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.     

ATTILA: atmospheric tracer transport in a Lagrangian model

The model ATTILA has been developed to treat the global-scale transport of passive trace species in the atmosphere within the framework of a general circulation model (GCM). ATTILA runs online within the GCM ECHAM4 and advects the centroids of 80.000 to 190.000 constant mass air parcels. Each trace constituent is thereby represented by a mass mixing ratio in each parcel. ATTILA contains state-of-the-art parameterizations of convection, turbulent boundary layer mixing and inter-parcel transport, and provides an algorithm to map the tracer concentrations from the trajectories to the ECHAM model grid. The transport characteristics of ATTILA are evaluated against observations and the standard semi-Lagrangian transport scheme of ECHAM by two experiments. (1) We simulate the distribution of the short-lived tracer radon (²²²Rn) in order to examine fast vertical transport over continents, and long-range transport from the continents to remote areas. (2) We simulate the distribution of radiocarbon (¹⁴C) from nuclear weapon tests in order to examine upper tropospheric and stratospheric transport characteristics. Contrary to the semi-Lagrangian scheme, ATTILA shows a greatly reduced meridional transport in the upper troposphere and lower stratosphere, and a reduced downward flux from the stratosphere to the troposphere, especially in mid-latitudes. Since ATTILA is a numerically non-diffusive scheme, it is able to maintain steep gradients, which compare better to the observations than the rather smooth gradients produced by the semi-Lagrangian scheme.     

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     

Turbulent diffusivity and diurnal variations in the atmospheric boundary layer

Diurnal variations of the vertical profiles of wind and temperature have been surveyed, and the diffusivity and the dimensionless gradient function in the atmospheric boundary layer have been estimated. Even in the middle of the atmospheric boundary layer (e.g., below a height of 442 m), the vertical wind profile normalized by the surface friction velocity has approximately a universal profile function different from that in the surface boundary layer. Under strong stability conditions, the dimensionless gradient function has a value of about 9.     

Turbulent dispersion of gaseous pollutants: Numerical study of ground level concentration as a function of Eddy diffusivity parametrization on the atmospheric boundary layer

Theoretical and Applied Climatology - A numerical model of the dispersion of gaseous atmospheric pollutants based on the K-theory has been proposed, and the numerical procedure has been validated...     

Wind flow over ridges in simulated atmospheric boundary layers

The flows over four two-dimensional triangular hills and three two-dimensional bell-shaped hills have been investigated in a simulated rural atmospheric boundary layer modelled to a scale of 1:300: Further measurements were made over two of the triangular hills in a simulated rural boundary layer of 1: 3000 scale and in a simulated urban boundary layer modelled to a scale of 1:400. The effect of the model hill surface roughness was also investigated. Flow measurements were restricted to the mean velocity U, RMS velocity fluctuations u and the energy spectra for the streamwise velocity component Measurements were made at a number of longitudinal positions in the approach flow, over the model hills and downstream of the model hills. For each model hill, the crest was the region of largest mean velocity and smallest velocity fluctuations. The largest mean velocities over the model hills occurred for hills of intermediate slope rather than for the steepest hills. A decrease in the scale of the simulated atmospheric boundary layer led to a reduction in the amplification factors at the hill crests, whereas an increase in the surface roughness of the approach flow resulted in increased amplification factors at the hill crests.     

Modeling of pollutant dispersion in sea breeze conditions using a Lagrangian model

Summary This work presents a numerical study of non-reactive pollutant dispersion in sea breeze conditions. Sea breeze circulation is investigated using a 3-D mesoscale meteorological model. Simulation was conducted for the area of Tarragona (Spain) which has an important petrochemical industry in the coastal region and complex terrain. Results from the meteorological model were used as input to a Lagrangian particle model in order to analyze the pollutant dispersion of an elevated plume emitting near the shoreline. The simulation was performed for 24 h and an analysis of the meteorological and concentration fields was untertaken for this time period. The results are compared with measured surface data. Good correlation exists between observed and simulated conditions indicating that the coupling of the meteorological and particle models provides a good tool for analyzing air pollution in complex situations.With 13 Figures     

An overwater stability criterion for the offshore and coastal dispersion model

The Offshore and Coastal Dispersion (OCD) model proposed and evaluated by Hanna et al. (1985) requires the Monin-Obukhov length to compute the stability class. Both wind shear and heat flux are needed for this computation; since these parameters are not normally observed, the stability length has been converted into a nomogram which consists of routinely measured wind speed and air-sea temperature difference. An analysis of the vertical turbulence intensity as a function of the stability length demonstrates that under neutral conditions, the stability scheme used in the OCD model is reasonable.     

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.     

Formation and characteristics of coastal internal boundary layers during onshore flows

The development and characteristics of coastal internal boundary layers were investigated in 28 tests. These were made at all seasons and in both gradient and sea-breeze flows but only during mid-day periods. Measurements of turbulence and temperature were taken from a light aircraft which flew traverses across Long Island at successive altitudes parallel to the wind direction. These were used to locate the boundary between modified and unmodified air as a function of height and distance from the coast. The same measurements plus tower measurements of wind, turbulence and temperature, pilot balloon soundings and measurements of land and water surface temperatures by a remote sensing IR thermometer were used to quantify the characteristics of the modified and unmodified air. The boundary layer slope was steep close to the land-water interface and became shallower with downwind distance. Growth of the boundary layer was initially slower with stable lapse rates upwind than with neutral or unstable conditions over the water. An equilibrium height was found in many tests except under conditions of free convection when the internal boundary layer merged into the mixed layer inland and with sea-breeze conditions. The equilibrium height depended on downwind conditions and was greater with low wind speeds and strong land surface heating than with stronger winds and small land-water temperature differences. Current theoretical models are not adequate to predict the height of the boundary layer at the altitudes and distances studied but reasonably good predictions were given by an empirical model developed earlier. Wind speed in the modified air averaged about 70% of that at the coast but turbulence levels were several times higher both near the surface and aloft. These findings have important implications for diffusion from coastal sites.     

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.     

Characterizing the dispersive state of convective boundary layers for applied dispersion modeling

Estimates from semiempirical models that characterize surface heat flux, mixing depth, and profiles of temperature, wind, and turbulence are compared with observations from atmospheric field studies conducted in Colorado, Illinois, Indiana, and Minnesota. Sodar observations are compared with tower measurements at the Colorado site, for wind and turbulence profiles. The median surface heat flux, as calculated using surface-layer flux-profile relationships and an energy budget model, was consistently overestimated by 20 to 80%. Several mixing-depth models were evaluated: (1) integration of the hourly surface heat flux and friction velocity, (2) solving for the time rate of change of profiles of virtual potential temperature, and (3) an interpolation scheme used by the U.S. Environmental Protection Agency in regulatory dispersion models. For the late afternoon, 80 to 90% of the estimates from the first and third models were within 40% of the observed values. For the morning hours after sunrise, all were less accurate. Temperature estimates from surface-layer flux-profile relationships compared well with observations within the mixed layer, but were too low for the inversion layer aloft. Wind profiles were derived using surface-layer flux-profile relationships, a windprofile power-law based on Pasquill stability category, and sodar measurements. The sodar measurements were superior to both types of model estimates. Turbulence profiles were derived from sodar measurements and from semiempirical similarity relationships based on mixing depth and Obukhov length. The scatter in the comparisons with the sodar observations is twice that seen in the comparisons with empirical profile relationships. Overall, it appears that uncertainty of as low as 20 to 30% in the characterization of the diffusion meteorology is the exception rather than the rule.On assignment from the National Oceanic and Atmospheric Administration, U. S. Department of Commerce.Disclaimer: Although the research described in this article has been supported by the United States Environmental Protection Agency, it has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.     

An isentropic vertical coordinate model: Design and application to atmospheric frontogenesis studies

Summary The isentropic vertical coordinate model developed at UCLA is briefly reviewed. The review includes an outline of the approach used to overcome technical difficulties in handling model layers with small mass.The model's performance is demonstrated by simulating the evolution of a middle-latitude baroclinic disturbance. During the evolution of the disturbance, sharp frontal zones are generated in the upper and middle troposphere with realistic tropopause folding. The extent to which different dynamical processes contribute to frontogenesis is analyzed.While the model successfully simulates frontogenesis in the upper and middle troposphere, it has a difficulty in simulating surface fronts. The difficulty arises due to the lack of degrees of freedom in surface temperatures since an isentropic vertical coordinate model requires a large number ofvertical layers to obtain a highhorizontal resolution at the lower boundary. This suggests the potential of a hybrid vertical coordinate, which approaches at upper levels and at lower levels.With 12 Figures     

A linked three-dimensional PBL and dispersion model in coastal regions

In this paper a 3-D mesoscale PBL dynamic prognostic model for a coastal region with complex terrain was developed to simulate the 3-D flow field under a local sea-land breeze circulation. The output from the PBL model was used as an input to an Eulerian numerical model which can be used to simulate finely the temporal and spatial distributions of an air pollutant (SO₂) during a sea-land breeze developing in a coastal region. With respect to its use as a diagnostic model, only a few data would be required to simulate the background winds controlled by a larger scale synoptic system, and then provide initial winds for the PBL model.Having linked the three models and defined the coefficients of turbulent diffusion in a simple form, an integrated 3-D numerical air quality model suitable for the coastal environment was designed. The period of May 29, 1986 was selected for simulating and analysing the distribution of air pollutants over the coastal area of Bohai-sea in Northern China. The results indicated that the calculated concentrations corresponded with the observed ones on the whole. Thus this linked model has been shown to be feasible and useful in practice.     

基于欧拉—拉格朗日模式的风沙跃移数值模拟方法

通过对沙粒的起动过程以及粒床相互作用机理进行参数化处理,利用计算流体力学方法与离散动力学模拟对风场和沙粒进行耦合迭代,建立了基于欧拉—拉格朗日模式的、适用于研究风沙流二维特征的风沙跃移数值模型。通过模拟沙粒集团以及单颗沙粒跃移运动的随机特征,有效描述了风沙流垂向以及沿程变化规律,并且能够输出任意时刻每颗沙粒的空间位置、速度和加速度等信息。输沙通量的空间分布与现场观测实验数据有较高的吻合度,摩阻风速和风速廓线两个宏观物理量的变化情况与风沙流形成和发展的物理实际相符,说明了本数值模型和计算方法的合理性。     

The atmospheric boundary layer — advances in knowledge and application

We summarise major activities and advances in boundary-layer knowledge in the 25 years since 1970, with emphasis on the application of this knowledge to surface and boundary-layer parametrisation schemes in numerical models of the atmosphere. Progress in three areas is discussed: (i) the mesoscale modelling of selected phenomena; (ii) numerical weather prediction; and (iii) climate simulations. Future trends are identified, including the incorporation into models of advanced cloud schemes and interactive canopy schemes, and the nesting of high resolution boundary-layer schemes in global climate models.     

Turbulent dispersion in a stable layer with a quadratic exchange coefficient

Dispersion from a line source into a stable boundary layer of thickness l is analysed through solution of the diffusion equation assuming an exchange coefficient K(z) (1 – z/l) ² and wind profiles u(z) z ⁿ, with n = 0,1. Estimates of ground-level concentrations are made by developing analytic formulae where this is possible. A general method of solution using Laplace transformation and Green's function techniques is developed as an alternative to the eigenfunction expansion method discussed previously (Robson, 1983).     

An experimental study of the airflow over a hill in the atmospheric boundary layer

Between 1975 and 1977, the Centre Scientifique et Technique du Bâtiment (CSTB) carried out a study of the overspeed effect over a hill in the surface boundary layer. The hill in question was situated in open country and had a drop of about 100 m for an average slope of 8%. The experimental equipment consisted of three 25 m high masts placed along the upwind slope of the hill. Each mast was equipped with four Gill Propeller anemometers. The data set analyzed consisted of 65 recordings of strong winds which had an average velocity greater than 6 m s^-1.Near the ground, local topographic effects and inhomogeneous roughness along the slope have the same influence on the overspeed effect as the mean slope of the hill. The overspeed is proportional to the upwind slope, but on the other hand, the turbulence structure does not seem to be disturbed by the hill.     

A similarity model for maximum ground-level concentration in a freely convective atmospheric boundary layer

A model of buoyancy- and momentum-driven industrial plumes in a freely convective boundary layer is proposed. The development combines the Lagrangian similarity models of Yaglom for non-buoyant releases in the convective surface layer with the Scorer similarity model for industrial plumes. Constraints on the validity of the extension of Yaglom’s model to the entire convective planetary boundary layer, arrived at by consideration of Batchelor’s formulation for diffusion in an inertial subrange, are often met in practice. The resulting formulation applies to an interval of time in which the entrainment of the atmosphere by the plume is balanced by the entrainment of the plume by the atmosphere. It is argued that during this interval, both maximum plume rise and ground contact are achieved. Further examination of the physical interrelationship with the Csanady-Briggs formulation serves to consolidate the model hypotheses, as well as to simplify the derivation of maximum ground-level concentrations. Experimental evidence is presented for the validity of the model, based on Moore’s published data.     

Turbulent characteristics of a shallow convective internal boundary layer

Turbulent characteristics of a 50 to 100 m deep convective internal boundary layer (I.B.L.) have been studied. The data were gathered at a flat coastal site (Näsudden on the island of Gotland, Sweden) during three consecutive days in May 1980 which were characterized by a steady, very stable stratified marine approach flow. The site is situated on a flat area ca. 1500 m from the shoreline. Only daytime runs have been analysed in the present paper. The sensible heat flux at the ground was typically 200 W m^-2 and was found to decrease more or less linearly with height throughout the I.B.L., being slightly negative at greater heights. The momentum flux was also found to decrease with height, but nevertheless shear production of turbulent kinetic energy was found to be large throughout the entire I.B.L. The analysis shows that the turbulent regime has a mixed character. Certain characteristics, such as the rate of growth of the I.B.L., appear to be almost entirely controlled by mechanical turbulence, while others, notably temperature variance and the spectrum of vertical velocity, scale remarkably well with w _* and z _i, in accordance with the results found in fully convective conditions during the experiments at Minnesota and Aschurch. Other turbulent characteristics, such as spectra of the horizontal wind components measured near the top of the I.B.L. tend to adhere to mixed-layer scaling in the high frequency range, exhibiting much increased energy in the lower (reduced) frequency range. Spectra of the velocity components from 10 m are shown to be in general agreement with findings from ‘ideal’, homogeneous sites (Kansas) when properly normalized, although the low frequency part of u- and v-spectra are slightly reduced compared to the case with deep convection.