Showing papers on "Wind shear published in 1984"

The structure and classification of numerically simulated convective storms in directionally varying wind shears

Abstract: Using a three-dimensional numerical cloud model, we investigate the effects of directionally varying wind shear on convective storm structure and evolution over a wide range of shear magnitudes. As with a previous series of experiments using unidirectional wind shear profiles (Weisman and Klemp), the current results evince a spectrum of storm types ranging from short lived single cells at low shears, multicells at intermediate shears, to supercells at high shears. With a clockwise curved hodograph, the supercellular growth is confined to the right flank of the storm system while multicellular growth is favored on the left flank. An analysis of the dynamic structure of the various cells reveals that the quasi-steady supercell updrafts are strongly enhanced by dynamically induced pressure gradients on the right flank of the storm system. We use this feature along with other related storm characteristics (such as updraft rotation) to propose a dynamically based storm classification scheme. Following...

Microburst Wind Structure and Evaluation of Doppler Radar for Airport Wind Shear Detection

Abstract: The horizontal and vertical structure of airflow within microbursts has been determined using Doppler weather radar data from the Joint Airport Weather Studies (JAWS) Project. It is shown that the downdraft typically associated with microbursts is about 1 km wide and begins to spread horizontally at a height below 1 km. The median time from initial divergence at the surface to maximum differential wind velocity across the microburst is five minutes. The height of maximum differential velocity is about 75 m, and the median velocity differential is 22 m/s over an average distance of 3.1 km. The outflow of the air is asymmetric, averaging twice as strong along the maximum axis compared to the mininum axis. Some technical requirements for a radar system to detect microbursts and to provide aircraft with early warnings of the onset of windshear are identified.

Lidar Observations of Mixed Layer Dynamics: Tests of Parameterized Entrainment Models of Mixed Layer Growth Rate

Abstract: Ground based lidar measurements of the atmospheric mixed layer depth, the entrainment zone depth and the wind speed and wind direction were used to test various parameterized entrainment models of mixed layer growth rate. Six case studies under clear air convective conditions over flat terrain in central Illinois are presented. It is shown that surface heating alone accounts for a major portion of the rise of the mixed layer on all days. A new set of entrainment model constants was determined which optimized height predictions for the dataset. Under convective conditions, the shape of the mixed layer height prediction curves closely resembled the observed shapes. Under conditions when significant wind shear was present, the shape of the height prediction curve departed from the data suggesting deficiencies in the parameterization of shear production. Development of small cumulus clouds on top of the layer is shown to affect mixed layer depths in the afternoon growth phase.

The Influence of Bubbles on Ambient Noise in the Ocean at High Wind Speeds

Abstract: Observations of ambient noise in the ocean at high wind speeds reveal significant departures in spectral shape from previously reported values at lower wind speeds. The observations were made in open ocean conditions in Queen Charlotte Sound, British Columbia, in bands centered at frequencies of 4.3, 8.0, 14.5 and 25.0 kHz; surface wind speeds up to 25 m s−1 were recorded. Ambient noise at 4.3 kHz displayed a logarithmic relationship with wind speed throughout the observed speed range, similar to that reported previously. However, at the two higher frequencies (14.5 and 25.0 kHz), the noise spectrum levels did not increase with increasing wind speed at the same rate and for winds above about 15 m s−1 the noise levels actually decreased with increasing wind speed. Similar though less extreme behavior was observed at 8.0 kHz. We attribute this effect to the presence of bubbles which are known to be entrained at the surface of the ocean. The hypothesis is explored with a model in which the noise sou...

The spatial and seasonal distribution of Martian clouds and some meteorological implications

Abstract: All Mariner 9 and Viking Orbiter images of Martian atmospheric clouds were analyzed to obtain temporal meteorological characteristics of Mars. The data comprised over 57,000 images which were separated into seven categories of surface obscuration. The classes included: lee waves, waves, plumes, streak clouds, cloud streets, localized fogs and hazes and were associated with particular formation mechanisms, probabilities of wind shear, static stability and surface shear stress and composition. Near surface winds followed the cap edges except in summer. Hemispheric asymmetries were attributed to the hydration state and global dust storm effects. Static stability was regionally and seasonally variable. Finally, clouds formed more easily in the northern than in the southern hemisphere in all corresponding seasons.

Scale Analysis of Marine Winds in Straits and along Mountainous Coasts

Abstract: The complicated wind regimes in straits which develop in response to difFerent large-scale pressure fields are investigated by scale analysis of the equations of motion. Adjustment of the mass and motion fields in straits O(lOs km) in width is governed by four nondimensional numbers: separate along- and cross-strait Rossby numbers, a strait drag coefficient, and a stratification parameter, which relates the internal Rossby radius of deformation to the width of the strait. The wind field is in approximate geostrophic balance with an imposed cross-channel pressure gradient. An along-channel pressure gradient is primarily balanced by ageostrophic acceleration of the wind field down the axis of the strait (the gap wind). Vertical motion and the accompanying horizontal divergence in the near-surface wind field can be large even for moderately stable stratification; as a consequence, there may be particularly abrupt transitions of the surface wind field at the exits of straits, where there is a rapid c...

Thunderstorm cloud height-rainfall rate relations for use with satellite rainfall estimation techniques

Abstract: Observational studies of thunderstorm cloud height-rainfall rate and cloud height-volume rainfall rate relations are reviewed with significant variations being noted among climatological regimes. Analysis of the Florida (summer) and Oklahoma (spring) relations are made using a one-dimensional cloud model to ascertain the important factors in determining the individual cloud-rain relations and the differences between the two regimes. In general, the observed relations are well simulated by the model-based calculations. The generally lower predicted rain rates in Oklahoma (as compared to Florida) result from lower precipitation efficiencies which are due to a combination of larger entrainment (related to larger vertical wind shear) and drier environment. The generally steeper slope of the Oklahoma rain rate height curves is shown to be due to a stronger variation in maximum vertical velocity with cloud top height, which, in turn, is related to the greater static stability in the range of cloud tops. The impact of the regime-to-regime variations on empirical rain estimation schemes based on satellite-observed cloud height or cloud temperature information is discussed and a rain estimation approach based on model-generated cloud-rain relations is outlined.

Wind shear terms in the equations of aircraft motion

Abstract: Conventional analyses of aircraft motion in the atmosphere have neglected wind speed variability on the scales associated with many atmospheric phenomena such as thunderstorms, low level jets, etc These phenomena produce wind shears that have been determined as the probable cause in many recent commercial airline ac cidents This paper derives the 6 deg equations of motion for an aircraft incorporating the variable wind terms The equations are presented in several coordinate systems (i e , body coordinates, inertia! coordinates, etc ) The wind shear terms, including the temporal and spatial gradients of the wind, appear differently in the various coordinate systems; these terms are discussed. Also, the influence of wind shear on inputs to computing the aerodynamic coefficients, such as the effects of wind velocity vector rotation on relative angular rates of rotation and on the time rate of change of angles of attack and sideslip, are addressed

A Synoptic Climatology of Northwest-Flow Severe Weather Outbreaks. Part II: Meteorological Parameters and Synoptic Patterns

Abstract: A climatology of meteorological parameters and synoptic patterns associated with severe weather outbreaks occurring in arm where the mid-tropospheric flow has a north of west component is presented. This climatology utilizes data and criteria previously described by Johns. A comparison of the northwest flow parameters and those associated with general severe weather is given. The importance of conditional instability and low-level warm advection in northwest flow situations is discussed. An explanation is offered for the location of the axes of highest frequency of northwest flow outbreaks. Furthermore, the varying nature of wind shear associated with severe weather is discussed and the importance of the directional contribution of wind shear to northwest flow severe weather is demonstrated.

Measurements on the origin of the wind-dependent ambient noise variability in shallow water

Abstract: Contributions of environmental quantities related to the noise source field and the propagation path are derived from the comparison of measurements of the wind‐dependent noise by omnidirectional receivers at a fixed North Sea station with shipborne measurements in the Baltic Sea. The influence of propagation loss on the wind‐dependent shallow water noise appears to be only marginal, even at extremely different sea areas. The quantity governing the noise spectrum level under uncontaminated conditions is the wind speed at the sea surface for which the second power law relation has been verified between 50 Hz and 20 kHz and above a ‘‘threshold’’ wind speed of ≊5 kts. Neither the characteristic height of the sea waves nor the wind turbulence at the reference height are relevant to the noise production, but both may indicate wind profile changes which originate an essential portion of the noise variability for a given wind speed. Further deviations from the second power law are attributed to a bubble layer effect under storm conditions reducing or enhancing the high frequency noise level thus yielding a spread of the average spectrum level of more than 20 dB.

Monthly mean values of the mesospheric wind field over Poker Flat Alaska

Abstract: Monthly-averaged horizontal and vertical mesospheric wind fields have been measured using the Poker Flat Radar at 65°N latitude during 15 months in 1980–81. The horizontal wind fields are reasonably consistent with previous observations and with some of the current theoretical models that take into account enhanced turbulence and eddy transport observed at mesospheric heights. However, the observed vertical wind field has a quasi-sinusoidal seasonal variation with peak values of 25 cm s−1 both downward near summer solstice and upward near winter solstice and is inconsistent with current models of mean circulation in the meridional plane. Because there are no other comparable vertical wind observations, the possibility of error in the vertical wind measurements was carefully considered and rejected. We conclude that the actual mean circulation is more complex than that implied by current models. Possible complicating factors include nonuniform zonal flow, multicellular meridional structure and an ...

A Wave-CISK Model of Squall Lines

Abstract: The wave-CISK model of Raymond incorporating lag effects in the updraft and downdraft is implemented as an initial value problem in physical space. Examples of both midlatitude and tropical squall lines are successfully simulated. Diagnosis of the model shows that condensational heating in the updraft is the primary driving mechanism of a mature squall line, with evaporative cooling and convective momentum transfer playing subsidiary roles. However, an instability involving the displacement of boundary layer air by downdrafts apparently plays an important role in squall line initiation. A weakness of the model is its inability to predict the direction of squall line propagation relative to the low level wind shear. This is traced to the insensitivity of the convective parameterization to midlevel entrainment. However, unlike strict two-dimensional squall lint models, the parameterization allows cross-stream mass transfer to occur in the context of overall slab symmetry. Such transfer is dynamical...

Modeling stochastic wind loads on vertical axis wind turbines

Abstract: The Vertical AXIS Wind Turbine (VAWT) is a machine which extracts energy from the wind. Since random turbulence is always present, the effect of this turbulence on the wind turbine fatigue life must be evaluated. This problem 1s approached by numerically simulating the turbulence and calcu– latlng, In the time domain, the aerodynamic loads on the turbine blades. These loads are reduced to the form of power and cross spectral densities which can be used In standard linear structural analysls codes. The relative Importance of the turbulence on blade loads 1s determined. The most common des]gn for Vertical Axis Wind Turb]nes (VAWT’S) was first patented in 1931 by Darrleus, a Frenchman. Th]s “egg beater” shaped machine consists of one or more blades with air– foil cross sections attached to the top and bottom of a central shaft, or tower. To m]nlmize bending stresses In the blades while the turbine rotates, the blades usually have a characteristic tropos– klen, or “splnnlng rope,” shape. Torque is produced when the turb]ne rotor turns In the wind. Fig. 1 shows the 17 meter research VAWT at Sandla National Laboratories, Albuquerque, NM This turb]ne has the most common configuration of VAWT’S currently being built, two blades w]th a central tower and guy cables supporting the top of the rotor. The aerodynamic analysls of the VAWT 1s compli– cated by several factors Since the VAWT blade rotates through 360 degrees relatlve to the lncl– dent wind during each rotation, the rate of change of angle of attack 1s often much greater than experienced ]n other airfoil applications. But because the speed of the blade 1s normally much greater than the w]nd speed, the angle of attack varies between posltlve and negative values around zero degrees. The higher the w]nd speed, the greater the angle of attack excursions. When the winds become h]gh enough, the angles of attack become large enough that the alrfoll begins to stall dynamically. Research In VAWT aerodynamics 1s currently lnvestlgatlng both dynamic stall and p]tch rate effects. Aerodynrunlc analys]s using the streamtube momentum balance approach was the earnest and slm lest approach used to est]mate Y VAWT performance The vortex llftlng llne ap– preach has produced some better results, in an average sense, over a wider range of wlnd2cond]tlons, but 1s computatlonally expensive Both methods suffer from the lack of a valldated method of predicting dynemlc stall. Except for emplrlcal llft and drag curves from wind tunnel testing, accurate pltchlng airfoil, dynamic stall models do not exist. Most VAWT’S experience stall over a s]gnlflcant portion of the normal operat]ng range The loss of llft due to stall in high winds 1s considered benef]clal because It llmlts the maximum power that the drive train must trans– mlt, thereby holding down the turbine cost. The structural analysls of the VAWT must deal with the fact that the structure is rotating. Corlolls and centrifugal effects make the modes of vlbra– tlon complex, but the system remains l]near. Free v]bratlon analysls and aero lastlc analysis 5 are well developed and valldated The forced v]bratlon analysis of the VAWT rotor ]s developed, but agreement w]th field data from operating turbines 1s sometimes poor, especlall~ In the high w]nd speed, significant stall, regime It is not yet clear whether the source of the error 1s In the structural analysls or ]n the calculation of the aerodynem]c loads The relatively good agree– ment of the structural analysis to the data gathered at low wind speeds would seem to indicate that the problem may l]e in the aerodynamics. Fig. 1 Darrleus Vertical AXIS Wind Turb]ne (VAWT) at Sand]a National Laboratories, Albuquerque, NM Another possible source of error In previous load calculations 1s the assumption of a steady w]nd. Until now, all the aerodynamic loads have been calculated based on a constant emblent wind. The loads which result from this calculation are shown In Fig. 2. Tangential forces are def]ned as the component In the dlrect]on tangent to the path of the blade as It rotates and normal forces are normal to this path. The tangential forces pro– duce a net torque about the central tower while the normal forces produce no net energy. When a constant wind lS assumed, the forces repeat exactly for each rotor revolution. The frequency content of these forces 1s therefore limlted to Integer mult;ples of the turbine rotational speed (abbreviated as “per rev” frequencies). This approach will produce zero excltat]on at any frequency other than the per rev frequencies. This lS not totally unreasonable because the per rev frequency content of the loads 1s produced by the mean wind and the rotation of the rotor, and ]s therefore large compared to the turbulence induced, stochastic loads However, data collected from VAWT’S operat]ng in high winds have shown slgnlf]cant response at the structural resonant frequencies as well as at the per rev frequencies. The spectral content of the blade stress response shown in Flg 3 could never be predicted with loads calculated assuming e steady wind. By calculating the aerodynamic loads due to a turbulent wind, the loads will contain all fre– quencles, not only the per rev frequencies. These stochastic loads are described by their power spectral densities (psd’s) and cross spectral densltles (csd’s). Because the relationship between incident wind speed and blade forces is

Seasonal variability of the temperature field off the south-west coast of India

Abstract: The temperature field in the coastal region off south-west India exhibits a wellmarked annual cycle. Around March the isotherms develop an upward tilt near the coast. The magnitude of the tilt increases continuously till August, then decreases and vanishes in November. To check the hypothesis that this feature is in response to the local wind, we have used the resultant wind data to determine the annual march of the wind stress. It is found that though weak during November–March, the monthly-mean longshore component of the wind stress is always conducive to coastal upwelling and follows a pattern similar to that of the isotherm tilt. We interpret this result to indicate that coastal processes in the area during April–October are controlled by the longshore component of the local wind stress in accordance with the classical model for a coastal upwelling system. During November–March, when the wind stress is weak, it appears that the influence of the longshore density gradient, which persists at the surface during this period, dominates over the effect of the wind.

The Wind Regime in Coastal Areas with Special Reference to Results Obtained from the Swedish Wind Energy Program

Abstract: The paper describes various aspects of the wind regime in coastal areas as obtained from several experimental programs along the Baltic coast of Sweden. The studies include the change with distance inland of mean wind structure as well as the turbulence structure in various conditions. It is found that wind spectra are usually well described by local similarity in its high frequency part, even in complex terrain. The low frequency parts of the spectra show clear ‘spectral lag effects’. The effects of a small slope, ca 5 m height change over 300 m travel distance, is clearly seen in some spectra, and it is shown that current ‘flow over a hill theory’ can be used to account for it. The change of mean wind speed throughout the entire boundary layer as the wind passes a low but wooded island (Gotland), ca 30 km wide, has been studied in a series of relatively strong wind, near neutral cases. Some unexpected features are found; in particular the wind speed decreases more rapidly with distance inland than predicted by current numerical models. Two cases with a low level jet are discussed in some detail. Arguments are presented for the phenomenon to be caused by frictional decoupling at the Latvian coast, about 200 km upwind — the jet being thus an analogy in space to the classical Blackadar nocturnal jet frequently observed in continental areas.

Oceanic winds measured from the seafloor

Abstract: Acoustic ambient noise in the ocean at frequencies of 4.3, 8.0, and 14.5 kHz has been shown to be highly correlated with surface wind speed. Measurement of noise at the sea floor yields estimates of wind speed to within ±1 m/s (r2≥0.845) when compared with coincident surface observations. The frequency dependence of the wind speed-noise relation reported by others (17 dB/decade) has been confirmed, except that separate calibrations are required above and below the speed characterizing whitecap formation. There is also some evidence of local hydrography influencing the absolute calibration.

Monthly estimates of wind speed and wind run for Australia

Abstract: This paper introduces the first comprehensive wind speed and wind run maps for Australia. Maps for January, April, July and October based on analyses for all twelve months are presented and discussed. They show, in turn, the distributions of mean 9 a.m. and mean 3 p.m. wind speed at a height of 10 m, the distribution of the ratio between mean daily wind speed and the mean of 9 a.m. and 3 p.m. wind speeds and, finally, the distribution of mean daily wind run at a height of 2 m. The wind speed and wind run maps have been produced from Laplacian smoothing spline surfaces fitted to wind speed observations and to wind run observations and estimates at approximately 500 Bureau of Meteorology stations. The ratio maps, which may be used to estimate wind run from 9 a.m. and 3 p.m. wind speeds, have been similarly produced from data at just 73 Bureau of Meteorology anemograph stations but with independent validation at 68 stations which record both wind speed and wind run. The fitted surfaces are functions of latitude, longitude and a suitable transformation of distance inland from the coast. The degree of data smoothing imposed by the surface fitting procedure has been chosen to minimize for each surface the mean square predictive error estimate as measured by generalized cross-validation.

Seasonal variation in wind speed and sea state from global satellite measurements

Abstract: The GEOS 3 altimeter, which collected data intermittently for nearly 4 years, has measured significant wave heights and surface wind speeds over most of the world's oceans. Using these data, we have constructed contour maps of spatial variations in sea state and wind speed for winter and summer. To obtain reliable averages in the southern oceans, we low-pass filtered the data using a two-dimensional Gaussian filter with a half width of 600 km. The wind speed maps show that the zonal surface wind patterns, such as the westerlies, the horse latitudes, the trade winds, and the doldrums, shift south by about 10° between winter and summer. As expected, the highest wind speeds and sea states occur during the winter months in the mid-latitudes, 30°–60°. The most striking feature of the maps, however, is the large asymmetry in the summer to winter variation between the two hemispheres. The largest seasonal variations in sea state and wind speed occur in the northern hemisphere oceans and especially in the North Atlantic, where there is almost a factor of 2 variation. In contrast, the summer to winter variation in wind speed and sea state in the southern hemisphere oceans is relatively small. For example, the summer to winter increase in wind speed at 50°S is less than 10%, while at 50°N it is more than 50%. This differing variability can be attributed to the asymmetric distribution of continental area between the two hemispheres and the low effective heat capacity of the continents relative to the oceans.

On the Measurement of the Turbulence Dissipation Rate from Rising Balloons

Abstract: In this paper we analyze the feasibility of turbulence measurements below a rising balloon. From a typical case study we show the ability of the helisonde, hung below a large diameter balloon, to resolve the vertical profile of the dissipation rate in the troposphere and stratosphere. We examine the potential effect of the balloon wake, and the results emphasize the importance of a wind shear measurement in order to interpret properly the basic data.

Simulation and Assessment of Wind Array Power Variations Based on Simultaneous Wind Speed Measurements

Abstract: A wind model for relating wind speeds based on simultaneous wind speed measurements at several sites is developed for both meteorological events and turbulence wind conditions. The wind model is used to perform an analysis and simulation of wind power variations from single and multiple wind turbine arrays for meteorological events and turbulence. The results indicate that serious reduction in operating reliability and economy can occur for meteorological event induced wind power variations when total wind power generation capacity exceeds spinning reserve levels on a utility. The effects of turbulence induced variations are shown to be small for 5% wind penetration on a large utility but quite serious for the same penetration level on a small utility.

Turbulent Wind and Tension Leg Platform Surge

Abstract: A procedure is presented for estimating surge response to turbulent wind in the presence of current and waves. The procedure accounts for the nonlinearity of the hydrodynamic forces with respect to surge and for the coupling of aerodynamic and hydrodynamic effects. It is shown that current wind spectra do not model correctly the wind speed fluctuations at very low frequencies and an alternative model of the wind spectrum, whose ordinate at the origin is consistent with fundamental principles, is presented. The equation of surge motion under turbulent wind in the presence of current and waves is solved for typical tension leg platforms, and it is shown that under extreme wave conditions the damping provided by the hydrodynamic forces precludes the occurrence of significant wind‐induced resonant amplification effects even if the drag coefficient in the Morison equation is very small (e.g., Cd=0.1). It is verified that for the platforms being investigated the use of a one‐minute wind speed to represent the e...

A spatial model of wind shear and turbulence

Abstract: The purpose of the spatial model considered in the present investigation is to generate the wind environment for use by others for flight simulation. Winds and gusts are provided over any finite area (e.g., aircraft body) from which aircraft loads and moments may be calculated. Three-dimensional autospectral information and correlation are contained in the data. It is pointed out that the three-dimensionality as contained in the spatial model affords much greater realism than widely used one-dimensional models. The resulting simulated wind is a nonlinear, non-Gaussian combination of real atmospheric winds and Gaussian, three-dimensional turbulence modulated by gust intensities which may vary freely as desired over space. The turbulence as represented by a product of a varying gust intensity and simulated turbulence is nonlinear and non-Gaussian.

The microburst - Hazard to aircraft

Abstract: In encounters with microbursts, low altitude aircraft first encounter a strong headwind which increases their wing lift and altitude; this phenomenon is followed in short succession by a decreasing headwind component, a downdraft, and finally a strong tailwind that catastrophically reduces wing lift and precipitates a crash dive. It is noted that the potentially lethal low altitude wind shear of a microburst may lie in apparently harmless, rain-free air beneath a cloud base. Occasionally, such tell-tale signs as localized blowing of ground dust may be sighted in time. Microbursts may, however, occur in the heavy rain of a thunderstorm, where they will be totally obscured from view. Wind shear may be detected by an array of six anemometers and vanes situated in the vicinity of an airport, and by Doppler radar equipment at the airport or aboard aircraft.

The response of the coastal boundary layer to wind and waves: Analysis of an experiment in Lake Erie

Abstract: Wind, waves, currents, and water levels were measured near Lake Erie's southeastern shore during fall 1981. Data were recorded continuously at a wave research tower 6 km offshore at three Waverider buoys deployed around the tower and at three current meter moorings along a transect between the tower and the shore. During four storm episodes, wave and current data were also recorded from a dense array of instruments along a transect across the surf zone. A statistical analysis of the deep water data was performed, and histograms of meteorological parameters, wave parameters, and water level slope were plotted as a function of wind direction. The atmospheric boundary layer was usually unstable; on the average, the water was 2–3°C warmer than the air. This was true for all wind directions. At the tower the highest waves were associated with the greatest fetch distances to the WSW and WNW directions. The longest period waves were also associated with these wind directions. The mean water level slope was opposite the mean wind direction. This discrepancy is most likely due to small leveling errors in the water level gages. The variation of currents with wind direction was also examined. The currents are strongest when the winds are from the west, the quadrant with the strongest wind speeds. However, there are strong currents (5–10 cm/s) for all wind directions. The deep water currents are basically shore parallel, with the onshore/offshore component averaging 39% of the root mean square longshore component. Analysis of the four storm episodes reveals the influence of the offshore currents on the flow regime within the surf zone. Quantitative estimates of the longshore momentum balance outside the surf zone show wind stress and pressure gradient to be the dominant dynamic terms. Inside the surf zone, the radiation stress, wind stress, and bottom stress terms all have comparable effects and are usually larger than the acceleration and pressure gradient terms. In between, there is evidence of longshore flow reversals in the breaker zone and the effect of seiche-induced offshore currents on the near-shore flow regime.