Horizontal Plume Spread Research Articles

Project Sagebrush: Revisiting the Value of the Horizontal Plume Spread Parameter σy

AbstractThe first phase of an atmospheric tracer experiment program, designated Project Sagebrush, was conducted at the Idaho National Laboratory in October 2013. The purpose was to reevaluate the results of classical field experiments in short-range plume dispersion (e.g., Project Prairie Grass) using the newer technologies that are available for measuring both turbulence levels and tracer concentrations. All releases were conducted during the daytime with atmospheric conditions ranging from neutral to unstable. The key finding was that the values of the horizontal plume spread parameter σy tended to be larger, by up to a factor of ~2, than those measured in many previous field studies. The discrepancies tended to increase with downwind distance. The values of the ratio σy/σθ, where σθ is the standard deviation of the horizontal wind direction, also trend near the upper limit or above the range of values determined in earlier studies. There was also evidence to suggest that the value of σy began to be independent of σθ for σθ greater than 18°. It was also found that the commonly accepted range of values for σθ in different stability conditions might be limiting, at best, and might possibly be unrealistically low, especially at night in low wind speeds. The results raise questions about the commonly accepted magnitudes of σy derived from older studies. These values are used in the parameterization and validation of both older stability-class dispersion models as well as newer models that are based on Taylor’s equation and modern PBL theory.

Re-formulation of plume spread for near-surface dispersion

Recent concerns about effects of automobile emissions on the health of people living close to roads have motivated an examination of models to estimate dispersion in the surface boundary layer. During the development of a new line source dispersion model, RLINE (Snyder et al., 2013), analysis of data from a tracer field study led to a re-examination of near-surface dispersion resulting in new formulations for horizontal and vertical plume spread presented in this paper. The equations for vertical spread use the solution of the two-dimensional diffusion equation, in which the eddy diffusivity, based on surface layer similarity, is a function of surface micrometeorological variables such as surface friction velocity and Monin–Obukhov length. The horizontal plume spread equations are based on Eckman's (1994) suggestion that plume spread is governed by horizontal turbulent velocity fluctuations and the vertical variation of the wind speed at mean plume height. Concentration estimates based on the proposed plume spread equations compare well with data from both the Prairie Grass experiment (Barad, 1958) as well as the recently conducted Idaho Falls experiment (Finn et al., 2010). One of the major conclusions of this study is that the plume spreads as well as the wind speed used to estimate concentrations in a dispersion model form a set of coupled variables.

Wind tunnel modeling of hourly observed atmospheric diffusion by oscillating blade cascade

Horizontal plume spread increases greatly with averaging time. For hourly averaging, plume spreads are much greater in the horizontal direction than in the vertical. The ratio of horizontal to vertical spread was found to be as large as 6. Conventional method cannot reproduce such anisotropic diffusion characteristic in wind tunnel experiments. The author has developed an oscillating blade cascade as a solution to the problem. Since the blade cascade generates strong secondary flow in the test section, a horizontal wind fluctuation of more than 11° has been achieved, which is similar to the standard deviation of wind direction averaged over 1 h under neutral condition. Distributions of mean wind velocity, turbulence intensity, and spectral structures were also similar to those observed in the atmospheric boundary layer. Hourly-averaged plume spread in horizontal and vertical directions could be simulated in the wind tunnel on 1:5000 scale, with appropriate selection of random motion amplitude of the blade cascade and ground roughness arrangement. Thus, the oscillating blade cascade method is useful to model large-scale plume diffusion from a continuous source such as thermal power plant stacks.

Study of Near-field Dispersion through Large Groups of Obstacles

The flow and dispersion around the obstacles is important to understand the pollution problem in the urban areas. This study presents the preliminary results of the experiment carried out at Dugway Proving Ground, Utah. It shows that the presence of obstacles has a significant effect on the structure of the plume. Both horizontal plume spread (σy) and vertical plume spread (σz) are higher in arrays of obstacles compared to without obstacle configuration. It is also observed that the concentrations in arrays of obstacles are generally lower than those without obstacles. The modeling results show that the Gaussian plume model for the time-averaged concentration in the near-field can provide important information with few input parameters. The information obtained from the controlled conditions of the model canopy can be useful for interpreting data from real urban areas.

Measuring methane emission rates of a dairy cow herd by two micrometeorological techniques

Two well-known micrometeorological techniques were used to measure methane emission rates from a herd of dairy cows freely grazing within a fenced paddock. The integrated horizontal flux technique (IHF) was preferable to the flux-gradient technique (FG) because it does not rely on similarity assumptions, and because it had the smaller measurement error. Both techniques were comparable, within estimated errors, with scaled-up animal emissions, determined by a tracer gas technique. The cows’ emissions, by IHF including 243 half-hourly data obtained during three campaigns in the midsummer, late summer and spring of 2002, averaged 330 (±153) g CH 4 per day per cow (±S.D.). For FG, source strength was proportional to measurement height, after application of a conventional source area weight function. This implies that the turbulent diffusivity of CH 4, a passive tracer, cannot be substituted by that of heat or momentum, as assumed by FG. We show that in our experimental situation, where the passive tracer has a finite source area, the ratio of FG to IHF results approximates the square of the turbulent Schmidt number (the ratio of momentum diffusivity to tracer diffusivity). This ratio increased with height and decreased with distance from the source area. We conclude that the Schmidt number is a function of position in the downstream airflow, and argue that the cause is vertical transport efficiency being affected by horizontal plume spread. FG may thus include systematic errors if one does not account for the position dependence of the Schmidt number.

Instantaneous Spread of Plumes in the Surface Layer

Abstract Data are presented from two recent tracer campaigns regarding relative diffusion of surface-level plumes. One study consists of tests performed amid flat, rural terrain near Galen, Montana, while other experiments were conducted above a poplar forest with uniform canopy density and height near Boardman, Oregon. In both cases, sulfur hexafluoride was released near the surface at a constant rate, and fast-response analyzers were used to measure plume concentrations along crosswind traverses and at fixed locations within 1 km of the source. This paper characterizes horizontal plume spread on near-instantaneous time frames during 29 tests, and the field data are used to test seven empirical and theoretical approaches for estimating relative diffusion coefficients using on-site wind data. Five of the equations utilize simple turbulence statistics to predict plume spread as a function of downwind distance; one method utilizes stability-based power-law formulas; and the last technique invokes second-ord...

Wintertime Dispersion in a Mountainous Basin at Roanoke, Virginia: Tracer Study

Abstract During January 1989, five nighttime SF6 tracer experiments were conducted in Roanoke, Virginia. The experiments were designed to help identify and understand the dispersion characteristics of a basin atmosphere during winter stagnation conditions. The basin studied was the Roanoke basin located on the eastern slopes of the Appalachian Mountains. This paper documents this tracer study and gives results from the experiment conducted on the night of 16–17 January 1989. A cold-air pool formed in the basin beginning after the evening transition period and filled to near the elevation of the lowest mountain barrier. A simple model of the ascent rate of the top of this cold-air pool is proposed. A sharp potential temperature jump was present at the top of this fully developed cold-air pool. Vertical measurements of tracer showed the initial ground-level plume to become elevated and ride over the top of the cold-air pool. Horizontal plume spread was enhanced over that expected from turbulent diffusion al...

A Modification of the Gaussian Dispersion Equation to Accommodate Restricted Lateral Dispersion in Deep River Valleys

Lateral plume dispersion in deep river valleys during neutral and stable lapse rate conditions can be exceptionally high due to the intense horizontal turbulence generated by prominent variations in the width, orientation and surface roughness of the valley walls. Use of the standard Gaussian dispersion equation to represent this enhanced lateral plume spread in a narrow valley may be inappropriate since consideration should also be given to the restriction of the horizontal plume spread due to impingement against the valley walls. The basic concept employed in the modification of the Gaussian dispersion equation was to assume that multiple eddy reflections occur between the valley walls in a manner similar to the vertical eddy reflections between the ground plane and an inversion layer aloft. The expressions are developed, but no attempt has been made to validate them with actual measured field data. 4 references, 3 figures, 1 table.

Wind tunnel observation of flow and diffusion under stable stratification

Neutral and four different levels of stable stratification (weak to strongly stable i.e. Ri δ = 0.048−0.248 ) were created in the stratified wind tunnel at NIES. The effects of stable stratification on the spread of a plume from a small, circular, ground level release were examined by flow visualization, trace gas concentration measurement, and flow field velocity and turbulence measurements. As stable stratification increased, flow field visualization showed that the turbulence was increasingly suppressed, especially near the surface, and the vertical plume spread was also inhibited with increasing stability. The normalized velocity power exponent, m, increased from 0.17 to 1.04 (for neutral to strong stability), and the u and w components of turbulent intensity were suppressed. The vertical concentration power exponent, p (from C ~ exp (−z p) ), varied from 1.3 for the neutral case, to 3.0 for the strong stable case. The observed values of p showed reasonably good agreement with the values of p estimated from the half-empirical diffusion equation. The vertical plume spread, σ z , vs distance downwind decreased with increasing stability, appearing as logical extensions of the Pasquill-Gifford chart. However, the horizontal plume spread, σ y , vs distance at first decreased with increasing stability then began to increase as the stability increased from moderate to strongly stable, approaching the neutral case curve. This behavior is caused by the suppression of vertical motion at the stronger stable stratifications which leads to increased plume meandering and plume collapse.

Dispersion from an elevated source in a convective boundary layer

This paper proposes a model for dispersion of pollutants emitted from an elevated source into a convective boundary layer. In our model we incorporate some of the most recent understanding of dispersion in the mixed layer. For example, we relate the horizontal plume spread σ y to the convective velocity scale w ∗. Our formulation for σ y emphasises the role of the mixed layer height z t and the associated time scale z t/w ∗ in controlling dispersion in the vertical direction. We have also attempted to account for the effects of self-induced plume turbulence on plume spread. An important ingredient of our model is the concept of plume impingement distributed around a mean distance x t at ground-level. The computation of x t has been guided by the recent work of Lamb (1979) and that of Briggs (1975). The model has been tested by comparing predictions with concentrations measured around the Dickerson and Morgantown power plants in Maryland. The results of the verification are very encouraging. This study shows that we can obtain rough estimates of ground-level concentrations by assuming that the pollutants are well mixed through the PBL. Then, the expression for the concentration C can be simply written as C = 0.9Q w ∗z tx , where Q is the emission rate and x is the distance from the source.