Showing papers on "Wind shear published in 1981"

Thermally driven wind variability in the planetary boundary layer above Lima, Peru

Abstract: Land-sea thermal forcing of the coastal wind near Lima, Peru, is examined as a mechanism to explain the observed wind variability there, in particular the similarity between monthly anomalies of sea surface temperature and wind speeds on the interannual time scales typical of E1 Nifio occurrences. Aerological and surface meteorological observations from Peru coastal sites are analyzed for two time scales: over 1520 year periods (1958-1977) for monthly averaged data and over a year (1976-1977) for daily data. Wind profile characteristics and annual cycles, and their relation to the cross-coastal temperature gradient are qualitatively and quantitatively consistent with the thermally forced boundary-layer model proposed by Lettau (1978) to explain the Peru coastal wind. The cross-coastal air temperature difference between Lima and Callao explained 55% of the variance in the surface wind speeds at Lima during 1976-1977. The mean annual cycles of the surface wind at Lima and other Peru coastal locations are in phase with the annual solar heating cycle where the seasonality of low cloudiness is large but with the annual cycle of the southeast trade circulation where it is not. This suggests that insolation over the desert is the principal source of thermal forcing variability. During E1 Nifio periods the alongshore wind and the crosscoastal circulation above Lima intensify, in conjunction with anomalously warm sea surface temperatures. The planetary boundary layer is destabilized, and the base of the winter inversion is eroded upward. The observations are consistent with a reduction of desert cloudiness during E1 Nifio events owing to air-sea exchange and a consequent increase in the thermal forcing of the wind.

Thermal structure and dynamics of the Jovian atmosphere 2. Visible cloud features

Abstract: Voyager IRIS data reveal strong similarities among a broad range of features which differ considerably in visual appearance. The atmosphere above anticyclonic features, including the major white ovals, the Great Red Spot, and a zone, are cold relative to the immediate surroundings in the upper troposphere and tropopause region. These results are consistent with upwelling and divergence in this part of the atmosphere. A hot spot and a barge, which are localized cyclonic features, are found to be warm relative to their surroundings, implying subsidence with accompanying convergence. In all cases, the thermal wind shear associated with the features indicates a decay of the vorticity with height in the upper troposphere and lower stratosphere. Vertical velocities inferred from the observed temperature perturbations imply an upper limit of vertical mixing times near the tropopause of approximately 20 years. Temperatures in the upper stratosphere above the anticyclonic features show considerable variation, but in most cases are found to be relatively warm.

Height and latitude structure of electric fields and currents due to local east-west winds in the equatorial electrojet

Abstract: Using the equivalent electrical circuit method for the theoretical treatment of the local wind effects on the equatorial electrojet, the height structures of electric fields and currents that are generated by height-varying east-west winds in the electrojet region have been calculated for theoretical model wind sturctures and for a variety of experimentally observed wind structures. The results bring out clearly the nature and extent of the local wind effects on the height and latitude structures of the equatorial electrojet. The more important conclusions of the present study are: (a) The vertical wind shears associated with tidal winds and gravity waves can generate significantly large electric fields and east-west currents in the equatorial electrojet (EEJ). (b) The wind-generated electric fields and currents are characterized by large height gradients, latitudinal gradients, and reversals of direction. (c) At the magnetic equator, wind-generated electric fields and currents are often small (10%–30%) in comparison with the eastward electric-field-generated polarization field and east-west currents around noon hours. (d) At geomagnetic latitudes of 3° and beyond the wind-generated currents and electric fields often exceed those caused by the eastward electric field. (e) Beyond 2° the neutral winds can drastically alter the intensity and latitudinal structure of the EEJ, depending upon the magnitude and height location of the wind shear. (f) The observed variabilities in the ‘width’ and latitude structure of the EEJ can arise from the effects of such east-west winds that have been observed experimentally.

Global satellite measurements of water vapour, wind speed and wave height

Abstract: The results of global measurements of atmospheric water vapor by the Seasat Scanning Multichannel Microwave Radiometer and wave height and wind speed by the Seasat altimeter (ALT) are reported. The 13.5 GHz ALT has a 3.125 ns pulsewidth and 1022 Hz repetition rate, and measures surface height to a resolution exceeding 10 cm celative to a reference ellipsoid. Full ALT data comprise 135 km equatorial groundtracks, with about a 50 cm difference of sea wave height compared to buoy reference measurements, and windspeed accuracy to within 0.25-1.58 m/sec up to 20 m/sec. Highest water vapor concentrations were observed in the tropics and the lowest at high latitudes. Wind speeds were highest for the north-east and south-east tradewinds in both the Atlantic and Pacific oceans. Average wave height is small in the summer North Hemisphere and the largest waves are in the winter Southern ocean, and lowest in western Atlantic and Pacific ocean areas where winds are lightest.

Velocity and Temperature Oscillations in Drainage Winds

Abstract: Spectra of the time series of wind speeds and temperatures measured in the outflow area of a large drainage region are presented. Peaks are found in these spectra at frequencies corresponding to periods of oscillation of ∼1.5 h. A simple model is given, which accounts qualitatively for some of the observed features.

A Three-Dimensional Numerical Simulation of Splitting Severe Storms on 3 April 1964

Abstract: A three-dimensional numerical storm model is used to investigate the observed splitting of several reflectivity echoes on 3 April 1964 in Oklahoma. Representative soundings from this day exhibit a nearly one-directional environmental wind shear vector and the presence of strong low-level wind shear. In the numerical simulation an initial cloud splits into two long-lived rotating storms, one that moves to the left of the mean winds and the other to the right. The left-moving storm develops more slowly than the right-moving one due to the deviation of the environmental wind hodograph from a straight line below 1 km. Further, the left mover eventually splits. Convergence induced by the cold, low-level storm outflow plays a major role in the development of both the first and second splits. However, the second split appears to be dynamically different than the first as the left-moving updraft remains essentially unchanged while a new updraft forms immediately adjacent to it. Because of the different p...

Effect of cross-winds on trains

Abstract: Low-mass, high-speed trains may be in danger of being overturned by strong crosswinds. This paper examines the aerodynamic data required to estimate overturning wind speeds. The results of wind tunnel tests and a moving model experiment, including the effect of the turbulent wind, are described.

Thermal structure and dynamics of Saturn and Jupiter

Abstract: High resolution Voyager IRIS measurements for Saturn and Jupiter are assembled in meridional cross sections of the retrieved upper tropospheric temperatures. The calculated thermal wind shear in the upper troposphere is highly correlated on both planets with the cloud top winds derived from imaging data. In contrast, temperatures below approximately 300 mbar are not simply related to the zonal jet structure. The upper tropospheric temperatures seem to have been more consistently correlated with cloud top winds than with major albedo features at the time of the Voyager encounters.

Wind Tunnel Design for Physical Modeling of Atmospheric Boundary Layer

Abstract: Wind in the lower atmosphere has had direct adverse and beneficial impacts of the entirety of humanity throughout the ages. The evolution of wind tunnels for the physical modeling of natural wind in the lower 500 m-1,000 m of the atmosphere is reviewed. Following comments of essential features of the atmospheric boundary layer requirements for physical modeling of natural winds in this layer are reviewed. Details of a meteorological wind tunnel designed by the writer to meet the primary requirements are presented. After some comparisons of meteorological variables measured in the wind tunnel with data obtained from atmospheric measurements, applications of wind tunnels to the solution of wind-engineering problems of current concern in civil engineering practice are examined.

Models for estimating offshore winds from onshore meteorological measurements

Abstract: To understand and estimate wind speed differences across the coastal zone, two models, one theoretical and another semi-empirical, have been developed and verified by available data sets. Assuming that: (1) mean horizontal motion exists across the coastal zone; and (2) the geostrophic wind does not change appreciably at the top of the planetary boundary layer (PBL), the equation of motion in the direction of the wind can be reduced so that 341-01, where U, H, and CD are wind speed, height of PBL, and drag coefficient over the sea and land, respectively. For practice, CD SEA has been modified from a formula with ULAND as the only input. HSEA may be estimated routinely from known HD LANDLAND and the temperature difference between land and sea, which can be provided by such means as remote sensing from meteorological satellites. For a given coast, Cmay be estimated also. This formula is recommended for weather forecasters. The semiempirical formula is based mainly on the power law wind distribution with height in the PBL. The formula states that 341-02. Simultaneous offshore and onshore wind measurements made at stations ranging from Somalia, near the equator, to the Gulf of Alaska indicated that values of a and b are 2.98 and 0.34 with a correlation coefficient of -0.95. For oceanographic applications, a simplified equation, i.e., 341-03, is also proposed.

Wind-induced water movements in the South Basin of Windermere

Abstract: SUMMARY. The results of direct current measurements in the South Basin of Windermere are presented and related to wind history and horizontal variations in near-surface water temperature. Currents were measured by Lagrangian methods using drift-bottles and depth-specific drogues. Except when sudden calms followed strong winds, internal seiche movements had little effect on horizontal transport in the epilimnion. The most important factor governing mass water movement was direct wind forcing. Variations in wind speed accounted for 93% of the variation in near-surface current speed. For wind speeds between 100 and 500 cm s−1 the wind factor (current speed/wind speed) decreased linearly with wind speed. At wind speeds above 500 cm s−1 the wind factor remained relatively constant around 1 %. Coriolis effects deflected near-surface currents 4–38° to the right of the wind. The degree of deflection was strongly correlated with the relative depth DE/D* (where DE was the depth of the epilimnion and D* the depth of frictional resistance). The influence of the Coriolis force also produced pronounced rotations, with depth, of the wind-driven current. The circulation pattern within the epilimnion was broadly that of a distorted conveyor belt moving at some angle to the wind axis. The strength of the transverse circulation was greatest at low wind speeds with a deep thermocline. Richardson-number calculations suggest that the thermocline generally acts as a low-friction boundary between a turbulent epilimnion and a relatively quiescent hypolimnion. Horizontal variations in water temperature, although ranging from only 0.2 to 1.0°C per km, served as a good indirect ‘tracer’ of the circulation pattern.

Lateral coherence of longitudinal wind components in strong winds

Abstract: A combination of lateral coherence measurements of wind speed at five locations suggests that the ‘decay constant’ is a monotonically increasing function of the ratio of separation to height, under neutral conditions.

Gravity wave activity in vertical winds observed by the Poker Flat MST Radar

Abstract: Over 30 days of nearly continuous observations of the vertical wind up to 20 km obtained using part of the MST radar at Poker Flat, Alaska, are presented in this report. These observations are typical of a longer 6-month set of observations and demonstrate several new features. The most obvious feature is the extent to which the magnitude of the vertical wind fluctuations is controlled by propagating planetary scale waves, which modulate the large-scale wind field. Enhanced wind and wind shear in intense baroclinic zones greatly increase gravity wave activity as seen in the vertical winds. Although shorter period fluctuations in vertical velocity on occasion show very regular sinusoidal oscillations with periods from 5 to 30 minutes, typically the vertical velocity fluctuations are irregular with no well-defined oscillation period. Steady vertical winds of up to 1 m/s lasting for several hours are observed at certain low altitudes when background winds and wind shear are enhanced. The occurrences are not unusual and are probably due to mountain lee wave activity.

Studies of Jupiter's lower ionospheric layers

Abstract: Fluxes of heavy ions necessary to form layers of enhanced ionization observed in the lower ionosphere of Jupiter are theoretically calculated, and possible formation mechanisms of the layers are investigated. Estimates of ion drift velocities and neutral wind speeds are made from the shape of the ionization layers, and are found to range from a few centimeters per second to meters per second. Zonal wind shear ranges from 50 m/s westward to 200 m/s eastward over a 70 km altitude range, while small meridional winds are sufficient to form the layer. Results indicate that if the layers are formed from sodium or sulfur ions from the Galilean satellite which are injected into the Jovian atmosphere, then the Na(+) flux must be 30,000 sq cm/s, and the S(+) flux must be 4000 sq cm/s in order to correlate with Pioneer 10 observations of the L(6) layer. At low altitudes of the L(6) and L(7) layers, the denser atmosphere makes diffusion very slow, and the vertical drift velocity of 1 cm/s requires a zonal wind of only several centimeters per second to drive it.

An airport wind shear detection and warning system using Doppler radar: A feasibility study

Abstract: A feasibility study was conducted to determine whether ground based Doppler radar could measure the wind along the path of an approaching aircraft with sufficient accuracy to predict aircraft performance Forty-three PAR approaches were conducted, with 16 examined in detail In each, Doppler derived longitudinal winds were compared to aircraft measured winds; in approximately 75 percent of the cases, the Doppler and aircraft winds were in acceptable agreement In the remaining cases, errors may have been due to a lack of Doppler resolution, a lack of co-location of the two sampling volumes, the presence of eddy or vortex like disturbances within the pulse volume, or the presence of point targets in antenna side lobes It was further concluded that shrouding techniques would have reduced the side lobe problem A ground based Doppler radar operating in the optically clear air, provides the appropriate longitudinal winds along an aircraft's intended flight path

Warm Rain Development in a Three-Dimensional Cloud Model

Abstract: A warm rainshower was simulated in a shallow, anelastic, three-dimensional cloud model with detailed microphysics. After many features of the drop growth process were successfully simulated in a symmetric cloud, wind shear effects on both microphysical and dynamic properties were investigated. A number of special characteristics appeared under wind shear conditions: 1) the development of stronger downdrafts and a shortened cloud lift; 2) weak rainfall occurs early in the process; 3) a sharp raindrop size distribution; 4) initial rainfall localized in a narrow area at the ground with a large rainfall intensity gradient near the upshear side; 5) greater movement of the cloud system during rainfall; and 6) formation of cyclonic-anticyclonic circulations near the cloud top. Many of these characteristics may be explained by the transport of horizontal momentum by updrafts/downdrafts, as shown by close correspondence with the detailed microphysical processes.

A wind tunnel boundary-layer simulation of wind flow over complex terrain: Effect of terrain and model construction

Abstract: The report presents the results of a wind-tunnel study of the flow of the natural wind over complex terrain. A 1:4000 undistorted scale model of Gebbies Pass in the South Island of New Zealand was prepared and tested in the boundary-layer wind tunnel in the Department of Mechanical Engineering at the University of Canterbury.

The onset of the summer monsoon during the FGGE 1979 experiment off the East African coast: A comparison of wind data collected by different means

Abstract: During FGGE 1979, from March to July, an extensive oceanographic experiment with ships and moored stations was carried out in the Somali Current. The development of the monsoon winds off Somalia during the time of that experiment is described in a comparative analysis of standard ship wind observations, moored buoy wind measurements, low-level cloud winds, and winds from land stations. The onset 1979 is found to be of the multiple type, with northward winds off Somalia beginning around May 5 but dying down into early June; the real onset of sustained high winds starts around June 10. Cloud level wind observation numbers off Somalia decrease drastically with the monsoon onset because of lack of clouds over the quickly developing cold upwelling areas. An intercomparison of cloud level and ship winds for the period May 16 to July 6 at five offshore points shows good agreement in directions but reduction of ship wind speeds against cloud level winds off northern Somalia after the onset, which may be explained by the increased vertical wind shear due to high air stability over the upwelled water and by geostrophic shear due to the strong gradients of sea surface temperature. A comparison of 3-day averages of buoy winds measured at 3-m height 30 km offshore, but still inland from the ship lane, with ship winds for the period March 3 to June 10 showed good agreement in directions but lower buoy wind speeds, which could partly be due to sensor height difference and partly due to horizontal wind shear towards the coast. Coastal stations and wind buoys near the coast are found not to be good indicators of the monsoon onset further out in the open ocean.

Prevailing Wind in the Meteor Zone (80–100 km) over Atlanta and its Association with Midwinter Stratospheric Warming

Abstract: The wind data generated by an all sky, continuous wave radio meteor wind facility at Atlanta (34°N, 84°W) is analyzed over the period of August 1974 through July 1975. Zonal and meridional components of the prevailing wind over the height range of 80–100 km, at 2 km intervals represent 5–10 day averages where the tidal components have been removed. Large southerly wind during winter and weak northerly wind during summer at 80–100,km altitude is consistent with other observations and mesospheric circulation models. Various phases of the 1974–75 midwinter stratospheric warming, including a pre-warming pulse during the second half of November 1974, are indicated to affect the prevailing Wind in the meteor zone over Atlanta in a consistent manner, by making use of the latitudinal and vertical compensation of temperature and also the movement of pressure systems in the stratosphere and above.

Enhancement and suppression of urban heat plumes over Johannesburg

Abstract: Observations of the atmospheric temperature structure over Johannesburg have revealed the existence of both well-developed and suppressed heat plumes. Suppressed plumes appear to be associated with the incidence in a stable atmosphere of positive wind shear and well-developed katabatic flow away from the city. A localised cool region in the atmosphere above the central city is invariably associated with suppressed plume development. A tentative identification of wind shear and katabatic flow as the major influences on such structures is strengthened by the observation that during conditions of negative windshear and reduced katabatic advection, significant heat plumes are developed. It is at present not possible to assess the extent to which the observations and causative processes are site-specific.

The effect of heavy rain on windshear attributed accidents

Abstract: In recent years several commercial aircraft accidents have occurred as an aircraft attempted landing in a thunderstorm cell. The horizontal and vertical windshear associated with the cell have been identified as the factors responsible for the accident. In addition to the wind shear encounter, several of these aircraft simultaneously penetrated a heavy rain cell. Heavy rain affects the aircraft by imparting a momentum penalty, and a drag penalty, due to the rain roughening of the airfoil. These penalties have been evaluated by a computer model that, when incorporated with a landing simulation program, was used to assess the relative influence of heavy rain versus wind shear. Using this model, an assessment was made of the influence of heavy rain on several wind shear attributed accidents.

Microbursts as an aviation wind shear hazard

Abstract: The downburst-related accidents or near-misses of jet aircraft have been occurring at the rate of once or twice a year since 1975. A microburst with its field comparable to the length of runways can induce a wind shear which endangers landing or liftoff aircraft; the latest near miss landing of a 727 aircraft at Atlanta, Ga. in 1979 indicated that some microbursts are too small to trigger the warning device of the anemometer network at major U.S. airports. The nature of microbursts and their possible detection by Doppler radar are discussed, along with proposed studies of small-scale microbursts.

Wind turbulence inputs for horizontal axis wind turbines

Abstract: Wind turbine response characteristics in the presence of atmospheric turbulence was predicted using two major modeling steps. First, the important atmospheric sources for the force excitations felt by the wind turbine system were identified and characterized. Second, a dynamic model was developed which describes how these excitations are transmitted through the structure and power train. The first modeling step, that of quantifying the important excitations due to the atmospheric turbulence was established. The dynamic modeling of the second step was undertaken separately.

The Influence of Gravity Waves on Radiometric Measurements: A Case Study.

Abstract: During the 1978 PHOENIX experiment at the Boulder Atmospheric Observatory in Colorado, the presence of atmospheric gravity waves was detected by various independent remote sensing instruments. Fluctuations in the zenith atmospheric radiation were measured at 22.235 and 55.45 GHz in the water vapor and oxygen absorption bands and compared with corresponding fluctuations of surface pressure and the height of FM-CW radar echo returns. These fluctuations are explained, qualitatively and quantitatively, in terms of an internal gravity wave generated by wind shear above the boundary layer. The analysis shows that the oscillations at 22.235 GHz are essentially due to fluctuations of water vapor in the antenna beam while those at 55.45 GHz are due to temperature variations.

Passive cyclic pitch control for horizontal axis wind turbines

Abstract: A flexible rotor concept, called the balanced pitch rotor, is described. The system provides passive adjustment of cyclic pitch in response to unbalanced pitching moments across the rotor disk. Various applications are described and performance predictions are made for wind shear and cross wind operating conditions. Comparisons with the teetered hub are made and significant cost savings are predicted.