Shinji Wakamatsu

Wind tunnel experiments on how thermal stratification affects flow in and above urban street canyons

The effects of atmospheric stability on flow in urban street canyons were studied using a stratified wind tunnel. We conducted experiments using a model that represented city streets with simply shaped block forms, while varying atmospheric stability across seven stages from stable (Rb=0.79) to unstable (Rb=−0.21). We used a laser Doppler anemometer (LDA) and a cold wire to measure the flow field and temperature within and above the street canyon. In addition to mean values of wind speed components and temperatures, we measured turbulence intensity, shear stress, and heat flux distribution. Our results led to the following conclusions: Cavity eddies that arose in the street canyon tended to be weak when the atmosphere was stable and strong when unstable. Stable atmospheric conditions led to a positive feedback effect in which the downward flow into the street canyon weakened due to buoyancy, which facilitated the formation of a more highly stable stratification. As a result, when stability exceeded a certain threshold (somewhere in the range of Rb=0.4–0.8), the wind speed in the street canyon dropped nearly to zero.

Numerical modeling of the nocturnal urban boundary layer

A numerical case study with a second-order turbulence closure model is proposed to study the role of urban canopy layer (UCL) for the formation of the nocturnal urban boundary layer (UBL). The turbulent diffusion coefficient was determined from an algebraic stress model. The concept of urban building surface area density is proposed to represent the UCL. Calculated results were also compared with field observation data. The height of the elevated inversion above an urban center was simulated and found to be approximately twice the average building height. The turbulent kinetic energy k, energy dissipation rate ε, and turbulence intensities 〈u2〉 and 〈w2〉 increase rapidly at the upwind edge of the urban area. The Reynolds stress 〈uw〉 displayed a nearly uniform profile inside the UBL, and the vertical sensible heat flux 〈wθ〉 had a negative value at the inversion base height. This indicates that the downward transport of sensible heat from the inversion base may play an important role in the formation of the nocturnal UBL.

Recent trends in precursor concentrations and oxidant distributions in the Tokyo and Osaka areas

Photochemical air pollution is still a serious environmental issue in Tokyo and Osaka areas. To clarify the past trend and present status of photochemical air pollution, the annual trends of daily maximum oxidant concentration covering Kanto (Tokyo and surrounding prefectures) and Kansai (Osaka and surrounding prefectures) areas were analyzed. Trends in the photochemical air pollution and NOx and VOC precursors showed identical characteristics both in Tokyo and Osaka areas. Trends in volatile organic hydrocarbons and NOx were analyzed; NOx is incresing and NMHC/NOx ratio is decreasing in the long term over both Kanto and Kansai areas, indicating an increase in ozone forming potential and a decrease in photochemical reactivity, which may be responsible for the air pollution patterns observed in the Tokyo and Osaka areas. The location of the daily maximum oxidant concentration has moved further from the emission areas.

Source discrimination of short-term hydrocarbon samples measured aloft.

Weighted least-squares fitting of 17 hydrocarbons was used to estimate ambient contributions from four source categories. The data set consisted of 192 samples collected from 300 to 1500 m over Tokyo, July 16-17, 1981. Six runs (flights), 1-1.5 h long, spaced throughout each day, constituted chemical snapshots of the urban air. Vehicles contributed 7.0%, gasoline vapor 10.5%, petroleum refinery 26.0%, paint solvents 27.2%, and unexplained sources 29.3% of the total hydrocarbon concentration (based on the 17 components measured and all samples). These coefficients are only representative of the days and conditions sampled and should not be interpreted as annual emission fractions. On a run-averaged basis the correlation between refinery emissions per hour, adjusted for dispersions, and ambient concentrations of total hydrocarbon attributable to petroleum refineries was r/sup 2/ = 0.899, indicating that the refinery profile was sufficiently unique to be sensed up to 70 km. The fractions for paint solvents, gasoline vapor, and unidentified sources were also consistent with wind trajectory observations. 20 references, 4 figures, 8 tables.

Long-Range Transport of Air Pollution under Light Gradient Wind Conditions

Abstract The long-range transport of air pollution on clew days under light gradient wind conditions is investigated from an analysis of all days with high oxidant concentrations in 1979 at locations in central Japan that are far from pollutant sources. Surface-level wind and pressure distributions over a 300×300 km area were analyzed, together with concentration isopleths of oxidants and suspended particles produced by photochemical reactions. It was found that the transport mechanism consists of. 1) land/sea breezes; 2) a steady onshore wind driven by the diurnal-mean land–sea temperature difference; 3) the generation of a strong thermal low in the inland mountainous region in the daytime; and 4) a subsidence inversion accompanied by a synoptic-scale high pressure system. The last three mechanisms work to combine land/sea breezes and slope and valley winds into one large-scale high-speed wind field that transports pollutants a long distance inland into the mountainous region.

Seasonal variation in atmospheric aerosols concentration covering northern Kyushu, Japan and Seoul, Korea

Abstract Atmospheric aerosols were collected from October 1990 to February 1992 in northern Kyushu (Tsushima and Ogori), Japan and Seoul, Korea, simultaneously using identical sampling system and chemical analysis methods. Observed data were analyzed with meteorological data to clarify the seasonal variations in aerosol concentration and composition. Dominant ion components of aerosol were SO42− in anion and NH4+ in cation. The concentrations of ion components were higher in winter than those in summer. Equivalent concentration ratio of particulate nitrate and sulfate ( NO 3 − SO 4 2− ) showed high value in Ogori having a peak in winter season. Equivalent concentration ratio of Ca2+ and sulfate ( Ca 2+ SO 4 2− ) is highest in Seoul and shows no seasonal variation. On the contrary, Tsushima and Ogori showed low value in summer season. In summer, this area is affected by the atmospheric high-pressure system which developed at the southern Kyushu sea area and southerly monsoon wind predominate. As the result of this, supply of Ca2+ from soil should be diminished in summer. Equivalent concentration ratio of particulate Cl− and Na+ ( Cl − Na + ) in August was half compared with the ratio observed in February. Episode analysis of June 1991 and February 1992 indicated the transportation of pollutants from the outside of Japan. Air masses from the Chinese continent have been concluded to bring the greatest amount of anthropogenic pollutants to Korea and Japan.

Studies on critical Reynolds number indices for wind-tunnel experiments on flow within urban areas

Reynolds-number dependence of flow fields within a modelled urban area was studied in a wind tunnel. We measured flow around a single model building and around model city blocks at various wind speeds, and studied Reynolds number indices more appropriate than the building Reynolds number. Our results led to the following conclusions. Firstly, the flow around the models in the wind tunnel was roughly divided into three parts according to the intensities of viscous stress and Reynolds stress as follows: (1) the flow in the vicinity of the ground or the surfaces of the model, where viscous stress became dominant under certain conditions; (2) the flow detached from the surfaces of the model, where Reynolds stress was always dominant; and (3) the flow around the separation bubble at the leading edge of the building model, where the influences of both viscous stress near the wall and the Reynolds stress in the separated boundary layer were mixed.Secondly, the critical Reynolds number of the flow in the modelled urban area could be defined by using both the roughness Reynolds number Rez0 (= z0u*/ν) and the dimensionless height z+ (= zu*/ν). Reynolds-number independence could be expected for whole flow fields in the modelled urban areas as long as the critical values of Rez0 and z+ were satisfied.

An observational study of the structure of the nocturnal urban boundary layer

The formation mechanism of the nocturnal urban boundary layer (UBL), especially in the winter nighttime, was investigated based on the extensive field observations conducted during November 1984 in Sapporo, Japan. A strong, elevated inversion formed over the Sapporo urban area and the inversion base height was approximately twice the average building height. Velocity fluctuations Σu, Σw and Reynolds stress % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaaca% WG1bWaaWbaaSqabeaacaaIXaaaaGGaaOGae8hiaaIaam4DamaaCaaa% leqabaGaaGymaaaaaaaaaa!3A9C!\[\overline {u^1 w^1 } \] had nearly uniform profiles within the nocturnal UBL and decreased with height above the UBL. On the other hand, temperature fluctuations Σt, and heat fluxes % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaaca% WG1bWaaWbaaSqabeaacaaIXaaaaGGaaOGae8hiaaIaeqiUde3aaWba% aSqabeaacaaIXaaaaaaaaaa!3B56!\[\overline {u^1 \theta ^1 } \] and % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaaca% WG3bWaaWbaaSqabeaacaaIXaaaaGGaaOGae8hiaaIaeqiUde3aaWba% aSqabeaacaaIXaaaaaaaaaa!3B58!\[\overline {w^1 \theta ^1 } \] had peaks at the inversion base and small values within the nocturnal UBL. The turbulent kinetic energy budget showed that the turbulent transport term and shear generation from urban canopy elements are important in the nocturnal UBL development; the role of the buoyancy term is small. The turbulence data analysis and application of a simple advective model showed that the mechanism of UBL formation may be controlled by the downward transport of sensible heat from the elevated inversion caused by mechanically-generated turbulence.

Observational study of stratospheric ozone intrusions into the lower troposphere

Abstract To clarify the dynamic behavior of stratospheric ozone intrusion processes into the lower troposphere intensive field observations were made during the middle ten days of May 1986 covering the northern Kyushu district. The data were evaluated in conjunction with spectrophotometic total ozone data. During the study the pattern of surface ozone concentration increase was similar to those for the increase in atmospheric pressure and the decrease in specific humidity. The vertical movement of locally observed ozone peaks was consistent with the movement of a northwesterly dry air zone. An ozone peak was often accompanied by a temperature inversion layer. These observational results showed two major stratospheric ozone intrusion mechanism. At first stratospheric ozone was transported into the upper troposphere due to the descending flow near the cold front and then subsided to the surface under the subsequent high pressure system.

C2–C5 hydrocarbon concentrations in Central Osaka

Abstract Hourly measurements of nine C 2 –C 5 hydrocarbons (HCs) were made over a period of 15 months in the center of Osaka City, Japan. The measurements were made by using an automated system consisting of a combination of a gas preconcentrator and a gas chromatograph. The concentrations of all components were high in early winter and low in summer. The highest concentration was of C 2 H 4 , while C 3 H 6 showed the lowest concentration throughout the year, and was rarely detected in summer. There were small annual variations in the concentrations of C 4 –C 5 HC. The chemical reactions that affect HC concentrations were also examined. The seasonal C 2 H 4 /C 2 H 2 and C 3 H 6 /C 2 H 2 ratios were compared, taking into account that C 2 H 4 , C 2 H 2 , and C 3 H 6 are mainly from vehicles and the differences in their reactivities. Despite the high reactivity of C 2 H 4 , the C 2 H 4 /C 2 H 2 ratio was low in winter and high in summer, with the result that the C 2 H 4 consumption by chemical reaction seemed small in the center of the city. The behavior of C 3 H 6 /C 2 H 2 is opposite to that of C 2 H 4 /C 2 H 2 , illustrating the more reactive characteristics of C 3 H 6 . Regarding the effect of photochemical reactions, it was observed that HC’s levels drop as NO 2 /NO x increases. Though the slope does not show a direct proportionality to HC reactivity, reactivity of C 3 H 6 , which is the most reactive HC among the nine measured HCs, showed the largest declines. The ratio of C 4 –C 5 HC to the C 2 –C 5 HC rose with increase in ambient temperature, while the ratio of C 2 –C 5 HC to the nonmethane hydrocarbons decreased. This phenomenon was correlated with ambient temperature, indicating higher emissions of higher molecular weight HCs in summer.