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We investigate the characteristics of a dust-devil-like vortex (DDLV) observed using thermal image velocimetry (TIV) at a sports ground in Tokyo. Thermal image velocimetry provides unique observations of the two-dimensional velocity distribution for a DDLV with high spatio–temporal resolution (i.e., tens of cm s−1) near the ground. Two DDLVs were detected, one each in summer and winter, and the quantitative features of the larger, stronger DDLV in the winter are examined. The size and strength of the detected DDLV, which are quantified using TIV, are within the ranges reported in past observations and numerical simulations of dust devils. The vortex appears at the boundary of a cold-air current near a 55-m building wall, and persists for more than 3 min.

Urban geometry and materials combine to create complex spatial, temporal and directional patterns of longwave infrared (LWIR) radiation. Effective anisotropy (or directional variability) of thermal radiance causes remote sensing (RS) derived urban surface temperatures to vary with RS view angles. Here a new and novel method to resolve effective thermal anisotropy processes from LWIR camera observations is demonstrated at the Comprehensive Outdoor Scale MOdel (COSMO) test site. Pixel-level differences of brightness temperatures reach 18.4 K within one hour of a 24-h study period. To understand this variability, the orientation and shadowing of surfaces is explored using the Discrete Anisotropic Radiative Transfer (DART) model and Blender three-dimensional (3D) rendering software. Observed pixels and the entire canopy surface are classified in terms of surface orientation and illumination. To assess the variability of exitant longwave radiation (M_LW) from the 3D COSMO surface (M_LW^3D), the observations are prescribed based on class. The parameterisation is tested by simulating thermal images using a camera view model to determine camera perspectives of M_LW^3D fluxes. The mean brightness temperature differences per image (simulated and observed) are within 0.65 K throughout a 24-h period. Pixel-level comparisons are possible with the high spatial resolution of M_LW^3D and DART camera view simulations. At this spatial scale (< 0.10 m), shadow hysteresis, surface sky view factor and building edge effects are not completely resolved by M_LW^3D. By simulating apparent brightness temperatures from multiple view directions, effective thermal anisotropy of M_LW^3D is shown to be up to 6.18 K. The developed methods can be extended to resolve some of the identified sources of sub-facet variability in realistic urban settings. The extension of DART to the interpretation of ground-based RS is shown to be promising.

Approaches to understanding urban atmospheric phenomena in cities include mesoscale models coupled with urban canopy models (UCMs). To improve the accuracy of these models, urban morphological parameters of higher spatial resolution are necessary. In this paper, we introduce an approach to derive 1-km scale urban parameters from globally available satellite images of Landsat 8, Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), Global Multi-resolution Terrain Elevation Data 2010 (GMTED2010), and nighttime light (NL) images from the Defense Meteorological Satellite Program Operational Linescan System (DMSP-OLS) for use in numerical weather models. Empirical equations derived from linear fittings were used to calculate urban geometric parameter plane area index from satellite images, which were then compared with values derived from real plane area index. To calculate urban geometric parameters average building height, an artificial neural network fitting was conducted. From the basic urban geometric parameters, other necessary urban parameters were derived. After validating the method, a weather simulation was conducted using the derived parameters for Jakarta, Indonesia. Results showed that urban parameters made from global datasets could improve the performance of the single-layer urban canopy model; and these estimated parameters can be used as a substitute when actual building information are lacking.

This study developed a top-down method for estimating global anthropogenic heat emission (AHE), with a high spatial resolution of 30 arc-seconds and temporal resolution of 1 h. Annual average AHE was derived from human metabolic heating and primary energy consumption, which was further divided into three components based on consumer sector. The first and second components were heat loss and heat emissions from industrial sectors equally distributed throughout the country and populated areas, respectively. The third component comprised the sum of emissions from commercial, residential, and transportation sectors (CRT). Bulk AHE from the CRT was proportionally distributed using a global population dataset, with a radiance-calibrated nighttime lights adjustment. An empirical function to estimate monthly fluctuations of AHE based on gridded monthly temperatures was derived from various Japanese and American city measurements. Finally, an AHE database with a global coverage was constructed for the year 2013. Comparisons between our proposed AHE and other existing datasets revealed that the problem of overestimation of AHE intensity in previous top-down models was mitigated by the separation of energy consumption sectors; furthermore, the problem of AHE underestimation at central urban areas was solved by the nighttime lights adjustment. A strong agreement in the monthly profiles of AHE between our database and other bottom-up datasets further proved the validity of the current methodology. Investigations of AHE for the 29 largest urban agglomerations globally highlighted that the share of heat emissions from CRT sectors to the total AHE at the city level was 40–95%; whereas that of metabolic heating varied with the city's level of development by a range of 2–60%. A negative correlation between gross domestic product (GDP) and the share of metabolic heating to a city's total AHE was found. Globally, peak AHE values were found to occur between December and February, while the lowest values were found around June to August. The northern mid-latitudes contributed most to the global AHE.

We used numerical simulations to investigate the general relationship between urban morphology and the intensity of wind gusts in built-up areas at the pedestrian level. The simulated urban boundary layer developed over a 19.2 km (length) $$\times $$ 4.8 km (width) $$\times $$ 1.0 km (height) simulation domain, with 2-m resolution in all directions, to explicitly resolve the detailed shapes of buildings and the flow at the pedestrian level. This complex computation was accomplished using the lattice Boltzmann method and by implementing a large-eddy simulation model. To generalize the results, a new parameter that expresses the intensity of gusts (the gust index, $${\tilde{U}}_{ max})$$ was defined as the local maximum wind speed divided by the freestream velocity. In addition, this parameter was decomposed into the mean wind-speed ratio, $${\tilde{U}} $$ and turbulent gust ratio, $${\tilde{U}}^{{\prime }}$$ to evaluate the qualities of gusts. These parameters were useful for quantitatively comparing the gust intensities within urban canopies at different locations or even among different experiments. In addition, the entire horizontal domain was subdivided into homogeneous square patches, in which both the simulated gust parameters and the morphological characteristics of building geometries were averaged. This procedure masked the detailed structure of individual buildings but retained the bulk characteristics of the urban morphology. At the pedestrian level, the gust index decreased with increasing building cover. Compared to $${\tilde{U}} $$ , the quantity $${\tilde{U}}^{{\prime }}$$ notably contributed to the index throughout the range of plan area index $$(\lambda _p)$$ values. The dependences of all normalized wind-speed ratios transiently changed at $$\lambda _p =~0.28$$ . In cases where $$\lambda _p < 0.28, {\tilde{U}} $$ decreased with increasing $$\lambda _p $$ , although $${\tilde{U}}^{{\prime }}$$ was almost constant. In cases where $$\lambda _p > 0.28, {\tilde{U}}$$ was almost constant and $${\tilde{U}}^{{\prime }}$$ decreased with increasing $$\lambda _p $$ . This was explained by the change in flow regimes within the building canyon. At a higher elevation above the canopy layer, $$\lambda _p $$ becomes less relevant to normalized wind-speed ratios, and instead the aerodynamic roughness length became important.

The applicability of outer-layer scaling is examined by numerical simulation of a developing neutral boundary layer over a realistic building geometry of Tokyo. Large-eddy simulations are carried out over a large computational domain $$(19.2\hbox { km} \times 4.8 \hbox { km}\times 1 \hbox { km}$$ in the streamwise, spanwise, and vertical directions, respectively), with a fine grid spacing (2 m) using the lattice-Boltzmann method with massively parallel graphics processing units. The simulation produces a ratio of the boundary-layer height to the average building height of more than 50. Results from simulations show that outer-layer features are maintained for turbulence statistics in the upper part of the boundary layer, as well as the width of predominant streaky structures throughout the entire boundary layer, despite the very large roughness. This is caused by the existence of very large streaky structures extending throughout the entire boundary layer, which follow outer-layer scaling with a self-preserving development. We assume the top-down mechanism in the physical interpretation of results.

Observations using a three-dimensional scanning coherent Doppler lidar in an urban area revealed the characteristics of streaky structures above a rough, inhomogeneous surface for a high-Reynolds-number flow. The study focused on two points: (1) the frequency of occurrence and conditions required for the presence of streaky structures, and (2) the universal scaling of the spacing of streaky structures ( $$\lambda )$$ . The horizontal snapshots of the radial velocity were visually classified into six groups: Streak, Mixed, Fishnet, No streak, Front, and Others. The Streak category accounted for more than 50% of all possible flows and occurred when the horizontal wind speed was large and the atmospheric stratification was near-neutral. The spacing ( $$\lambda )$$ was estimated from the power spectral density of the streamwise velocity fluctuations along the spanwise direction. The spacing $$\lambda $$ decreased with an increase in the local velocity gradient. Furthermore, it was revealed that the local velocity gradient normalized by the friction velocity and the boundary-layer height ( $$z_i )$$ comprehensively predicts $$\lambda /z_i $$ under various experimental and environmental conditions, in terms of the scale of motion (i.e., indoor and outdoor scales), thermal stratification (i.e., from weakly unstable to stable stratification), and surface roughness (i.e., from flat to very rough surfaces).

As urban population is forecast to exceed 60% of the world’s population by 2050, urban growth can be expected. However, research on spatial projections of urban growth at a global scale are limited. We constructed a framework to project global urban growth based on the SLEUTH urban growth model and a database with a resolution of 30 arc-seconds containing urban growth probabilities from 2020 to 2050. Using the historical distribution of the global population from LandScanTM as a proxy for urban land cover, the SLEUTH model was calibrated for the period from 2000 to 2013. This model simulates urban growth using two layers of 50 arc-minutes grids encompassing global urban regions. While varying growth rates are observed in each urban area, the global urban cover is forecast to reach 1.7 × 106 km2 by 2050, which is approximately 1.4 times that of the year 2012. A global urban growth database is essential for future environmental planning and assessments, as well as numerical investigations of future urban climates., Design Type(s)modeling and simulation objective • population modeling objective • time series design • population dynamics analysis objectiveMeasurement Type(s)GrowthTechnology Type(s)computational modeling techniqueFactor Type(s)temporal_interval • geographic locationSample Characteristic(s)water body • area of protected biodiversity • public road • road • populated place Machine-accessible metadata file describing the reported data (ISA-Tab format)

Sea breeze is closely related to the living environment in coastal-urban regions. Therefore, the framework for sea-breeze detection is crucial. We proposed a framework to derive a two-dimensional distribution of sea-breeze front (SBF) using geostationary satellite images. The framework includes the application of morphological snake algorithm into visible band images for automated detection of the cumulus cloud-lines associated with SBF. The verification was undertaken by comparing the passage time of the cloud-line with the arrival time of the SBF estimated from ground observation. The results show that the cloud-line was reasonably able to represent the arrival time of the SBF with a bias time of 20 min and within a distance of 3 km. The mean penetration speed obtained from the cloud-lines passage was slightly slower than that estimated from the arrival time of SBF (1.4 ms−1 and 1.5 ms−1, respectively). In certain cases, the cloud-lines passage was found to be behind the SBF more than 5 km, particularly in the city center. This indicated the enhanced updraft due to urban-heat-island circulation potentially caused the delayed of cloud-line propagation in certain prevailing wind condition.

Air temperature trends (1960–2009) based on stations in cities, minus those based on global surface temperature datasets, are defined herein as urban heat island (UHI) trends. Urban climate was examined globally by comparing UHI trends with indices of geophysical factors, including background climate, latitude, and diurnal temperature range (DTR) and indices of artificial factors, including anthropogenic heat emission (AHE) and population indices. Surprisingly, a better relationship was found between UHI trends and DTR—an integrated geophysical index representing thermal inertia—than with the indices of artificial factors. Thus, while an increase in sensible heat (mechanism 1) triggers UHI formation, this study infers that large thermal inertia (mechanism 2) contributes significantly on UHI. The correlation of UHI trends with other indices can be explained by both mechanisms. The capacity of urban areas to absorb heat largely explains the urban heat island (UHI) effect observed in global cities. Alvin CG Varquez and Manabu Kanda from Tokyo Institute of Technology, Japan, assess the trends in urban heat island effect from 1960 to 2009 in 286 cities globally. They find the UHI effect has increased in most cities by 1.38 °C per century on average, with a larger effect at night and a more pronounced trend at higher latitudes. Most UHI trends can be explained by the thermal inertia, a higher resistance to temperature changes of urban surfaces, with heat trapped during the day being released at night. Human factors such as heat emissions and population were not correlated to the trend, but further studies are needed to fully disentangle the interactions between all factors.

Scintillometer measurements of turbulent fluxes of momentum and sensible heat in the roughness sublayer over a regular array of cubes in an outdoor environment were tested with direct measurement from sonic anemometers. The dissipation rate, e, and temperature structure parameter, C T 2 , obtained from the scintillometer agreed well with those from four sonic anemometers located along the scintillometer path. The fluxes measured by the scintillometer also corresponded well to those from the line-averaged eddy covariance approach, although this agreement was greatly influenced by the choice of the zero-plane displacement length and the form of the similarity function used in the scintillometer software. A guide for choosing the appropriate similarity function for the urban roughness sublayer is proposed.

Average horizontal wind velocity in an urban canopy is mainly determined by a balance between flow deceleration caused by the drag force of buildings and flow acceleration from the momentum flux gradient in the canopy. To express the transport of momentum in an urban canopy, mixing length is often used to calculate diffusivity there. A new parametrization for mixing length is introduced for a one-dimensional multilayer urban canopy model (UCM). A database from large-eddy simulations using actual urban morphology for Tokyo is used for this parametrization. The derived mixing length is described as a function of the non-dimensional height raised to the power of $$q$$ , where $$q < 1$$ . The $$q$$ value and constants of the function also depend on the selection of canopy height. The mixing length profile is closely related to that of the average plane area index of the buildings in the study area. Recalculation of mean horizontal wind velocity using the new parametrization of mixing length for Tokyo slightly improved the multilayer UCM results.

We consider the effects of detailed urban roughness parameters on a sea-breeze simulation. An urban roughness database, constructed using a new aerodynamic parametrization derived from large-eddy simulations, was incorporated as a surface boundary condition in the advanced Weather Research and Forecasting model. The zero-plane displacement and aerodynamic roughness length at several densely built-up urban grids were three times larger than conventional values due to the consideration of building-height variability. A comparison between simulations from the modified model and its default version, which uses uniform roughness parameters within a conventional method, was conducted for a 2-month period during summer. Results showed a significant improvement in the simulation of surface wind speed but not with temperature. From the 2-month study period, a day with an evident sea-breeze penetration was selected and simulated at higher temporal resolution. Sea-breeze penetration weakened and was more delayed over urbanized areas. The slow sea-breeze penetration also lessened heat advection downwind allowing stronger turbulent mixing and a deeper boundary layer above urban areas. Horizontal wind-speed reduction due to the increased urban surface drag reached heights of several hundreds of metres due to the strong convection.

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.

Urban land surface schemes have been developed to model the distinct features of the urban surface and the associated energy exchange processes. These models have been developed for a range of purposes and make different assumptions related to the inclusion and representation of the relevant processes. Here, the first results of Phase 2 from an international comparison project to evaluate 32 urban land surface schemes are presented. This is the first large-scale systematic evaluation of these models. In four stages, participants were given increasingly detailed information about an urban site for which urban fluxes were directly observed. At each stage, each group returned their models' calculated surface energy balance fluxes. Wide variations are evident in the performance of the models for individual fluxes. No individual model performs best for all fluxes. Providing additional information about the surface generally results in better performance. However, there is clear evidence that poor choice of parameter values can cause a large drop in performance for models that otherwise perform well. As many models do not perform well across all fluxes, there is need for caution in their application, and users should be aware of the implications for applications and decision making. Copyright  2010 Royal Meteorological Society

The objective of this study is to examine the differences and similarities in the annual trends of the urban surface energy balance (SEB) among long-term field measurements. Four datasets analyzed for the study were collected in the following experiments or observational sites: Comprehensive Outdoor Scale Model experiments (COSMO), the Kugahara site in Tokyo, Japan (Ku04), and the Sperrstrasse and Spalenring sites in Basel, Switzerland (BuU1 and BuU2). A new variable, the forcing radiation QFR, has been proposed to replace the conventional net radiation Q* for the normalization of the SEB components. Here, QFR is defined as the sum of net shortwave radiation and downward longwave radiation. Because QFR does not include the upward longwave radiation, it is independent of the surface temperature, which is determined by the energy partitioning process. Therefore, QFR is independent of the energy partitioning process itself. With the use of QFR, the characteristics of the daytime normalized components of the SEB equation (i.e., upward longwave radiation, QL↑/QFR, turbulence fluxes (QH + QE)/QFR, and heat storage ΔQS/QFR) were investigated. The above energy fluxes normalized by the forcing radiation depended on the friction velocity u*. An increase of u* predominantly enhanced (QH + QE)/QFR and reduced both QL↑/QFR and ΔQS/QFR. When this u* dependency on the SEB was taken into consideration, the annual variations of these three flux ratios from cities located in similar latitudes and longitudes (i.e., BuU1 and BuU2, Ku04, and COSMO) were very similar. At BuU1 and BuU2, QL↑/QFR showed larger seasonal amplitudes than at COSMO and Ku04. The seasonal variations of (QH + QE)/QFR were roughly out of phase with respect to those of QL↑/QFR, resulting in relatively small seasonal variations of ΔQS/QFR. Furthermore, the effects of urban canyon geometry on the SEB were examined by comparing the SEB for the roofs to that for the canyon. The three-dimensional urban canyon geometry enhanced the heat storage efficiency relative to the forcing radiation ΔQS/QFR of the canyon in comparison with that of roofs or flat surfaces. This observation was explained by the continuous movement of sunlit areas on the walls and streets.

The objective of this study is to examine the basic features of the surface energy balance (SEB) using the data obtained from the Comprehensive Outdoor Scale Model (COSMO). COSMO is an idealized miniature city that has no vegetation, no human activity, and no heterogeneity of the surface geometry. The basic features of the SEB such as energy balance closure, the ensemble mean of the diurnal variation of the energy balance, and the daytime and daily statistics of the energy balance were investigated. The following were the main findings of the study: 1) A surface energy imbalance was observed. The sum of sensible and latent heat fluxes estimated by the eddy correlation method underestimated the available energy by 1% during the daytime and by 44% during the night. 2) Large heat storage in the daytime and small radiative cooling at night sustained positive sensible heat fluxes throughout the night in all seasons and in all sunshine conditions. 3) The daytime ratio of heat storage ΔQS to net radiation Q*, ΔQS/Q*, depended on the friction velocity u* and decreased with increasing u*. 4) The values of ΔQS/Q* tended to be larger in winter than in summer. The annual averaged value of this ratio was approximately 0.6. 5) The large volumetric heat capacity of the surface materials and the resulting large energetic hysteresis produced nonzero total daily values of heat storage. The total daily values of heat storage largely depended on the weather (i.e., sunshine condition and with or without rainfall) and showed positive and negative values on clear-sky days and rainy days, respectively.

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).

Sea surface temperature (SST) and air temperature were measured in situ in Tokyo Bay. These measurements were made with high spatial and temporal resolutions between November 2006 and September 2007. The analysis of these data revealed 1) the seasonal and diurnal variations of SST and air temperature, and 2) the physical process by which Tokyo Bay lowers urban air temperature in summer. The following were the major findings obtained: 1) the diurnal amplitude of SST was as large as 5.5°C; 2) abrupt increases of SST occurred at the head and mouth of the bay that were due to heated water discharge and the Kuroshio, respectively; 3) the values of the satellite-based objectively analyzed SSTs were higher than those of the in situ SSTs, especially in winter; 4) the relationship between SST and air temperature was classified into three seasonal modes—winter, transient, and summer—and each mode was associated with the seasonal stability condition of the near-surface water; 5) the strong southwesterly wind over the bay in summer decreased the SST mainly because of increased turbulent mixing at the water surface, thereby increasing downward sensible heat flux up to −100 W m−2; 6) the lower SSTs in summer lowered the air temperature, but only for the urban atmosphere near the coast, and no effect was detected at 20 km inland; and 7) the horizontal gradient of air temperature over the land intensified with increasing wind speed.

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.