J5.2
Physical modeling and its role towards improving understandings of transport and dispersion as relevant to urban -coastal area
Petra Klein, Univ. of Oklahoma, Norman, OK; and B. Leitl and M. Schatzmann
Due to an ongoing trend of urbanization, the majority of the world's population lives in cities. Transport and dispersion of atmospheric pollutants in an urban environment are thus of great concern, but the complex nature of these processes still poses a challenge to the research community. Of particular interest are the processes inside the roughness sublayer (RSL) – the lowest part of the urban boundary layer where both the effects of urbanization on atmospheric processes are most significant and, in turn, city dwellers are most directly affected by increased pollution levels. Within the RSL, both mean transport and diffusion are significantly altered by the presence of buildings, and relatively high emission rates can be found as the majority of urban air pollutants result from traffic and industrial processes. Accidental or intentional releases of air toxics are also of concern and can pose a great threat to urban populations. Dispersion patterns in urban areas are known to be spatially rather inhomogeneous: while mixing of pollutants may be enhanced in some areas, flow regions with low ventilation rates can develop in street canyons or buildings wakes, and in such regions pollutants may accumulate and reach harmful concentration levels. More recent studies have also shown, that tall buildings may enhance vertical mixing and that urban plumes are rather instationary. High resolution information – both in time and space – is thus of crucial importance for evaluating potential risks associated with releases of pollutants within the urban RSL. To obtain such information basically three different options exist: field measurements, physical modeling in wind tunnels, and numerical modeling. While field measurements have the advantage of providing the most realistic representation of the actual conditions in a study area, they are rather costly, do not offer the flexibility to study different scenarios, allow only deployment of a limited amount of sensors, and the atmospheric conditions cannot be controlled which limits the statistical significance of the collected data. Numerical modeling on the other hand has become a rather inexpensive tool to study atmospheric phenomena and gives the user the flexibility to study different scenarios with different release configurations and variable boundary conditions. However, it is still difficult to resolve the complexity of urban flow and dispersion patterns with numerical models and typically only mean concentrations are computed. Physical modeling of dispersion phenomena on the other hand has been proven to be a useful tool to provide reliable, high-resolution data of both flow and dispersion inside the urban RSL and to study the instantaneous nature of these processes. The paper provides an overview of recent wind-tunnel studies of urban transport and dispersion phenomena and summarizes important findings. Using data from the Joint Urban 2003 studies, a comparison of full-scale and wind tunnel concentration data is also presented. The analysis will focus not only on mean values but also include a discussion of plume intermittency and concentration fluctuations in urban areas. To better describe these parameters is relevant for a number of practical applications like e.g. dispersion predictions for air toxics for which peak values become more important than mean concentrations.
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Joint Session J5, Plenary: Modeling for Emergency Response and Air Quality in Urban-Coastal Areas
Thursday, 13 September 2007, 8:30 AM-10:30 AM, Kon Tiki Ballroom
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