Aircraft measurement of ozone turbulent flux in the atmospheric boundary layer

In May 1995, the “Chimie-Creil 95” experiment was undertaken in the north of France. The field data are first used to validate the methodology for airborne measurement of ozone flux. A certain number of methodological problems due to the location of the fast ozone sensor inside the airplane are, furthermore discussed. The paper describes the instrumentation of the ARAT (Avion de Recherche Atmosphérique et de Télédétection), an atmospheric research and remote-sensing aircraft used to perform the airborne measurements, the area flown over, the meteorological conditions and boundary layer stability conditions. These aircraft measurements are then used to determine ozone deposition velocity and values are proposed for aerodynamic, bulk transfer coefficients (ozone and momentum). The paper also establishes the relationship between the normalised standard deviation and stability parameters (z/L) for ozone, temperature, humidity and vertical velocity. The laws obtained are then presented.     

Numerical simulations of air pollutant dispersion in a stratified planetary boundary layer

A numerical model is described for computing pollutant concentration distributions downwind from a source. It is based on the three-dimensional dispersion equation governing the time-dependent advective and diffusive transport of air pollutants and is solved numerically by a mixed Lagrangian-Eulerian finite-difference scheme. The model includes the vertical wind shear, the turning of the actual wind, and vertical variations of the vertical eddy diffusivity. In this paper the model is used to simulate the pollutant dispersion process in a stratified planetary boundary layer. The vertical profiles of horizontal mean wind and vertical eddy diffusivities are calculated numerically from a planetary boundary layer model. The influence of the ground roughness and the atmospheric stability on the pollutant distribution is investigated. The results indicate that both parameters essentially determine the air pollutant dispersion process.     

A turbulent energy model for diffusion in the convective boundary layer

A turbulent energy model developed by the authors to describe atmospheric flows is used to study diffusion in the convective boundary layer. The model is based on the turbulent energy transport equation coupled with eddy diffusivity expressions for momentum and heat transfer. The diffusion model assumes equality of the eddy diffusivity for heat and mass and Gaussian diffusion in the cross-stream direction. The model is shown to reproduce satisfactorily the main features of diffusion in convective flows, and its predictions compare well with the measurements of the laboratory experiments of Willis and Deardorff, as well as with field data.     

The ground level extent of a negatively buoyant plume in a turbulent boundary layer

Experiments have been undertaken in a water flume to measure the extent of a negatively buoyant plume issuing from a continuous, area source under a turbulent boundary layer. The lateral growth of the plume is compared with a semi-empirical analysis and with measurements of a full-scale experiment. The upwind extent of the plume was also obtained.     

A numerical study on the vertical dispersion of passive contaminants from a continuous source in the atmospheric surface layer

A numerical solution of the diffusion equation is obtained for the vertical diffusion from a continuous source. The wind speed profile and the eddy diffusivity in this equation are expressed in terms of the surface-layer similarity relations obtained in the well-known Kansas experiments. A comparison is made with vertical concentration measurements obtained during the Prairie Grass and Porton experiments. The numerical results are in agreement with these experimental data.     

An efficient Lagrangian stochastic model of vertical dispersion in the convective boundary layer

We consider the one-dimensional case of vertical dispersion in the convective boundary layer (CBL) assuming that the turbulence field is stationary and horizontally homogeneous. The dispersion process is simulated by following Lagrangian trajectories of many independent tracer particles in the turbulent flow field, leading to a prediction of the mean concentration. The particle acceleration is determined using a stochastic differential equation, assuming that the joint evolution of the particle velocity and position is a Markov process. The equation consists of a deterministic term and a random term. While the formulation is standard, attention has been focused in recent years on various ways of calculating the deterministic term using the well-mixed condition incorporating the Fokker–Planck equation. Here we propose a simple parameterisation for the deterministic acceleration term by approximating it as a quadratic function of velocity. Such a function is shown to represent well the acceleration under moderate velocity skewness conditions observed in the CBL. The coefficients in the quadratic form are determined in terms of given turbulence statistics by directly integrating the Fokker–Planck equation. An advantage of this approach is that, unlike in existing Lagrangian stochastic models for the CBL, the use of the turbulence statistics up to the fourth order can be made without assuming any predefined form for the probability distribution function (PDF) of the velocity. The main strength of the model, however, lies in its simplicity and computational efficiency. The dispersion results obtained from the new model are compared with existing laboratory data as well as with those obtained from a more complex Lagrangian model in which the deterministic acceleration term is based on a bi-Gaussian velocity PDF. The comparison shows that the new model performs well.     

CFD simulation of the atmospheric boundary layer: wall function problems

Accurate Computational Fluid Dynamics (CFD) simulations of atmospheric boundary layer (ABL) flow are essential for a wide variety of atmospheric studies including pollutant dispersion and deposition. The accuracy of such simulations can be seriously compromised when wall-function roughness modifications based on experimental data for sand-grain roughened pipes and channels are applied at the bottom of the computational domain. This type of roughness modification is currently present in many CFD codes including Fluent 6.2 and Ansys CFX 10.0, previously called CFX-5. The problems typically manifest themselves as unintended streamwise gradients in the vertical mean wind speed and turbulence profiles as they travel through the computational domain. These gradients can be held responsible—at least partly—for the discrepancies that are sometimes found between seemingly identical CFD simulations performed with different CFD codes and between CFD simulations and measurements. This paper discusses the problem by focusing on the simulation of a neutrally stratified, fully developed, horizontally homogeneous ABL over uniformly rough, flat terrain. The problem and its negative consequences are discussed and suggestions to improve the CFD simulations are made.     

Evaluation of the turbulent parameters of the unstable surface boundary layer outside Businger's range

The dispersion coefficient, σ_z, involves two important turbulent parameters, the friction velocity and the Monin-Obukhov length. We have used Businger's universal functions to compute those parameters which were developed within the range, − 2 ⩽ z / L ⩽ 0. Our purpose is to show that Businger's universal functions perform well outside that range. Correlation coefficients are satisfactory when the roughness length is used as a key to detect good runs. A wider range can be considered and given as follows: − 8 ⩽ z / L ⩽ 0. Two different ranges are defined and given by 3.3 ⩾ z₀ ⩾ 0.4 cmand 6.1 ⩾ z₀ ⩾ 0.2 cm. The iteration method used in this paper can be considered to be quickly convergent and computationally efficient, except for free convection.     

The relationship between the convective boundary layer and dispersion from tall stacks

This paper discusses the structure of the convective planetary boundary layer in relation to dispersion of pollutants. As tall stack plumes are brought down to the ground primarily during unstable conditions, we have investigated the conditions under which the PBL is convective. Our analysis indicates that the convective PBL occurs more often than is commonly thought. The second part of the paper describes some aspects of dispersion in the convective PBL. We propose a simple model capable of providing factor-of-two type estimates of ground-level concentrations. We also discuss some of the factors which complicate attempts to obtain more accurate results. In this context, we point out the problems associated with using model-predicted ensemble means to estimate observed concentrations.     

A wind tunnel study of dense gas dispersion in a stable boundary layer over a rough surface

Measurements of the vertical entrainment velocity into two-dimensional dense gas plumes over fully rough surfaces were carried out as part of a co-operative research programme with wind tunnel facilities in the USA. This paper presents results obtained for stable boundary layer conditions in the EnFlo wind tunnel at the University of Surrey; a companion paper treats the neutral boundary layer case. Mean velocity and temperature, turbulent normal and shear tresses, temperature fluctuations and heat fluxes were measured and used to demonstrate that a moderately stable atmospheric boundary layer had been successfully simulated in the tunnel. Entrainment velocities, W_E, were then deduced from the streamwise development of the concentration field, non-dimensionalised with respect to the friction velocity in the undisturbed flow, u^*, and correlated with the plume Richardson number, Ri^*. Higher non-dimensional entrainment speeds, W_E/u^*, were observed for Ri^*>5 in the stable boundary layer than in the neutral boundary layer, the difference growing with increasing Richardson number. Emission velocity ratios, W₀/u^*, were however larger in the stable experiments, and exceeded one at about Ri^*=18. Entrainment in the stable boundary layer appeared therefore to be more sensitive to emission velocity ratio than in the neutral case. Entrainment behaviour for Ri^*⩽5 followed that found in the neutral boundary layer. In this regime, use of the neutral boundary layer entrainment speed correlation is unlikely to lead to the over-prediction of plume dilution rates in moderately stable boundary layers.     

A wind tunnel study of dense gas dispersion in a neutral boundary layer over a rough surface

Measurements of the vertical entrainment velocity into two-dimensional dense gas plumes over fully rough surfaces were carried out as part of a co-operative research programme with wind tunnel facilities in the USA. This paper presents results obtained for neutral boundary layer conditions in the EnFlo wind tunnel at the University of Surrey; a companion paper treats the stable boundary layer case. Entrainment velocities, W_E, were deduced from the streamwise development of the concentration field, non-dimensionalised with respect to the friction velocity in the undisturbed flow, u^*, and correlated with the plume Richardson number, Ri^*. Results for Richardson numbers in the range Ri^*<15 were found to be well fitted by the empirical expression: W_E/u^*=0.65/(1+0.2Ri^*). Flow visualisation studies showed layered plume structures with a sharp upper interface at higher Richardson numbers and in this regime turbulent motion below the interface became progressively more intermittent as Ri^* increased. Measured turbulence levels collapsed within such high Richardson number plumes and flow and dispersion were significantly affected by molecular processes. Up-welling above the source was observed when the emission speed exceeded the approach flow friction velocity, though there was no clear evidence that this affected plume behaviour away from the immediate vicinity of the source.     

A mixture model for the PDF of a diffusing scalar in a turbulent flow

This paper offers physical and statistical arguments that lead to the representation of the probability density function of a scalar diffusing in a turbulent flow as a mixture of two probability density functions. Particular attention is given to the high-concentration tails which are consistently observed to be described accurately by a generalized Pareto distribution. A 4-parameter construct, using a mixture of beta densities, is shown to provide an adequate representation of measured concentration distributions of wind-tunnel, grid turbulence, plume data. By using either measured or theoretically predicted moments to conduct statistical inference, it is expected that the proposed model will be flexible enough to apply to a wide variety of experimental data sets, including those obtained from large-scale environmental flows.     

Thermal valley inversion impact on the dispersion of a passive pollutant in a complex mountainous area

This contribution deals with the large eddy simulations of a complete diurnal cycle of atmospheric flow within a schematic deep valley for two seasons (i.e. winter and summer). The scale of interest is characteristic of an urban site located in a complex area, with special focus on low wind conditions. The results of the numerical simulations describe the influence of the season on the mechanisms responsible for the formation and the destruction of the inversion layer, and its impact on pollutant trapping within the valley.     

Air pollution in a convective boundary layer

Detailed profiles of the concentration and the concentration flux for pollutants released continuously from a point source near the ground are presented. Although there are no observational data of the concentration flux and the covariance of temperature and concentration, the distributions of concentration and eddy diffusivity derived from this study are in good agreement with those of laboratory experiments. This study also shows that the covariance of temperature and concentration is important in producing a countergradient concentration flux in a convective boundary layer.     

On the influence of the urban roughness sublayer on turbulence and dispersion

The concept of the urban roughness sublayer is discussed and this lowest atmospheric layer over a rough surface is shown to have a non-negligible vertical extension over typical urban surfaces. The existing knowledge on the turbulence and flow structure within an urban roughness sublayer is reviewed, focusing on the height dependence of turbulent fluxes and a scaling approach for turbulence statistics, such as velocity variances, in the above-roof part of the roughness sublayer. Finally, the implication of this turbulence and flow structure upon dispersion characteristics is investigated. The most prominent difference of explicitly taking into account the roughness sublayer in a dispersion simulation (as compared to assuming a `constant flux layer') is a clearly enhanced ground level concentration far downwind from the source. For the example of a tracer release experiment over a (sub) urban surface (Copenhagen) it is shown that introducing the roughness sublayer clearly improves the model performance.