used to model the evolution of the boundary layer as it travels from the upwind rural area to the built urban area. We then use a model for the urban canopy layer to relate micrometeorological variables above the urban canopy to those within it. The variables within the urban canopy are then used to estimate the dispersion of pollutants emitted at street level. The predictions of these models are compared with those of the Operational Street Pollution Model (OSPM), which is widely used to model concentrations within urban areas.