Wind Direction Fluctuations Research Articles

On plume meandering under stable stratification

Under stable stratification and light winds large horizontal wind direction fluctuations have been observed quite often leading to meandering of pollutants plume downwind of elevated point sources. Due to this plume meandering long term concentration levels can be reduced by a factor of 4 or more, compared to straight plume conditions. Some possible mechanisms causing plume meandering under stable conditions are discussed, including gravity waves and vortices with horizontal or vertical axis.

Atmospheric dispersion in the arctic: Winter-time boundary-layer measurements

The winter-time arctic atmospheric boundary layer was investigated with micrometeorological and SF6 tracer measurements collected in Prudhoe Bay, Alaska. The flat, snow-covered tundra surface at this site generates a very small (0.03 cm) surface roughness. The relatively warm maritime air mass originating over the nearby, partially frozen Beaufort Sea is cooled at the tundra surface resulting in strong (4 to 30 °C · (100 m)-1) temperature inversions with light winds and a persistent weak (1 to 2 °C · (100 m)-1) surface inversion with wind speeds up to 17 m s-1. The absence of any diurnal atmospheric stability pattern during the study was due to the very limited solar insolation. Vertical profiles were measured with a multi-level mast from 1 to 17 m and with a Doppler acoustic sounder from 60 to 450 m. With high wind speeds, stable layers below 17 m and above 300 m were typically separated by a layer of neutral stability. Turbulence statistics and spectra calculated at a height of 33 m are similar to measurements reported for non-arctic, open terrain sites and indicate that the production of turbulence is primarily due to wind shear. The distribution of wind direction recorded at 1 Hz was frequently non-Gaussian for 1-hr periods but was always Gaussian for 5-min periods. We also observed non-Gaussian hourly averaged crosswind concentration profiles and assume that they can be modeled by calculating sequential short-term concentrations, using the 5-min standard deviation of horizontal wind direction fluctuations (Σθ) to estimate a horizontal dispersion coefficient (Σ y ), and constructing hourly concentrations by averaging the short-term results. Non-Gaussian hourly crosswind distributions are not unique to the arctic and can be observed at most field sites. A weak correlation between horizontal (Σ v ) and vertical (Σ w ) turbulence observed for both 1-hr and 5-min periods indicates that a single stability classification method is not sufficient to determine both vertical and horizontal dispersion at this site. An estimate of the vertical dispersion coefficient, Σ z , could be based on ΣΦ or a stability classification parameter which includes vertical thermal and wind shear effects (e.g., Monin-Obukhov length, L).

Methods for Estimating the Mean and the Standard Deviation of Wind Direction

In a recent paid, the author reviewed methods for estimating the mean and the standard deviation of wind direction. Some comments on this subject are now added. If large sporadic eddy fluctuations do not occur in a sampling duration, we can exactly calculate an arithmetic mean and standard deviation of wind direction by using an approach suggested by Mitsuta. This is via a single-pass procedure without considering the discontinuity in wind direction scale. By using the Mitsuta approach it is found that Mardia's method provides a good estimator of the standard deviation of wind direction even in the case when the range of fluctuations of wind direction is wide.

Relating error bounds for maximum concentration estimates to diffusion meteorology uncertainty

The intent of this paper is to relate the magnitude of the error bounds of data, used as inputs to a Gaussian dispersion model, to the magnitude of the error bounds of the model output, which include the estimates of the maximum concentration and the distance to that maximum. The research specifically addresses the uncertainty in estimating the maximum concentrations from elevated buoyant sources during unstable atmospheric conditions, as these are most often of practical concern in regulatory decision making. A direct and quantitative link between the nature and magnitude of the input uncertainty and modeling results has not been previously investigated extensively. The ability to develop specific error bounds, tailored to the modeling situation, allows more informed application of the model estimates to the air quality issues. In this study, a numerical uncertainty analysis is performed using the Monte-Carlo technique to propagate the uncertainties associated with the model input. Uncertainties were assumed to exist in four model input parameters: (1) wind speed, (2) standard deviation of lateral wind direction fluctuations, (3) standard deviation of vertical wind direction fluctuations, and (4) plume rise. For each simulation, results were summarized characterizing the uncertainty in four features of the ground-level concentration pattern predicted by the model: (1) the magnitude of the maximum concentration, (2) the distance to the maximum concentration, and (3) and (4) the areas enclosed within the isopleths of 50% and 25% of the error-free estimate of maximum concentration. The authors conclude that the error bounds for the estimated maximum concentration and the distance to the maximum can be double that of the error bounds for individual model input parameters. The model output error bounds for the areas enclosed within isopleth values can be triple the error bounds of the input. It was not our intent to cover all possible combinations for the error in the input parameters. Ours was a much more limited goal of providing a lower bound estimate of model uncertainty in which we assume the input is reasonably well characterized and there is no bias in the input. These results allow estimation of minimum bounds on errors in model output when considering reasonable input error bounds.

Accuracy of various diffusion and stability schemes over Washington, D.C.

One year of continuous 8-h samples at three locations in and around Washington, D.C. in conjunction with tracer releases at three sites 14–40 km from the samplers were used to evaluate the accuracy of various stability and dispersion techniques. The combination of the Pasquill-Gifford dispersion curves and a stability category determined from the wind-direction fluctuation showed the least biased calculations. The precision of all methods was similar. When measured and calculated concentrations were paired in space and time, the scatter ranged over two orders of magnitude. The calculations were sensitive to the meteorological data, such that when the spatial density of meteorological sites was increased, the calculations improved. Corrections to account for enhanced dispersion over an urban area were only appropriate when the calculations were based on a rural meteorological site. Turner's stability classification method (1964, J. appl. Met. 3, 83–91) did not provide enough unstable cases, especially during the daytime, and hence resulted in overcalculation. Briggs' urban dispersion curves (1973, Air Resources Laboratories, TN) resulted in undercalculation by a factor of five. Greatest differences between dispersion methods were observed for night-time samples.

環境予測のための風胴実験における可視化の応用例

Wind tunnel experiments are frequently carried out as a useful method for atmospheric environ-mental prediction.Specially gas diffusion in the complex terrain and around ground structures entails the model experiment. This paper describes simulation of field meteorological condition in the wind tunnel and examples of diffusion phenomena clarified with use of flow and diffusion visualization. Meteorological factors reproduced are atmospheric stability and wind direction fluctuation, the simulation technique of which is now under technical development in our institute.

Short-range atmospheric dispersion over a heterogeneous surface—I. Lateral dispersion

Results are presented from a series of diffusion trials in which a tracer gas, sulphur hexafluoride, was released from an elevated point source over the heterogeneous surface of the Canadian Precambrian Shield. The experimental procedures used in the sampling and analysis of the tracer gas, and in the collection of supporting meteorological information, are described briefly. Physical and meteorological data are presented for 39 successful trials. Under steady-state meteorological conditions, the crosswind concentration distributions were approximately normal in shape. Lateral Gaussian dispersion parameters were extracted from the profiles and compared statistically with the predictions of a number of standard schemes. Best agreement was found with Pasquill's (1976) method, which uses observed wind direction fluctuations to deduce the lateral spread of the plume. The success of Pasquill's method for the rough, heterogeneous surface of the Shield suggests that it may be quite generally applicable. The spread of the plume was found to scale best with meteorological measurements made near the release height, where the flow is characteristic of the heterogeneous surface as a whole. When scaled using Pasquill's procedure, lateral spread is independent of the type of vegetative cover beneath the plume. Similarly, normalized dispersion over a snow-covered surface progresses at the same rate as over a snow-free surface

Stability studies with the help of acoustic sounding

A Monostatic acoustic sounder has been operating at the National Physical Laboratory, New Delhi (India) for several years. Analytical studies of the data on the atmospheric boundary layer obtained at Delhi for the period May 1977 to April 1982 are reported. A systematic classification of the observed structure has also been proposed to give information on the prevailing Pasquill stability category which determines pollution dispersion, This was done by comparing the echo structure type with observations of fluctuations in the horizontal wind direction,

Application of the micrometeorological mass balance method to the determination of ammonia loss from a grazed sward

AbstractThe micrometeorological mass balance method has been assessed with a view to its use in the determination of ammonia (NH3) loss from grazed swards. The method involves the measurement of wind speed and the concentration of NH3 in air at different heights above the sward at its windward boundary and a position leeward of the grazed area. The flux of NH3 is calculated from these measurements and a continuous record of wind direction. Quantitative recovery of NH3 from air flows up to 10 litre min−1 was achieved using a simple trap containing dilute orthophosphoric acid and a gas dispersion tube. Wind speed measured at a height of 0.25 m at six different positions above a previously grazed sward varied by <2%. The NH3 concentration at the same positions was affected by the distance between the windward edge of the sward and the sampling position (i.e. the fetch) and by heterogeneity in the distribution of sources of NH3 (urine‐ or dung‐affected areas). However, the normalised horizontal flux through 0.25 m (wind speedXNH3 concentration/fetch) varied by less than ±8.1% probably due to mixing through fluctuations in wind direction (approximately ±30° on the mean) as air passed over the sources of NH3 during each measurement period. Plots of wind speed or NH3 concentration versus logarithm of height indicated that each approximated a linear relationship. This facilitated the calculation of the NH3 flux per unit land area and reduced the total error to about 10%. The total loss of NH3 derived by summing losses during individual sampling intervals of 2 to 18 h within a 24 h period was essentially the same as that derived by averaging wind speeds and NH3 concentrations measured continuously over the same 24 h period. The flux of NH3 from a ryegrass sward grazed by yearling steers ranged from 0.01 to 0.14 kg N ha−1 h−1 during a 2 day grazing period within a 28 day rotation and during the 5 days following removal of animals. A pronounced diurnal variation was observed in the flux of NH3, the maximum occurring between 13.00 and 20.00 hours on each day. Rainfall and low rates of evapotranspiration reduced the flux to <0.02 kg N h−1 h−1. The total loss of NH3 during 28 days was 20.7 kg N ha−1.

Eolian Sandstones in Upper Triassic-Lower Jurassic Red Beds of the Fundy Basin, Nova Scotia

ABSTRACT Eolian sandstones occur interbedded with alluvial-fan and braided-river red beds in the Upper Triassic Wolfville Formation and at the base of the Lower Jurassic McCoy Brook Formation in Nova Scotia. Extensive semiaridity and aridity in the subtropical rift valley favored deposition of eolian sandstones, caliche paleosols, alluvial-fan conglomerates, braided-river alluvium, and playa mudstones. In postdrift configuration, paleowinds blew to the southwest, west, and northwest down the valley, reflecting the direction of trade winds at 25°N paleolatitude. A three-step hierarchy of surfaces truncates cross-beds. First-order bedding planes of low relief extend for at least hundreds of meters, truncating all eolian structures. These surfaces were sculptured by desert flash floods and then deflated to ventifact-bearing bimodal regs. Second-order surfaces bound cross-bed sets and are mostly concave-up and dip downwind, recording shifts in migration direction of individual dunes or dunes superimposed on larger dune structures. Third-order surfaces within cross-bed sets formed on the lee sides of dunes due to fluctuations in wind direction and strength. Barchans initially developed on the alluvial-fan and braided-river alluvium, later coalescing into barchanoid transverse ridges, as evidenced by the internal geometry of the eolian sandstones. /P>

Measurements of wind loads on full-scale film plastic clad greenhouses

Full-scale measurements of the surface pressures on film plastic clad greenhouses have been made under natural wind conditions. The structures, which are of curved-arch construction, are representative of commercial greenhouses in the United Kingdom and include single- and multi-span buildings. The experimental procedure is described and detailed results are presented together with an outline of the data-analysis technique which takes into account short-term fluctuations in wind direction to give pressure coefficients consistent with use in the British Standards Code of Practice procedure. A simplification of the detailed results is presented in the Appendix in a form convenient for use in design.

Correlation between mixing height and surface turbulence

Diurnal variation of mixing height and standard deviation of horizontal wind-direction fluctuation ( σ ϑ ) have been studied for Visakhapatnam city (17° 42′ N, 82° 18′ E) for the months January, April, August and October, typical of winter, pre-monsoon, monsoon and post-monsoon seasons, respectively. The study has revealed that the variation of mixing height almost follows σ ϑ values which reflect turbulence.

Terrain roughness and atmospheric stability at a typical coastal site

Directional dependence of horizontal wind direction fluctuations (Σθ) is studied at the coastal site of Madras Atomic Power Project, Kalpakkam with significant inhomogeneity in roughness element distribution around the location of measurement. Σθ is measured by a potentiometric wind vane mounted on a 30 m meteorological tower. Values of Σθ showed as high as threefold variation for the same atmospheric stability depending on the effective roughness length of the upwind sector. Average Σθ values separated for sea- and land-breeze conditions, when correlated with Pasquill stability categories showed a monotonic decrease with increasing stability for land breeze but was found to increase for change from D to F category during sea breezes presumably due to the influence of an internal boundary-layer development.

Seasonal variation of wind direction fluctuations vs Pasquill stabilities in complex terrain

The authors have studied the seasonal variation of Σθ (the standard deviation of wind direction fluctuations) vs Pasquill stabilities over complex terrain. It is found that the values of Σθ are quite high in the month of April in contrast with other months. Values are also high when compared with those estimated over flat terrain and at a coastal site.

Statistics of wind direction fluctuations in the surface layer over plant canopies

Height dependency of statistics of horizontal and vertical wind direction fluctuations over plant canopies have been studied, using observational data in the surface layer over paddy fields. Vertical wind directions just above plant canopies have significant positive average values and large standard deviations. The rotary spectral analysis indicates that the shear flow over paddy fields is predominantly a clockwise wind oscillation in the high frequency range.

Short range dispersion experiments in an urban area

We present results from a series of nine tracer experiments studying urban dispersion at distances under 600 m from a ground level point source in Worcester, Massachusetts. Gaussian forms provide a good fit to the lateral concentration distributions. The measured parameters were compared with models based on stability classification and wind direction fluctuations and to other urban and rural experiments. The comparison showed general consistencies with urban models and data, with some evidence for short range enhancement of the plume. The growth of the cross wind dispersion coefficient ( σ y ) and of the standard deviation of wind direction fluctuation ( σ θ ) with sampling time were investigated and showed considerably slower growth than is commonly assumed.

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.

A comparison of short-term dispersion estimates resulting from various atmospheric stability classification methods

Four methods of classifying atmospheric stability class are applied at four sites to make short-term (1-h) dispersion estimates from a ground-level source based on a model consistent with U.S. Nuclear Regulatory Commission practice. The classification methods include vertical temperature gradient, standard deviation of horizontal wind direction fluctuations (sigma theta), Pasquill-Turner, and modified sigma theta which accounts for meander. Results indicate that modified sigma theta yields reasonable dispersion estimates compared to those produced using methods of vertical temperature gradient and Pasquill-Turner, and can be considered as a potential economic alternative in establishing onsite monitoring programs.

Diurnal variation of horizontal wind direction fluctuations in complex terrain at Geysers, Cal.

Horizontal diffusion in the surface layer is dependent on the standard deviation of wind direction fluctuations σθ. Diurnal variation of this parameter in complex terrain was studied for the July 1979 Geysers, Cal., experiment using data from a network of 11 short meteorological towers in the 25 km2 Anderson Creek watershed Valley side slopes are roughly 20 ° and maximum terrain difference is about 1 km.

A Statistical Model for Titanium Pollution Transport and Dispersion in the Atmosphere of St. Louis

Abstract Multiple-regression analysis techniques are applied to gain insights into the influence meteorological variables have in the transport and dispersion of stack particulate titanium pollution on the urban scale. Meteorological variables utilized in the analysis procedure are wind direction, categorized into sectors, wind speed, low-level vertical temperature gradient, and the standard deviation of the horizontal wind direction fluctuations. Physical interpretations of the associations found in the St. Louis urban setting between titanium concentrations and the derived meteorological variables corroborate present theoretical understanding of stack plume dispersion characteristics.