Citation:

Pirhalla, M., D. Heist, S. Perry, W. Tang, AND L. Brouwer. Simulations of dispersion through an irregular urban building array. ATMOSPHERIC ENVIRONMENT. Elsevier Science Ltd, New York, NY, 258:118500, (2021). https://doi.org/10.1016/j.atmosenv.2021.118500

Impact/Purpose:

Following a release of a harmful substance within an urban environment, the presence of buildings and street canyons create complex flow regimes that affect dispersion and effluent concentrations. Field, laboratory, and modeling experiments that simulate urban releases are critical in advancing dispersion models and improve the tools useful for emergency preparation and response. This paper offers some observations and suggestions for Gaussian dispersion modeling based on this mock urban modeling exercise and may be useful for those interested in examining harmful releases within urban environments, where human exposures could be the greatest.

Description:

While some fast-response dispersion models can capture the effects of individual buildings, research is required to further refine urban characterizations, especially within the presence of a nonhomogeneous array of structures. This project leverages the configuration of buildings used in a full-scale, mock-urban field study to examine concentrations of a tracer in a series of wind tunnel and Embedded Large Eddy Simulations (ELES) experiments. The behavior, propagation, and magnitude of the plumes were compared and examined to identify microscale effects. After demonstrating excellent comparisons between the wind tunnel and ELES via lateral and vertical concentration profiles, it is shown that the plume remained Gaussian in form, even within an array of buildings and network of street canyons. Despite some variability in the initial plume dispersion, which strongly depended on the structures immediately adjacent to the release, the near-surface plume spread rapidly to its widest point in the array by the first three street canyons downwind of the source. ELES modeling showed, under slightly oblique incoming wind directions considerable off axis channeling of the plume occurred, indicating how building structures can cause plume drift from the otherwise expected centerline axis, especially with greater wind obliquity. Additionally, AERMOD, was used to gauge the performance of a fast-running Gaussian-type dispersion model and inform best practices for use within this group of buildings. Using an urban wind speed profile and other parameters that may be locally available after a release, AERMOD was shown to qualitatively represent the ground-level plume, but underestimate peak concentrations throughout, overestimate lateral plume spread, and was challenged in the near-field. Adding a turbulence profile from the ELES data into AERMOD’s meteorological input improved model estimates of lateral plume spread and centerline concentrations, although peak concentrations values were still underestimated in the far-field.

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