A field study of diffusion around a model cube in a suburban area

To investigate diffusion around a building in a suburban area, a field observation was conducted on a model cube with a centrally located rooftop level source in September 1992 in Sapporo, Japan. The results show that high concentrations were observed both upwind and downwind of the source on the roof, although the mean velocity U was positive. The values of normalized concentration at locations upwind and downwind of the source were lower than those obtained from wind tunnel data conducted at moderated turbulence levels. At ground level, the mean concentrations along the model centre line show the highest value near the cube and decay rapidly in the downstream direction. The relationship between the instantaneous concentrations and instantaneous velocity was investigated using two fast-response concentration detectors and an ultrasonic anemometer. It was found that when reverse flow occurred on the roof, the tracer gas was detected upwind of the source.     

The effects of source size on concentration fluctuations in plumes

The effects of source size on plume behaviour have been examined in a 1.2 m wind tunnel boundary layer for isokinetic sources with diameters from 3 to 35 mm at source heights of 230 mm and at ground level. Experimental measurements of mean concentration and the variance, intermittency and probability density functions of the concentration fluctuations were obtained. In addition, a fluctuating Gaussian plume model is presented which reproduces many of the observed features of the elevated emission. The mean plume width becomes independent of source size much more rapidly than the instantaneous plume width. Since it is the meandering of the instantaneous plume which generates most of the concentration fluctuations near the source, these are also dependent on source size. The flux of variance in the plume reaches a maximum, whose value is greatest for the smallest source size, close to the source and thereafter is monotonically decreasing. The intermittency factor reaches a minimum, whose value is lowest for the smallest source, and increases back towards one. Concentration fluctuations for the ground-level source are much less dependent on source size due to the effects of the surface.     

A wind tunnel study of turbulent dispersion over two- and three-dimensional gentle hills from upwind point sources in neutral flow

A study of turbulent dispersion over hills for upstream, elevated sources was conducted, based on wind tunnel tracer gas (CO₂) experiments over a gentle 2-D ridge and a 3-D circular hill, both having a cosine-square cross-section. The concentration measurements were made with four different source locations for each hill case (2-D or 3-D), and the study focused on dispersion parameters under the influence of the presence of the hills in order to provide a better understanding of the mechanisms involved.The wind tunnel measurements show that, in the case of gentle hills, the topographic impact on turbulent dispersion from upstream sources is only moderate and is more pronounced for the 3-D than for the 2-D hill. The perturbation in mean flow introduced by the hills, including streamline divergence/convergence, is shown to dominate the changes in the dispersion due to the hills in this case. The plume spread, both in the lateral and the vertical, is enhanced over the upwind hill foot and reduced over the hill top in response to the mean flow slow-down and speed-up at these places, and is further enhanced or reduced due to streamline divergence/convergence in the vertical over the hills as well as in the horizontal over the 3-D hill. These results are also compared with cases of turbulent dispersion over more steep hills (Snyder and Britter, 1987).     

Mean temperature and mean concentration distributions over a physically modelled three-dimensional heat island for different stability conditions

Results of flow visualization, and the mean temperature measurements over a physically modelled three-dimensional heat island in a wind tunnel capable of simulating stratified atmospheric boundary layers are presented. Concentration measurements of Kr85 released from an upwind two-dimensional continuous line source show good correlation with flow visualization and mean temperature distributions. The results indicate the unique features of three-dimensional flow over a heat island - lateral low-level convergence, upward vertical motions, and upper-level horizontal divergence.     

凹坑地形风流结构对污染物散布的模拟研究

利用建立的三维非静力高分辨率高阶湍流闭合模式与随机游动扩散模式研究了一个深凹露天矿区污染物散布的规律,同时在风洞中进行了示踪实验。结果表明,由于凹坑内复环流结构的存在,使得坑内污染物浓度较大,且浓度最大值出现在源的上风侧。数值试验与风洞试验结果吻合较好。     

Tracer experiments with turbulently dispersed air ions

Unipolar air ions released into the wind constitute a tracer which can be measured with high resolution. An ion source produces a cloud with homogeneous charge density, insensitive to source strength, dependent on time since formation only. It is well suited for tracing concentration changes due to turbulence, less suited for cloud size tracing. A tight array of 8 sensors has been used to examine turbulently dispersed ions. High-resolution records are presented and discussed. The highest concentrations measured could be undiluted source material. The frequency distribution within a plume did not differ from that in a multitude of puffs. The distribution seems to be log-normal with a geometric standard deviation of about 2.45. The time resolution used corresponds to volume resolutions of 40, 225, and 650 cm³. Sample size had no apparent effect.     

The persistence of pollutants downwind of a point source following termination of the emission

Experimental studies have shown that tracers injected over short time intervals can often be measured downwind near the ground for a much longer time than would be expected considering the duration of the release and the wind speed. A two-dimensional advection-diffusion model was used in a numerical study to investigate this effect. The results from the numerical study showed that the rate of decrease in concentrations downwind at a receptor after a source has stopped emitting was more sensitive to magnitude of the wind speed and alongwind mixing coefficient than the rate of increase in concentrations when the source first starts emitting. The lower the wind speed or the greater the mixing, the more slowly the concentration decreased as the region upwind of the receptor continued to contribute tracer until it was depleted by advection and vertical diffusion. These atmospheric processes are able, qualitatively, to account for the slowly decreasing air concentrations.     

“两湖盆地”秋季黑碳气溶胶的分布及来源特征

应用耦合了黑碳(Black Carbon,BC)区域源追踪方法的中尺度天气-化学模式(WRF-Chem),对位于长江中游的湖南湖北盆地(以下简称两湖盆地)秋季(2015年10月)黑碳的分布、区域来源和日变化特征进行研究。结果表明,由于两湖盆地所处地理位置及地形的特殊性,在不同的风场条件下,本地源和外来源对两湖盆地BC的贡献占比变化较大。两湖盆地秋季BC的来源主要有3种情形:1)北风控制下的传输型:地面风速大,BC质量浓度相对低,上风向源对两湖盆地近地面BC的贡献占比达30.6%,其中汾渭地区的贡献最大,本地源的贡献占比为61.7%。2)地面风速小、大气稳定条件下的静稳型:近地面BC质量浓度高,本地源的贡献占比高达79.6%。3)东北风控制下的传输型:本地源和外来源的贡献共同使得两湖盆地BC维持较高质量浓度,本地源的贡献占比为64.1%,外来源的贡献主要包括安徽省、汾渭地区及长江三角洲的输送。在垂直方向上,传输型的本地源贡献占比仅在300 m以下较高;静稳型的本地源贡献占比较高,可延伸到1 500 m以上(本地源的贡献占比在近地面约为75%、至1 500 m约为50%)。在日变化上,本地源与外来源对两湖盆地BC的贡献占比呈相反变化,夜间和早间本地源的贡献占比较高而外来源的贡献占比较低,午后本地源的贡献占比减小而外来源的贡献占比明显增大;相对静稳型而言,传输型的这一日变化特征更为显著。     

Plume Dispersion in Low-Wind-Speed Conditions During Project Sagebrush Phase 2, with Emphasis on Concentration Variability

Eight short-range, open-terrain SF₆ tracer tests in low wind speeds were conducted during Phase 2 of Project Sagebrush using continuous releases. Four tests were made during very unstable conditions in July and August 2016, and four during very stable conditions in October 2016. All tests featured 10-min averaging and 1-Hz sampling of tracer concentrations together with an extensive suite of meteorological measurements. We find that the uncertainty in well-mixed daytime measurements of tracer concentrations, using the absolute value of the relative percentage difference in collocated duplicate samplers, approaches a downwind limit of about 7–8%. Concentration variability in collocated sampling, due to stochastic factors and independent of measurement uncertainty, increases the total observational uncertainty closer to the source from about 20% (daytime) to 40% (very stable conditions). Longer averaging periods moderately reduce the concentration variability. The data indicate that the large increase in concentration variability is linked with the suppression of turbulent mixing, small eddy length scales, and meandering in very stable conditions. These results should be considered when comparing observations with model predictions in evaluations.     

Numerical Study of Dual-Plume Interference in a Turbulent Boundary Layer

Direct numerical simulation is used to investigate the interference arising from the dispersion of passive scalar plumes released from a pair of point sources in a fully-developed wall-bounded shear flow. Four different lateral separations of the two sources for both near ground-level and elevated releases are considered. The downwind evolution of the correlation between the plume concentrations along the centreline between the two sources and the behaviour of the lateral profiles of the correlation at various locations downwind of the two sources are examined in detail. Differences in the exceedance probability over a high concentration level for a single plume and the total plume are highlighted and studied, and the effects of destructive and constructive interferences on the exceedance probabilities for the total plume are used to explain these differences. One significant result is that all higher-order (third-order and above) moments of the total concentration can be inferred from the application of a clipped-gamma distribution using the information embodied in only the first- and second-order concentration moments of each single plume, and in the cross-correlation coefficient of the instantaneous concentration of the two plumes.     

上海地区地面风向对空气污染物浓度的影响

利用2000年6月至2002年12月上海地区空气污染物浓度和地面风向资料，统计了月平均污染物浓度随偏西风频率的分布，两者具有一致的趋势。首要污染物：PM10平均浓度风玫瑰图显示高浓度对应偏西风向的出现，各季节均表现出此特性。由于冬、夏半年主导上海地区的天气系统的不同，分冬半年和夏半年统计了各污染物浓度日变化随风向日变化的均值和标准差，当风向由非西风转为偏西风或连续受偏西风影响时，污染物浓度有所上升，不同污染物间，PM10浓度对地面风向的改变较SO2和NO2更敏感。     

亚洲夏季风爆发与热源强迫下的热带大气Rossby波

对南海夏季风爆发前的风场和局地对流凝结加热强迫作用进行了合成分析, 表明南海夏季风爆发前局地对流凝结加热作用对与南海夏季风爆发密切相关的低频涡对的产生起着重要作用.然后, 从动力学的角度研究了带有外部热源强迫的赤道β平面上的Rossby波, 利用Gardner-Morikawa变换和摄动展开方法, 得到了热源强迫下描写赤道Rossby的mKdV方程, 进而得到了不同热源形式下切变气流中赤道非线性Rossby波的解析解.结果表明, 不同类型的热源对赤道Rossby波的结构起着非常重要的作用, 同时也为南海夏季风爆发前低频涡对的形成给出了一种动力学解释.     

Origin of July Antarctic precipitation and its influence on deuterium content: a GCM analysis

The NASA/GISS GCM is used to estimate the evaporative contributions of several oceanic regions (defined by temperature) to Antarctica's July precipitation. Tracer diagnostics in the GCM suggest that the weighted average evaporative source temperature for Antarctic precipitation as a whole is about 12°C. The average source temperature for local precipitation there varies from 9° C to 14° C. To examine the effect of evaporative source on water isotope concentration, the GCM also follows a global deuterium (HDO) tracer and deuterium tracers evaporating from each oceanic region. The results suggest that although evaporative source temperature does affect the concentrations of the individual HDO tracers, differences in evaporative source do not explain the scatter in the roughly linear relationship between condensation temperature and isotope concentration. Offprint requests to: R Koster     

Analysis of wind tunnel dispersion data over two-dimensional obstacles

An analysis of wind tunnel data of dispersion from elevated point sources over a flat floor and gently sloping, 2-D hills is performed. (The data were obtained by Khurshudyan et al., 1981.) Formulas for the mixing lengths and the Lagrangian time scales are tested, suitable for use in various dispersion models. Some expressions for the vertical first moments of the plume concentration distribution suitable for shear flow (Hunt, 1985) are also tested.Then, a normalization is suggested, based on the source mean flow and turbulence parameters, for the ground-level concentration maximum value and position along the plume centerline. Using this normalization, the maximum position is almost constant, regardless of source height variation and of whether the hill is present or not, at least for source positions upstream or over the top.The maximum values allow the determination of normalized terrain amplification factors TAF_N, which are shown to be in most cases closer to one than the corresponding TAF obtained without normalization.     

Test of the climatological dispersion model in the strait of canso area,Nova Scotia

Abstract

The Climatological Dispersion Mode1 is used to estimate mean annual and mean monthly concentrations of sulphur dioxide in the Strait of Canso area of Noua Scotia. It is found that the wind distribution at the southern end of the Strait is more likely to produce a reliable estimate of the concentration of pollutants from the taller stacks, whereas the surface wind distribution from a central location in the Strait can be used, because of low‐level channelling, to estimate concentrations of pollutants from the lower sources. Data on winds aloft over the Strait, are utilized in support of this thesis. The procedure does not apply well to the summer months; otherwise the resulting patterns of predicted concentrations of sulphur dioxide are generally consistent with observed values. More accurate and sensitive instruments are needed in future studies of long‐term average conditions.     

Elemental composition of particulate material sampled from the Arctic haze aerosol

Thirty-six aerosol filter samples collected in tropospheric Arctic haze layers, in the stratosphere, and in the marine boundary layer during the 1983 Arctic Gas and Aerosol Sampling Program were analyzed for trace elements using instrumental neutron activation analysis. Average crustal dust concentrations were 540 ng/m³ and 330 ng/m³ for samples collected in Arctic haze over the North American and Norwegian Arctic, respectively. An average marine salt concentration of 120 ng/m³ was obtained for haze samples collected above the marine boundary layer on both sides of the Arctic.Meteorological and wind trajectory information were used to identify specific haze transport pathways, which brought relatively unmixed aerosol from the central Soviet Union into the AGASP sampling areas. Results from individual filters collected within these transport zones are discussed, with emphasis on certain trace metal ratos which have been proposed by other researchers as discriminators of aerosols from different source regions. Our aircraft-collected data are compared with previously-collected ground-based measurements, and show reasonably good agreement for most tracer elements and ratios. Specifically, we have determined the As/Sb ratio tracer, named by other researchers as the most effective elemental discriminator of aerosol from the central Soviet Union, to be approximately 5–6. This relatively high tracer value is consistent with previous ground-based findings. A significantly lower V/Sb ratio was observed throughout this study, possibly indicating a change in the source signature.     

Diurnal variations of radon and meteorological variables near the ground

Radon is an excellent tracer for the study of transport processes in the lower atmospheric boundary layer. Analyses of the radon data measured on a 300-m meteorological tower at Philadelphia show that the diurnal variation of atmospheric turbulence is closely related to the meteorological variables. A model of variation of radon concentration with mean wind speed and low-level vertical temperature difference is derived. It indicates that radon concentration is inversely proportional to the mean wind speed and directly proportional to the temperature difference. These predictions are in good agreement with the measurements.     

街谷大气扩散试验研究

为研究机动车辆排出的污染物在大气中的扩散规律，在北京做了小风条件下的街谷示踪试验。当楼顶风速u接近或大于1米/秒时，街谷内可形成一稳定的原生涡；u＜0.6米/秒时原生涡将消失。对于楼之间空间较小的街谷，背风面和迎风面的示踪剂浓度平均比值可达8。浓度值沿楼层高度无明显变化；由于快车路旁松墙的阻挡和抬升作用，可能造成沿高度方向楼层中段的浓度偏高。在街谷外，除下风方路面上有一按下风距离的负幂指数衰减的浓度分布外，上风方路面上也有一按较大负幂指数衰减的分布。根据上述试验，给出了用以预测街谷中机动车辆排出的惰性气体污染物的扩散模式；模式中，对原生涡和小尺度湍流，做了分别处理。     

Concentration fluctuations according to fluctuating plume models in one and two dimensions

Concentration probability density functions (pdfs) calculated according to fluctuating plume models in one- and two-dimensions, representing the limiting cases of one-dimensional dispersion from a line source or a point source in strongly anisotropic turbulence and of axisymmetric dispersion from a point source in isotropic turbulence, are discussed and analyzed in terms of the location of the sampling point within the mean plume and of the ratio, s/m, of the standard deviations for relative dispersion and meandering.In both cases, the pdfs cover the finite concentration range from zero to C ₀, the centreline concentration of the instantaneous plume. The main difference between them is that whereas the 2-D pdf is always unimodal, the 1-D pdf has a singularity at C ₀ which under some circumstances results in a bimodal form. However, the probability associated with this singularity is not always significant. Differences of practical importance in the shape of the pdfs occur mainly for centreline or near-centreline sampling locations when meandering is not too much larger than relative dispersion (1 < m ²/s² < 10) and for sampling locations a distance of order s from the centreline when relative dispersion is not too much larger than meandering (1 < s ²/m² < 5).Comparison against wind tunnel measurements not too far downstream of a line source in grid turbulence shows that the 1-D model reproduces the essential features and trends of the measurements. Under appropriate circumstances the measurements show the bimodal pdf predicted by the 1-D model (but not by the 2-D model) confirming that the effect of the anisotropy in the source distribution is observable.Present address: School of Mechanical Engineering, Aristotle University, Thessaloniki, 54006 Thessaloniki, Greece.     

Source of low frequency modulation of ENSO amplitude in a CGCM

We study the relationship between changes in equatorial stratification and low frequency El Niño/Southern Oscillation (ENSO) amplitude modulation in a coupled general circulation model (CGCM) that uses an anomaly coupling strategy to prevent climate drifts in the mean state. The stratification is intensified at upper levels in the western and central equatorial Pacific during periods of high ENSO amplitude. Furthermore, changes in equatorial stratification are connected with subsurface temperature anomalies originating from the central south tropical Pacific. The correlation analysis of ocean temperature anomalies against an index for the ENSO modulation supports the hypothesis of the existence of an oceanic “tunnel” that connects the south tropical Pacific to the equatorial wave guide. Further analysis of the wind stress projection coefficient onto the oceanic baroclinic modes suggests that the low frequency modulation of ENSO amplitude is associated with a significant contribution of higher-order modes in the western and central equatorial Pacific. In the light of these results, we suggest that, in the CGCM, change in the baroclinic mode energy distribution associated with low frequency ENSO amplitude modulation have its source in the central south tropical Pacific.