Neutral Atmospheric Boundary Layer Research Articles

Fixed‐point values of contaminant cloud dilution

AbstractThe contaminant concentration values that are experienced by a fixed observer are represented by an expected‐mass‐fraction function. It is shown that the α‐β prescription of moments enable the expected‐mass‐fraction to be approximated. The release of a puff of contaminant in the neutral atmospheric boundary layer is used as an illustrative example.

A prognosis for the sudden release of contaminant in an environmental flow

AbstractA diffusing contaminant cloud is characterized by its location, size and state. The state of contaminant concentration reduction is the main concern in this paper and is represented by the expected‐mass‐fraction function. A bi‐Gaussian probability density function for concentration is developed and shown to describe measured probability density functions throughout the bulk of a variety of steady flows with continuous contaminant sources. The bi‐Gaussian density function is shown to be directly related to the α‐β prescription of moments and to enable the approximate prediction of the expected‐mass‐fraction function. A puff of contaminant released in the neutral atmospheric boundary layer is used as an illustrative example.

Similarity in vita-detected events in a nearly neutral atmospheric boundary layer

Horizontal velocity data recorded in the atmospheric boundary layer at Boulder, Colorado and Cabauw, the Netherlands under near-neutral conditions, are analysed here by the Variable Interval Time Average (VITA) method. It is found that event frequency shows similarity with vita threshold and averaging time over a significant range of parameter values, enabling us to devise an objective method for (i) deciding on the choice of these hitherto arbitrarily selected parameters, and (ii) determining values for a well-defined ‘characteristic’ frequency. The conditional averages obtained from the events in the similarity region, when suitably normalized, are found to be extraordinarily similar to those obtained from low Reynolds number laboratory flows, indicating the existence of similar coherent events at widely varying Reynolds numbers. The characteristic frequency shows little variation with height beyond about 20 m, and a scaling that is poor with height and friction velocity but not inconsistent with outer variables.

Numerical Modeling of the Turbulent Fluxes of Chemically Reactive Trace Gases in the Atmospheric Boundary Layer

Turbulent fluxes of chemically reactive trace gases in the neutral atmospheric boundary layer (ABL) were simulated with a one-dimensional, coupled diffusion-chemistry model. The effects of rapid chemical reactions were included with a suite of second-order turbulence equations in which additional chemical terms were used to describe contributions to flux by rapid chemical production and loss. A total of 69 chemical reactions were incorporated to describe basic atmospheric photochemistry coupled with chemistry for isoprene and its oxidation products. Daytime flux Profiles of O3, NO, N02, OH, isoprene, and other depositing gases were simulated with assumed rates of NO emission from soil, isoprene emission rates appropriate for a deciduous forest, and initial concentrations of various chemical species typical of a remote area. Results show that chemical reactions can influence vertical fluxes by producing sources or sinks in the atmosphere and by changing mean concentrations. Magnitudes of NO and NO2 fluxes decrease with height at a much greater rate than predicted by a nonreactive model. The NO emitted from soil can quickly be converted to N02, and the upward NO flux can decrease by as much as 80% at a height of 100 m. The magnitude of NO2 flux decreases sharply with height because of the NO-to-NO2 conversion, but NO2 deposition near the surface tends to be enhanced by an increase in NO2 concentration near the surface NO emission source. The Profile of 03 flux simulated with forced entrainment at the top of the ABL closely matches the profile derived from a field experiment, and the flux throughout the ABL increases slightly because mean O3 concentrations are increased by chemical production associated with isoprene emissions. Simulated profiles of isoprene flux closely agree with results of a nonreactive model and appear to be controlled primarily by surface emission and vertical turbulent mixing. Chemical reactions appear to have a substantial effect on vertical concentration gradients, diffusivities, and deposition velocities for NO2, NO3, and N2O5. The reactions have a negligible effect on the deposition velocities for O3, HCHO, CH3OOH, HNO2, H2O2, and HNO3.

Wind-tunnel simulation of gas dispersion over complex terrain: Comparison of two length-scale studies

Abstract The effect of the length scale on wind-tunnel simulation is evaluated by comparing the results obtained from similar experiments performed in two differently sized wind tunnels. One experiment was carried out in the EPA (U.S. Environmental Protection Agency) meteorological wind tunnel (3.7 m wide, 2.1 m high and 18.3 m long). A similar study at a geometrical scale reduced to 1: 5 was performed at VKI (von Karman Institute) L-2B wind tunnel. Similar approaching flow (a simulated neutral atmospheric boundary layer), source characteristics and obstacle features were used. Three hill shapes were tested: a cone, a triangular ridge with conical edges and a 2D triangular ridge. Mean velocity, streamwise turbulence intensity and concentration profiles (for three source heights) were measured, as in the reference study, and the compared results are presented. The normalized concentration values are comparable in the two experiments and similar conclusions can be drawn from the two scale experiments concerning plume behaviour with respect to the aspect ratio and the location of the maximal concentration values. However, the results are not fully conclusive about the scale effect.

Wind and turbulence profiles in a simulated wind tunnel boundary layer

A system of Honeycomb Flat Plate (HFP) grid and cylindrical rods has been developed to accelerate the growth of a thick (32 cm) turbulent boundary layer, artificially, over rough floor of a low speed short test-section (0.61 m x 0.61 m) wind tunnel. Simulated profiles of wind velocity, longitudinal turbulence intensity and Reynolds stress are shown to have similarity to those of a neutral atmospheric boundary layer over a typical rural terrain. Longitudinal spectrum of turbulence measured at 10,30 and 100 mm above tunnel floor is shown to compare well with atmospheric spectrum and agree closely with the Kolmogoroff's -2/3 law in the inertial sub-range of the spectrum. Based on the length scale of longitudinal turbulence estimated from the spectrum, a scale of 1 :900 has been proposed for laboratory modeling of environmental problems wherein the transport of mass in a neutral atmospheric surface layer IS solely due to eddies of mechanical origin.

The Structure of Turbulence in the Near-neutral Atmospheric Boundary Layer

Abstract In this paper, the idea of local similarity, previously used to study turbulence in the stably stratified boundary layer, is used to investigate the structure of turbulence in the neutral atmospheric boundary layer and to relate this structure to the turbulent kinetic energy balance. It is found that in the lower half of the boundary layer, the turbulence exhibits local similarity and that the turbulence kinetic energy budget is essentially a balance between shear production and dissipation. In the upper half of the boundary layer, where turbulent transport is a significant term in the turbulence kinetic energy balance, local similarity breaks down. This is consistent with the second-order moment equations, which suggest that local similarity arises in situations where the turbulent and pressure transport terms are small. Results from a large-eddy simulation of the neutral boundary layer are also investigated using local similarity. The simulated turbulence agrees well with the observations for t...

Off‐target glyphosate deposits from aerial silvicultural applications under various meteorological conditions

AbstractAerial spray applications of the herbicide glyphosate were made over a forest canopy under various meteorological conditions. A ‘Thru Valve Boom’ dispersal system carried by a Cessna 188 fixed‐wing aircraft flying at 49 m s−1 was used to generate an aqueous spray cloud with a volume median diameter of 150 μm. Glyphosate deposits from multiple overlaid crosswind line sources released at 10 m above ground level were measured on ground sheets and artificial foliage at downwind distances between 50 and 400 m. Trials were conducted in stable, neutral and unstable atmospheric boundary layers with average wind speeds between 2·2 and 5·7 m s−1 and vertical intensities of turbulence between 0·07 and 0·16. Linear regression lines fitted to logarithmically transformed measurements and downwind distances (x) gave statistically significant correlation coefficients (P = 0·01), and were compared by ANOVA. Glyphosate deposits on ground sheets and artificial foliage were attenuated at rates inversely proportional to x to the power 1·7‐4·3. Regression line comparison showed that, in general, deposits on ground sheets decreased with increasing wind speed and intensity of turbulence, and some statistically significant differences were found in slopes and elevations of regression lines from different trials. However, deposits at the 50‐m station increased with wind speed due to the large‐drop cloud component. Regression line comparison for deposits on artificial foliage showed that, in general, they were highest in the intermediate wind speed‐neutral stability case and similar in the high wind speed‐unstable and low wind speed‐stable boundary layers, although deposits at the 50‐m station also increased with wind speed.

An experimental study on the evolution and dispersion of a cloud of gas heavier than air

Abstract This paper presents an experimental study of the evolution and dispersion of a cloud of gas heavier than air in a simulated neutral atmospheric boundary layer. First, a comparative study of instantaneous releases for two density cases of 1 and 1.87 are presented. The results were obtained by laser tomography technique. Then, we report some statistics on the fluctuating concentration field in a two-dimensional plane. The measurements were obtained by a technique based on visualization and digital image processing. The method is well suited for this type of study, provided that the number of observations is sufficient.

Evaluation of a Turbulence Closure Scheme Suitable for Air-Pollution Applications

Abstract A computationally turbulence closure scheme is formulated and evaluated. The scheme includes a correction to the redistribution terms in order to account for the influence of the underlying surface. Care is taken in order to ensure realizability in simulated fields. The formulation is implemented in a mesoscale model containing prognostic equations for mean momentum, heat and turbulent kinetic energy. All other second moments are obtained through diagnostic expressions. A one-dimensional version of this highly simplified scheme is evaluated against turbulence measurements from stable, neutral and convective atmospheric boundary layer. Inclusion of wall effects is shown to improve the performance of the closure when applied to the atmospheric boundary layer. It gives a better prediction of ratios between velocity variances, of importance particularly for air pollution applications. Discrepancies found in the upper 40% of the convective boundary layer suggest that the additional computational effor...

Fluid modeling applied to atmospheric diffusion in complex terrain

Wind-tunnel and towing-tank studies conducted over the past 10 years at the U.S. Environmental Protection Agency's Fluid Modeling Facility (FMF) of flow and diffusion in complex terrain are reviewed. A primary impetus for this work was EPA's Complex Terrain Model Development Program (CTMDP)—designed to develop reliable atmospheric dispersion models applicable to large pollutant sources in complex terrain, with primary emphasis on plume impaction during night-time stable conditions. The FMF interacted closely with the model developers participating in the CTMDP and provided support in various ways through the conduct of a wide range of laboratory studies. Work at the FMF prior to the inception of the program provided the basic framework for the model—the dividing-streamline concept— and the focal point around which the field program was designed. At the beginning of the program, the FMF provided direct support as an aid to planning the details and strategies of the field experiments and testing the limits of applicability of the dividing-streamline concept. Later work included exercises of ‘filling in the gaps’ in the field data, furthering the understanding of the physical mechanisms important to plume impaction in complex terrain and in stably stratified flows in general, testing various modeling assumptions, providing data for ‘calibration’ of various modeling parameters, and testing the ability of the laboratory models to simulate full-scale conditions. Simultaneously, the FMF responded to the needs of the regulatory arm of EPA, the Office of Air Quality Planning and Standards (OAQPS), by providing guidance concerning expected terrain effects and by conducting demonstration studies. These latter studies were concerned primarily with simulation of diffusion in the neutral atmospheric boundary layer. Finally, several supplemental studies were conducted, broadening and expanding upon the specific requests of the model developers and the OAQPS. The highlights of the FMF complex-terrain research work are described herein.

Estimation of maximum surface concentrations from sources near complex terrain in neutral flow

A wind tunnel study was conducted to determine maximum ground-level concentrations for a variety of source positions (locations and heights) both upstream and downstream of two model hills, an axisymmetric hill (maximum slope 24°) and a two-dimensional ridge (maximum slope 16°), immersed in a simulated neutral atmospheric boundary layer. Terrain amplification factors derived from these measurements were used to construct contour plots showing regions or ‘windows’ of enhanced ground-level concentration. These windows of enhanced ground-level concentration are shown to be a useful guide for estimating the effects of complex terrain on pollutant dispersion or, conversely, for determining source locations near complex terrain which minimize the enhancement of ground-level concentration.

Application of a quasi-nonhydrostatic parameterization for numerically modeling neutral flow over an isolated hill

A parameterization of the nonhydrostatic pressure was modified and adapted to a nonlinear numerical model of the neutral atmospheric boundary layer. A hydrostatic model and the quasinonhydrostatic version were used to simulate neutral flow over a symmetrical hill of uniform roughness. Mean-flow quantities and some turbulence characteristics of the flow from both models are presented. These results were compared with observations, analytic theory, and other numerical models. The quasi-nonhydrostatic method produced qualitative features commonly observed in such flows that the hydrostatic model could not simulate. For instance, the observed velocity reduction at the hill base and the speedup at the summit both were simulated by the quasi-nonhydrostatic model. However, computation of vertical velocities from the incompressible continuity equation is inadequate above regions of recirculation and presents a limitation to the method.

Evaluation of an alternative method for numerically modeling nonhydrostatic flows over irregular terrain

Modeling nonhydrostatic atmospheric flow requires the solution of the vertical equation of motion and a prognostic or diagnostic equation for pressure. If the nonhydrostatic components of the flow are relatively small, they can be approximated and incorporated into a purely hydrostatic model, which usually is conceptually simpler and computationally more efficient. A method to do this for a linear model of local thermally-induced circulations is further developed and adapted to a non-linear numerical model of the neutral atmospheric boundary layer. A hydrostatic model and the quasi-nonhydrostatic version were used to simulate neutral flow over simple terrain features. One set of observations taken over a simple change in roughness and another set taken over a change in both roughness and terrain were simulated by both models to assess the capabilities of the quasi-nonhydrostatic technique.

Application of theE– ϵ Turbulence Closure Model to the Neutral and Stable Atmospheric Boundary Layer

Abstract In the E – ϵ turbulence model an eddy-exchange coefficient is evaluated from the turbulent kinetic energy E and viscous dissipation ϵ. In this study we will apply the E – ϵ model to the stable and neutral atmospheric boundary layer. A discussion is given on the equation for ϵ, which terms should be included and how we have evaluated the constants. Constant cooling rate results for the stable atmospheric boundary layer are compared with a second-order closure study. For the neutral atmospheric boundary layer a comparison is made with observations, large-eddy simulations and a second-order closure study. It is shown that a small stability effect can change the neutral atmospheric boundary layer quite drastically, and therefore, it will be difficult to observe a neutral boundary layer in the atmosphere.

Windbreak Effectiveness for Storage Pile Fugitive Dust Control: A Wind Tunnel Study

Results of wind tunnel experiments to determine the optimal size and location of porous windbreaks for controlling fugitive dust emissions from storage piles in a simulated neutral atmospheric boundary layer are presented. Straight sections of windbreak material were placed upwind of two nonerodible, typically shaped piles and were also placed on the top of one of the piles. Wind speed near the pile surface is considered here as the primary factor affecting particle uptake. Wind speed distributions about the piles with and without porous windbreaks are presented. Relative wind speed reduction factors are described and efficiencies based on the relationship between wind speed and.particle uptake are given. The largest and most solid windbreak caused the greatest wind speed reduction, but similar wind speed reductions were also obtained from several smaller windbreaks. A 50 percent porous windbreak of height equal to the pile height and length equal to the pile length at the base, located one pile height from the base of the pile was found to be quite effective in reducing wind speeds over much of the pile. Windbreaks placed on the top of a flat-topped pile caused large wind speed reductions on the pile top, but small, if any, reductions on the windward pile face. Windbreak effectiveness decreased as the angle between the windbreak and the wind direction decreased

Fluid modeling of dense gas dispersion over a ramp

The basic nature of the transport and dispersion of a dense gas plume in the simulated neutral atmospheric boundary layer of a wind tunnel was investigated, both in flat terrain and over a ramp. Measurements were made of the concentration fields downstream of a ground-level, circular source; these measurements consisted of longitudinal ground-level, vertical, and crosswind profiles at various distances downwind. Both neutrally buoyant (air) and negatively buoyant (CO 2) source gases were used so that the specific effects of the density difference could be observed. Similarly, measurements were made in both flat terrain and over the ramp (14° slope followed by an elevated plateau) so that specific effects of the terrain could be observed. Flow visualization was done to ascertain that the dense plume was turbulent, hence, that the effects of molecular properties were insignificant. For the particular value of the buoyancy parameter used in these experiments, the plume buoyancy was significant; the resulting dense plume was significantly wider in the lateral direction and much narrower in the vertical direction, yet the longitudinal ground-level concentration profile downwind was essentially identical to that from the neutral plume. The lateral concentration profiles of the neutral plumes were essentially Gaussian in character, whereas the dense gas plumes exhibited top-hat distributions for considerable distances from the source. The vertical concentration profiles of the neutral plumes were not Gaussian, but displayed variations of the form C/ C mx = exp (− Az n ) with n ⋍ 1.5. On the other hand, the dense gas plumes displayed vertical variations of the form C/ C mx = exp [− z/ z ], where z is the centroid of the distribution. The net effect of the ramp on the dense gas plume was a small reduction in ground-level concentration (less than a factor of two, even for a source relatively close to the base of the ramp). This reduction was quite similar to that observed for the neutral plume.

A wind tunnel study of the flow structure and dispersion from sources upwind of three-dimensional hills

The flow fields around moderately steep hills of triangular cross section and varying crosswind aspect ratio and around a bell-shaped hill were examined by using models immersed in a simulated neutral atmospheric boundary layer in a meteorological wind tunnel. The triangular hills ranged from an axisymmetric cone to a two-dimensional ridge. Concentration patterns resulting from sources of three heights placed upwind of each of these hills were examined to determine plume deformations and terrain amplification factors. The separated flow fields, increasing in size with increasing aspect ratio, appeared to have dominating influences on the entire flow structure; changes in several flow parameters were plausibly explained in terms of the notion that the effective hill shape was the hill-plus-recirculation region rather than the actual hill shape. The concentration measurements showed strong distortions of plume shapes effected by the hills, with convergence in vertical planes and divergence in horizontal planes. Plumes from elevated sources approached the hill surfaces much more closely the smaller the aspect ratio; streamline displacements were generally within the limits suggested by potential flow theory. The terrain amplification factor A, defined as the ratio of the maximum surface concentration in the presence of the hill to the maximum in flat terrain, was found to decrease with increasing aspect ratio. For the half-hill-height sources, values of A ranged from 4 for the bell-shaped hill down to 1.5 for the two-dimensional ridge; for the hill-height source, from 1.8 for the cone down to 0.6 for the two-dimensional ridge. The latter value is suspect, however, because larger concentrations are expected downwind of the reattachment point, a region not probed in the current study.

Mathematical modeling of Turbulent Reacting Plumes—II. Application to the NONO 2O 3 system

Calculations performed with the Turbulent Reacting Plume Model (TRPM) developed in Part I are compared with the experimental data of Builtjes (1981, Netherlands Organization for Applied Scientific Research, Div. of Technology for Soc., Ref. No. 81-013563) for the reaction between NO in a point source plume and ambient O 3, taking place in a wind tunnel simulating a neutral atmospheric boundary layer. The comparison shows the TRPM capable of quantitatively predicting the retardation imposed on the evolution of nonlinear plume chemistry by incomplete mixing.

Mesoscale circulations in a hydrostatic model: coastal convergence and orographic lifting

A hydrostatic two-dimensional K -theory model (Δ x = 4 km, 10 σ levels) is described and used to simulate mesoscale circulations developing in a neutral atmospheric boundary layer when wind blows over or along a 80 km wide sea gulf. On-shore geostrophic wind of 10 m s -1 induces rising motion ˜ 1 cm/s at 300 m level above the coastline due to roughness difference (coastal convergence) and∼1.6 cm s -1 further inland over sloping land (orographic lifting). Off-shore wind produces sinking motion. When the geostrophic wind 10 m s -1 blows along the coastline with lower pressure over the sea, the coastal convergence induces rising motion∼0.5 cm s -1 25 km off-shore at 600 m level, weak sinking motion just over the coastline below 200 m, and the topography induces sinking motion further inland. The results are opposite for reversed wind direction. The two effects are additive. DOI: 10.1111/j.1600-0870.1985.tb00277.x