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Yansen Wang, Cheryl L. Klipp, Dennis M. Garvey, David A. Ligon, Chatt C. Williamson, Sam S. Chang, Rob K. Newsom, and Ronald Calhoun

a topic of research. Due to the interaction of high roughness and thermally generated turbulence, the urban atmospheric boundary layer is complex and difficult to study. Many observational studies have focused on intraurban surface fluxes using standard micrometeorological towers. Very few observations have been made above the building height. Most published urban boundary layer (UBL) field studies have concentrated on convective conditions ( Jackson 1978 ; Ching 1985 ; Godowitch 1986

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Donald A. Burrows, Eric A. Hendricks, Steve R. Diehl, and Robert Keith

sources are modeled explicitly but require information on the atmospheric conditions, especially turbulence due to mesoscale circulations and boundary layer processes. For mesoscale models, estimates of the heat and momentum fluxes due to urban effects are needed, as is a characterization of the city in terms of surface roughness or drag coefficient. Urban-scale models could be used to provide better information to mesoscale models regarding the local influence of large building areas of a city. Of

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Cheryl Klipp

1. Introduction The complexity of urban environments makes interpretation of urban boundary layer data difficult at best, yet it is in urban and suburban areas where the largest population impacts of pollution and accidental releases of hazardous substances occur. The surface heterogeneity produces a complex vertical layering of multiple internal boundary layers and a horizontal patchwork of local microclimates ( Oke 1995 ). The atmosphere immediately above these heterogeneities is not a

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Stevens T. Chan and Martin J. Leach

that to simulate urban dispersion scenarios successfully under light and highly variable winds it is necessary to use appropriate time-dependent forcing and turbulence from the larger-scale flow through the inflow boundary. Their results also indicate that inflow turbulence is as important, if not more so, than building-induced mechanical turbulence in dispersion scenarios under the above conditions. Although high-resolution CFD models are very useful for emergency planning, vulnerability analyses

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P. Ramamurthy, E. R. Pardyjak, and J. C. Klewicki

building height in scale-model cities immersed in a neutral stability flow. For neutral conditions over a large range of building frontal and plan areas, Macdonald et al. (2002) have identified the basic shapes for mean and turbulence profiles within and above regular, idealized arrays of buildings. During stable conditions, many questions still exist regarding the effect of upstream stability on flow within cities. This is likely a result of the high variability of urban morphologies, land uses, and

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Steve R. Diehl, Donald A. Burrows, Eric A. Hendricks, and Robert Keith

“filtration.” The model, which is based on similitude curve fits ( Seinfeld and Pandis 1998 ; Slinn 1977 ), is a function of particle size and density, surface characteristics, surface layer meteorological conditions, and vegetation characteristics. Each ground cell of the flow grid can have its own surface/vegetation qualities, including canopy type, a condition that allows the deposition rate to be modeled on a cell-by-cell basis. The primary limitation of the model is the assumption that the boundary

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Eric A. Hendricks, Steve R. Diehl, Donald A. Burrows, and Robert Keith

the modified set of the RANS equations. Then, MESO uses the RUSTIC steady-state wind field and eddy viscosities to predict the transport and diffusion of a released agent. These eddy viscosities change significantly with atmospheric stability, strongly affecting the dispersion. As an example of this, simulated surface concentrations are shown in Fig. 2 for stable (surface upwind heat flux of −30 W m −2 ) and unstable (+250 W m −2 ) conditions in the Oklahoma City CBD for a surface release. In

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M. A. Nelson, E. R. Pardyjak, J. C. Klewicki, S. U. Pol, and M. J. Brown

1. Introduction Urban areas affect the mean and turbulent flow characteristics of the atmospheric surface layer (ASL). This is realized through a variety of mechanisms such as enhanced form drag, heat storage, vortex shedding, etc. ( Roth 2000 ). The urban boundary layer is defined as the region of the ASL where the effects of the urban landscape can be detected ( Oke 1987 ). Understanding the mean flow structure and turbulence characteristics of ASL flow in and through urban areas is essential

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M. A. Nelson, E. R. Pardyjak, M. J. Brown, and J. C. Klewicki

flow within the UCL, field experiments in real-world UCLs are needed to verify that the idealized conditions simulated in numerical and laboratory UCL studies capture the essence of the UCL flow in real cities. In recent years several UFM studies have been performed in various cities around the world that include measurements within the UCL ( Nakamura and Oke 1988 ; Rotach 1993 , 1995 ; Louka et al. 2000 ; Nielsen 2000 ; Longley et al. 2004 ; Dobre et al. 2005 ; Rotach et al. 2005 ); however

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Julia E. Flaherty, David Stock, and Brian Lamb

modeling, boundary conditions, modeling cases, and grid uncertainty. The results of this modeling investigation are presented in section 4 . Conclusions and future work are presented in section 5 . 2. Field measurements The Joint Urban 2003 (JU03) campaign was a major field study conducted in the summer of 2003 in Oklahoma City, Oklahoma ( Allwine et al. 2004 ). Meteorological data were collected continuously throughout Oklahoma City, while tracer releases were performed during 10 main intensive

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