Evaluation of different methods of simulating the effects of a meandering upwind velocity on urban dispersion

Turbulent fluctuations on the mean inflow velocity into urban areas greatly influence the dispersion. The magnitude of these fluctuations is most pronounced at low wind speeds and the net effect is to produce a plume that is more uniform and distributed further horizontally. In a forecast mode, direct simulation of these fluctuations is not possible due to the limits of predictability of turbulence. Thus, dispersion models have generally resorted to parameterize this effect by its empirical characteristics.

Using Joint Urban 2003 sampler data and a fast-running urban dispersion modeling system, an evaluation of different methods for simulating the meandering upwind velocity was conducted. Some potential methods used were direct simulation using upwind velocity measurements, post-processing by a weighting method, using a preset stochastic wind input, or simply increasing the horizontal diffusivity. The relative effects of these methods versus using a fixed mean upwind velocity were assessed in the comparison to Joint Urban 2003 sampler data. Recommendations are made on speed, fidelity and operational tradeoffs between different methods.