P.G. Mestayer

Pollutant dispersion and thermal effects in urban street canyons

A numerical model has been built to simulate the small-scale atmospheric flows within the urban canopy, based on the lower atmosphere classical assumptions and the “standard κ-ϵ” two-equation turbulence model. This model is used here to study the flows and vertical exchanges of pollutants within the street and at the interface with the atmospheric layer above the roofs, in the asymptotic case of infinitely long street canyons. A thorough study of the influence of the street geometrical aspect ratio leads to a refinement of the flow separation in three regimes, popularized by Oke (1988, Energy Bldg 11, 103–113), taking into account not only the influence of the buildings on the flow of the surface layer just above the roofs but also the structure of the recirculating flow within the street. The time evolution of pollutant concentration within the street canyon and at the pedestrian level is analysed as a function of the geometry and pollutant doses are presented. The number and arrangement of vortex structures within the street canyon largely influence the vertical exchange rates. A preliminary study shows that the differential heating of street surfaces can largely influence the flows capability to transport and exchange pollutants. In fact, differential heating can even shift the in-street flow structure from one regime to another, e.g., from a one-vortex flow to a flow with several contra-rotative vortices.

Parameterization of the urban water budget with the submesoscale soil model

Abstract This paper presents the hydrological component of the Submesoscale Soil Model, urbanized version (SM2-U). This model is an extension of the rural Interactions between Soil, Biosphere, and Atmosphere (ISBA) soil model to urban surfaces. It considers in detail both rural and urban surfaces. Its purpose is to compute the sensible heat and humidity fluxes at the canopy–atmosphere interface for the computational domain lower boundary condition of atmospheric mesoscale models in order to simulate the urban boundary layer in any weather conditions. Because it computes separately the surface temperature of each land use cover mode while the original model computes a unique temperature for the soil and vegetation system, the new version is first validated for rural grounds by comparison with experimental data from the Hydrological Atmospheric Pilot Experiment-Modelisation du Bilan Hydrique (HAPEX-MOBILHY) and the European Field Experiment in a Desertification Threatened Area (EFEDA). The SM2-U water budge...

Influence of Geometry on the Mean Flow within Urban Street Canyons – A Comparison of Wind Tunnel Experiments and Numerical Simulations

A comparison between numerical simulations and wind tunnel modelling has been performed to examine the variation with streamwise aspect ratio (width/height, W/H) of the mean flow patterns in a street canyon. For this purpose a two-dimensional (2-D) cavity was subjected to a thick turbulent boundary layer flow perpendicular to its principal axis. Five different test cases, W/H = 0.3, 0.5, 0.7, 1.0 and 2.0, have been studied experimentally with flow measurements taken using pulsed-wire anemometry. The results show that the skimming flow regime, with a large vortex in the canyon, occurred for all the cases investigated. For the cavities with W/H ≤ 0.7 a weaker secondary circulation developed beneath the main vortex. The narrower the canyon, the smaller the wind speed close to the cavity ground, giving increasingly poor ventilation qualities. The corresponding numerical results were obtained with the Computational Fluid Dynamics (CFD) code CHENSI that uses the standard k-ε model. The intercomparison showed good agreement in terms of the gross features of the mean flow for all the geometries examined, although some detailed differences were observed.

Two Major Experiments in the Humidity Exchange over the Sea (HEXOS) Program

Abstract Humidity Exchange over the Sea (HEXOS) is an international program for the study of evaporation and spray droplet flux from sea to air. The program includes measurements in the field, simulation studies in wind tunnels, interpretive studies such as flow distortion modeling, boundary-layer modeling and development of parameterization for use in synoptic, and climatic models of the atmosphere and ocean. The HEXOS Main Experiment (HEXMAX) was carried out in October and November of 1986 at the Dutch offshore research plat-form Meetpost Noordwijk (MPN) and from ship, aircraft, and shore stations in the vicinity. Evaporation, wind stress, and heat flux were determined at all stations using combinations of eddy correlation, dissipation, and profile methods. Concurrent measurements of spray and aerosol distributions and other relevant parameters, and the regular occurence of favorable winds and weather make the HEX-MAX dataset unique in its completeness and in the range of conditions covered. Some prelim...

Investigating the Surface Energy Balance in Urban Areas – Recent Advances and Future Needs

Recent advances in understanding of the surface energy balance of urban areas, based on both experimental investigations andnumerical models, are reviewed. Particular attention is directedto the outcome of a COST-715 Expert Meeting held in April 2000,as well as experiments initiated by that action. In addition, recentcomplete parameterisations of urban effects in meso-scalemodels are reviewed. Given that neither the surface energybalance, nor its components, normally are directly measuredat meteorological stations, nor are there guidelines for theset-up of representative meteorological stations in urbanareas, this paper also provides recommendations to closethese gaps.

Eulero-Lagrangian simulation of raindrop trajectories and impacts within the urban canopy

Abstract An integrated approach is presented for numerical simulation of the wind-driven rain impacts on building surfaces. The numerical code combines Eulerian simulations of the turbulent flows, Lagrangian random-flight. simulations of heavy particle trajectories, and impacting water rate computations. The flow is modelled using a version of CHENSI, a code based on the two-equation κ-ϵ model developed to simulate flows in urban canopies and dispersion within streets and around buildings. Particle trajectories are computed by means of a Markov chain modified to model the effects of turbulence, gravity and inertia. Three different models are derived from the work of Edson and Fairall (1994, J. geophys. Res. 99, 25,296–25,311) and Mostafa and Mongia (1987, Int. J. Heat Mass Transfer 12, 2585–2593), and tested for application to raindrops that range in diameter from 0.2 to 2 mm. The distribution of impacting drops on the various boundaries of the calculation domain is used in combination with a rain distribution model to compute the amount of water that is absorbed by the street and building surfaces. A resulting set of simulations is compared to experimental data and semi-empirical formulations. The extension of the method to assess the impact of other atmospheric hydrometeors, e.g. snowflakes or fog drops, is discussed.

THE IMPACT OF THE HEXOS PROGRAMME

In HEXOS, a programme of coordinated laboratory, field and model studies, an international group of participants has extended the range of measurements of evaporation from the sea and has investigated the role of droplets in the transfer of water to the atmosphere. Predictions of a rapid rise in the evaporation coefficient at wind speeds above 15 m s-1 have not been substantiated. Wind stress measurements showed a relationship with wave age. New methods were developed for coping with flow distortion.

Urban roughness mapping – validation techniques and some first results

Because of measuring problems related to evaluation of urban roughness parameters, a new approach using a roughness mapping tool has been tested: evaluation of roughness length z(o) and zero displacement z(d) from cadastral databases. Special attention needs to be given to the validation of the tool, and the roughness formulas therein. After a roughness formula validation using laboratory data, three other validation tests have been applied sucessfully, using roughness classifications, comparison with an established roughness model, and spatial averaging consistency tests. The latter is an interesting, new approach which allows to rank the performance of the otherwise largely similar models. Further results of interest are the sensitivity of the Raupach model to some of its parameters, and the need to include the drag of forest, town and obstacle group edges in an effective, spatially averaged roughness

Climatology of Cities

There is no generally-accepted theory of the climatology of cities, yet. The urban climate is altogether an old subject of investigations, since the book of Howard (1833), and a new one because of this lack of generally-accepted framework: new techniques and new approaches are likely to produce new breakthroughs.

Measurements of Humidity and Temperature in the Marine Environment during the HEXOS Main Experiment

Accurate measurement of fluctuations in temperature and humidity are needed for determination of the surface evaporation rate and the air-sea sensible heat flux using either the eddy correlation or inertial dissipation method for flux calculations. These measurements are difficult to make over the ocean, and are subject to large errors when sensors are exposed to marine air containing spray droplets. All currently available commercial measurement devices for atmospheric humidity require frequent maintenance. Included in the objectives of the Humidity Exchange over the Sea program were testing and comparison of sensors used for measuring both the fluctuating and mean humidity in the marine atmosphere at high wind speeds and development of techniques for the protection of these sensors against contamination by oceanic aerosols. These sensors and droplet removal techniques are described and comparisons between measurements from several different systems are discussed in this paper. To accomplish these goals, participating groups devised and tested three methods of removing sea spray from the sample airstream. The best performance was given by a rotating semen device, the “spray Ringer.” Several high-frequency temperature and humidity instruments, based on different physical principles, were used in the collaborative field experiment. Temperature and humidity fluctuations were measured with sufficient accuracy inside the spray removal devices using Lyman-α hygrometers and a fast thermocouple psychrometer. Comparison of several types of psychrometers (using electric thermometers) and a Rotronic MP-100 humidity sensor for measuring the mean humidity illustrated the hysteresis of the Rotronic MP-100 device after periods of high relative humidity. Confidence in the readings of the electronic psychrometer was established by in situ calibration with repeated and careful readings of ordinary hand-held Assman psychrometers (based on mercury thermometers). Electronic psychrometer employing platinum resistance thermometers perform very well.