Showing papers on "Wind profile power law published in 1990"

Wave Dependence of Sea-Surface Wind Stress

Abstract: Distribution of the wind stress over the oceans is usually estimated by using a bulk formula. It contains the squared 10-m wind speed multiplied by the drag coefficient, which has been assumed in many cases to be a weak function of the 10-m wind speed. Over land the important role of thermal stratification has been clearly recognized, but over the sea the influence of wind waves is less well documented. This paper presents evidence showing the likelihood that the influence of the wind waves can also be large. Charnock proposed an expression for the marine atmospheric boundary layer roughness parameter, z0, which depended only on the wind friction velocity, u☆ and the acceleration of gravity, g. Toba and Koga have recently proposed an alternative expression for flow over growing wind waves, which are in local equilibrium with the wind, given by a form including the wind-wave spectral peak frequency explicity. The criterion for local equilibrium of the wave field with the wind is its consistency wi...

Some measurements of turbulence over an antarctic ice shelf

Abstract: Profiles of wind speed, temperature and turbulent fluxes of heat and momentum in the lowest 32 m have been measured at a station on an antarctic ice shelf. During the antarctic winter the surface layer often shows strong static stability, with temperature gradients as large as 1 Km−1 in the lowest few metres. the surface inversion is destroyed during periods of high wind speed but the wind profile shows significant deviation from the expected logarithmic form under such conditions. Measurements of stress at 5 m indicate that the roughness length of the snow surface is about lO−4 m. At 5 m height, the variations of the dimensionless wind shear, ϕm, and potential temperature gradient, ϕT, agree with previously determined forms of the Monin-Obukhov similarity functions. Above 5 m, the behaviour of ϕm and ϕT is only qualitatively similar and surface-layer similarity theory does not provide a good description of the profiles. Turbulence length scales have been deduced from vertical velocity power spectra. Under near-neutral conditions, the ratio of turbulence length scale to measurement height is observed to decrease with increasing height of measurement. This observation is consistent with the variation of turbulence length scale with height implied by the measurements of ϕm. It is suggested that the stability of the overlying atmosphere restricts the depth of the turbulent boundary layer and hence the length scales of turbulence within this layer. Increasing stability causes a decrease of turbulence length scales at all levels. The ratios of turbulence kinetic energy to stress and temperature variance to heat flux are examined. Measurements are somewhat scattered, but the distribution of values varies little with height or stability. the form of the distribution suggests that large-scale motions, possibly internal gravity waves, may be playing an important role in boundary-layer processes.

Wind erosion within a simple field

Abstract: Elementary analysis of the wind erosion process within a flat and uniform field is presented. A relationship is derived that describes the variation of the horizontal component of mass flux, fx, downwind of a distinct field boundary. The derived functional relationship is then verified and completed through the use of field data. The generalized form of the equation contains two parameters, fj^j^ and b, that describe different aspects of the wind erosion process. The quantity f^^^ is the maximum horizontal flux for a given height. The length scale b represents the distance at which f,^ attains a value of 63.2% of f^x-

Effects of wind shear and turbulence on wind turbine power curves

Abstract: It is a common practice to use wind speeds at hub height in determining wind turbine power curves. Although the possible influence of other variables (sub as turbulence and wind shear) is generally neglected in power curve measurements, we discovered the importance of other variables in an analysis of power curves for three 2.5 MW wind turbines. When the power curves were stratified by turbulence intensity. Such a large sensitivity to turbulence was not expected, and further analyses were conducted to determine if other factors accompanying the change in turbulence level could cause or contribute to the observed sensitivity of the power curves to turbulence. In summary, the sensitivity of the observed power curves was largely due to two factors: (1) an actual sensitivity to turbulence in determining the power curve and (2) the deviation of the disk-averaged velocity from the hub-height velocity under low turbulence conditions that were most prevalent at the site. An examination of the wind shear profiles over the height of the rotor disk revealed that low turbulence conditions were characterized by strong shear in the lower half of the rotor disk and weak or negative shear in the upper half. Implications of this analysis aremore » that significant errors in power curve measurements can result if the effects of wind shear and turbulence are ignored. 7 refs., 6 figs.« less

Stochastic wind simulation for erosion modeling

Abstract: The purpose of this study was to develop a wind simulator to furnish wind direction and sub-hourly wind speed to users of wind speed information, particularly for wind erosion modeling. We analyzed the Wind Energy Resource Information System data to determine scale and shape parameters of the WeibuU distribution for each of the 16 cardinal directions for each month at 704 locations in the United States. We also summarized wind direction distributions, ratio of daily maximum to daily minimum wind speed, and hour of maximum wind speed by month for each location. This summary of historical wind statistics constitutes a compact data base for wind simulation. Equations were formulated and procedures developed and used with the compact data base and a random number generator to simulate wind direction and sub-hourly wind speed. Cumulative wind speed distributions, calculated from the WeibuU parameters, and wind speeds simulated at one-hour intervals for 1000 days agreed well. The model reflects historical day-to-day wind variation and wind speed variations within a day. It will be useful to those needing wind speed and wind direction information and will provide the wind simulator requirements in a wind erosion prediction system.

Aerodynamics of a portable wind erosion tunnel for measuring soil erodibility by wind

Abstract: Portable wind erosion tunnels must satisfy several aerodynamic criteria to ensure that the flow within them acceptably reproduces the atmospheric flow causing natural wind erosion. We define these criteria and use them to assess the flow and turbulence in two alternative designs of portable wind erosion tunnel: the first has a working section with an approximately triangular, 'tent-shaped' cross section, while the second has a conventional, rectangular working section. The measurements were made with Pitot-static tubes and X-configuration hot-wire anemometers, over stable (non-eroding) rough surfaces, mainly mowed grass of height 1 cm. We found that, with careful attention to flow conditioning elements such as honeycombs and tripping fences, an acceptable flow can be achieved in the rectangular tunnel. The flow in the tent-shaped tunnel is less satisfactory, exhibiting departures from the logarithmic wind profile law which depend on the surface roughness.

On the Wave Dependence of Sea-Surface Wind Stress --A Study by Using Data from an Ocean Data Buoy Station*--

Abstract: Analysis is made of wind and wave data, which were obtained during the passage of Typhoon 8013 at an Ocean Data Buoy Station south of Honshu operated by the Japan Meteorological Agency, in order to investigate the wave dependence of sea-surface roughness parameter in the situation where wind waves are dominant with less significant swells. The data fit better the wave-dependent expression of the wind stress,z 0 σ p/u*=γ, than to Charnock's formula,gz 0/u*2=β, wherez 0 is the roughness length,σ p the angular frequency of the spectral peak of wind waves,u* the friction velocity of air,g the acceleration of gravity,γ andβ are non-dimensional constants. The results are very similar to those of our previous study using data from an oil producing platform in the Bass Strait, Australia, although the type of observation system and the synoptic situation of the winds and wind waves were totally different.

Are Radiosonde Time Scales Appropriate to Characterize Boundary Layer Wind Profiles

Abstract: One approach under investigation for obtaining regional-scale surface fluxes of water vapor, heat, and momentum from complex terrain involves the applicability of flux-profile relationships in the atmospheric boundary layer (ABL). Mean humidity, temperature, and wind speed profiles in the ABL can be measured by means of radiosondes. A disadvantage, however, of this method is the relatively quick passage of the sonde through the ABL compared to the characteristic time scales or memory of the turbulence. Remote sensing with sodar (Sound Detection and Ranging) allows the measurement of mean wind profiles over time scales which are perhaps more appropriate for the turbulence. Wind profiles measured by both radiosondes and sodar over the Landes Forest in southwestern France, as part of the HAPEX-MOBILHY experiment, are compared under conditions of neutral stability. The sodar wind profiles were measured by a team from the CRPE (Centre de Recherches en Physique de l'Environnement, Saint-Maur-des-Fosses...

Airflow over a two‐dimensional escarpment. II: Hydrostatic flow

Abstract: Hydrostatically dominated forced flow over the asymmetric topography of the Darling Scarp, Western Australia, is modelled using a hydrostatic numerical mesoscale model and compared with observations. Simulations reproduce the essential features of wave overturning and the development of a shooting hydraulic flow. This response is dependent on the wind profile but not on the existence of a critical level. the high backsheared environment leads to a high ratio of the Brunt-Vaisala frequency to wind speed throughout the profile, resulting in short wavelengths, high nonlinearity and overturning. With the ratio of the halfwidth of the topography to the boundary-layer depth being small, the flow fields are shown to be sensitive to the boundary layer.

The influence of wind and temperature gradients on sound propagation, calculated with the two‐way wave equation

Abstract: A method is introduced to calculate the influence of wind and temperature gradients in stratified media on sound propagating above an absorbing ground surface. It is based on the ‘‘two‐way wave equation’’ for the Fourier transforms of the sound pressure P and its derivative V. The vector containing P and V is stepwise extrapolated through the medium in the direction perpendicular to the ground surface, fulfilling the boundary conditions at the ground surface, at a top level, and at the source height. The propagation equations for P and V appear as simple plane‐wave equations, and computer (CPU) time within each layer is very low. Therefore, many thin layers (in the order of centimeters if desired) can be applied, and any complicated gradient can be used. Calculations for a homogeneous atmosphere, with a computer program based on this model, show an excellent agreement with previous models. When a wind profile is present, results are mainly compared with measurements by Parkin and Scholes [P. H. Parkin and W. E. Scholes, J. Sound Vib. 1, 1–13 (1964); 2, 353–374 (1964)]. They show very good agreement in the no‐wind and downwind cases. In the upwind situation, agreement is very good below 500 Hz. Above this value the model does not predict sound to penetrate into the shadow region, while Parkin and Scholes found (low) sound levels.

The influence of atmospheric stratification on scatterometer winds

Abstract: Scatterometers measure surface roughness which is empirically related either to surface stress or to the equivalent neutral stability wind. The importance of atmospheric stability effects for the analysis of these data is studied. For low wind speeds and neutral to slightly stable conditions, neutral stability wind is quite sensitive to stability. A variational analysis procedure for the scatterometer data, which adjusts both the near-surface velocity and temperature, is developed. In simulation tests, temperature analysis increments are found to be small. Also, the differences in the wind analyses due to differences in the temperature background field are small. However, if stability effects are not accounted for, there will be small systematic errors in the wind analysis.

Wind models for the NSTS ascent trajectory biasing for wind load alleviation

Abstract: New concepts are presented for aerospace vehicle ascent wind profile biasing. The purpose for wind biasing the ascent trajectory is to provide ascent wind loads relief and thus decrease the probability for launch delays due to wind loads exceeding critical limits. Wind biasing trajectories to the the profile of monthly mean winds have been widely used for this purpose. The wind profile models presented give additional alternatives for wind biased trajectories. They are derived from the properties of the bivariate normal probability function using the available wind statistical parameters for the launch site. The analytical expressions are presented to permit generalizations. Specific examples are given to illustrate the procedures. The wind profile models can be used to establish the ascent trajectory steering commands to guide the vehicle through the first stage. For the National Space Transportation System (NSTS) program these steering commands are called I-loads.

Measurement of a zonal wind profile on Titan by Doppler tracking of the Cassini entry probe

Abstract: A program, called the Cassini mission, intended to study the Saturn system by utilizing a Saturn orbiter and a probe descending to the surface of Titan, is discussed. Winds are expected to cause perturbations to the probe local horizontal velocity, resulting in an anomalous drift in the probe location and a shift in the frequency of the probe telemetry, due to the Doppler effect. By using an iterative algorithm, in which the time variation of the probe telemetry frequency is monitored throughout the descent, and the probe trajectory is updated to reflect the effect of wind on the probe location, a highly accurate relative wind profile can be recovered. By adding a single wind velocity, measured by independent means, an absolute wind profile can be obtained. However, the accuracy of the zonal winds recovery is limited by errors in trajectory, and frequency.

Stochastic prediction techniques for wind shear hazard assessment

Abstract: The threat of low-altitude wind shear has prompted development of aircraft-based sensors that measure winds directly on the aircraft's intended flight path. Measurements from these devices are subject to turbulence inputs and measurement error, as well as to the underlying wind profile. Stochastic estimators are developed to process on-board Doppler sensor measurements, producing optimal estimates of the winds along the path. A stochastic prediction technique is described to predict the hazard to the aircraft from the estimates as well as the level of uncertainty of the hazard prediction. The stochastic prediction technique is demonstrated in a simulated microburst wind shear environment. Use of the technique in a decision-making process is discussed. >

An analytical study of the frictional response of coastal currents and upwelling to wind stress

Abstract: In this paper we present several theoretical results concerning currents forced by the wind in coastal regions, for a shallow sea and for a very deep sea. We investigate the time behavior and the spatial structure of the stream function and the momentum components, i.e., the onset, the transient, the asymptotic, and the periodic behavior of the currents and of the upwelling, forced by winds of different spatial and time structures. Results show that the intensity of the along coast jet initially grows linearly under a δ in time wind impulse, quadratically under a Heaviside in time wind impulse, and cubically under a linearly growing wind impulse. The asymptotic state is such that the intensity of the current vanishes if the wind impulse has a finite duration, while the intensity of the sea current has a final finite amplitude if the wind intensity goes to some finite value. If the wind stress is periodic in time, there is upwelling only when the period of the forcing is longer than a characteristic time scale, which is the sum of the inertial period and the friction e-folding time. Otherwise there are waves which propagate away from the region where the wind stress is acting. The spatial structure is such that the upwelling occurs in a horizontal region of the order of the Rossby deformation radius, corrected by the effect of friction (and by the effect of periodicity, when the wind stress is periodic in time). However, the horizontal gradient of the wind stress can be more important than the Rossby deformation radius in determining the horizontal extent of the upwelling region.

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

Abstract: 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.

The correction of infrasound signals for upper atmospheric winds

Abstract: Infrasound waves propagate in the atmosphere by a well known mechanism produced by refraction of the waves, return to earth, and reflection at the surface into the atmosphere for subsequent bounces. A figure illustrates this phenomenon with results from a ray trace model. In this instance three rays are returned to earth from a region centered at about 50 kilometers in altitude and two from a region near 110 kilometers in altitude. The control of the wave refraction is largely dominated by the temperature-height profile and inversions; however, a major influence is also produced by the atmospheric wind profile. Another figure illustrates the considerable ray differences for rays moving in the wind direction (to the right) and in the counter direction (to the left). It obviously can be expected that infrasonic signal amplitudes will be greatly influenced by the winds in the atmosphere. The seasonal variation of the high altitude atmospheric winds is well documented. A third figure illustrates this with average statistics on the observed zonal wind in the region of 50 plus or minus 5 kilometers in altitude. The results are based upon a survey by Webb; Webb terms this parameterization the Stratospheric Circulation Index (SCI). The very strong seasonal variation has the ability to exert a major seasonal influence on infrasonic signals. The purpose here is to obtain a method for the correction of this effect.