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The presence of variable heat fluxes between building walls and air within a street canyon, due to the non-uniform sun-light radiation at the street level, poses a key challenge for modeling the heat exchanges with local three-dimensional numerical approaches. In absence of trees, regions with temperature distribution much higher than air temperature, and therefore significant temperature gradients, might arise, while the trees shadowing effects on the walls mitigate these temperature peaks. Heat exchange processes between building walls and external air within the canyon is an important topic in air quality modeling, considering that the thermal structure largely affects the dynamics and pollutant concentration. The influence of trees is analyzed by considering different shadowing characteristics and crown shapes. The study is approached via the computational fluid dynamics finite-volume method, with large eddy simulations (LES) to model turbulence within the canyon and Boussinesq approach to model heat transfer [1]. Consequences on pollutant concentrations are analyzed. The thermal boundary conditions on the building surfaces are set by analyses on the basis of previous experimental results obtained from infrared camera during a UHI experiment [2], for the latitude and longitude of the case study, the orientation, the time of the day. A map with the maximum temperatures reached within the urban area analyzed as a function of the time instant is obtained. We propose a new formulation for the exchanges as a way to link thermal features in street canyons to pollutant concentrations [3]. Model simulations have been validated by previous studies done under the EU funded project iSCAPE (www.iscapeproject.eu). Temperature measured by distributed sensors, surface temperature using infra-red camera, together with measurements of particulate matter, ozone and traffic-related compounds concentrations will be used for the validation of the model. The model will give a tool to design and optimize the key characteristics within the city district (vegetation, geometry of buildings, etc.) in order to minimize the effects of pollutant concentration hotspots within an urban heat island.

In several contexts of risk assessment, prediction of safety distances is a key issue. Toxic or overpressure safety distances can be evaluated using both results from atmospheric 3D models (CFD, mass consistent, ...) and conventional approach results as Gaussian or integral models. Indeed, atmospheric dispersion of different types can be used in the same scope of application: hazardous materials, unobstructed environment, context of use (land use planning or emergency situation). In this context, the SAPHEDRA project offers an innovative dedicated evaluation procedure for hazardous phenomena consequence modelling tools to be used for risk assessment studies. The present paper describes the main steps of this procedure, the importance of defining the scope of application and the possible application to both above mentioned approaches with a view to increasing compatibility among atmospheric modelling evaluation. Based on a two-phase release example, which is much more complex than a gas release, this paper highligts different physics that both integral models and CFD tools evaluations have to adress to demonstrate the tools capability for a specific scenario.

Reliable estimates of atmospheric deposition of nitrogen and sulphur are essential for assessing the risks of eutrophication and acidification to sensitive ecosystems. Atmospheric chemistry and transport models are frequently used to provide these estimates. Several models could be used to estimate deposition rates and, therefore, it is desirable that the models’ abilities to estimate atmospheric deposition are evaluated and compared. Since these models are used to study future scenarios (e.g. emission abatement), it is also desirable that the model estimates respond realistically to changing conditions (e.g. emissions, boundary conditions, meteorology, etc.), something that can only be tested by evaluating the models’ abilities to estimate past changes in deposition. As part of the EURODELTA/TFMM trend modelling exercise, wet deposition of sulphur and nitrogen in Europe was simulated by seven models for the period 1990-2010. The model estimates of annual deposition rates and their temporal trends were compared and evaluated for two ten year periods (1990-2000 and 2000-2010) using data from EMEP background measurement stations. Model estimates of wet deposition differ greatly for all species and all models tended to underestimate the observed values, apart from one model that tended to overestimate sulphur deposition. Most of the observed and modelled trends of reduced nitrogen are not significant, reflecting the smaller emission changes for ammonia compared with those of nitrogen oxides and sulphur dioxide. Observed and modelled trends of wet deposition of oxidised nitrogen were more significant, with decreasing trends at most of the measurement stations. The most significant observed and modelled trends were found for sulphur wet deposition, the majority of which were decreasing trends, reflecting the reported European sulphur dioxide emission reductions of approximately 70% for the modelling period. Most of the models underestimated the trends in wet deposition of sulphur and oxidised nitrogen over the two ten year periods, although model performance differed greatly. Model performance was better and less variable for the relative trends (trends with respect to the values at the beginning of the period) than for the absolute trends. This suggests that relative trends could be used to produce more reliable deposition estimates for future scenarios.

This paper deals with aerodynamic effects of avenue-like tree planting on flow and traffic-originated pollutant dispersion in urban street canyons by means of wind tunnel experiments and numerical simulations. Several parameters affecting pedestrian level concentration are investigated, namely plant morphology, positioning and arrangement. We extend our previous work in this novel aspect of research to new configurations which comprise tree planting of different crown porosity and stand density, planted in two rows within a canyon of street width to building height ratio W/H = 2 with perpendicular approaching wind. Sulfur hexafluoride was used as tracer gas to model the traffic emissions. Complementary to wind tunnel experiments, 3D numerical simulations were performed with the Computational Fluid Dynamics (CFD) code FLUENT (TM) using a Reynolds Stress turbulence closure for flow and the advection-diffusion method for concentration calculations. In the presence of trees, both measurements and simulations showed considerable larger pollutant concentrations near the leeward wall and slightly lower concentrations near the windward wall in comparison with the tree-less case. Tree stand density and crown porosity were found to be of minor importance in affecting pollutant concentration. On the other hand, the analysis indicated that W/H is a more crucial parameter. The larger the value of W/H the smaller is the effect of trees on pedestrian level concentration regardless of tree morphology and arrangement. A preliminary analysis of approaching flow velocities showed that at low wind speed the effect of trees on concentrations is worst than at higher speed. The investigations carried out in this work allowed us to set up an appropriate CFD modelling methodology for the study of the aerodynamic effects of tree planting in street canyons. The results obtained can be used by city planners for the design of tree planting in the urban environment with regard to air quality issues.

The focus is the analysis Of urban Digital Elevation Models (DEMs) with image processing techniques. A brief review of existing methods to derive sky view factors, building energy consumption and space syntax shows how well established parameters that relate to airflow and dispersion (Such as the height-to-width of urban canyons and the aerodynamic roughness length) can be calculated. Other measures of urban directionality and periodicity, inspired by traditional image processing, are also introduced, such as the Radon, Hough and Fourier transforms and the variance plot. Analyses of three case Study sites in London, Toulouse and Berlin are compared, showing considerable variation in the chosen parameters, Results Suggest that the DEM format is an extremely versatile tool to investigate the urban intermediate scale, allowing analyses that Would be very difficult Or impossible to carry Out using traditional vectorial models.

This paper discusses the performance of the temperature perturbation-type ADMS-Temperature and Humidity Model (ADMS-TH) and the Computational Fluid Dynamics (CFD)-based model ENVI-met for the prediction of urban air temperature using measurements collected in the city of Lecce (IT) in summer 2012. The goal is to identify the most important factors influencing numerical predictions. Direct comparisons with measured data and statistical indices show that modelled results are within the range of acceptance. Daily trends are well captured although an underestimation of maximum temperature is observed. In ADMS-TH this is due to an underestimation of sensible heat fluxes during daytime, while in ENVI-met it can be attributed to an underestimation of turbulent momentum and thermal diffusivity. Overall, ADMS-TH did predict the temperature cycle with higher accuracy than ENVI-met and its performance was particularly good during the night. ENVI-met required an ad-hoc tuning of surface boundary conditions to predict nocturnal cooling, satisfactorily. ©

A procedure is described for analysing the transport equations for Reynolds stresses written in a streamline coordinate system, starting from the fields of first- and second-order moments of wind velocity measured in a terrain-following system over topography. In the analysis, the equations are split into two parts: the first contains the terms that can be calculated directly from measurements; the second contains third-order moments that are parameterized using suitable models. To evaluate the error associated with both parts, a Monte-Carlo technique that takes into account the experimental errors is proposed. An example of the application of this method for the Reynolds shear stress equation, using wind-tunnel data for non-separating flow over a two-dimensional valley, is reported. The comparison between the measured and modelled parts is fair near the surface, while at higher levels, the modelled part can be shown to miss a correct treatment of the third-order moments. In the frame of this analysis, the use of the correct derivative transformation has been found to be significant even for moderately sloping topography.

A field study conducted to investigate the flow and turbulence structure of the urban boundary layer (UBL) over an industrial/suburban area is described. The emphasis was on morning and evening transition periods, but some measurements covered the entire diurnal cycle. The data analysis incorporated the dependence of wind direction on morphometric parameters of the urban canopy. The measurements of heat and momentum fluxes showed the possibility of a constant flux layer above the height $$z\approx 2{H}$$ , wherein the Monin-Obukhov Similarity Theory (MOST) is valid; here $$H$$ is the averaged building height. For the nocturnal boundary layer, the mean velocity and temperature profiles obeyed classical MOST scaling up to $$\sim 0.5\Lambda \left( {\sim 6{H}}\right) $$ , where $$\Lambda $$ is the Obukhov length scale, beyond which stronger stratification may disrupt the occurrence of constant fluxes. For unstable and neutral cases, MOST scaling described the mean data well up to the maximum measured height $$(\sim 6{H})$$ . Available MOST functions, however, could not describe the measured turbulence structure, indicating the influence of additional governing parameters. Alternative turbulence parameterizations were tested, and some were found to perform well. Calculation of integral length scales for convective and neutral cases allowed a phenomenological description of eddy characteristics within and above the urban canopy layer. The development of a significant nocturnal surface inversion occurred only on certain days, for which a criterion was proposed. The nocturnal UBL exhibited length scale relationships consistent with the evening collapse of the convective boundary layer and maintenance of buoyancy-affected turbulence overnight. The length and velocity scales so identified are useful in parameterizing turbulent dispersion coefficients in different diurnal phases of the UBL.

Motivated by air quality and numerical modelling applications as well as recent theoretical advancements in the topic, a field experiment, dubbed transition flow experiment, was conducted in Phoenix, Arizona to study the evening transition in complex terrain (shift of winds from upslope to downslope). Two scenarios were considered: (i) the flow reversal due to a change of buoyancy of a cooled slab of air near the ground, and (ii) the formation of a transition front. A suite of in-situ flow, turbulence and particulate matter (PM) concentration sensors, vertically profiling tethered balloons and remote sensors were deployed, and a mesoscale numerical model provided guidance for interpreting observations. The results were consistent with the front formation mechanism, where it was also found that enhanced turbulence associated with the front increases the local PM concentration. During the transition period the flow adjustment was complex, involving the arrival of multiple fronts from different slopes, directional shear between fronts and episodic turbulent mixing events. The upward momentum diffusion from the incipient downslope flow was small because of stable stratification near the ground, and full establishment of downslope flow occurred over several hours following sunset. Episodic frontal events pose challenges to the modelling of the evening transition in complex terrain, requiring conditional parametrizations for subgrid scales. The observed increase of PM concentration during the evening transition has significant implications for the regulatory enforcement of PM standards for the area.

This chpater is devoted to illustrate meteorological processes in urban areas that are relevant to urban air quality. Of most significance are the impacts of the urban morphology on the mean flow and turbulence, which determines the transport and dispersion of pollutants and therefore their concentration.

This paper is devoted to the study of pollutant concentration distribution within urban-like geometries. By applying efficiency concepts originally developed for indoor environments, the term ventilation is used as a measure of city " breathability" It can be applied to analyse pollutant removal within a city in operational contexts. This implies the evaluation of the bulk flow balance over the city and of the mean age of air. The influence of building packing density on flow and pollutant removal is, therefore, evaluated using those quantities. Idealized cities of regular cubical buildings were created with packing density ranging from 6.25% to 69% to represent configurations from urban sprawl to compact cities. The relative simplicity of these arrangements allowed us to apply the Computational Fluid Dynamics (CFD) flow and dispersion simulations using the standard k- e{open} turbulence model. Results show that city breathability within the urban canopy layer is strongly dependent from the building packing density. At the lower packing densities, the city responds to the wind as an agglomeration of obstacles, at larger densities (from about 44%) the city itself responds as a single obstacle. With the exception of the lowest packing density, airflow enters the array through lateral sides and leaves throughout the street top and flow out downstream. The air entering through lateral sides increases with increasing packing density.At the street top of the windward side of compact building configurations, a large upward flow is observed. This vertical transport reduces over short distance to turn into a downward flow further downstream of the building array. These findings suggest a practical way of identifying city breathability. Even though the application of these results to real scenarios require further analyses the paper illustrates a practical framework to be adopted in the assessment of the optimum neighbourhood building layout to minimize pollution levels.

A morphometric analysis of a southern European city and the derivation of relevant fluid dynamical parameters for use in urban flow and dispersion models are explained in this paper. Calculated parameters are compared with building statistics that have already been computed for parts of three northern European and two North American cities. The aim of this comparison is to identify similarities and differences between several building configurations and city types, such as building packing density, compact versus sprawling neighborhoods, regular versus irregular street orientation, etc. A novel aspect of this work is the derivation and use of digital elevation models (DEMs) for parts of a southern European city. Another novel aspect is the DEMs’ construction methodology, which is low cost, low tech, and of simple implementation. Several building morphological parameters are calculated from the urban DEMs using image processing techniques. The correctness and robustness of these techniques have been verified through a series of sensitivity tests performed on both idealized building configurations, as well as on real case DEMs, which were derived using the methodology here. In addition, the planar and frontal area indices were calculated as a function of elevation. It is argued that those indices, estimated for neighborhoods of real cities, may be used instead of the detailed building geometry within urban canopy models as those indices together synthesize the geometric features of a city. The direct application of these results will facilitate the development of fast urban flow and dispersion models.

The present paper is aimed at the analysis of flow and pollutant dispersion in a portion of the Canal Grande (Grand Canal) in Venice (Italy) by means of both Computational Fluid Dynamics (CFD) FLUENT simulations and wind tunnel experiments performed at the University of Gävle (Sweden). For this application, Canal Grande can be viewed as a sort of street canyon where the bottom surface is water and bus boat emissions are the major source of pollution. Numerical investigations were made to assess the effect of the water surface on air flow and pollutant concentrations in the atmosphere. One of the challenges has been to deal with the interface between two immiscible fluids which requires ad-hoc treatment of the wall in terms of the numerical scheme adopted and the grid definition which needs to be much finer than in typical numerical airflow simulations in urban street canyons. Preliminary results have shown that the presence of water at the bottom of the street canyon modifies airflow and turbulence structure with direct consequences on concentration distribution within the domain.

This paper is devoted to the study of flow and traffic exhaust dispersion in urban street canyons with avenue-like tree planting. The influence of tree planting with different crown porosity was investigated. Wind tunnel experiments for perpendicular approaching flow showed that avenue-like tree planting cause increases in exhaust concentrations at the leeward wall as tree crowns reduce the vortex found in the outer regions of the tree-free street canyon and the vertically entering volume flow rate at the canyon-roof top interface. This results in less ventilation and consequently larger concentrations in proximity of the leeward wall. At the windward wall, decreases in concentration are due to the upward moving stream in front of the leeward wall which extends farther into the skimming above roof flow and is better mixed. The clean air entrained in front of the windward wall mixes with air inside the street canyon leading to smaller concentrations. Experiments performed in the wind tunnel with different tree crown porosities did not indicate substantial changes in the flow and concentration fields. The porous model crowns investigated behaved almost like impermeable objects when arranged in a sheltered position and wind speeds are relatively small as in the street canyon. The above described experiments have been also investigated by means of numerical simulations with the CFD code FLUENT (TM), rarely applied to this type of problems. The standard k-epsilon turbulence model and the Reynolds Stress Model were used for flow while the Eulerian advection diffusion scheme has been used for dispersion. Both models reproduced qualitatively the main aspects found in wind tunnel experiments, even though they underestimated flow velocities. Improvement of CFD dispersion performance was obtained by increasing the diffusivity through the turbulent Schmidt number Sc(t). Overall we found that the k-epsilon model failed to capture the complex structure of dispersion process in the presence of tree planting as it would require unphysical low Sc(t) values. On the other hand the RSM turbulence model agreed fairly well with experiments by slightly reducing the standard Sc(t). The results obtained in this work by combining wind tunnel experiments and CFD based simulations to investigate this novel aspect of research suggest ways to obtain quantitative information for assessment, planning and implementation of exposure mitigation using trees in urban street canyons

This paper is devoted to the study of flow within a small building arrangement and pollutant dispersion in street canyons starting from the simplest case of dispersion from a simple traffic source. Flow results from the commercial computational fluid dynamics (CFD) code FLUENT are validated against wind tunnel data (CEDVAL). Dispersion results from FLUENT are analysed using the well-validated atmospheric dispersion model ADMS-Urban. The k-epsilon turbulence model and the advection-diffusion (AD) method are used for the CFD simulations. Sensitivity of dispersion results to wind direction within street canyons of aspect ratio equal to 1 is investigated. The analysis shows that the CFD model well reproduces the wind tunnel flow measurements and compares adequately with ADMS-Urban dispersion predictions for a simple traffic source by using a slightly modified k-epsilon model. It is found that a Schmidt number of 0.4 is the most appropriate number for the simulation of a simple traffic source and in street canyons except for the case when the wind direction is perpendicular to the street canyon axis. For this last case a Schmidt number equal to 0.04 gives the best agreement with ADMS-Urban. Overall the modified k-epsilon turbulence model may be accurate for the simulation of pollutant dispersion in street canyons provided that an appropriate choice for coefficients in the turbulence model and the Schmidt number in the diffusion model are made.

This paper looks at the application of Computational Fluid Dynamics (CFD) and integral approaches to the study of effects of obstacles on pollutant dispersion from a point source placed within an idealised urban area (MUST). This study is part of a modelling exercise within the COST Action 732. Numerical results are compared with wind tunnel data. We use the CFD code FLUENT and the dispersion model ADMS-Urban. The CFD model predicts concentrations more accurately than the integral model. However, both models results satisfy accepted statistical criteria, showing that those criteria should not be the only way of evaluating a model.

Air quality modelling near airports has received attention due to the impact of emissions from aircrafts near ground level. This work is part of the model inter-comparison study undertaken for the Department of Transport in connection with air quality near Heathrow Airport. Results formed part of a submission to the UK government in July 2006. The Emissions and Dispersion Modelling System (EDMS) was used. Our contribution required the setting up and running of EDMS for Heathrow Airport and its surroundings to simulate the year 2002. NO(X), NO(2) and PM(10) were chosen for the study; these being of particular concern.

Until recently, urban air quality modelling has been based on operational models of an integral nature. The use of computational fluid dynamics (CFD) models to address the same problems is increasing rapidly. Operational models e.g. OSPM, AERMOD, ADMS-Urban have undergone many comprehensive formal evaluations as to their “fitness for purpose” while CFD models do not have such an evaluation record in the urban air quality context. This paper looks at the application of both approaches to common problems. In particular, pollutant dispersion from point and line sources in the simplest neutral atmospheric boundary layer and line sources placed within different regular building geometries is studied with the CFD code FLUENT and the atmospheric dispersion model ADMS-Urban. Both the effect of street canyons of different aspect ratios and various obstacle array configurations consisting of cubical buildings are investigated. The standard k – e turbulence model and the advection–diffusion (AD) method (in contrast to the Lagrangian particle tracking method) are used for the CFD simulations. Results from the two approaches are compared. Overall CFD simulations with the appropriate choice of coefficients produce similar concentration fields to those predicted by the integral approach. However, some quantitative differences are observed. These differences can be explained by investigating the role of the Schmidt number in the CFD simulations. A further interpretation of the differences between the two approaches is given by quantifying the exchange velocities linked to the mass fluxes between the in-canopy and above-canopy layers.

The urban heat island (UHI) phenomenon may produce several cascade effects on citizens’ health, energy consumption and air quality. Numerical modelling is recognised to be a powerful tool for the analysis of the UHI, although the question of which model to use (as implied in the ‘fit-for-purpose’ approach) much depends on the application and on the result of satisfactory validation against field measurements. In this paper, two different modelling approaches are applied, namely the integral-semi-Gaussian model ADMS-TH and the CFD-based model ENVI-met, to assess the UHI phenomenon in a city of south Italy (Lecce). Modelling results are validated against field measurements collected during summer 2012. The results suggest that the integral model has the ability of capturing the UHI cycle at city scale, while CFD modelling did not provide any substantial improvements in terms of local geometric effects on temperature distribution.

Dispersion models require as input various geometrical parameters to calculate the flow field and dispersion characteristics in the urban environment. As a result of recent advances in digital photogrammetry and remote sensing, databases of the actual 3-D geometry of city centre areas are now increasingly available. In this work we outline a procedure to reduce this large amount of data to a structured input for urban pollution dispersion models, i.e. to extract the important flow and dispersion parameters from the urban databases. Based on a review of the scientific literature, we have identified a number of parameters relevant to the modelling of pollution dispersion and atmospheric flows in urban areas. These parameters are: the plan and frontal area densities, the plan and frontal area density as a function of height, the distribution of heights, their standard deviation, the aerodynamic roughness length and the sky view factor. These parameters are obtained by analysing urban Digital Elevation Models (DEMs) which are regularly spaced grids of elevation values. Examples of the parameters calculated from high-resolution databases (with pixel size of about 1 m) for three European (London, Toulouse and Berlin) and two North American (Salt Lake City and Los Angeles) cities are presented and discussed. The calculated aerodynamic roughness length was smaller for the European cities than for the North American ones. A multiplicative correction factor kappa to the aerodynamic roughness length is proposed to include the effect of the variability of the building heights.

An analysis of fractional speed-up induced by small topographic features in atmospheric stable conditions is presented. The effects of both roughness and slope changes, under different stability conditions, are modelled in the framework of the linear theory. Under near-neutral conditions the adopted models of wind speed-up induced by topographic slope and roughness variation reproduce the experimental results fairly well. In more stable cases, the model works well in the inner layer, while at the upper levels some discrepancies are encountered.

The past half century has seen an unprecedented growth of the world’s urban population. While urban areas proffer the highest quality of life, they also inflict environmental degradation that pervades a multitude of space-time scales. In the atmospheric context, stressors of human (anthropogenic) origin are mainly imparted on the lower urban atmosphere and communicated to regional, global, and smaller scales via transport and turbulence processes. Conversely, changes in all scales are transmitted to urban regions through the atmosphere. The fluid dynamics of the urban atmospheric boundary layer and its prediction is the theme of this overview paper, where it is advocated that decision and policymaking in urban atmospheric management must be based on integrated models that incorporate cumulative effects of anthropogenic forcing, atmospheric dynamics, and social implications (e.g., health outcomes). An integrated modeling system juxtaposes a suite of submodels, each covering a particular range of scales while communicating with models of neighboring scales. Unresolved scales of these models need to be parametrized based on flow physics, for which developments in fluid dynamics play an indispensible role. Illustrations of how controlled laboratory, outdoor (field), and numerical experiments can be used to understand and parametrize urban atmospheric processes are presented, and the utility of predictive models is exemplified. Field experiments in real urban areas are central to urban atmospheric research, as validation of predictive models requires data that encapsulate four-dimensional complexities of nature.

Hydro-meteorological risks due to natural hazards such as severe floods, storm surges, landslides and droughts are causing impacts on different sectors of society. Such risks are expected to become worse given projected changes in climate, degradation of ecosystems, population growth and urbanisation. In this respect, nature-based solutions (NBSs) have emerged as effective means to respond to such challenges. A NBS is a term used for innovative solutions that are based on natural processes and ecosystems to solve different types of societal and environmental challenges. The present paper provides a critical review of the literature concerning NBSs for hydro-meteorological risk reduction and identifies current knowledge gaps and future research prospects. There has been a considerable growth of scientific publications on this topic, with a more significant rise taking place from 2007 onwards. Hence, the review process presented in this paper starts by sourcing 1608 articles from Scopus and 1431 articles from the Web of Science. The full analysis was performed on 146 articles. The analysis confirmed that numerous advancements in the area of NBSs have been achieved to date. These solutions have already proven to be valuable in providing sustainable, cost-effective, multi-purpose and flexible means for hydro-meteorological risk reduction. However, there are still many areas where further research and demonstration are needed in order to promote their upscaling and replication and to make them become mainstream solutions.

As part of the COST 732 action more than a dozen different research groups have modelled the MUST experiment, as simulated in a wind tunnel. The model evaluation guidance developed within COST 732 recommends 'exploratory data analysis' as one of the elements in model validation. Experience has shown that such exploratory analysis is crucial to reveal shortcomings of models that might otherwise pass unnoticed. Conditions are best for detecting common patterns and anomalies if you have a situation where several models are put into a common framework – like the case at hand. The available material provides a unique opportunity to identify and explore patterns within model performance.

This paper deals with the development of a simple urban model for flow and dispersion in the urban canopy layer (UCL). The flow module of the model calculates spatially-averaged wind profiles adopting a technique recently proposed in the literature, which is based on a balance equation between the obstacle drag force and the local shear stress. Spatially-averaged wind profiles are used as input for a newly proposed dispersion model which solves the advection-diffusion equation at neighbourhood scale. In the model, the effects of the buildings within the UCL are taken into account by means of morphological parameters λf and λp (the ratios of plan area and frontal area of buildings to the lot area). Spatially-averaged mean concentrations output by the developed model are compared with numerical results obtained from the computational fluid dynamics (CFD) model FLUENT. In particular, two configurations of constant height UCL have been considered, which refer to as λ p = λf = 0.16 and λ p = λf = 0.44. The originality of the study is that the dispersion model itself integrates the equations without explicitly resolving the flow around individual buildings but still accounts for their effects. The computational costs are much reduced which makes it suitable for the predictions of concentrations over the neighbourbood scale in an operational context.

This paper reports on a comprehensive model evaluation protocol for urban scale flow and dispersion models that has been developed within the framework of the COST action 732 on Quality Assurance and Improvement of Micro-Scale Meteorological Models. It briefly discusses the different components forming model evaluation with particular emphasis on model validation and on the implementation of the protocol for a specific test case: the MUST (Mock Urban Setting Test) experiment. The protocol was first developed with building-resolving models in mind, but more traditional integral models have also been included. Currently the Action is finalising the MUST exercise results and will suggest the best approach for further model evaluation and for the standardization of CFD modelling practise for micro-scale meteorological applications.

Flow and traffic-originated pollutant dispersion in an urban street canyon with avenue-like tree planting have been studied by means of wind tunnel and CFD investigations. The study comprises tree planting of different crown porosity, planted in two rows within a canyon of street width to building height ratio W/H = 2 and street length to building height ratio L/H= 10 exposed to a perpendicular approaching boundary layer flow. Numerical simulations have been performed with the commercial CFD code FLUENT(tm) by employing the RSM turbulence model. In the presence of tree planting, both measurements and simulations show considerable larger pollutant concentrations in proximity of the leeward wall and slightly lower concentrations in proximity of the windward wall in comparison to the tree-less street canyon. In particular, FLUENT slightly underestimated pollutant concentrations in proximity of the leeward wall in all cases studied, while near the windward wall there is no general tendency towards underestimation or overestimation. Overall, numerical computations compare qualitatively well with experimental data. Results from commonly used statistical tests also suggest the CFD predictions to he satisfactory. Results obtained in this work by combining wind tunnel experiments and CFD based simulations in a novel aspect of research suggest ways to obtain quantitative information for planning and implementation of exposure mitigation using trees in urban street canyons.

We are presenting an initial evaluation of the outcomes of the FAIRMODE & AQUILA intercomparison exercise (IE) on spatial representativeness (SR). To the best of our knowledge, this study provides the first attempt to quantitatively compare the range of methods used for estimating the spatial representativeness of air quality monitoring stations in Europe. As a common working basis, a shared dataset has been selected comprising modelling data and auxiliary information from the city of Antwerp. Based on this, 11 teams from 10 different countries provided their SR estimates for PM10 and NO2 at one traffic site, and for PM10, NO2 and O3 at two urban background sites. The main objective of this exercise was to evaluate the possible variety of spatial representativeness results obtained by applying the range of different contemporary approaches to a jointly used example case study. The results of the IE revealed a considerable range of variation between the different SR estimates - not only in terms of the extent and position of the SR perimeters, but also in the technical procedures and the extent of input data effectively used. These outcomes do also underline the need for (i) a more harmonized definition of the concept of “the area of representativeness” and (ii) consistent and transparent criteria used for its quantification.

This paper presents the impact of different scenarios of future emissions from ships taking into account the implementation of the Directive 2012/33/EU as far as the sulphur content in fuels is concerned in the port of Brindisi. This is one of the most important of the Adriatic Sea, located in the south-east coast of Italy. Ship emissions are estimated through the MEET methodology using appropriate emission factors for manoeuvring and hoteling phases. Numerical simulations of NOx, SO2 and primary PM emissions and dispersion are performed by means of the mesoscale model BOLCHEM coupled with ADMS-Urban dispersion model. This study extends our previous work presented at the 15th Harmo Conference in Madrid, by considering different sulphur content in the fuel oil to analyse the impact of future emission scenarios on the foreseen reduction of pollutant concentrations and assess the potential improvement of air quality in the port area.

This paper reports on the stages forming a model evaluation protocol for urban flow and dispersion models proposed within the COST Action 732 on “Quality Assurance and Improvement of Micro-Scale Meteorological Models”. It discusses the different components forming model evaluation with emphasis on validation and implementation of the protocol for the test case Mock Urban Setting Test (MUST). The protocol was proposed with building-resolving models in mind, but integral models have also been included. The suggested approach can be used for further micro-scale model evaluation and for the standardisation of their applications.

The EURODELTA III (ED-III) exercise aimed to perform a comprehensive chemistry transport model inter-comparison study exploiting the data from four intensive measurement campaigns carried out by EMEP. The campaigns were held in different seasons (1–30 June 2006; 8 January–4 February 2007; 17 September–15 October 2008; 25 February–26 March 2009) thus allowing to test the influence of different meteorological conditions on models’ results. Seven models simulated the air quality over the whole Europe: CHIM (CHIMERE; version chim2013), EMEP (rv 4.1.3), LOTO (LOTOSEUROS, V1.8), CAMX (CAMx, v5.41 VBS), MINNI (version 4.7), CMAQ (V5.0.1) and RCG (v.2.1). Except CMAQ, all the models performed simulations over the same domain with the same horizontal spatial resolution. They also used the same input data (emissions, meteorology and boundary conditions) as much as possible. This work presents and discusses the behaviour of the models with regard to the criteria defined in the EU Directive on Air Quality 2008/50/EC for the air concentrations of PM10, PM2.5, O3, NO2 and SO2 and to the meteorological conditions. The wet deposition of sulphate (S) (WSOx), of oxidized and reduced nitrogen (N) (WNOx and WNHx, respectively) and the air concentrations of the deposited species were also investigated. Furthermore, a comparison of the capacities of air quality models to simulate carbonaceous aerosols (elemental (EC) and organic carbon (OC)) in Europe was conducted, given the diversity in modelling natural precursor emissions and formation and evolution of organic species, both natural and anthropogenic. In addition to EMEP data, the evaluation of models’ output included AirBase data and meteorological data from more than 2000 synoptic stations. The simulated concentrations of organic aerosol (OA) were compared to measurements available from two intensive measurement field campaigns carried out in a joint framework of EMEP and EUCAARI (the European Integrated Project on Aerosol Cloud Climate and Air Quality Interactions) project in 2008 and 2009. © 2018 Hungarian Meteorological Service. All Rights Reserved. ARIANET srl-Aria Technologies SA; Combustion Ltd.; Eurelettronica ICAS; Lombard and Marozzini srl

The model trend analysis for PM10 and PM2.5, performed within the Eurodelta-Trends experiment, covers 21 year, from 1990 through 2010, with particular focus on the period 2001-2010 for which appropriate amount of PM observations is available. Eight chemical transport models (CTM) participated in the multi-modal trend analysis: EMEP/MSC-W, CHIMERE, LOTOS-EUROS, MINNI, MATCH, WRF-Chem, CMAQ and Polyphemus (out of which six models performed trend runs for the 21-year period). The average modelled trends are somewhat smaller than the observed, though the models identify significant PM trends at more sites in the period 2001-2010. There are considerable difference in the PM trends between the regions/countries and in different seasons. Investigation of the changes in PM chemical composition during the investigated period shows that the models differ in terms of relative contribution of the individual PM components to the PM trends. For the 2001-2010 period, the effcct of inter-annual meteorological variability appears more important relative to emission changes. Finally, we look at PM trends/changes during the 1990-2010 period.

A heatwave (HW) is a large-scale meteorological event characterised by persistent and extremely high-temperature condition. At the local scale, the urban heat island (UHI) is another thermal-related phenomenon defined as an urban area warmer than its surrounding regions due to different surfaces’ capabilities to absorb and store heat. However, the assessment about the effect produced on UHI by HW events is not homogeneous. Indeed, regarding the capability of HWs to influence the urban-rural temperature difference, several studies report different conclusions describing both an exacerbation and a reduction of UHI during HW events. In this context, the present study analyses in situ long records of temperature measurements (20 years) to provide observational shreds of evidence of UHI modification under HW conditions. We examine data from the European Climate Assessment & Dataset and World Meteorological Organization computing the UHI index (UHII) to quantify the UHI effect intensity in 37 European cities during the last 20 summers. The results show an UHII intensification for 28 of the 32 cities affected by positive UHI during extremely high temperatures at night, while substantial variations are not observed during the daytime. The time evolution of UHI during a HW highlights that a more significant and persistent urban-rural temperature gradient explains the UHI intensification. Finally, the relationship between the large and local-scale temperature phenomena reveals that continental high-temperature periods are often associated with prominent temperature differences between small-scale urban and rural environments, assessing the impact of large-scale features on thermal stress at the local scale.