Showing papers on "Wind shear published in 1970"

Case Studies of a Convective Plume and a Dust Devil

Abstract: The paper describes the characteristics of a convective plume and a dust devil from measurements made at 5.66 and 22.6 m above a flat uniform site in Kansas. The velocity fluctuations were measured with a continuous-wave, three-component sonic anemometer and the temperature fluctuations with a fine platinum wire thermometer. The data show that the plume is basically a non-rotating system; it is more tilted in the downwind direction than the dust devil, travels at a lower velocity than the mean wind speed at 0.5 m, and requires vertical stretching for its maintenance in the presence of wind shear. The dust devil shows a down-draft in the middle, travels at a higher velocity than the mean wind at 32 m, and derives much of its stability from rotation. Both systems tend to transport momentum upward, against the velocity gradient, which probably accounts for the very low and sometimes negative stresses observed during unstable conditions.

A three‐dimensional numerical investigation of the idealized planetary boundary layer

Abstract: The nonlinear equations of motion are integrated numerically in time for a region of x‐y‐z space of volume 3h × h × h, where h turns out to be a height slightly above the level where the wind first attains the geostrophic flow direction. Only the ideal case is treated of a horizontal lower boundary, neutral stability, horizontal homogeneity of all dependent mean variables except the mean pressure, and statistically steady state. The resulting flow patterns are turbulent and the eddies transport required amounts of momentum vertically. Topics which are investigated include the relative directions of stress, wind shear and wind; differences in Ekman wind spirals for the neutral numerical case and a stable atmospheric case; profiles of dimensionless turbulence statistics; effect of allowing the mean density to be either constant or to decrease with height; effect of the wind direction or latitude upon the turbulence intensities; and characteristic structure of the eddies in the planetary boundary layer.

A model of airflow above changes in surface heat flux, temperature and roughness for neutral and unstable conditions

Abstract: A numerical model of airflow in the lowest 50–100 m of the atmosphere above changes in surface roughness and temperature or heat flux has been developed based on boundary layer approximations, the Businger-Dyer hypotheses for the non-dimensional wind shear and heat flux and a mixing length hypothesis.

Wind Speeds as Measured by Cup and Sonic Anemometers and Influenced by Tower Structure

Abstract: Wind tunnel and field experiments have shown that the fast-response three-component sonic anemometer is a highly accurate wind speed sensor. When sonic anemometers were used as reference sensors for wind speed, slower response cup anemometers were found to consistently overestimate the wind speed. Despite measures taken during a field program in Kansas to minimize tower influence on wind measurements, the errors due to the tower effect on the windward side are inferred to be about ±5% of the observed wind speed ratios of cup to sonic anemometers. When the observed speed ratios are compared with the errors due to tower influence, the overspeeding of the cup anemometer is estimated to be about 10% of the reference wind speed.

The Vertical Mean Wind Profile Over the Ocean for Light to Moderate Winds

Abstract: Analysis of 299 wind profile observations collected at the Massachusetts Institute of Technology oceano-graphic research platform provides a detailed picture of the behavior of wind profile parameters. A plot of roughness length vs velocity suggests the existence of classical hydrodynamic phenomena such as Jeffreys’ minimum wind speed and the Kelvin-Helmholtz instability. The friction velocity appears to be, in general, a linear function of the wind velocity, except in regions where discontinuities exist. It is suggested that the air flow regime over the ocean in the wind range of 2-10 m sec−1 (measured at a height of 10 m) is best described as a region of constant drag coefficient (CD = 1.6 × 10−3) modified by the superposition of almost singular departures at a number of discrete wind speeds, apparently caused by the onset of distinct instabilities at the water's surface.

A Correlation of Field Observations of Plume Rise

Abstract: Data from 137 sets of plume observations, comprising nearly 1 500 data points, are correlated with two simple formulae. These formulae, one for the buoyancy-dominated rise region and the other for the stratification-dominated levelled-off region of a plume, represent an approximate form of the entrainment theory of Hoult, et al. (1968)1 for the case of uniform atmospheric stratification and zero wind shear. The observations, which are those of the Tennessee Valley Authority and of Bringfelt (1968),6 were made of plumes whose source strengths ranged from 0.4 to 111 Mw and which were emitted from stacks of heights between 21 and 183 m. The two formulae are found to correlate the data equally well over all values of the stack exit and meteorological parameters, provided only that the bulk mean velocity of the stack gases exceeds the mean wind speed by at least 20%. The ratio of observed to calculated plume rise is found to be distributed log normally about the mean value. The median rise at large distances d...

Estimation of Maximum Wind Speeds in Tornadoes

Abstract: A method is proposed for rapidly estimating the maximum value of the azimuthal velocity component (maximum swirling speed) in tornadoes and waterspouts. The method requires knowledge of the cloud-deck height and a photograph of the funnel cloud—data usually available. Calculations based on this data confirm that the lower maximum wind speeds suggested by recent workers (roughly one-quarter of the sonic speed for sea-level air) are more plausible for tornadoes than the sonic speed sometimes cited a decade ago. Comparison in a few cases with maximum wind speeds determined either by spray-tracking on motion pictures of a waterspout or by another method reported by the authors elsewhere (using the tephigram for air at the outer edge of the whirl) yields compatible results. DOI: 10.1111/j.2153-3490.1970.tb00513.x

A case study of warm air advection over a melting snow surface

Abstract: When relatively warm, moist air moves over a snow surface, sensible heat and moisture are extracted from its lower layers and used to melt the snow. The depth of the cooled layer depends on horizontal wind speeds and the presence of high vertical wind shear. The mechanism for air mass modification appears to be turbulent mixing.

Breaking of wind waves and the sea surface wind stress

Abstract: In the conventional treatment of the coefficient of sea surface wind stress by plotting it against 10-m wind speed, there are inevitable discrepancies among results of various investigators. The reason is considered to lie primarily in the fact that the state of the sea surface or of waves is disregarded, which may have great influence on the sea surface wind stress. Former concepts concerning the conditions which control the sea surface wind stress are discussed, and it is shown that a more universal expression may be obtained by plotting the coefficient against a kind of roughness Reynolds number:Re 2 *=u*H/ν, whereu* is the friction velocity of air, ν the kinematic viscosity of air, andH the characteristic wave height.H is used here to treat some data in wind-wave tunnels, as a tentative variable, one step towards a more rigorous approach to the problem. This variableRe 2 *, orRe 4 *=u *w /L/v w =2πgu *w /v w n 1, where the subscript ω represents values for water,L andn 1 the characteristic wave length and frequency, respectively, is also the condition describing the air entrainment or the breaking of wind waves. In this case, these Reynolds numbers are interpreted as the quantity describing the intensity of turbulence of the water surface itself. It is shown, using data from our wind-wave tunnel experiments, that the breaking commences asRe 2 * reaches 1×103, or asRe 4 * reaches 3×103. Simultaneously, the stress-coefficient begins to increase sharply at this value ofRe 2 *. This phenomenon is understood as an increased momentum transfer from the air to the water through “boundary penetration of turbulence” caused by the breaking of wind waves. Further, it is suggested that there is a possibility that this excess momentum transfer does not increase wave momentum, but reinforces drift current.

The influence of mean wind shear on the propagation of Kelvin waves

Abstract: The long Kelvin waves observed in the equatorial lower stratosphere transport sufficient westerly momentum upwards to account for the westerly acceleration of the quasi-biennial oscillation, provided that this momentum is absorbed in the westerly shear zone of the mean wind. The behavior of linearized Kelvin waves as they propagate through such a shear zone is deduced by scaling the wave equations with the ratio of the latitudinal scale to the zonal scale (the aspect ratio) which is a small parameter for the observed waves. The resulting system is reduced to a single elliptic equation which is solved numerically for various configurations of the mean zonal wind. It is found that passage through a westerly shear zone with no critical level modifies the waves as follows: (1) Both the latitudinal and vertical scales of the waves decrease. (2) The upward momentum transport becomes concentrated towards the equator. (3) The latitudinally averaged vertical momentum transport remains nearly constant. If the zonal wind profile contains a critical level the waves are nearly totally absorbed at that level. Furthermore, the wave amplitude has a pronounced maximum at the equator just below the critical level. For cases in which the zonal wind has lateral as well as vertical shear it was found that the Kelvin waves are effectively absorbed at the height of the critical level at the equator even when the critical level height rises rapidly away from the equator. DOI: 10.1111/j.2153-3490.1970.tb01520.x

Stratified Flow over Extended Obstacles and Its Application to Topographical Effect on Vertical Wind Shear

Abstract: An investigation is made of the two-dimensional flow of a stratified fluid over an extended obstacle, characterized by a source disturbance, for an infinite medium and for a layer of finite depth. The problems are posed as initial-boundary value problems, and steady-state solutions for the flow field are obtained in the limit of large time. The results show an unattenuated system of jets or shear layers extending far upstream from the obstacle, which occurs whenever a system of lee waves is present. For the finite depth case detailed calculations are made, starting with realistic mean values for the atmosphere. The final established profile indicates a variable shear and density profile far upstream, characterized by variable local Richardson numbers. The wind shears induced are also of the order of shears due to thermal wind effects, though quite apart from these effects. The study therefore points to the importance of topographical features on vertical wind shear in the troposphere.

On the Variation of Maximum Wind Gusts with Height

Abstract: The effects of time-averaging and length of record on the variance of horizontal wind speed are considered through power spectral techniques. These effects are applied to a height profile of intensity of turbulence (ratio of the standard deviation of horizontal wind speed to mean wind speed) derived from data published by Deacon. For stationary strong-wind regimes dominated by “mechanical turbulence,” maximum gusts are determined as a function of height and averaging time.

Barotropic Stability and Tropical Disturbances

Abstract: This paper attempts to determine under what conditions horizontal shear in the mean zonal flow can provide the initial source of energy for the traveling disturbances of low latitudes. A three-zone barotropic model is constructed in order to examine the stability of an idealized mean zonal current. The width and total wind shear associated with this mean current are varied. The form of growing disturbances and their amplification rates are found. A stability analysis is also carried out for a basic flow which has a hyperbolic tangent variation with latitude. Results obtained by numerical integration for this basic flow are similar to those found previously with the three-zone model. In discussing his easterly wave model, Yanai indicates a basic flow which has a total wind shear of about 8 m sec–1 occurring over approximately 6° of latitude. Results obtained for a basic flow with these characteristics show that the fastest growing wave has a wavelength near 2500 km and an e-folding time of about 7...

Ducting of acoustic-gravity waves in the atmosphere with spatially periodic wind shears

Abstract: A mechanism for horizontal ducting of acoustic gravity waves in the atmosphere is studied. We assume a background horizontal wind with a periodic vertical shear of the form υo sin K1z. This wind may interact with the acoustic-gravity wave to cause horizontal ducting of the wave. The ducting condition is derived. Trapping depth for a typical atmosphere is computed.

Observations of Reynolds Stresses in wind waves

Abstract: Data are presented concerning Reynolds Stresses in wind waves obtained from time series records of horizontal and vertical velocity components of motion beneath the ocean free surface. The stresses, of the order of 25 dyne cm−2, are generally positive indicating horizontal momentum transfer downward through the dynamic wind wave regime. The magnitude of the observed stress increases with wind speed and sea state. The co-spectra show strong negative peaks which appear at the ambient wave frequencies and indicate that the correlations or eddy stresses of the gross wave motions are responsible for the momentum flux. This is a corroboration of results reported previously by the writer in this journal.

Diurnal Variation of Onshore Wind Speed Near a Coastline

Abstract: Surface wind data taken from 19 km offshore to 14 km inland during several days of onshore wind occurrence were reduced and analysed. The mean kinetic enemy per unit mass and its changes normal to the coastline were computed directly from the wind data. Analysis of these data shows two well defined regimes of diurnal variation in wind speed. The marine air has a nighttime speed maximum and a daytime minimum. As the air moves inland, the speed distribution becomes bimodal with the primary maximum occurring in the daytime and the secondary maximum at night. As expected Intuitively and predicted by theory, the speed changes most abruptly near the change in surface roughness (the coastline). Also as predicted by theory, complete dynamic equilibrium with the new lower boundary is not achieved until the air is ∼5–12 km downwind from the coastline.

Comparison of tower influence on wind velocity for NASA's 150-meter meteorological tower and a wind tunnel model of the tower

Abstract: Tower influence of wind speed and direction measurements, comparing 150 meter meteorological tower and scale wind tunnel model

Applicability Of A Photochemical Box ModelOver Complex Coastal Areas

Abstract: The aim of this paper is to show that a Photochemical Box Model (P.B.M) is able to describe the air pollution diurnal profiles within a typical complex coastal area. Due to the simplicity of the box model physical structure, P.B.M. is well suited for rapid assessment. In principle, it is best designed to treat low wind conditions, in presence of sunlight. It can manage point, line and area sources. The emission sources are homogeneously distributed across the surface of the volume and the volume is mixed the whole time. In this first study, atmospheric diffusion and wind shear have been neglected.

On the Mean Path of a Buoyant Chimney Plume in Non-Uniform Wind

Abstract: Based on the turbulent entrainment hypothesis of Morton et al. and the eddy diffusivity hypothesis, approximate theories have been developed for the rise of a buoyant chimney plume in the atmospheric boundary layer, incorporating wind shear effects by assuming a power law type of wind velocity profile, U∝Zp, for the boundary layer. The two theoretical approaches suggest that the mean path of a hot plume in a neutral atmospheric boundary layer can be represented by a power law Z*∝X*n,where the exponent n is related to the wind velocity profile exponent p by n=⅔(1+p), and is less than the value of ⅔ reported in the case of uniform wind. Wind tunnel test results show fair agreement with the theoretical results.

Analysis of Jimsphere wind profiles viewed in the flight time domain of a Saturn vehicle

Abstract: Jimsphere wind profiles gust variance and spectrum densities for horizontal wind speeds viewed by Saturn vehicle

Evaluation of roughness lengths at the NSSL- WKY meteorological tower

Abstract: Wind and temperature sensors installed on NSSL-WKY /Oklahoma City/ meteorological tower to evaluate roughness lengths

Vertical Wind Component Estimates up to 1.2 km above Ground

Abstract: Vertical wind components were computed up to 1.2 km from 37 wintertime and 10 summertime balloon observations between 0900 and 1200 local time utilizing the accurate and high-resolution cinetheodolite/Jimsphere system. The mean ascent rate of the Jimsphere was computed from all observations taken on a particular day. The ascent rate was found to be 5.16 m sec−1 for the winter and 5.10 m sec−1 for the summer months. The individual variations of a given observation from the mean ascent rate were assumed to be the vertical component. Variations in balloon ascent caused by variation in drag, anomalous variation in atmospheric density, balloon response to the wind, and aerodynamically induced motions are discussed. Vertical wind components ranged from 10–25 cm sec−1 in a stable atmosphere and 55–100 cm sec−1 under unstable conditions depending on wind speed.

CAT at a subsidence inversion - A case study

Abstract: CAT at subsidence inversion in presence of strong wind shear, noting radar echo pattern as Kelvin-Helmholtz instability indicator

The effect of wind shear on a puff

Abstract: This paper corrects Hall's work in the light of more complete experiments which have established that a thermal is merely a special case of a buoyant puff. The general theory, presented here, applies to any isolated buoyant or non-buoyant puff in shearing surroundings of uniform potential density. This theory is then applied to the case of constant mass deficit and constant wind shear, and solutions are obtained in the form of algebraic equations and simple definite integrals. When these solutions are evaluated numerically, using experimentally determined constants, it is found that the ratio of the coefficients of vertical transport of momentum and heat attributable to the puff lies in the range 0.27 σ to 0.7 σ approximately, where σ is the (small) proportion of the area of the horizontal plane through any height which is occupied by puffs. Also, realistic variation of the angle of spread of puffs is found to produce a very considerable effect on the depth of the convective layer. Only narrow puffs rise far.

Fluctuations of horizontal wind speed near the ground in convective conditions

Abstract: Simultaneous records of temperature fluctuations and wind speed at several heights have been obtained with instruments suspended from captive balloons flying 225 m above level, uniform land. Variations in the horizontal wind speed near the ground appear to be associated with bursts of temperature fluctuation at heights of more than about 100 m in a way which suggests that convective plumes, or ‘thermals,’ become organized within the lowest few metres of the atmosphere.

Factors affecting uredospore liberation in Puccinia antirrhini

Abstract: Air sampling within an annular field plot of rusted snapdragons, and in a wind tunnel, has shown that uredospores of Puccinia antirrhini are readily liberated by wind shear at speeds = 2 m/sec. At constant temperature and wind speed, most uredospores are liberated in the light, mainly in the morning. Liberation in dry air is related logarithmically to wind speed; it is increased by the onset of rainfall or overhead sprinkling, and by bombarding sori with water droplets in the wind tunnel.

Factors affecting uredospore liberation in Puccinia antirrhini.

Abstract: Air sampling within an annular field plot of rusted snapdragons, and in a wind tunnel, has shown that uredospores of Puccinia antirrhini are readily liberated by wind shear at speeds = 2 m/sec. At constant temperature and wind speed, most uredospores are liberated in the light, mainly in the morning. Liberation in dry air is related logarithmically to wind speed; it is increased by the onset of rainfall or overhead sprinkling, and by bombarding sori with water droplets in the wind tunnel.

Study Of Atmospheric Dispersion Under Low Wind Speed Conditions

Abstract: This paper presents the analysis of atmospheric SF6 tracing experiments carried out under low wind speed conditions associated with strong thermal stability on a flat terrain. Analysis of meteorological data, processed in the form of probability densities, reveals that unsteady character of the current increase the greater the stability. The spectral densities of the three wind components deduced from sonic anemometers show that the horizontal current is dominated by large-scale turbulent structures, and that there is a weakening of energy levels in the neighborhood of 0, 01 Hz. Tracings confirm that the levels of ATC (Atmospheric Transfer Coefficient) are greater the stronger the stability. Release impact is as closely linked to the increase in pollutant residence time as to higher instantaneous concentration levels. Lastly, ATCs deduced from a gaussian model being developed at IPSN gives good agreements with the experimental values.