Lagrangian simulation of wind transport in the urban environment

Fluid element trajectories are computed in inhomogeneous urban-like flows, the needed wind statistics being furnished by a Reynolds-averaged Navier-Stokes (RANS) model that explicitly resolves obstacles. Performance is assessed against pre-existing measurements in flows ranging from the horizontally uniform atmospheric surface layer (no buildings), through regular obstacle arrays in a water-channel wall shear layer, to full-scale observations at street scale in an urban core (the Oklahoma City tracer dispersion experiment Joint Urban 2003). Agreement with observations is encouraging, e.g. for an Oklahoma City tracer trial in which sixteen detectors reported non-zero concentration, modelled concentration lies within a factor of two of the corresponding observation in nine cases (FAC2 = 56%). Although forward and backward simulations offer comparable fidelity relative to the data, interestingly they differ (by a margin far exceeding statistical uncertainty) wherever trajectories from source to receptor traverse regions of abrupt change in the Reynolds stress tensor. Copyright (C) 2009 Royal Meteorological Society