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In the framework of the Urban Dispersion International Evaluation Exercise (UDINEE) project coordinated by the European Commission's Joint Research Centre, a case study was conducted of the Joint Urban 2003 (JU2003) experimental campaign in the central area of Oklahoma City, USA. The UDINEE project concerned the cases of puff dispersion of the JU2003 campaign, which are of special interest to scenarios related to security studies, such as explosions of radiological dispersal devices. Starting from the fact that puff-dispersion variability is substantial, especially in complex urban areas, even for puffs released under similar meteorological conditions, a methodology is presented for assessing this variability, which is applied to the dispersion of puffs in two of the intensive operation periods of the JU2003 campaign. Lagrangian and Eulerian dispersion models are applied for the simulations. For the Lagrangian model, variability is assessed by repeating the computations a large number of times. For the Eulerian model, variability is assessed by constructing probability density functions of concentrations on the basis of the dispersion-model results. Peak concentrations, dosages, puff-arrival times and puff durations are considered. Percentiles calculated by the Lagrangian model for all the above quantities and by the Eulerian model for peak concentrations and dosages are compared with the measurements. The results are encouraging since in several cases the measured and computed ranges of values overlap. [ABSTRACT FROM AUTHOR]

A new approach is proposed to predict concentration fluctuations in the framework of one-particle Lagrangian stochastic models. The approach is innovative since it allows the computation of concentration fluctuations in dispersing plumes using a Lagrangian one-particle model with micromixing but with no need for the simulating of background particles. The extension of the model for the treatment of chemically reactive plumes is also accomplished and allows the computation of plume-related chemical reactions in a Lagrangian one-particle framework separately from the background chemical reactions, accounting for the effect of concentration fluctuations on chemical reactions in a general, albeit approximate, manner. These characteristics should make the proposed approach an ideal tool for plume-in-grid calculations in chemistry transport models. The results are compared to the wind-tunnel experiments of Fackrell and Robins (J Fluid Mech, 117:1-26, ) for plume dispersion in a neutral boundary layer and to the measurements of Legg et al. (Boundary-Layer Meteorol, 35:277-302, ) for line source dispersion in and above a model canopy. Preliminary reacting plume simulations are also shown comparing the model with the experimental results of Brown and Bilger (J Fluid Mech, 312:373-407, ; Atmos Environ, 32:611-628, ) to demonstrate the feasibility of computing chemical reactions in the proposed framework. [ABSTRACT FROM AUTHOR]

We propose a two-dimensional Lagrangian analytical solution for relating source strength and concentration profiles within and above a plant canopy. The new solution describes passive scalar dispersion under conditions of local advection through a fetch correction function in a one-dimensional Lagrangian analytical dispersion model. The model is capable of predicting absolute concentration profiles of passive scalars for different fetches for situations in which the reference concentration is known or the background concentration is available. Tests of the model showed good agreement with measurements from field and wind-tunnel experiments. [ABSTRACT FROM AUTHOR]

Profiles of velocity variances based on observations in flat rural areas are well established, and are used for modelling turbulent dispersion in all types of regions including those of complex terrain and urban areas. Surface-based and balloon observations are used to assess the profiles in both rural and urban areas. It is shown that, with good meteorological inputs for the locality of friction velocity and surface sensible heat flux, the profiles are equally well suited to urban areas. The sensitivity of the profiles to the input meteorological data, in particular using numerical weather prediction (NWP) data, is discussed. This highlights the limitations of NWP data for dispersion modelling and stresses the importance of schemes for modelling urban meteorology. [ABSTRACT FROM AUTHOR]

This study proposes a two-dimensional Lagrangian stochastic dispersion model for estimating spatial and temporal variation of scalar sources, sinks, and fluxes within a forest canopy. Carbon dioxide and heat dispersion experiments were conducted for field testing the model. These experiments also provided data for field testing a newly developed one-dimensional Lagrangian analytical dispersion model. It was found that these two models produce similar scalar source-sink and flux distribution patterns. Comparing with CO2 flux measurements, the one-dimensional model performed as well as the two-dimensional model even when the fetch is short (≈100 m). To drive these Lagrangian models, velocity statistics through the canopy volume must be specified a priori. The sensitivity of the computed sources, sinks, and fluxes to the description of the flow statistics was further examined. All in all, we found good agreement between model predicted and eddy-correlation measured CO2 and sensible heat fluxes. [ABSTRACT FROM AUTHOR]

The characteristics of turbulence within a forest are spatially heterogeneous and distinct from those associated with the surface boundary layer. Consequently, the size and probability distribution of ’flux footprints‘ emanating from sources below a forest canopy have the potential to differ from those observed above forests. A Lagrangian random walk model was used to investigate this problem since no analytical solution of the diffusion equation exists. Model calculations suggest that spatial characteristics of ’flux footprint‘ probability distributions under forest canopies are much contracted, compared to those evaluated in the surface boundary layer. The key factors affecting the statistical spread of the ’flux footprint‘, and the position of the peak of its probability distribution, are horizontal wind velocity and the standard deviations of vertical and horizontal velocity fluctuations. Consequently, canopies, which attenuate mean horizontal wind speed, or atmospheric conditions, which enhance vertical velocity fluctuations, will contract flux footprint distributions mostly near the floor of a forest. It was also found that the probability distributions of the ’flux footprint‘ are narrower when horizontal wind velocity fluctuations are considered, instead of the simpler case that considers only vertical velocity fluctuations and mean horizontal wind velocity. [ABSTRACT FROM AUTHOR]