This study has looked at the development of the internal boundary layer (IBL) over a block array close to a sharp change in surface roughness and its effect on dispersion from a ground level source for ratios of the downstream distance to the roughness length of less than 300. This was done by comparing a Large-Eddy Simulation (LES) with inflow boundary conditions against a LES with inlet-outlet periodic boundary conditions and data from a wind tunnel experiment. In addition to established methods, an alternative approach based on the vertical Reynolds stress was used to evaluate the depth of the IBL as it developed over the array which enabled the location of the interface to be more clearly defined. It was confirmed that the IBL growth rate close to the change in surface roughness could be described by a power law profile, similar to the power law formula used in previous studies for a ratio of the downstream distance to the roughness length greater than 1000. An analysis of mean concentration and turbulent scalar fluxes suggested that the presence of the IBL constrained the vertical development of the plume from a ground level source and so led to trapping of material in the canopy layer.

Many models exist for predicting the atmospheric transport and dispersion of material following its release into the atmosphere. The purpose of these models may be to support air quality assessments and/or to predict the hazard resulting from releases of harmful materials to inform emergency response actions. In either case it is essential that the user understands the level of predictive accuracy that might be expected. However, contrary to expectation, this is not easily determined from published comparisons of model predictions against data from dispersion experiments. The paper presents and reviews the methods adopted and issues involved in comparing the predictive performance of atmospheric transport and dispersion models to experimental data, by reference to a number of experimental data sets and comparison results. It then presents an approach which is designed to make the performance of atmospheric dispersion models more transparent, through clearly defining the basis on which the comparison is made, and comparing the performance of the chosen model to that of a reference model. Such an approach establishes a clear baseline against which the accuracy of models can be evaluated and the performance benefits of more sophisticated approaches quantified. The use of a simple analytic reference model applicable to continuous ground level releases in open terrain and urban areas is shown as a proof-of-principle.

A synthetic-turbulence and temperature-fluctuation-generation method is developed and embedded in large-eddy simulations to investigate the effects of weak stable stratification (i.e. Richardson number Ri ≤ 1) on turbulence and dispersion following a simulated ruralto-urban transition. The modelling approach is validated by comparing predictions of mean velocity, turbulent stresses, and point-source dispersion against data from a wind-tunnel experiment that simulates a stable atmospheric boundary layer (Ri = 0.21) approaching a regular array of uniform rectangular blocks. The depth of the internal boundary layer (IBL) that develops from the leading edge of the block array is determined using the wall-normal turbulent stress method proposed by Sessa et al. (J Wind Eng Ind Aerodyn 182:189-291, 2018). This shows that the depth and growth rate of the IBL are sensitive to the thermal stability and the turbulence kinetic energy (TKE) prescribed at the inlet, such that the IBL depth reduces as the TKE of the inflow is reduced while maintaining the same Ri, or as the Ri is increased while maintaining the same inflow TKE. When a ground level line source is introduced it is found that increasing Ri evidently reduces the vertical scalar fluxes at the canopy height, while increasing the mean concentrations within the streets. Furthermore, as with IBL development it is found that for a given value of Ri the effect of stratification becomes more pronounced as the inflow level of TKE is reduced, affecting scalar fluxes within and above the canopy, and volume-averaged mean concentrations within the streets.

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