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1.
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. 相似文献
2.
Hurricane intensity and track are strongly affected by air-sea interactions. Classified as following swells, crossing swells, and opposing swells, the observed wave height was parameterized by using the 10-m wind speed collected on 5 buoys by the National Buoy Data Center during 13 hurricane events. The path information of these 13 hurricanes was obtained from the National Hurricane Center Best Track (NHC-BT). Results show that the wave height increases exponentially with the 10-m wind speed, and the wave height reaches the maximum value, 11.2 m (8.1 m), when 10-m wind speed is 40 m s-1 under the following and crossing (opposing) swell conditions. We find that the wave steepness (the ratio of wave height to wave length) is proportional to the -2/3 power of the wave age (the ratio of wave phase velocity to 10-m wind speed). The parameterizations of friction velocity and drag coefficient are tested using the buoy data collected in moderate to high wind under following, crossing and opposing swell conditions. A wave age dependent equation for drag coefficient is found more accurate and suggested for future usage in numerical models. Further, these algorithms also suggest that wind-swell orientation needs to be considered to retrieve accurate surface drag under high winds and strong swells. 相似文献
3.
On the Impact of Trees on Dispersion Processes of Traffic Emissions in Street Canyons 总被引：4，自引：0，他引：4
Wind-tunnel studies of dispersion processes of traffic exhaust in urban street canyons with tree planting were performed and
tracer gas concentrations using electron capture detection (ECD) and flow fields using laser Doppler velocimetry (LDV) were
measured. It was found that tree planting reduces the air exchange between street canyons and the ambience. In comparison
to treeless street canyons, higher overall pollutant concentrations and lower flow velocities were measured. In particular,
for perpendicular approaching wind, markedly higher concentrations at the leeward canyon wall and slightly lower concentrations
at the windward canyon wall were observed. Furthermore, a new approach is suggested to model porous vegetative structures
such as tree crowns for small-scale wind-tunnel applications. The approach is based on creating different model tree crown
porosities by incorporating a certain amount of wadding material into a specified volume. A significant influence of the crown
porosity on pollutant concentrations was found for high degrees of porosity, however, when it falls below a certain threshold,
no further changes in pollutant concentrations were observed. 相似文献
4.
Abstract During the Labrador Ice Margin Experiments, LIMEX ‘87 in March 1987 and LIMEX ‘89 in March and April 1989, the Canada Centre for Remote Sensing (CCRS) CV‐580 aircraft collected synthetic aperture radar (SAR) image data over the marginal ice zone off the east coast of Newfoundland, Canada. One aspect of these experimental programs was the observation of ocean waves penetrating into the marginal ice zone (MIZ). Based upon directional wavenumber spectra derived from SAR image data, the wave attenuation rate is estimated using SAR image spectra and compared with predictions from a model developed by Liu and Mollo‐Christensen (1988). The wave and ice conditions were considerably different in LIMEX ‘87 and LIMEX ‘89. However, the model‐data comparisons are very good for all ice conditions observed. Both the model and the SAR‐derived wave attenuation rates show a characteristic roll‐over at high wavenumbers. A model for the eddy viscosity is proposed, using dimensional analysis, as a simple function of ice roughness and wave‐induced velocity. Eddy viscosities derived from SAR and wave buoy data for the wave attenuation rate show a trend that is consistent with the proposed model. 相似文献
5.
The effects of a pressure jump and a following internal gravity wave on turbulence and plume diffusion in the stable planetary
boundary layer are examined. The pressure jump was accompanied by a sudden increase in turbulence and plume dispersion. The
effects of wave perturbations on turbulence statistics are analysed by calculating fluxes and variances with and without the
wave signal for averaging times ranging from 1 to 30 min. The wave signals are obtained using a band-pass filter. It is shown
that second-order turbulence quantities calculated without first subtracting the wave perturbations from the time are greater
than those calculated when the wave signal is separated from the turbulence. Estimates of the vertical dispersion of an elevated
tracer plume in the stable boundary layer are made using an elastic backscatter lidar. Plume dispersion observed 25 m downwind
of the source increases rapidly with the arrival of the flow disturbances. Measured plume dispersion and plume centreline
height correlate with the standard deviation of the vertical velocity but not with the wave signal. 相似文献
6.
D. Finn R. G. Carter R. M. Eckman J. D. Rich Z. Gao H. Liu 《Boundary-Layer Meteorology》2018,169(1):67-91
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. 相似文献
7.
大涡模式垂直分辨率在夹卷及示踪物垂直传输模拟结果分析中的作用 总被引：1，自引：0，他引：1
基于敦煌野外观测资料和大涡模式,研究了垂直方向不同尺度湍涡对夹卷及示踪物垂直传输的影响,明确了模式垂直分辨率在模拟结果分析中的作用。结果表明:垂直方向上小尺度湍涡对夹卷作用贡献更大,小尺度湍涡较多时夹卷层相对更暖,而夹卷层厚度、夹卷强度和风速变化受垂直方向湍涡尺度影响较小。当垂直分辨率为50 m时,越往夹卷层上部,上升气流和下沉气流分布较多且强度较大;分辨率为10、20和30 m时,夹卷层各高度垂直速度、位温和示踪物浓度分布较接近。另外,垂直方向湍涡尺度对示踪物垂直传输高度影响不大,而对示踪物的空间分布有一定作用。当大尺度湍涡较多且强度较强时,越有利于将高浓度的示踪物向上传输。综合考虑到模式采用较高分辨率模拟时产生的噪音及计算时间等问题,认为模式采用30 m的垂直分辨率,既能较好地模拟出夹卷层平均结构特征,又能模拟出夹卷层湍流的精细分布,是较为理想的选择。 相似文献
8.
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。 相似文献
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11.
Nicholas E. Graham Daniel R. Cayan Peter D. Bromirski Reinhard E. Flick 《Climate Dynamics》2013,40(5-6):1335-1360
A dynamical wave model implemented over the North Pacific Ocean was forced with winds from three coupled global climate models (CGCMs) run under a medium-to-high scenario for greenhouse gas emissions through the twenty-first century. The results are analyzed with respect to changes in upper quantiles of significant wave height (90th and 99th percentile HS) during boreal winter. The three CGCMs produce surprisingly similar patterns of change in winter wave climate during the century, with waves becoming 10–15 % smaller over the lower mid-latitudes of the North Pacific, particularly in the central and western ocean. These decreases are closely associated with decreasing windspeeds along the southern flank of the main core of the westerlies. At higher latitudes, 99th percentile wave heights generally increase, though the patterns of change are less uniform than at lower latitudes. The increased wave heights at high latitudes appear to be due a variety of wind-related factors including both increased windspeeds and changes in the structure of the wind field, these varying from model to model. For one of the CGCMs, a commonly used statistical approach for estimating seasonal quantiles of HS on the basis of seasonal mean sea level pressure (SLP) is used to develop a regression model from 60 years of twentieth century data as a training set, and then applied using twenty-first century SLP data. The statistical model reproduces the general pattern of decreasing twenty-first century wave heights south of ~40 N, but underestimates the magnitude of the changes by ~50–70 %, reflecting relatively weak coupling between sea level pressure and wave heights in the CGCM data and loss of variability in the statistically projected wave heights. 相似文献
12.
On the number concentration of aerosols in towns 总被引：1，自引：0，他引：1
A simple model of the aerosol plume generated by a town is developed under the assumption that the generation rate of particles is proportional to the town population. Thus the volume concentration of aerosols in towns is predicted for various meteorological conditions. The effects of localized sources of aerosols, such as industrial chimney stacks, are considered. 相似文献
13.
A series of sixteen atmospheric tracer experiments using sulfur hexafluoride (SF6), chemical smoke and meteorological balloons was conducted to explore the transport of airborne contaminants in the boundary layer over the ocean surface and in the separating boundary layer over an isolated island cape. The immediate objective of the tests was to determine the impact of local pollutant sources on a background air quality sampling program conducted in the South Pacific from elevated towers on Tutuila Island, American Samoa. In addition to satisfying this objective, the tests are of interest in that they illustrate the local behavior of pollutants in a complex natural atmospheric flow.Offshore tracer tests indicated that the crosswind dispersion of pollutants over the ocean surface can be approximately modeled using the simple Gaussian plume model. The observed crosswind dispersion of the tracer corresponded to that expected under neutrally stable atmospheric conditions, consistent with the near equilibration of the ocean surface and air temperature in the South Pacific. Local, or near-field, tests indicated that tracer released into the wake downwind of the leading edge of the cape mixed rapidly to a height of about 8 m above the surface (i.e., 30–40% of the cape height). Due to decoupling between the boundary layer over the cape and the freestream flow, however, very little of the tracer was observed above this height. This suggests that the impact of local pollutant sources (i.e., on the cape) would be minimized if the proposed sampling towers were elevated significantly above an 8 m altitude (e.g., twice that height). 相似文献
14.
Surendra S. Parasnis 《Boundary-Layer Meteorology》1991,54(3):277-285
Wind characteristics in the lowest 340 m (agl) of the atmospheric boundary layer at Pune (18°32N, 73°51E, 559 m ASL) have been investigated using the pilot balloon wind observations obtained during the summer monsoon seasons of 1976, 1979 and 1980. Variations in the zonal and meridional components of wind at the surface, 40, 150 and 340 m (agl) have been described. Frequency distribution of the zonal component showed an unimodal character at the surface, which gradually approached a multimodal character at 340 m. The distribution pattern of the wind components was, by and large, normal. Spectral analysis of the wind components showed that the spectral energy was predominantly shared by 5–7 and 2–3 day periods. 相似文献
15.
Mathematical model for hermitized atmospheric dispersion in low winds with eddy diffusivities as linear functions of down wind distance 总被引：1，自引：0，他引：1
Summary In the present paper, an attempt is made for generalized the atmospheric diffusion operator. This can be accomplished by employing
the realizability procedure, to identify a surface operator, that ensures self-adjointness’ of the atmospheric diffusion operator.
The dispersion modeling in low wind speeds assumes importance because of the high frequency of occurrence and episodic nature
of these poor diffusion conditions. A steady-state mathematical model for hermitized model has been calculated for the dispersion
of air pollutants in low winds by taking into account the diffusion in the three coordinate directions and advection along
the mean wind. The eddy diffusivities have been parameterized in terms of downwind distance for near source dispersion (Arya,
1995). The constants involved in this parameterization are the squares of intensities of turbulence. An analytical solution
for resulting advection-diffusion equation with the physically relevant boundary conditions has been obtained. The solution
has been used to simulate the field tracer data collected at IIT Delhi in low wind convective conditions. 相似文献
16.
Atmospheric tracer dispersion experiments have been carried out to measure the statistical characteristics (variance, frequency distribution, spectrum) of the concentration downwind of a pair of partly overlapping plumes. By releasing different tracer substances from each source, it was possible to identify the contributions of the two sources at a given measurement point, both separately and jointly, and thus to compare and interpret the joint statistics in terms of those from the individual sources.Statistics for the individual sources agree well with, and support, existing wind tunnel and theoretical results. Nondimensionalization of the data using the mean concentration and the lateral width of the plume as concentration and length scales successfully removes much of the variation due to changes in atmospheric and surface conditions.Measurements of the correlation between the concentration contributions from separated sources are consistent with recent wind tunnel measurements. Entirely new measurements of the frequency distribution of the combined concentration from a pair of sources show that in many situations, high concentrations relative to the mean occur much less frequently than for an isolated source. Generally the extent of the reduction in frequency of occurrence is inversely related to the degree of correlation between concentrations from the two sources. 相似文献
17.
Cesar Villanoy Laura David Olivia Cabrera Michael Atrigenio Fernando Siringan Porfirio Aliño Maya Villaluz 《Climatic change》2012,112(2):493-505
Coral reefs and other coastal ecosystems such as seagrasses and mangroves are widely recognized to provide protection against
the devastating effects of strong waves associated with tsunamis and storms. The predicted warming climate brings to fore
the role of these ecosystems in providing protection against stronger typhoons that can result in more devastating waves of
greater amplitude. We performed a model simulation of storm generated waves on a Philippine reef, which is located along the
path of tropical storms, i.e., at least 10 typhoons on the average pass through the study site yearly. A model to simulate
wave propagation was developed using Simulating Waves Nearshore (SWAN) and DELFT3D-WAVE computer simulation software. Scenarios
involving local monsoonal wind forcing and storm conditions were simulated. In addition, as climate change may also result
to increased relative sea level, a 0.3 m and 1 m rise in sea level scenarios were also used in the wave model simulations.
Results showed that the extensive reef system in the site helped dissipate wave energy that in turn reduced wave run-up on
land. A significant reduction in wave energy was observed in both climate change, i.e., stronger wind and higher sea level,
and non-climate change scenarios. This present study was conducted in a reef whose coral cover is in excellent condition (i.e.,
50 to 80% coral cover). Estimates of coral reef growth are in the same order of magnitude as estimates of relative sea level
rise based on tide gauge and satellite altimeter data, thus it is possible that the role of reefs in attenuating wave energy
may be maintained if coral reef growth can keep up with the change in sea level. Nonetheless, to maintain reef growth, it
is imperative to manage coral reef ecosystems sustainably and to eliminate the stressors that are within human control. Minimizing
activities such as illegal and destructive blast and poison fishing methods, pollution and siltation, is crucial to minimize
the impacts of high-energy waves that may increase with climate change. 相似文献
18.
N. E. Veremei Yu. A. Dovgalyuk V. Gopalakrishnan K. F. Komarovskikh P. Murugavel S. D. Pawar A. A. Sinkevich 《Russian Meteorology and Hydrology》2015,40(12):777-786
Studied are the effects of severe aerosol pollution of the atmosphere on the parameters of a cumulonimbus cloud (including its charge structure) and on precipitation. Considered is the example of the cloud that developed on May 11, 2009 near the town of Kharagpur (India) under conditions of severe aerosol pollution of the atmosphere due to dust transport from the desert. The in situ observations of the evolution of the cumulonimbus cloud of large vertical and horizontal extent and of its electric conditions were carried out on that day. It is found that the distribution of electric charges in the cloud was characterized by inverted polarity (i.e., the main positive charge is in the bottom of the cloud and the negative one is in the upper part of the cloud that contradicts usually observed cases). Using the small-dimension numerical model conducted are numerical experiments on the simulation of aerosol effects produced on the evolution of dynamic, microphysical, and electric structure of the cloud under study, namely, the cloud development under background conditions and in case of high aerosol concentration. It is assumed that aerosol particles possess ice-forming properties. It is obtained that the dynamic, microphysical, and electric structures of the cloud are significantly transformed under the influence of high aerosol concentrations; precipitation generation also significantly intensifies; polarity in the distribution of electric charge varies that agrees with the data of in situ observations. 相似文献
19.
R. Salerno 《Theoretical and Applied Climatology》1992,45(1):19-35
Summary Dispersion and transport over complex terrain have been recognized as an important research field. In July 1988, for studying atmospheric dispersion in a mountain area, a tracer experiment has been performed in south alpine valleys near a mountain (Campo dei Fiori, 1226 m.a.s.l.). In this area air masses circulation in wind breeze conditions is frequent. This paper analyses the meteorological situation and the tracer dispersion during those experiments. First a reconstruction of the three-dimensional wind fields overCampo dei Fiori area, taking into account the effects of orography, surface thermal gradients, atmospheric stability and energy of air masses, has been performed. Then the tracer concentrations have been evaluated by means of suitable models and the results have been discussed.With 11 Figures 相似文献
20.
Ensemble simulations with a coupled ocean-troposphere-stratosphere model for the pre-industrial era (1860 AD), late twentieth century (1990 AD) greenhouse gas (GHG) concentrations, the SRES scenarios B1, A1B, A2, as well as stabilization experiments up to the Twenty-third century with B1 and A1B scenario GHG concentrations at their values at 2100, have been analyzed with regard to the occurrence of major sudden stratospheric warmings (SSWs). An automated algorithm using 60°N and 10 hPa zonal wind and the temperature gradient between 60°N and the North Pole is used to identify this phenomenon in the large data set. With 1990 CO2 concentrations (352 ppmv), the frequency of simulated SSWs in February and March is comparable to observation, but they are underestimated during November to January. All simulations show an increase in the number of SSWs from the pre-industrial period to the end of the twenty-first century, indicating that the increase of GHG is also reflected in the number of sudden warmings. However, a high variability partially masks the underlying trend. Multi-century averages during the stabilization periods indicate that the increase of SSWs is linear to the applied radiative forcing. A doubling of SSWs occurs when the GHG concentration reaches the level of the A2 scenario at the end of the twenty-first century (836 ppmv). The increase in SSWs in the projections is caused by a combination of increased wave flux from the troposphere and weaker middle atmospheric zonal winds. 相似文献