The Evaluation of a Diagnostic Wind Field Model Applied to the Urban Canopy Layer

This paper presents the evaluation of a model that constructs the wind fields within a complex building structure using a technique popular in emergency response models that require fast turnaround times (Brown, M.J., 2004). The model constructs the wind field using two steps: 1) the wind field is initialized with the known flow fields corresponding to the individual buildings that comprise the structure, and 2) the initial flow fields are patched together by insisting that the final flow field is mass consistent. We evaluated the model indirectly by comparing the concentrations associated with the flows with measurements made in a tracer experiment conducted among buildings located in the University of California, Riverside. The basis of this type of comparison is that dispersion in the midst of buildings is controlled by the mean flow rather than turbulence; the model being evaluated predicts the mean flow. The dispersion model was a Lagrangian particle model. The tracer experiment, conducted in June 2001, involved the release of SF6 from the top of a trailer located in an urban parking lot. The concentrations were sampled at two arcs at 10 m and 20 m downwind of the source as well as six locations around the trailer. A sonic anemometer measured winds on the second arc at a height of 3 m. The wind field model in combination with the dispersion model captured the main features of the observed concentration fields, which showed the effects of horizontal spread induced by building-induced channeling. The performance of the model was comparable to a semi-empirical model that used the measured distribution of horizontal winds and turbulent velocities. The results were relatively insensitive to the turbulence parameterization or the formulation of the Lagrangian particle model. This confirmed the basis for the indirect evaluation of the wind field model, which is that the mean flow rather than turbulence controls dispersion within the urban canopy.