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

their complex radiative, thermodynamic, and aerodynamic characteristics ( Oke 1988 ; Arya 2001 ). Additionally, the quasi-random behavior of turbulent eddies that arise from mechanical and thermal interactions of the flow with buildings are difficult to predict and also very sensitive to slight perturbations in the inflow. Because of these inherent complexities and for the sake of simply predicting general hazard area definitions, the common approach has been to use models that provide a solution

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

) models [e.g., the “Fluent” ( Fluent 2005 ) and Flame Acceleration Simulator (FLACS; Hanna 2004 ) models]. The CFD models attempt to solve for the flow at relatively high resolution to capture eddy formation using intensive calculations, and they take significant amounts of computer time for fairly small geographic regions. These models are very useful for examining the time-averaged structure of the flow at fairly small scales and for describing the turbulent shedding that occurs, but, in general

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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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Stevens T. Chan and Martin J. Leach

Dispersion of Hazardous Material Releases (2003) . Among the recommended priorities for improving modeling capabilities, the report states “New dispersion modeling constructs need to be further explored and possibly adapted for operational use in urban settings. This includes advanced, short execution time models, slower but more accurate computational fluid dynamics and large-eddy simulation models, and models with adaptive grids.” Under the sponsorship of the U.S. Department of Energy (DOE) and U

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

observed during 9 of the 10 intensive observation periods (IOPs; De Wekker et al. 2004 ). The LLJ and its associated wave motions have been observed in many other investigations in the Great Plains of the United States. Using a Doppler lidar in the Cooperative Atmosphere–Surface Exchange Study 1999 (CASES-99), Banta et al. (2002) , Newsom and Banta (2003) , and Blumen et al. (2001) recently showed that nocturnal LLJs were often at or below 100 m above ground level. As shown in the continuous

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

, of these works only Rotach (1995) and Dobre et al. (2005) present velocity spectra measured within the UCL. The spectra presented in Rotach (1995) were limited to the upper region of the UCL with the lowest measurement at zH −1 ≈ 0.7. Dobre et al. (2005) present a single representative along-street velocity spectrum obtained much deeper in the UCL ( zH −1 ≈ 0.23). However, they only used the velocity spectrum to demonstrate the range of the most energetic flow scales as justification

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Julia E. Flaherty, David Stock, and Brian Lamb

building geometry and require minimal parameterizations as compared with a Gaussian model. Knowledge gained from computational efforts can be used for guidance in urban design to explore pollutant “hot spots,” minimizing personal exposure, and ensuring proper positioning of air intakes for building heating–ventilation–air conditioning systems. In recent years, researchers have conducted CFD studies on various geometries. For example, there have been numerous investigations with single buildings, such

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

atmospheric stability on turbulence statistics deep within an urban street canyon. J. Appl. Meteor. Climatol. , 46 , 2074 – 2085 . Richards , P. J. , S. Fong , and R. P. Hoxey , 1997 : Anisotropic turbulence in the atmospheric surface layer. J. Wind Eng. Ind. Aerodyn. , 69–71 , 903 – 913 . Rooney , G. G. , 2001 : Comparison of upwind land use and roughness length measured in the urban boundary layer. Bound.-Layer Meteor. , 100 , 469 – 486 . Rotach , M. W. , 1993a : Turbulence

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Julia E. Flaherty, Brian Lamb, K. Jerry Allwine, and Eugene Allwine

urban landscapes, how these processes combine and interact with one another is less clear. In recent work, field studies, wind tunnel experiments, and numerical models have been used to investigate dispersion around individual buildings (e.g., Calhoun et al. 2004 ; Meroney et al. 1999 ), in single street canyons (e.g., Caton et al. 2003 ; Sagrado et al. 2002 ), and through small multibuilding industrial and urban areas (e.g., Guenther et al. 1990 ; Scaperdas and Colvile 1999 ). Additionally

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