Showing papers on "Wind profile power law published in 1988"

Coefficients for sea surface wind stress, heat flux, and wind profiles as a function of wind speed and temperature

Abstract: Surface layer coefficients for wind profiles, wind stress, and heat flux in typical open sea conditions are briefly reviewed. Businger-Dyer flux-gradient relationships and a Charnock wind stress formula fit the empirical data and are dimensionally consistent. These have been solved by an iterative method, and the results are presented in a tabular form suitable for climatological calculations from marine wind and temperature data.

Typhoon Structure as Revealed by Aircraft Reconnaissance. Part II: Structural Variability

Abstract: This is the second of two papers on the structure of northwest Pacific tropical cyclones as revealed by U.S. Air Force aircraft reconnaissance. This paper describes the varying structure of the tropical cyclone's outer-radius wind profile in relation to its inner-core intensity or minimum sea level pressure (MSLP) and eye-size characteristics. We explore this inner- to outer-radius structural relationship and its variability for the full range of cyclone central pressures, outer-core (1°–2.5° radius) wind strengths, and for radial extent of 15 m s−1 (30 kt) and 25 m s−1 (50 kt) surface winds. Results show that outer-radius wind strength and inner-core intensity can vary greatly and that there is only a weak relationship between these parameters. However, if information is available on whether an eye-wall cloud exists and what the size of the eye is, then a significant reduction in the wide variance between MSLP and outer wind radius is observed.

Numerical study of orographic-convective precipitation over the eastern Arabian Sea and the Ghat Mountains during the summer monsoon

Abstract: When the western coast of India lies in the path of the low-level west-southwest wind crossing the Arabian Sea during the summer monsoon season, deep convection frequently develops over the ocean off the coast. In such a situation, the maximum rainfall occurs near the coast, not over the Western Ghats. In order to study the physics underlying orographic-convective precipitation over this area, a two-dimensional compressible moist cloud model is applied. The model is written in terrain-following coordinates and includes the Coriolis force and a planetary boundary layer parameterization. The initial fields of thermodynamic variables are specified using observed data gathered upstream of the offshore precipitating systems over the Arabian Sea. Two wind profiles are considered: vertically uniform and nonuniform flows. The latter profile represents a monsoonal westerly jet at low levels and easterlies in the layer above 5 km. Three cases are considered for each wind profile by including or omitting mo...

Measurements of the angle between the wind vector and wind stress vector in the surface layer over the North Sea

Abstract: Measurements of surface layer turbulence, in addition to full meteorological and oceanography quantities, over the North Sea indicated that the stress vector on long time scales is often aligned with a direction slightly different from the mean wind flow. When stratifications were near neutral, the angle difference between the stress and wind exhibited a dependence on the heat and momentum fluxes. In general, the stress vector was observed to be to the left of the flow during stable stratifications, while for unstable stratifications, it was to the right. This finding was consistent using two independent sets of wind stress data, i.e., from MARSEN (1979) and the North Sea Platform Winter Exercise (1985).

The Kettles Hill Project: Field observations, wind-tunnel simulations and numerical model predictions for flow over a low hill

Abstract: Field observations of the influence of topography on steady, neutrally-stratified boundary-layer flow were carried out in February 1981 and March 1984 on Kettles Hill near Pincher Creek, Alberta, Canada. The primary measurements were of wind speed at 3,6, and 10 m levels at stations in linear arrays along and across the major axis of this gentle, 1 km long and 100 m high, elliptical hill. Wind profile measurements up to heights of 200 m were made with TALA kites and tethersondes on the hilltop and at a reference site located about 3.7 km west of the hilltop. In addition, AIRsondes were flown and tracked from the reference site to provide additional data. The field observations provided the basic data for a comparison with wind-tunnel and numerical model simulations of the same flow. The wind-tunnel investigation was carried out in the Atmospheric Environment Service Boundary-Layer Wind Tunnel while the numerical model used was MS3DJH. For ‘horizontal’ profiles of normalized mean wind speed at given heights above the prototype terrain, model results agree reasonably well with the field data. The wind-tunnel predictions are slightly high in most cases. For vertical profiles of wind speed up to 200 m above the hilltop, the numerical and wind-tunnel values are higher than were observed. The sensitivity of the normalized wind speed at the hilltop to deviations from non-logarithmic upwind profiles is demonstrated with data from the March 1984 experiment. A comparison of prototype with numerical-model mean-wind-direction perturbations at the 10 m level shows reasonable agreement except near the summit of the hill.

A study of wind speed modification and internal boundary-layer heights in a coastal region

Abstract: Wind profile data within the first two kilometres of a coast have been used to study the wind field modification downstream of this surface discontinuity The land area is generally very flat, having an overall roughness length of 004 m A wind model, suitable for practical applications and inexpensive to run, has been tested against the data and was found to give satisfactory results Knowing the climatological statistics of wind and stratification, eg, at the coast, the model may thus be used to estimate, on a climatological basis, how the wind field is modified with distance inland, at least in areas with only minor topography This type of information is of great importance when locating wind turbines It is in these cases also important to know the statistics of the internal boundary-layer (IBL) height, as the turbulence intensity may be quite different in and above the IBL, which in turn may influence load and fatigue calculations Using the wind profile data, the IBL height was clearly discernible in the majority of cases Having very unstable stratification over land, the IBL height could, however, not be determined from the wind profiles, as the wind in these cases did not decrease inland This result was also obtained using the wind model A simple model of the type z IBL = a · x b, was instead tested, and was shown to give reasonable results

A systematic error in MST/ST radar wind measurement induced by a finite range volume effect: 1. Observational results

Abstract: Wind measurement by MST/ST radars may be accompanied by a systematic error due to a finite range volume effect which works when a thin turbulent layer is simultaneously located in several adjacent range volumes. The error occurs when the layer coincides with a cross section through the range volume which is not symmetric with respect to the center of the beam. The finite range volume effect appears as a false vertical shear of horizontal wind in a vertical scale of the order of a few hundred meters, even if the ambient wind field is uniform. The false wind shear sometimes exceeds 40 ms−1 km−1 in magnitude or the critical value to induce the Kelvin-Helmholtz instability. Also the effect leads to a false temporal variation of the wind measurement, although the wind field does not change at all. The false wind shear with a magnitude less than 40 ms−1 km−1 cannot be discriminated from a true one in the observed data. It seems hard to indicate directly that the finite range volume effect appears as theoretically conceived. Judging from wind velocity and echo intensity data obtained by the MU radar in Japan, this effect appears quite frequently in the atmosphere. The small vertical scale wind shear as well as the temporal variation found only at a specific range should be treated with great care except when the ambient wind field is weak, where the finite range volume effect is not so important.

Aerodynamic Analysis of the Darrieus Wind Turbines Including Dynamic-Stall Effects

Abstract: Comparisons have been made between aerodynamic performance predictions using two dynamic-stall methods and experimental data for a 17-m Sandia wind turbine. These dynamic-stall models are the incidence delay methods originating from Boeing-Vertol and from the Massachusetts Institute of Technology, which are based on numerical correlations of the dynamic-stall delay with pitch rate parameter. The MIT model calculations show that aerodynamic coefficients are underestimated in comparison with those of the Gormont model, even if the shape of the curves is quite the same. The MIT model provides a good prediction of the dynamic-stall regime. This is characterized by a plateau oscillating near the experimental data of the rotor power vs wind speed at the equator

Turbulent diffusivity and diurnal variations in the atmospheric boundary layer

Abstract: Diurnal variations of the vertical profiles of wind and temperature have been surveyed, and the diffusivity and the dimensionless gradient function in the atmospheric boundary layer have been estimated. Even in the middle of the atmospheric boundary layer (e.g., below a height of 442 m), the vertical wind profile normalized by the surface friction velocity has approximately a universal profile function different from that in the surface boundary layer. Under strong stability conditions, the dimensionless gradient function has a value of about 9.

A systematic error in MST/ST radar wind measurement induced by a finite range volume effect: 2. Numerical considerations

Abstract: A finite range volume effect causes a systematic error in MST/ST radar wind measurement when a thin turbulent layer is simultaneously located in several adjacent range volumes. This error appears as a false vertical shear of horizontal wind or as a false temporal wind variation at some ranges even if the ambient wind field is uniform with height and does not change at all. Also, because of this effect the observed Doppler power spectrum becomes asymmetric, and a noticeable error is induced in estimation of echo power, mean Doppler velocity, and spectral width. The present investigation will show that these errors are well explained by a simple numerical model which is made to simulate an actual observational situation. The observed wind velocity is more reliable at ranges where the echo intensity is relatively larger compared with adjacent ranges. The finite range volume effect is negligibly small for weak ambient wind velocities less than approximately 10 ms−1 and/or for an antenna beam width less than one degree.

Three-Dimensional Response of a Shear Flow to Elevated Heating

Abstract: The three-dimensional response of a shear flow to elevated heating is investigated using linear theory. The basic wind profile is allowed to reverse directions at a certain height. Effects of shear, evaporative cooling, and the stratosphere are investigated. Morphology and spatial scales of the V-shaped features found in this study are consistent with observations and nonlinear numerical modeling results, but are explained by energy propagation associated with the thermally forced gravity wave. The response of a unidirectional shear flow to latent heating at the level of the wind reversal is an axisymmetric pattern of upward vertical motion, which is in direct response to the heating. Away from the level of the wind reversal, V-shaped patterns of upward motion are produced with vertices pointing upwind. The updraft core in the vertical plane in the direction of the storm movement is almost erect for a relatively weak shear flow. In accord with two-dimensional solutions, the critical level plays a...

Wind Spectra And Gust Factors Over Water

Abstract: Spectra of winds in hurricane conditions are needed for the analysis of compliant structures intended for deep water. Spectra of wind measurements made in Hurricanes Eloise and Frederic as well as in North Sea gales were calculated and compared to various models. All of the spectra were reduced to a single form using a nondimensionalisation based on total variance. The variance, in turn, could be accurately predicted given the mean wind speed and significant wave height. A model spectrum which behaves like isotropic turbulence in the high frequency limit and tends to a constant at low frequencies fit the averaged data well. Given the model spectrum and the assumption of Gaussian turbulence, it is possible to calculate gust factors for any averaging period and wind speed.

A case study of Kelvin-Helmholtz waves within an off-shore stable boundary layer: Observations and linear model

Abstract: A case study of Kelvin-Helmholtz waves which were observed by two aircraft in a warm off-shore stable boundary-layer flow over the North Sea is presented. During the one-hour flight mission within an area of 40 × 40 km2, the waves were intermittent both in space and time. They were centered around two levels, at 90 and 330m, where inflection points in the mean profile of the cross-wave wind component occurred together with Richardson numbers smaller than the critical value of 0.25. Observed wave amplitudes were on the order of 0.1 K for the potential temperature, 0.15ms-1 for the vertical wind component, 0.3ms-1 for the cross-wave wind component and 0.15ms-1 for the along-wave wind component. Horizontally averaged vertical wave transports were down-gradient. Based on the observed wind and temperature profiles, wave simulations with a linear model are performed. Different diffusion coefficient estimates are tested. The model produces two types of Kelvin-Helmholtz waves with maximum amplitudes at the above mentioned two heights. The modeled wavelengths are about 30% shorter than the observed ones. Adjusting the modeled to the observed temperature variations, the modeled vertical wind variance and the vertical transports agree well with the observations, whereas the modeled horizontal wind variances are smaller than the observed ones.

The streamwise Kolmogoroff constant

Abstract: Observations over grassland of the turbulent kinetic energy in a band of frequencies in the inertial subrange of the spectrum of the streamwise wind component are related to the stress indicated by the wind profiles. The object is to determine the effective Kolmogoroff constant, α UB which accords with the assumption of balance between turbulent energy production and dissipation. The mean from 60 half-hour runs made under stability conditions ranging from neutral to moderate instability is α UB = 0.62.

Local topographic influence on low-level wind at Scott Base, Antarctica

Abstract: Measurements of wind and temperature at Scott Base, Antarctica, and from a nearby elevated site are used to examine the relationship between the surface wind and the wind flow in the free air above the terrain. During stable conditions with temperature inversions, the surface wind at Scott Base is from the northeast for a wide range of flow directions above the terrain. This suggests that the Scott Base northeasterly results from deflection of the prevailing flow by local topography. During neutral or unstable temperature stratification, the surface wind may blow from other directions, and closely follows the direction of the flow above the topography. These results are consistent with simple physical arguments based on buoyancy considerations.

On the energy output estimation of wind turbines

Abstract: During the operation of the German test field for small Wind Energy Conversion Systems (WECS) on the island of Pellworm five wind turbines were tested following recommendations of the International Energy Agency (IEA) expert group. Possible errors in the estimation of a tested wind turbine's total energy output at a potential installation site are investigated. Different wind speed frequency distributions (the measured one, the Rayleigh and the two-parameter Weibull distribution) are used to calculate the total energy output. The differences between the various distributions are mostly below 10 per cent. An improvement of the energy output estimate by a Weibull-instead of a Rayleigh distribution was not found. It is also shown that the use of the recommended 10 min averages or any other average overestimates the WECS' efficiency, up to 14 per cent on average depending on turbulence intensity. Wind power instead of wind speed is the appropriate parameter for power performance testing. Spectra of wind power and electrical power output show three areas of different correlation. A resistance length for wind turbines is shown to be dependent on the WECS operation status.

The position of the lowest levels in the boundary layer of atmospheric circulation models

Abstract: The error associated with the position of the lowest wind level in atmospheric boundary‐layer modelling is studied in connection with vertical resolutions typical of parametrization schemes for atmospheric circulation models. The test case is the neutrally stratified steady‐state boundary layer. Finite‐difference and finite‐element schemes of two types are used: in one case the lowest wind level is centred with respect to the surface and the lowest internal level where the shear stress is calculated, in the other case the lowest wind level is set very close to the surface (5 m). It is found that schemes of the latter type underestimate the friction force at the lowest level and consequently overestimate the wind and the surface stress. This error is largest at low resolution since it is due to the uncentring of the lowest wind level with respect to the stress levels. The error in schemes of the former type is different, and is associated with the determination of the surface stress from a wind at...

The Extrapolation of Vertical Profiles of Wind Speed within the Marine Atmospheric Surface layer Using the p Formula

Abstract: Values of p for the exponent-type wind profile formulation, used in vertical extrapolations of wind speed, were derived for the marine atmospheric surface layer. Nomograms were constructed providing p values as dependent on a single elevation measurement of the air temperature, wind speed, and the surface water temperature. The range of p values in the unstable surface layer is between 0.02 to 0.2, while for stable situations the range is 0.1 to possibly ∼1.0. The values of p converge to about 0.2 for high wind speeds.

Simple Modeling Procedure for Estimation of Cyclonic Wind Speeds

Abstract: A new and simple modeling procedure is presented for the probabilistic definition of maximum wind speed due to tropical cyclones. Using a fluid dynamic model, the history of the cyclonic winds is first reconstructed for places of interest on the coast from information of paths and intensities. A probabilistic treatment then permits an estimation of the maximum wind speed associated with different return periods, using various probability distributions. The results from this model are similar to those obtained from more elaborate me¸thods. The model has been successfully used as part of a large computation scheme to improve coastal wind predictions in Mexico.