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The spatial variability of flow and turbulence properties above an outdoor scale model of regular cube roughness under neutral stability is investigated using eight simultaneously employed sonic anemometers which are characterized by an extra short transducer span (0.05 m) and high sampling rate (50 Hz). Measurements are conducted in a layer between the top of the cubes with a plan area density of 0.25 and two times the height of the roughness, $$H$$ . Large spatial variability (horizontal and vertical) is observed at the two lowest measurement levels ( $$z=H$$ and $$1.25H$$ ) for all variables considered which include mean wind, Reynolds stress, integral statistics, (co)spectra, budget of turbulent kinetic energy and a spatial quadrant analysis of the momentum transfer. The spatial inhomogeneity almost disappears at $$z = 1.5H$$ where normalized variables attain homogeneous surface layer values for neutral stratification. The present results support a new conceptual framework to explain the turbulence behavior just above the canopy consisting of two sublayers which develop in response to the strongly varying surface. Although an individual measurement location below $$z = 1.5H$$ is unable to produce representative turbulence statistics above the present bluff body roughness, averaging across a sufficient number of measurement locations to achieve complete spatial sampling of all surface characteristics (canyon spaces, rooftops, etc.), produces representative statistics even at heights which are probably still within the roughness sublayer with values close to the respective inertial subrange predictions.

This study conducted large-eddy simulations (LES) of fully developed turbulent flow within and above explicitly resolved buildings in Tokyo and Nagoya, Japan. The more than 100 LES results, each covering a 1,000 $$\times $$ 1,000 m $$^{2}$$ area with 2-m resolution, provide a database of the horizontally-averaged turbulent statistics and surface drag corresponding to various urban morphologies. The vertical profiles of horizontally-averaged wind velocity mostly follow a logarithmic law even for districts with high-rise buildings, allowing estimates of aerodynamic parameters such as displacement height and roughness length using the von Karman constant $$=$$ 0.4. As an alternative derivation of the aerodynamic parameters, a regression of roughness length and variable Karman constant was also attempted, using a displacement height physically determined as the central height of drag action. Although both the regression methods worked, the former gives larger (smaller) values of displacement height (roughness length) by 20–25 % than the latter. The LES database clearly illustrates the essential difference in bulk flow properties between real urban surfaces and simplified arrays. The vertical profiles of horizontally-averaged momentum flux were influenced by the maximum building height and the standard deviation of building height, as well as conventional geometric parameters such as the average building height, frontal area index, and plane area index. On the basis of these investigations, a new aerodynamic parametrization of roughness length and displacement height in terms of the five geometric parameters described above was empirically proposed. The new parametrizations work well for both real urban morphologies and simplified model geometries.

We examine the similarity of turbulent organized structures over smooth and very rough wall flows. Turbulent flow fields in horizontal cross-sections were measured using particle image velocimetry, and the characteristics of turbulent organized structures over four types of surfaces were investigated. Measurements were conducted at several measurement heights across the internal boundary layer. The length and width of turbulence structures were quantified using a two-point correlation method. We selected two thresholds of two-point correlation coefficients to consider both large-scale and small-scale structures; the validity of these choices was examined through the analyses using proper orthogonal decomposition. For large-scale structures, the length and aspect ratios (streamwise length/spanwise width) of structures were highly correlated with the velocity gradient for each measurement height and boundary-layer thickness. This relationship was also examined in the results of previous studies, and the scaling of the aspect ratio with the non-dimensional velocity gradient again showed the importance of the velocity gradient, with slight differences found between smooth and rough surfaces. In contrast, the small-scale structures exhibited weak dependency on the velocity gradient and boundary-layer thickness. Instantaneous snapshots of turbulent organized structures at the same shear level also displayed differences in small-scale structures, but the structures of the organized motions resembled each other, as in the results of the two-point correlation method.

Instantaneous flow structures “within” a cubical canopy are investigated via large-eddy simulation. The main topics of interest are, (1) large-scale coherent flow structures within a cubical canopy, (2) how the structures are coupled with the turbulent organized structures (TOS) above them, and (3) the classification and quantification of representative instantaneous flow patterns within a street canyon in relation to the coherent structures. We use a large numerical domain (2,560 m × 2,560 m × 1,710 m) with a fine spatial resolution (2.5 m), thereby simulating a complete daytime atmospheric boundary layer (ABL), as well as explicitly resolving a regular array of cubes (40 m in height) at the surface. A typical urban ABL is numerically modelled. In this situation, the constant heat supply from roof and floor surfaces sustains a convective mixed layer as a whole, but strong wind shear near the canopy top maintains the surface layer nearly neutral. The results reveal large coherent structures in both the velocity and temperature fields “within” the canopy layer. These structures are much larger than the cubes, and their shapes and locations are shown to be closely related to the TOS above them. We classify the instantaneous flow patterns in a cavity, specifically focusing on two characteristic flow patterns: flushing and cavity-eddy events. Flushing indicates a strong upward motion, while a cavity eddy is characterized by a dominant vortical motion within a single cavity. Flushing is clearly correlated with the TOS above, occurring frequently beneath low-momentum streaks. The instantaneous momentum and heat transport within and above a cavity due to flushing and cavity-eddy events are also quantified.

We investigate the spatial characteristics of urban-like canopy flow by applying particle image velocimetry (PIV) to atmospheric turbulence. The study site was a Comprehensive Outdoor Scale MOdel (COSMO) experiment for urban climate in Japan. The PIV system captured the two-dimensional flow field within the canopy layer continuously for an hour with a sampling frequency of 30 Hz, thereby providing reliable outdoor turbulence statistics. PIV measurements in a wind-tunnel facility using similar roughness geometry, but with a lower sampling frequency of 4 Hz, were also done for comparison. The turbulent momentum flux from COSMO, and the wind tunnel showed similar values and distributions when scaled using friction velocity. Some different characteristics between outdoor and indoor flow fields were mainly caused by the larger fluctuations in wind direction for the atmospheric turbulence. The focus of the analysis is on a variety of instantaneous turbulent flow structures. One remarkable flow structure is termed 'flushing', that is, a large-scale upward motion prevailing across the whole vertical cross-section of a building gap. This is observed intermittently, whereby tracer particles are flushed vertically out from the canopy layer. Flushing phenomena are also observed in the wind tunnel where there is neither thermal stratification nor outer-layer turbulence. It is suggested that flushing phenomena are correlated with the passing of large-scale low-momentum regions above the canopy.

A large-eddy simulation of the atmospheric boundary layer, large enough to contain both an urban surface layer and a convective mixed layer, was performed to investigate inner-layer and outer-layer scale motions. The objective was to determine the applicability of Monin–Obukhov similarity theory to inner-layer motions, to investigate the influence of outer-layer motions on surface-layer structure, as well as to assess the interaction of the two scales of motion. The urban surface roughness consisted of square-patterned cubic buildings of dimension H (40 m). A spatial filter was used to decompose the two scales in the inertial sublayer. The horizontal square filter of size 10H was effective in separating the inner-layer (surface-layer height ≈ 2 H) and outer-layer scales (boundary-layer height δ ≈ 30H), where the Reynolds stress contribution of the inner layer dominates in the logarithmic layer (depth 2H). Similarity coefficients for velocity fluctuations were successfully determined for inner-layer motions in the surface layer, proving the robustness of Monin–Obukhov similarity for surface-layer turbulence. The inner-layer structures exhibit streaky structures that have similar streamwise length but narrower spanwise width relative to the streamwise velocity fluctuation field, consistent with observations from an outdoor scale model. The outer-layer motions to some extent influence the location of ejections and sweeps through updraft and downdraft motions, respectively, thus, disturbing the homogeneity and similarity of inner-layer motions. Although the horizontal averages of the variances and covariance of motions reveal that the Reynolds stresses are dominated by inner-layer structures, the localized influence of the interaction of outer-layer horizontal and inner-layer vertical motions on the Reynolds stress is not insignificant.

Convection over the building envelope is a critical determinant of building cooling load, but parameterization of convection in building energy models and urban computational fluid dynamics models is challenging. An experimental investigation intended to clarify the heat transfer mechanism of a convective wall boundary layer (WBL) on a leeward, vertical building wall was conducted at the Comprehensive Outdoor Scale Model (COSMO) facility for urban atmospheric research. Comparison of mean and turbulent temperature fluctuation intensity profiles showed that the dominant regime of the WBL flow was turbulent natural convection. Implications for parameterization of convective heat fluxes in urban areas are discussed.

Large-eddy simulation (LES) is conducted to investigate the mechanism of pollutant removal from a two-dimensional street canyon with a building-height to street-width (aspect) ratio of 1. A pollutant is released as a ground-level line source at the centre of the canyon floor. The mean velocities, turbulent fluctuations, and mean pollutant concentration estimated by LES are in good agreement with those obtained by wind-tunnel experiments. Pollutant removal from the canyon is mainly determined by turbulent motions, except in the adjacent area to the windward wall. The turbulent motions are composed of small vortices and small-scale coherent structures of low-momentum fluid generated close to the plane of the roof. Although both small vortices and small-scale coherent structures affect pollutant removal, the pollutant is largely emitted from the canyon by ejection of low-momentum fluid when the small-scale coherent structures appear just above the canyon where the pollutant is retained. Large-scale coherent structures also develop above the canyon, but they do not always affect pollutant removal.

We investigate the coherent structure of atmosphere turbulence over very large roughness within a fully rough, high Reynolds number turbulent flow. The horizontal distributions of coherent turbulence were determined by multipoint measurements of velocity fluctuations using sonic anemometers in a comprehensive outdoor scale model experiment for urban climate (COSMO). COSMO is made up of 512 cubical obstacles, each 1.5 m on a side, arranged in a rectangular pattern on a flat 50 m × 100 m concrete plate. A total of 15 sets of sonic anemometers were aligned horizontally within the logarithmic layer above this site. The velocity fluctuations observed in COSMO were decomposed into active and inactive contributions by applying a spatial-filtering method, and which used a simple moving average along the spanwise direction of the predominant flow as a filter function. The size of the filter should be between the sizes of the active and inactive fluctuations. This method potentially eliminates the considerable portion of low frequency modes included in the horizontal velocity fluctuation, while preserving well the Reynolds stress. The structural characteristics of the active turbulence were qualitatively similar to those measured over various surface configurations. Overall, the observed structures of the active turbulence are composed of very large streaks of low momentum fluid elongated in the streamwise direction with some sub-structures included in the streaks. The sub-structures were the main cause of the ejections, which accompany horizontal vortices. The active motion, including the streaky structures, did not reproduce the lower frequency peak of the bi-modal distribution of the horizontal velocity spectra, but reproduced the higher frequency mode that robustly follows inner-layer similarity (i.e. Monin–Obukhov similarity).

Characteristics of a flow in the boundary layer over a sequence of small localized urban canopies with various heights were studied by wind-tunnel experiment under neutral stratification, where “small” means that the length (fetch) is insufficient for the flow to fully adapt to the surface geometry. The sequence was composed of two taller building groups, such as commercial buildings, and two roughness areas of lower obstacles, such as houses. In the sequence, the flow passed over boundaries from the building groups to the roughness areas, named downward boundaries, and boundaries from the roughness areas to the building groups, named upward boundaries. The flow was quite different depending on the boundary types. The focus was placed on equilibrium-like states of the flow and it was found that the flow behind the boundaries were in the states in short fetches.Relating to the downward boundary, flow over the roughness areas was obtained in the range L/H = 7.5 to 63, where H is the height of the upstream array and L is the distance from the end of the upstream array to the measurement position. It was found that the profiles of the Reynolds stress exhibited double-layered structure separated by kinks. In the range L/H > 19, the profiles had constant forms above the kinks and the flow was in some sort of equilibrium state. It was discussed schematically and found that the double-layered structure was explained well based on the three sources of turbulence: turbulence transported from the flow over the upstream building group, turbulence caused by the downward boundary, and turbulence caused by the surface obstacles.Relating to the upward boundary, flow over the building group (cubic array with a frontal area index of 0.25) was obtained. Two adaptation processes to equilibrium state were found: adaptation of transversal average of the streamwise velocity (U) and adaptation of transversal shear of U. The latter was in balanced state in shorter fetch than the former.

The main objective of this study is to examine the robustness of the inner-layer scaling similarity of near-wall turbulence. The turbulent boundary layer of interest is over a very rough surface with a very high Reynolds number and significant outer-layer disturbances. This is not consistent with the canonical turbulent flows studied in laboratories, but it is common in urban areas. The investigation was conducted using the comprehensive outdoor scale model (COSMO) facility. COSMO is composed of a regular array of 1.5 m concrete cubes on a 50×100 m2flat concrete base. This unique facility allows us to obtain the turbulent dataset within the vertical constant stress region under near-neutral stratification at high Reynolds numbers. The turbulent spectra and the standard deviation of velocity fluctuations from COSMO were compared with the values obtained over rural and urban surfaces, and in wind-tunnel experiments.The results confirmed that the inner-layer scaling similarity was robust for the wall-normal fluctuations and the Reynolds stress, independent of the roughness types and the outer-layer conditions. The inner-layer scaling similarity failed for the horizontal velocity fluctuations owing to the influence of the outer-layer disturbance. The relative importance of outer-layer turbulence to inner-layer-scale eddies in the horizontal velocity fluctuations was successfully quantified in terms of the roughness scale normalized by the outer-layer scale.

This paper shows the effects of building height variation on momentum and heat transfer characteristics using Comprehensive Outdoor Scale Model (COSMO) . Turbulent fluctuations and radiations were monitored on the two different type arrays at the same time to investigate the essential effects of building height heterogeneity; the normal model included an aligned array of only 15cm concrete cubes, and the variable model had a geometrically same array but with two building heights of 15cm±50% (i. e. 7.5cm and 22.5cm) . The uneven building array produced relatively strong mixing, and consequently the large drag coefficient. On the other hand, it showed a little difference in sensible heat transfer; there may exist a trade-off between the mixing intensity and the surface heating. The net radiations on the two sites were almost the same so that the building height variation was likely to have little influence on the heat balance as far as the current experimental condition is concerned.

Urban climate experimental results from the Comprehensive Outdoor Scale Model (COSMO) were used to estimate roughness lengths for momentum and heat. Two different physical scale models were used to investigate the scale dependence of the roughness lengths; the large scale model included an aligned array of 1.5-m concrete cubes, and the small scale model had a geometrically similar array of 0.15-m concrete cubes. Only turbulent data from the unstable boundary layers were considered. The roughness length for momentum relative to the obstacle height was dependent on wind direction, but the scale dependence was not evident. Estimated values agreed well with a conventional morphometric relationship. The logarithm of the roughness length for heat relative to the obstacle height depended on the scale but was insensitive to wind direction. COSMO data were used successfully to regress a theoretical relationship between κB−1, the logarithmic ratio of roughness length for momentum to heat, and Re*, the roughness Reynolds number. Values of κB−1 associated with Re* for three different urban sites from previous field experiments were intercompared. A surprising finding was that, even though surface geometry differed from site to site, the regressed function agreed with data from the three urban sites as well as with the COSMO data. Field data showed that κB−1 values decreased as the areal fraction of vegetation increased. The observed dependency of the bulk transfer coefficient on atmospheric stability in the COSMO data could be reproduced using the regressed function of Re* and κB−1, together with a Monin–Obukhov similarity framework.

This paper shows the seasonal dependency of vertical profile of air temperature, CO2, and H2O within and above a suburban canopy and the turbulent diffusivity of the scalars. The results were derived from field measurements of the vertical profiles and fluxes, which were conducted in a residential area (the mean height of canopy is 7.3 m) of Tokyo, Japan. The vertical profile of CO2 in the daytime drastically varied with the season; the CO2 concentration within the canopy was lower than that above the canopy in summer but was slightly higher in winter. The seasonal change of the daytime CO2 profile is likely due to the effect of photosynthesis by vegetation in backyards. The normalized turbulent diffusivity of air temperature and CO2 depended on height and atmospheric stability, which is similar to that for vegetated canopies. In contrast, the normalized turbulent diffusivity of H2O was relatively smaller than that of heat and CO2. Although the correlation coefficients indicated that the transfer efficiencies of CO2 were generally smaller than that of heat, the magnitude of normalized turbulent diffusivity of air temperature and CO2 was almost same.

The present study investigated the vertical profiles of the turbulent statistics over the reduced urban scale model in a neutral stratification. The experiment was conducted under the planetary boundary layer so that the corrected data was always affected by the outer layer turbulence more or less. We compared the surface layer turbulence statistics in the scale model with that in urban, which are velocity spectra, momentum co-spectra and nondimensional velocity variances and so on. A scale effect was apparent in the statistics related with the horizontal velocity variance. The scale effect came from the discrepancy of the ratio of outer layer (i. e. planetary boundary layer) scale with the surface layer scale of the scale model and with that of urban. Although the former scale is always same order, the latter scale is different in urban and in the scale model. Therefore we considered that the scale effect is an evidence of the influence of the outer layer turbulence in the surface layer.

The spatial variability of both turbulent flow statistics in the roughness sublayer (RSL) and temperature profiles within and above the canopy layer (CL) were investigated experimentally in a densely built-up residential area in Tokyo, Japan. Using five towers with measuring devices, each tower isolated from the others by at least 200 m, we collected high-frequency measurements of velocity and temperature at a height z=1.8 z H, where z H, the mean building height in the area, is 7.3 m. Also, temperature profiles were measured from z=0.4 to 1.8 z H. The ‘areal mean’ geometric parameters that were obtained for the areas within 200 m of each tower were fairly homogeneous among the tower sites. The main results are as follows: (1) The spatial variability of all RSL turbulent statistics, except the sensible heat flux, was comparable to that reported in a pine forest. Also, the variability decreased with increasing friction velocity. (2) The spatial variability of the RSL sensible heat flux was larger than that reported in a pine forest. Also, the variability depended on the time of the day and became larger in the morning. The difference among the sites was well related to the areal fraction of vegetation. (3) The spatial variability of the CL temperature profile depended on the time of the day and became larger in the morning. Nevertheless, the spatial standard deviation of CL temperature was always below 0.7 K. (4) It is suggested that the “warming-up” process in the morning when heat storage is dominant increases the spatial variation of RSL sensible heat flux and CL temperature according to the local properties around each tower and the variation decreases once there is further convective mixing in the midday

The vertical profiles and horizontal distribution of atmospheric turbulence were observed at an outdoor urban scale model site. The model dimensions were 50 x 100m, with 1.5-meter cubic roughness blocks arranged uniformly. Eight sonic anemometers were arranged vertically or horizontally in the site to obtain the vertical profiles or horizontal distribution. In this study, we focused on the atmospheric stability effect on the vertical profiles and the horizontal distribution of the turbulent structure over the model site.The quadrant analysis was conducted using the data set of the vertical distribution. The analysis showed the dependence of the turbulent motion on the atmospheric stability (z'/L). The sweep mode is mostly dominant at all height in the surface layer when the stability is stable. In case of the unstable atmospheric condition, the ejection mode can crossover the sweep mode at the higher portion in the surface layer.The streak pattern was observed in a turbulent flow over the building like roughness from the measurement of the horizontal distribution. The width was about 3 or 6 times larger than that of the blocks. This result strongly implies the importance of the horizontal distribution the turbulent structure over urban area.

In this study we focused on the relationships between the vertical profiles of scalar within and above a suburban canopy and the cold-air subsidence locally generated within the canopy. The results were derived from the wintertime field measurements of the vertical profiles of CO2, air temperature, water vapor, and turbulent flows, which were conducted in a residential area (the mean height of canopy is 7.3 m) of Tokyo, Japan. Under stably stratified conditions, the CO2 concentration above the canopy decreased with height, while that within the canopy kept almost same level. This indicates that the CO2 emitted from the houses accumulated within the canopy. Such behavior was not found in H2O profile. The vertical profile of air temperature and observations on the surface temperature using a thermal infrared camera suggested that the cold air generated at roof level moved down to the ground level. The effect of ‘cold air subsidence’ within the canopy is the most plausible reason for the nighttime vertical profiles of CO2 and H2O.

Turbulent organized structures (TOS) above building arrays were investigated using a large-eddy simulation (LES) model for a city (LES-CITY). Square and staggered building arrays produced contrasting behaviour in terms of turbulence that roughly corresponded to the conventional classification of ‘D-type’ and ‘K-type’ roughness, respectively: (1) The drag coefficients (referred to the building height) for staggered arrays were sensitive to building area density, but those for square arrays were not. (2) The relative contributions of ejections to sweeps (S2/S4) at the building height for square arrays were sensitive to building area density and nearly equalled or exceeded 1.0 (ejection dominant), but those for staggered arrays were insensitive to building area density and were mostly below 1.0 (sweep dominant). (3) Streaky patterns of longitudinal low speed regions (i.e., low speed streaks) existed in all flows regardless of array type. Height variations of the buildings in the square array drastically increased the drag coefficient and modified the turbulent flow structures. The mechanism of D-type and K-type urban-like roughness flows and the difference from vegetation flows are discussed. Although urban-like roughness flows exhibited mixed properties of mixing layers and flat-wall boundary layers as far as S2/S4 was concerned, the turbulent organized structures of urban-like roughness flows resembled those of flat-wall boundary layers.

Recent observational studies have described the non-closure of the energy balance when the eddy co-variance (EC) method is used for the measurements. We investigated this problem using a numerical simulation of a heterogeneous surface region. A typical daytime boundary layer was simulated, using the large eddy simulation (LES) method in which horizontal heterogeneity was imposed on the ground surface heating as a one-dimensional sinusoidal variation. This horizontal heterogeneity is expected to produce a mesoscale circulation.We decomposed the total vertical heat flux into the EC turbulent flux, the heat flux due to a mesoscale circulation (hereafter, mesoscale flux), and the “residual flux”. The sum of the mesoscale flux, and residual flux accounts for the energy imbalance if we estimate the total flux only from the EC method.The numerical results demonstrated that larger amplitude of surface heating caused larger mesoscale flux, but smaller residual flux. As a result, the energy imbalance became minima at some weak amplitude of surface heating.The residual flux was caused by the turbulent organized structure (hereafter, TOS), which is a cluster of thermals moving, with a larger time scale than that of individual plumes. The larger surface heating amplitude weakened the TOS due to the following two mechanisms; (1) the TOS is organized in roll due to the strong horizontal pressure gradient, (2) the higher horizontal wind speed, parallel to the mesoscale circulation, advects the TOS faster then the ergodicity works better.The other cases with gepstrophic winds, resulted in the decrease of the energy imbalance with increasing wind velocity.

We investigated the ‘local’ and ‘global’ similarity of vertical turbulent transfer of heat, water vapour, and CO2 within an urban surface layer. The results were derived from field measurements in a residential area of Tokyo, Japan during midday on fair-weather days in July 2001. In this study, correlation coefficients and quadrant analysis were used for the evaluation of ‘global’ similarity and wavelet analysis was employed for investigating ‘local’ similarity. The correlation coefficients indicated that the transfer efficiencies of water vapour and CO2 were generally smaller than that of heat. Using wavelet analysis, we found that heat is always efficiently transferred by thermal and organized motions. In contrast, water vapour and CO2, which are passive quantities, were not transferred as efficiently as heat. The quadrant analyses showed that the heat transfer by ejection exceeded that by sweep, and the ratios of ejection to sweep for water vapour and CO2 transfer were less than that for heat. This indicated that heat is more efficiently transferred by upward motions and supported the findings from wavelet analysis. The differences of turbulent transfer between heat and both CO2 and water vapour were probably caused both by the active role of temperature and the heterogeneity in the source distribution of scalars

A Simple 3-dimensional Urban canopy Model for Meso-scale simulation (SUMM) was proposed. The model predicts the energy balance of constituent faces (roof, floor and 4 vertical walls) without time-consuming iterations. In the model, complex 3-dimensional radiation processes (multi-reflection processes) are theoretically solved, resulting in a large reduction of computational time. As for the turbulent transfers, the network of resistance is adopted. We also conducted outdoor scale model experiments having the similar geometric structure with SUMM, to construct the databases (energy balances and surface temperatures) for model validation, and to examine unknown model parameters (e. g. turbulent transfer coefficients).

Large-eddy simulations have been performed for fully developed turbulent flow within and above explicitly resolved simple cube arrays. The results from our model, hereafter LES-CITY, are shown to agree with laboratory experiments. We investigated the systematic influence of cube density on turbulent flow characteristics by performing numerical experiments for cube areal densities from 0 to 44%. The following results were obtained: (1) The dispersive momentum flux was quite large within the canopy layer due to a mean stream re-circulation, whereas it was smaller above the canopy. The spatial variation of temporally averaged momentum in the roughness sub-layer was 20% or less of the total kinematic surface drag. (2) The temporally and spatially-averaged flow structure confirmed the existence of conventionally described canyon flow regimes; isolated, interfacial, and wake. However, the intermittency of the canyon flow for all cube densities was quite large and the stream patterns were never persistent. (3) Turbulent organized structures (TOS) similar to those observed in turbulent surface-layer flows were simulated, which are characterized by longitudinally-elongated low speed streaks and the corresponding shorter streamwise vortices. The streaks in sparse and dense canopy flows were likely to be aligned to the street line and to the roof lines, respectively. Such heterogeneity of TOS partially accounts for the large spatial variation of momentum flux. (4) In contrast to the mixing layer analogy of vegetation flows, the TOS and the resulting turbulent statistics of urban flow above the canopy resembled those in surface layers. The recirculation within the canopy significantly influenced the turbulent statistical properties.

Large Eddy Simulations have been performed for fully developed turbulent flow within and above explicitly resolved simple cube arrays. The model (LES-CITY) was validated in comparison with laboratory experiments. The systematic influence of cubic density on turbulent flow characteristics was investigated through numerical experiments in a wide range of cubic area density (0 to 0.44).The following results were obtained. 1) The intermittency of flow: the temporally and spatially averaged flow structure confirms the existence of conventionally described canyon flow regime. However, the intermittency of the canyon flow for any cubic densities is large and the stream patterns are never persistent. 2) Turbulent Organized Structures (TOS): TOS are characterized by longitudinally elongated low speed streaks and corresponding streamwise vortices. The streaks in sparse and dense canopy flows are likely to be aligned to the street-line and to the roof-line, respectively.

The spatial representativeness of turbulent fluxes on the basis of point measurements has been investigated through a theoretical approach and numerical experiments using Large Eddy Simulation (LES) Model. These investigations have been done for the daytime atmospheric boundary layers heated uniformly over the flat surface with no synoptic vertical motion under several geostrophic winds from 1 to 4 (ms-1). The following results were obtained;(1) The spatial representativeness of turbulent fluxes based on the time-series analysis are shown to be problematic: Their horizontally averaged value has a negative imbalance irrespective of the density of measurements points, and their spatial variability increases, in accordance with averaging time, or under lower horizontal wind.(2) The imbalance intensities are closely related to turbulent organized structure (TOS). The points over the updraft regions are likely to have positive imbalance due to highly activated thermal plumes. The temporal and spatial change of TOS pattern cause low-frequency trends in time-series data, which can account for the large horizontal scatter of flux estimates.(3) High-pass filter excluding the temperature trend is theoretically useful to improve the representativeness of point measurements, although the filter choice actually needs a great care not to damage the turbulent components.

The present study uses observational data from a residential site in Tokyo, Japan and a rice paddy in Chiba Pref., Japan, to investigate the relative facility with heat, water vapor, and carbon dioxide are transported by turbulence in the surface layer. The ratios of linear correlation coefficients |rwT/ruw| and |rwq/ruw| increase with stability and the ratios at residential area are generally smaller than at rice paddy due to bluff-body effects. At rice paddy, the ratios of linear correlation coefficient |rwc/rwq| is generally smaller than unity. Meanwhile at the residential area, the distribution of the ratio |rwc/rwq| scatters to |z'/L|. This inequality may be caused by the complex source patterns of the urban surface and the vertical profile within the canopy. Inequalities of transfer between T, c, and q will imply that the diffusion coefficient between them have different value respectively.

Large eddy simulations with meshes fine enough to resolve the roads have been performed to investigate the characteristics of convective boundary layer over Otemachi and the Imperial Palace as an example of urban area. Nondimensional profiles of mean temperature and turbulent intensities agree well with experimental data, and it is shown that the vertical convection and the entrainment are more effective when the ground surface is heated uniformly. It is also shown that many thermals grow up from the asphalt roads while the down stream region are generated around the Imperial Palace.

A new numerical model has been developed to investigate the hydrological and micrometeorological phenomena near around the 3D arbitrary-shaped boundary. This model has the following features.1) Adaptive grid method is adopted to generate the grid system. Using this method, orthogonality and density of the grid are easily and arbitrarily controlled. 2) Momentum, heat, and vapor fields are solved simultaneously. For the parameterization of turbulence, LES model is adopted. 3) Distribution of insolation on the ground surface is considered. The temperature of the ground surface is determined from the energy budget.The flow near the isolated mountain is calculated and the results get agreement qualitatively with the experimental data.

A new numerical model of three dimensional hydro-metorological process has been developed for investigating the initiation and development of convective clouds considering the effect of air-soil interactions. This model consists of three parts, that is, atmosphere, constant flux layer and soil. For the subgrid turbulence process, ‘ Large Eddy simulation (LES)’ model is adopted with a timedependent turbulence energy equation. Soil temperature distribution is determined time-dependently and locally to satisfy both energy and moisture balances.Microphysical processes of cloud and rain generation are described by the Kessler parameterization.The results show that the initiation of convection is effected strongly by the soil moisture distribution and the air-soil interaction intensify the development of convective clouds.

Heterogeneous surface conditions modify the flow structure of the convective boundary layer (CBL). We investigated this impact focusing on the spatial representativeness of eddy-covariance (EC) point observations. The typical daytime boundary layer was simulated using large eddy simulation (LES), where horizontal heterogeneity was imposed on the ground surface heating as one-dimensional sinusoidal variation. The effect of variation of the wavelength and amplitude on the vertical heat transport was examined.The surface budged based on the point EC method is examined excluding the net vertical heat transport due to the mesoscale circulations invoked by surface heterogeneity. The mesoscale circulation is represented using a phase average. The result found that our simulated EC point measurements still underestimate this energy budget. However, this deficit gets close to zero when the surface heterogeneity is strong. This underestimate can be strongly attributed to the turbulent organized structure (TOS) developed parallel to the mesoscale circulation.