Simulations of atmospheric diffusion process under various atmospheric stability conditions were carried out with a thermally stratified wind tunnel of IHI. We took Pasquill-Gifford Chart as a template of typical plume spread width. Simulated plume spread widths were good agree with characteristic lines of each atmospheric stability condition that was categorized by Pasquill in the chart. Turbulent intensities were widely distributed according with stability conditions, as real atmospheric phenomenon. We conclude that fundamental plume spread patterns of each Pasquill's stability category are able to be simulated in the wind tunnel.

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A numerical simulation method using turbulence closure techniques and Lagrangian particle dispersion model has been developed to predict atmospheric flows and stack gas diffusion under the complex terrain and non neutral stability conditions. Calculation results of flow and tracer gas diffusion were compared with those of wind tunnel experiments that were made under the same complex terrain and neutral stability conditions . Then calculation results were also compared with those of field experiments that were made under the same complex terrain and unstable stability conditions. Although some discrepancies between the results of calculations and experiments were observed, these were restricted and good agreements were obtained.

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The simulation method of numerical calculation of turbulent boundary layer over complex rough surfaces was reviewed. Turbulent boundary flows over rough surfaces calculated by different methods were examined. Firstly, the handling of roughness as a surface boundary condition and the modeling of roughness effect were discussed. Secondly, the generation methods of turbulent fields at the inflow boundary were presented and the effect of the inflow boundary condition was discussed.

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Experiments on simulating the atmospheric convective boundary layer (CBL), capped by a strong temperature inversion and affected by surface shear, were carried out in a thermally stratified wind tunnel. A quasi-stationary, horizontally evolving CBL was reproduced in the tunnel, with two overall Richardson numbers Rib in the lower part of the layer and Rit in the inversion layer up to -0.6 and 22.6, respectively, and the shear/buoyancy dynamic ratio u*/W*, in the range of 0.29-0.46. The comparison of the wind tunnel data with the results of atmospheric and water tank studies of CBL shows the crucial dependence of the turbulence statistics in the upper part of the layer on the parameters of plume impingement and entrainment, as well as the modification of the CBL turbulence regime by the surface shear.

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This review considers observational and experimental knowledge of turbulence characteristics and organized motions in and above building clusters, drawing from both atmospheric and laboratory data. The former describe the structure of the roughness sublayer in which the roughness has a direct dynamical influence, whereas the latter resolve the flow structure within building clusters in some detail, Topics considered include the turbulent velocity fields and organized motions above the cluster layer, properties of the wake flow within bulding clusters, and the nature of the separation bubble on the building roof. Overall, there are three types of turbulent flows which play important roles in the transport of heat and pollutants in and above building clusters: the organized motions transport heat near the cluster layer and decrease unstable conditions in the urban area, the turbulent wake flows determine the maximum values of concentration at ground-level, and the separations on the building roof control transport and dispersion of tracer gas emitted from the rooftop.

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We used a thermally stratified wind tunnel to investigate the effects of atmospheric stability on flow and dispersion in street canyons. Using a model that represented city streets with simply shaped block forms, we conducted experiments while varying atmospheric stability across 7 stages from stable (Rb=0.79) to unstable (Rb=0.21). Model scale was assumed to be 1: 300. We used a laser Doppler anemometer (LDA) and a cold wire to measure flow fields within and above street canyons . In addition to mean values of wind speed components and temperatures, we measured turbulence intensity, shear stress, and heat flux distribution. A tracer gas (C₂H₆) was released from a line source in the street canyon center to measure concentration distribution inside the street canyon and study its ventilation mechanism. Results indicated the following: Cavity eddies that arise in street canyons tend to be weak when the atmosphere is stable and strong when unstable. Owing to flow changes that depends on the atmospheric stability, air pollution concentrations in street canyons are high when the atmosphere is stable and low when unstable. Air pollutants released in street canyons recirculate from canyon tops to interiors, with the proportion varying according to atmospheric stability.

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In recent years in Japan, many electric power producers tend to construct small scalethermal power stations with wet-type cooling tower because of its cheapness compared with conventional sea-water-cooled system. However exhaust gas from wet-type cooling lower has much moisture and relatively high temperature, water vapor inside the plume becomes condensed when mixed with cool ambient air and this makes exhaust plume visible. This visible plume problem is one of the new environmental problem, because large visible plume which come down to the ground level can bring droplets which could be harmful and can also cause loss of light, low visibility and icing of roads, etc.
In this study, for the purpose of prediction of behavior of visible plume, numerical simulation model which is able to take account of the effect of heat transfer due to phase change of water and gravity effects on the large droplets was developed. Its calculation results were compared with field observation data set of existing cooling towers.

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The results of CFO analyses of mesoscale climate in the Tokyo area are presented . Here, the urban climate in the Tokyo area is analyzed with various land-use conditions in order to examine their effects on velocity and temperature fields. Furthermore, climatic change from the end of the Japanese Medieval Period (Tempo period in the Tokugawa Dynasty: 1830-1844) to the present is also analyzed. By comparing the results of these analyses, We effects of urbanization on the local climate in the Tokyo area are investigated.

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The local wind generated by the heterogeneity of the surface condition is investigated. The heterogeneity of the surface causes to the difference of sensible heat flux into the atmosphere. If the horizontal scale of the each surface is large enough, the thermal boundary layer depth is no more homogeneous. Then the local wind is induced dueto the pressure gradient force driven through the hydrostatic relation. The local wind in the Kanto plain in summer sometimes has large scale, because three different type of the thermal lows are combined. The relation of the accumulation of cold air and local wind in winter night is also investigated in the Kanto plain.

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The orographically-enhanced local severe wind is reviewed, focusing on the mechanism of the downslope wind. Theory of shallow water flow aver a ridge and internal gravity wavein a stratified atmosphere are presented to comprehend the local severe wind. Deformation by three-dimensional effect is also discussed . Numerical simulation of the “ Yamajikaze”, a downslope wind in Shikoku, is shown to suggest the predictability of the downslope wind.

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To understand the observed characteristics of transboundary air pollutants transport over the East Asia, an atmospheric transport model which includes detailed chemical reactions, the STEM (Carmichael et al, [3]), was applied. It was found that wind pattern variations associated with a synoptic scale pressure system are extremely important for the transport of pollutants in this region. An on-line transport model coupled with Regional Atmospheric Modeling System (RAMS, Pielke et al., [9]) are also applied during the rainy season (June). On-line transport model explained well the sulfate concentration variation, and indicates that the strong concentration gradients exists at the northern side of rainy meso-front (typically at Korea and Kyushu Japan). It was found that the location of rainy meso-front plays an important role for sulfate concentration level in Japan and Korea.

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