3D Measurements of Transient Dispersion in an Urban Canopy Model: A Green's Function Approach*

Predicting turbulent dispersion from street-level sources within the urban canopy is important for determining emergency response, assessing air quality, and providing input to neighborhood scale pollution models. Model improvements are challenged by the available validation data: either due to flow condition uncertainties for field measurements, or because conventional laboratory techniques are limited to point-wise and planar data. This work uses magnetic resonance imaging to obtain three-dimensional velocity and concentration fields for a transient release in a scale model of Oklahoma City circa 2003. A passive scalar is injected at ground level and data are obtained on a volumetric Cartesian grid at 12 temporal phases encompassing several blocks of the downtown business district. The source's Green's function is extracted at each voxel using a regularized optimization procedure. The Green's function generalizes the results to different release profiles, and statistics such as the plume residence time are determined objectively. Plume residence time is substantially longer than characteristic advection time scales in street canyons and corners between buildings that fill rapidly with contaminant but wash out slowly. Comparisons to the Joint Urban 2003 fields tests are given.