Turbulent dispersion in the atmospheric surface layer

By non-dimensionalizing a trajectory-simulation (TS) model of turbulent dispersion, it is shown that the dimensionless concentration z ₀cu_*/kQ (cu _*/kQ) due to a continuous line (area) source of strength Q in the atmospheric surface layer depends only on z/z ₀, x/z ₀, z ₀/L and z _s/z₀, where z _s is the source height. The TS model is used to tabulate concentration profiles due to ground-level line and area sources. Concentration profiles generated by the TS model for elevated sources are shown to be inconsistent with the Reciprocal Theorems of Smith (1957) and it is suggested that this is because the flux-mean gradient closure scheme inherent in the Reciprocal Theorem is invalid for an elevated source.     

An atmospheric dispersion model with continuous vertical variation of dispersion parameters

Summary A dispersion model is proposed to predict the continuous vertical variation of the dispersion parameters _y and _z in case of hot pollutant release to the atmosphere. In such a case, the plume rises far above the ground and is subject to varying levels of turbulence. The framework in this paper can be divided into three approaches: (1) determination of the eddy diffusivitiesK _y (z, _y ) andK _z (z, _z ) as functions of height above ground and plume dimensions, (2) determination of both the plume rise and its vertical velocity using a modified version of Brigg's formula, and (3) numerical solution of actual problems with buoyant plumes at each time step. The model results have been applied to a case of pollutant release from fire destruction of a chemical storehouse roof.With 15 Figures     

The pitfalls of thresholding atmospheric dispersion data

Atmospheric dispersion data are invariably corrupted by random noise and perhaps baseline drift giving rise to unreal negative values. This paper shows that the indiscriminate use of thresholding can give rise to large errors in estimates of some key statistical parameters.     

放射性核素大气弥散模式研究综述

放射性核素大气弥散模式可以模拟正常工况和事故工况下,放射性核素在不同尺度下的大气输送与扩散,为核电站选址、辐射环境监测和核事故应急提供科学依据。归纳了目前广泛用于模拟核素大气弥散的各种模式,介绍了这些模式的研究进展和适用范围。结果表明：对于区域范围小于2 km的弥散,一般可采用CFD（Computational flu...     

放射性核素大气弥散模式研究综述

放射性核素大气弥散模式可以模拟正常工况和事故工况下,放射性核素在不同尺度下的大气输送与扩散,为核电站选址、辐射环境监测和核事故应急提供科学依据。归纳了目前广泛用于模拟核素大气弥散的各种模式,介绍了这些模式的研究进展和适用范围。结果表明：对于区域范围小于2 km的弥散,一般可采用CFD（Computational fluid dynamics）湍流模式;对于20 km范围内的局地扩散,一般可采用高斯模式;而对于中尺度（20—200 km）和大尺度（200 km以上）弥散,则采用拉格朗日模式或欧拉模式。为了避免各种模式本身的局限性,采用嵌套模式将是较好的选择。最后,对放射性核素大气弥散模式发展前景进行了讨论。     

三维大气扩散粒子分裂(ADPS)模式

本文描述了基于PIC方法的三维大气扩散粒子分裂(ADPS)模式,并用解析解和1988年的广州地区现场观测资料验证了本模式.模式根据局地均匀和定常假设引入大粒子概念、烟团扩散原理和大粒子分裂技术,并运用了嵌套网格.ADPS模式用于模拟气态污染物在大气中的散布,同时也具备模拟TSP扩散的效能.     

The dispersion of chemically reactive species in the atmospheric boundary layer

Summary The role of turbulence in the dispersion of atmospheric pollutants that react with linear (decay) and nonlinear (second-order) chemical reactions is examined. The most relevant processes that drive the reactivity of species emitted in a surface area or released by a point source are studied by deriving the dimensionless scaling numbers from equations for the atmospheric turbulent reacting flow. The first number is the ratio of the time scale of turbulence to the time scale of the chemical reaction, namely the Damköhler number. The second number is the ratio of the concentrations of the species present in the chemical transformations. In this paper, model results and experimental studies of turbulent reacting flows in the atmospheric boundary layer are presented to show the modifications and control exerted by turbulence on the atmospheric chemistry as a function of these numbers and processes. We also discuss how the chemical transformation is affected when species are in a state of chemical equilibrium.By studying the plume dispersion of a reactant, that decays with a simple chemical reaction, one can analyse the dependence of concentration fluctuations on the Damköhler number. The study is extended to plumes that react nonlinearly. In such reacting systems, the large gradients and segregation of species result in a significant reduction in the reaction rates. Because of this modification, the chemistry of species related to NO_x and HO_x can be very different from the chemistry in conditions where the species are uniformly mixed. The lack of complete observational evidence is hampering our understanding of these processes and our evaluation of numerical modelling results. Finally, we discuss briefly how to represent, in the form of a parameterization, the effect that turbulence can have on the reactivity of species emitted by a point source or an area source.     

中尺度大气运动中孤立重力波特征的研究

通过对斜压大气(基本风场有垂直切变)运动中重力内波的研究发现:对于小振幅扰动(弱非线性问题),考虑波动之间的非线性相互作用以后,控制重力内波波包(调制波)的演变方程是非线性Schrodinger方程.然后通过对冷锋爆发冷涌过程的计算表明:冷涌主体前沿的宽度约10km,行进速度约为13/s,随着冷涌的增强,其宽度变窄.这些计算结果与实际的观测结果比较一致.     

A multi-scale urban atmospheric dispersion model for emergency management

To assist emergency management planning and prevention in case of hazardous chemical release into the atmosphere,especially in densely built-up regions with large populations,a multi-scale urban atmospheric dispersion model was established.Three numerical dispersion experiments,at horizontal resolutions of 10 m,50 m and 3000 m,were performed to estimate the adverse effects of toxic chemical release in densely built-up areas.The multi-scale atmospheric dispersion model is composed of the Weather Forecasting and Research （WRF） model,the Open Source Field Operation and Manipulation software package,and a Lagrangian dispersion model.Quantification of the adverse health effects of these chemical release events are given by referring to the U.S.Environmental Protection Agency＇s Acute Exposure Guideline Levels.The wind fields of the urban-scale case,with 3 km horizontal resolution,were simulated by the Beijing Rapid Update Cycle system,which were utilized by the WRF model.The sub-domain-scale cases took advantage of the computational fluid dynamics method to explicitly consider the effects of buildings.It was found that the multi-scale atmospheric dispersion model is capable of simulating the flow pattern and concentration distribution on different scales,ranging from several meters to kilometers,and can therefore be used to improve the planning of prevention and response programs.     

Identification of motion parameters from spatio-temporal atmospheric temperature patterns

In near-calm conditions it is difficult to make direct measurements of atmospheric advection reliably and cheaply, particularly at many points over a large area. An alternative indirect method is examined using time-series measurements of atmospheric temperature (or any other convenient conserved field variable) at points on a spatial grid.System identification methodology is applied to analyze atmospheric temperature data obtained in such near-calm conditions during an experiment with a low-flying helicopter. A three-parameter numerical model of atmospheric advection has been identified in the temperature data, the advection parameters being an eddy diffusion coefficient and horizontal components of the wind velocity. It has been demonstrated by analysis of all the assumptions, and using simulated data, that in this case the standard least-square procedure can be applied to recover sensible parameter estimates, even though the model is of the error-in-variables type and parameters appear to change abruptly at certain moments of time (but by a reasonably small amount). The parameter estimates have been validated against independent data.     

On the measurement of atmospheric winds

The measurement of atmospheric winds using a cup anemometer to measure speed and a wind vane to measure direction, recording the data on paper charts is commonplace. Standard Meteorological Service criteria stipulate that the wind charts so recorded are read (averaged over one hour) by taking the dominant wind direction over an hour and the wind run then gives the speed over that hour. However, fluctuations of wind direction can lead to erroneous results. A vector average wind obtained using two orthogonally mounted propeller anemometers is described here, and comparisons are drawn between this and the first-mentioned technique. Prevailing winds are shown to be approximately the same for the two systems, but minor components can differ considerably. It is also shown that the integration time of the wind will have a marked effect on results.     

An experimental study of two-dimensional atmospheric gas dispersion near two objects

Two-dimensional gas dispersion in the atmosphere was investigated experimentally using an atmospheric boundary-layer wind tunnel. Argon gas, which is one and one half times heavier than air, was injected two-dimensionally and uniformly near two square objects at the rates of 3.55 and 4.93 m³/min per lateral meter from the bottom of the wind tunnel floor through a porous metal plate. The gas injection source was located at one object distance upstream of the first square object. Gas concentrations and velocities of the flow field were measured using an aspirating hot-film probe and a low velocity flow analyzer, respectively. The gas was dispersed no higher than approximately 2.0 object heights near the objects, and dispersed no higher than 3.5 object heights at the location of downwind distances of nine object lengths for the injection rates tested. The gas concentrations between the injection source and the first object were about 4 to 5 times higher than the gas concentrations of the other locations. The gas concentrations were 2 to 3% in the recirculation zone between the two objects, and were less than 2% behind the second object. The air-gas mixtures in the recirculation zone between the two objects and within the wake recirculation bubble behind the second object were well mixed and no gravitational settlement was detected in those regions. However, as the mixture moved down-stream, the gravitational settlement occurred beyond the wake recirculation bubble. This paper demonstrates that it is possible to model a two-dimensional heavy gas release in the atmosphere using wind-tunnel simulation with properly matched similitude parameters.     

The treatment of atmospheric dispersion data in the presence of noise and baseline drift

Recently large quantities of data from many different field experiments have become available to facilitate the examination of various proposed models of atmospheric dispersion. However, these data sets are invariably corrupted by some form of random noise and, also, significant baseline drift is often recorded. Consequently, these problems require careful consideration and treatment before the data can be used in model testing. In many papers, the noise is simply treated by thresholding but this is unacceptable since many valid readings are discarded. This paper examines the performance of two different noise removal methods that are more soundly based, both physically and mathematically. The first is a Wiener filter with certain modifications, and the second is a maximum entropy inversion technique. A comparison of the results of applying these methods is presented, with the emphasis on the practical treatment of the numerical and computational problems that arise. The problem of baseline drift is treated initially by applying a number of subjective fits. Subsequently the noise removal methods are applied. In general, it is found that the maximum entropy method performs better than the Wiener filter for the data sets examined here.     

Comparison of Kalpana-1 atmospheric motion vectors with other observations

The operational derivation of atmospheric motion vectors (AMVs) using infrared (10.5–12.5 μm) and water vapor (6.3–7.1 μm) channels of successive geostationary satellite images started in the 1980s. Subsequently, AMVs have become an important component for operational numerical weather prediction throughout the globe for the last decade or so. In India, at the Space Applications Centre, Indian Space Research Organisation, the operational derivation of AMVs (infrared winds and water vapor winds) from the Indian geostationary satellite Kalpana-1 has been initiated a few years back. Recently, an L-band radar lower atmosphere wind profiler (LAWP) has been installed at the National Atmospheric Research Laboratory, Gadanki located at (13.58°N, 79.28°E) for continuous high-resolution wind measurements in the lower atmosphere. In this study, a comparison of Kalpana-1 AMVs with wind measurements from LAWP and radiosonde has been carried out for a period of one and a half years. The performances of Kalpana-1 AMVs are also assessed by a separate comparison of Meteosat-7 AMVs, derived at the European Organisation for the Exploitation of Meteorological Satellites, with wind measurements from LAWP and radiosonde. Both sets of comparison show that AMVs from Kalpana-1 and Meteosat-7 are comparable over the Indian Ocean region.     

Equatorial solitary waves of tropical atmospheric motion in shear flow

Starting from the primary equations, the author derives the KdV equation which describes solitary Rossby waves in the tropical atmosphere, and indicates that, because these waves are ageostrophic, they differ from the quasigeostrophic solitary Rossby waves studied by Redekopp et al. Owing to nonlinear action, these waves are also different from traditional linear waves of the tropical atmosphere. The author believes that the stationary tropical atmospheric waves reflect the characteristics of solitary waves in that the energy does not disperse.     

Effects of turbulent dispersion of atmospheric balance motions of planetary boundary layer

New Reynolds' mean momentum equations including both turbulent viscosity and dispersion are used to analyze atmospheric balance motions of the planetary boundary layer. It is pointed out that turbulent dispersion with r 0 will increase depth of Ekman layer, reduce wind velocity in Ekman layer and produce a more satisfactory Ekman spiral lines fit the observed wind hodograph. The wind profile in the surface layer including tur-bulent dispersion is still logarithmic but the von Karman constant k is replaced by k1 = 1 -2/k, the wind increasesa little more rapidly with height.     

On the dispersion of particles in the atmosphere

The method of moments is used to predict the spatial and temporal dispersion of a cloud of particles released in the atmosphere. The moment equations involve an eddy diffusivity, and a turbulence parameterization is developed to obtain the tensor eddy diffusivity which takes into account the anisotropy of the flow. The parabolic equations for the moments of the distribution are solved by a finite element method. The differences between the data requirements for Lagrangian statistical models and those for eddy diffusivity models are discussed in relation to the comparison of model predictions with observations.     

Experimental investigation of atmospheric dispersion over the Swiss Plain — Experiment “SIESTA”

Between November 15 and 30, 1985, an international mesoscale transport experiment was performed on the Swiss Plain. Seven meteorological groups from Denmark, Germany, Italy and Switzerland measured diffusion properties of near neutral planetary boundary layers during six completely overcast days: four Bise (north-east wind) situations, one transitional situation and one west-wind situation. Diffusion was measured using SF₆ as tracer, which was released at the meteotower of the Gösgen nuclear power plant at 6 m above ground level. One hundred automatic samplers plus a mobile gas chromatograph were used to measure the concentration fields at distances up to 90 km downwind. Meteorological parameters were observed using radar-tracked constant-level balloons, tethered balloon soundings, sonic anemometers, an acoustic sounder and several meteorological ground stations, including a 110 m mast. All data were collected on a magnetic tape with free access to interested persons.The aim of the experiments was to obtain knowledge about the general nature of the turbulence, advection and atmospheric dispersion during neutral weak-wind situations over complex terrain. The collected data set will be useful for testing mesoscale transport and diffusion models. The results clearly show the channelling effect of the Jura mountains and the hilly prealpine region. An interesting result is that the SF₆ plumes showed intensive horizontal spread but only limited diffusion in the vertical direction.     

On the vertical extent of atmospheric feedbacks

This study addresses the question: what vertical regions contribute the most to water vapor, surface temperature, lapse rate and cloud fraction feedback strengths in a general circulation model? Multi-level offline radiation perturbation calculations are used to diagnose the feedback contribution from each model level. As a first step, to locate regions of maximum radiative sensitivity to climate changes, the top of atmosphere radiative impact for each feedback is explored for each process by means of idealized parameter perturbations on top of a control (1?×?CO₂) model climate. As a second step, the actual feedbacks themselves are calculated using the changes modelled from a 2?×?CO₂ experiment. The impact of clouds on water vapor and lapse rate feedbacks is also isolated using `clear sky' calculations. Considering the idealized changes, it is found that the radiative sensitivity to water vapor changes is a maximum in the tropical lower troposphere. The sensitivity to temperature changes has both upper and lower tropospheric maxima. The sensitivity to idealized cloud changes is positive (warming) for upper level cloud increases but negative (cooling) for lower level increases, due to competing long and shortwave effects. Considering the actual feedbacks, it is found that water vapor feedback is a maximum in the tropical upper troposphere, due to the large relative increases in specific humidity which occur there. The actual lapse rate feedback changes sign with latitude and is a maximum (negative) again in the tropical upper troposphere. Cloud feedbacks reflect the general decrease in low- to mid-level low-latitude cloud, with an increase in the very highest cloud. This produces a net positive (negative) shortwave (longwave) cloud feedback. The role of clouds in the strength of the water vapor and lapse rate feedbacks is also discussed.