Showing papers on "Wind shear published in 1969"

Modification of mean flow and turbulent energy by a change in surface roughness under conditions of neutral stability

Abstract: A theory is developed which describes the adjustment of the flow of a hydrostatically neutral fluid in the lower portion of a fully-turbulent boundary layer, after an abrupt change in surface roughness. The model is based on the hypothesis that the horizontal shear stress is proportional to the turbulent energy. The theory postulates that the flow is primarily governed by the dominant terms of the horizontal-momentum, continuity, and turbulent-energy equations. The model was solved by numerical techniques on a digital computer. Unlike previous models there are no a priori assumptions about the distribution of velocity or stress, the behaviour of the nondimensional wind shear, mixing length, or momentum-exchange coefficient in the transition region. The theory, in contrast to earlier theories, suggests the distribution of turbulent energy, as well as velocity. An inflection point is predicted in the transition velocity-profile. The nondimensional wind shear is found to differ significantly from unity in the transition region. These predictions agree with observation.

Long-wavelength turbulence and the heating of the solar wind.

Abstract: Solar wind model for studying long wavelength turbulence as heat source for alpha particles and protons in solar plasma

Experiments on the generation of small water waves by wind

Abstract: The generation and growth of small water waves by a turbulent wind has been investigated in a laboratory channel. The evolution of these oscillations with fetch was traced from their inception with amplitudes in the micron range under conditions of steady air flow. The experiments revealed that the waves are generated at all air velocities in small bursts consisting of groups of waves of nearly constant frequency. After travelling for some distance downstream, these wavelets attain sufficient amplitude to become visible. For this condition, a wind speed critical to raise waves is well defined. After the first wavelets appear, two new stages of growth are identified at longer fetches if the air speed remains unchanged. In the first of these, the wave component associated with the spectral peak grows faster with fetch than any other part of the wave spectrum of the initial waves until its amplitude attains an upper limit consistent with Phillips's equilibrium range, which appears to be universal for wind waves on any body of water. If the air flow is not changed, then the frequency of this dominant wave remains constant with fetch up to equilibrium. This frequency tends to decrease, however, with increasing wind shear on the water. In the second stage of growth, only the energy of wave components with spectral densities lower than the equilibrium limit tend to increase with fetch so that the wave spectrum is maintained near equilibrium in the high-frequency range of the spectrum.The origin of the first waves and the rate of their subsequent growth was examined in the light of possible generating mechanisms. There was no indication that they were produced by direct interaction of the water surface with the air turbulence. Neither could any significant feedback of the waves into the turbulence structure be detected. The growth of the waves was found to be in better agreement with theoretical predictions. Under the shearing action of the wind, the first waves were found to grow exponentially. The growth rates agreed with the estimates from the viscous shearing mechanism of Miles (1962a) to a fractional error of 61% or less. A slight improvement was obtained with the viscous theory of Drake (1967) in which Miles’ model is extended to include the effect of the drift current induced by the wind in the water. Since the magnitude of the water currents observed in the tunnel is very small, this improvement is not significant.

A criterion for determining air-flow separation from wind waves

Abstract: Based on laboratory studies of wind-wave interactions, a criterion for determining air-flow separation from wind waves is proposed. The air separates from waves having a phase velocity less than the shear velocity, which characterizes the turbulent wind field. The successful application of this criterion to the analysis of oceanic data is herewith demonstrated. DOI: 10.1111/j.2153-3490.1969.tb00479.x

Power spectrum, structure function, vertical wind shear, and turbulence in troposphere and stratosphere

Abstract: It has been shown that a power law exists between vertical velocity differences\(\overline {\Delta v} \) measured over a layer thickness Δh, of the form\(\overline {\Delta v} = a_0 (\Delta h)a_1 \). This expression may be related directly to a structure function and to a power-spectrum function. Statistical derivations of the exponenta1 from detailedvertical wind profile measurements show resemblance with the spectral functions obtained for conditions of clear air turbulence.

The Nature of Clear Air Turbulence: A Review

Abstract: The National Committee for Clear Air Turbulence in a 1966 resolution (U. S. Department of Commerce, 1966), has adopted the following definition of clear air turbulence:

Wind shear and turbulence radar indicator

Abstract: A wind shear and turbulence radar indicator in which velocity information is obtained from a coherent memory filter and presented rapidly and conveniently on an intensity modulated PPI scope as the doppler radar antenna rotates in azimuth. Regions of wind shear and turbulence can be identified immediately by a characteristic signature on this display.

A Study of the Relation of Clear Air Turbulence to the Mesoscale Structure of the Jet Stream Region

Abstract: The relationship of clear air turbulence to mesoscale features of the temperature, wind, and ozone concentration fields and to the Richardson number is examined for a case of widespread, persistent CAT observed by a specially instrumented aircraft of the Air Force Cambridge Research Laboratories. The CAT occurred mainly in a sloping layer of about 2 km depth located just above the jet stream core and encompassed a thin inversion layer and a deeper near-adiabatic layer above. The Richardson number was 0.25 or less throughout most of the turbulent region, and the more intense CAT tended to be located near the upper boundary of the inversion, extending into both the inversion layer below and the near-adiabatic layer above.

Development of a nonrecirculating wind-tunnel configuration insensitive to external winds.

Abstract: The extreme size of facilities required for aerodynamics testing of V/STOL vehicles has prompted an investigation of a wind-tunnel configuration other than the conventional closed-return type. Because the open-return type selected is vulnerable to atmospheric wind effects, an extensive experimental program to minimize wind disturbances has been run on scale models. One ^4-scale model and two -^-scale models were used to study test section flow distribution and speed changes due to winds from various directions. A complete •§*j--scale configuration was immersed in the flow of a large wind tunnel simulating winds up to 30 mph. Results are presented for variations of tunnel components, test section airspeed, wind speed and direction, and presence of nearby buildings. Wind effects cannot be sufficiently attenuated by inlet shape modifications or addition of screens and honeycomb. Complete enclosure of the inlet in a protective housing, combined with a vertical cylindrical exhaust stack, will reduce wind disturbances to a satisfactory level. For configurations developed in this investigation, test-section disturbances have been held within rbl% of dynamic pressures as low as 2 psf with a 20-mph wind blowing from any direction.

The pattern of surface wind flow in Antarctica

Abstract: The main characteristics of surface winds are tabulated for 34 Antarctic stations. Using these data, supplemented by traverse records, the average wind flow is interpolated for each region and presented as a map showing the pattern of surface wind flow for the whole continent. Attention is focused on the flow in relation to surface contours. Statistics are presented for surface slope, wind speed, temperature, seasonal variations of speed and temperature, diurnal variation (including power spectra) of the wind speed and times of maximum and minimum speed at coastal and inland stations, wind frequency versus direction, the occurrence of calms, the deviation of the plateau wind from the downslope direction, the wind direction near the front of ice shelves, the proportion of cloud cover, and wind chill factors. In all cases data are grouped according to the environs of the stations in an attempt to isolate systematic differences depending on location: coastal stations near the foot of the ice slope and fully exposed to katabatic flow, coastal stations on offshore islands, coastal stations on peninsulas, coastal stations on extensive rock areas, ice shelf stations and inland stations.

Meteorological Analysis of Clear Air Turbulence in the Stratosphere

Abstract: This paper is presented in two parts. In Part I, prepared by Mr. Mitchell a detailed meteorological analysis of several Project HICAT flights is presented along with some observations and conclusions derived therefrom. In Part II, Mr. Prophet describes a method of interpreting vertical temperature structure in terms of wave motion and turbulence intensity.

The effect of wind shear on the decay constant of meteor echoes

Abstract: For a meteor trail, rotational motion caused by wind gradients leads to errors in the decay constant of the radar echo. Two cases are assessed. In the first, large wind shears produce gross motion ...

Satellite Studies of Clouds and Cloud Bands Near the Low-Level JET1

Abstract: TIROS-VII and -VIII photographs are used to study synoptic and mesoscale cloud patterns during 6 days with strong southerly flow in the Central United States. Cases with nocturnal thunderstorms show a tendency for thunderstorms to occur in the downstream portion of the jet. Through advection of moisture and turbulent breakdown of the nocturnal inversion, the low-level jet plays an important role both in the formation of stratus and in its manner of dissipation. Longitudinal cumulus cloud bands with spacing of 10 to 15 km occur with slightly superadiabatic lapse rates and moderate wind shear in agreement with theoretical results by Kuo.

Analysis of wind data from a south carolina coastal forest

Abstract: : The report summarizes the mean wind and turbulence characteristics in and above a jungle-like South Carolina coastal forest. The vegetation was black gum-red maple. Data on leaf area density estimates as well as stem densities are tabulated. The mean vector wind had two notable features: a nearly constant speed in the lowest two-thirds of the canopy, and a turning of wind direction with height in the same layer. All three components of the turbulence intensity were larger below the canopy, especially the longitudinal component. Variance spectra of the vector wind speed indicated that an inertial subrange existed at all heights above and below the canopy, and that as far as could be determined the energy dissipation rate was nearly constant with height.

Clear Air Turbulence Detection Methods: A Review

Abstract: Of the various methods proposed for CAT detection, only ground-based ultrasensitive radars have demonstrated partial success. However, their size and cost makes an operational system of doubtful economic feasibility. Nevertheless a limited system is proposed in the Appendix. It can now be predicted with some assurance that a 20 db improvement in sensitivity of airborne radars will permit CAT detection at a range of about 10 n. mi., although not with 100% confidence. Serious efforts should thus be made to advance radar technology sufficiently to provide the required improvement. Meanwhile we recommend the use of chaff to mark turbulent regions and appropriately modified existing airborne radars to measure the turbulence intensity. The radar-CAT research has also provided new insight into the nature of CAT. In particular, vertical wind shear in excess of 10−2 sec−1 appears to be a suitable criterion for its occurrence; while this generally implies Richardson (Ri) numbers less than 1, such a yardstick does not appear to improve the specification of CAT over that attainable with the shear alone. The implication is that any method capable of monitoring wind shear will be a valuable tool in CAT warnings. Also, the greater the thermal stability, the greater the possible shear and the more violent the CAT when it is finally released. This suggests that the magnitude of the refractivity structure constant C n 2 , and thus the radar reflectivity, should be a useful CAT indicator in the upper troposphere, but only at the time and place of its origin. The radar results also imply that ground-based bistatic radio tropo-scatter techniques should be at least equally effective in detecting CAT. These techniques also offer the promise of measuring winds and wind shear.

Of bird navigation

Abstract: Radar observations often show nocturnal migrants well oriented under opaque layers of cloud, and perhaps even inside or between cloud layers. Some homing pigeons can orient towards home from unfamiliar territory under overcast skies. In searching for non-visual sensory capabilities of birds it is important to consider not only their sensory physiology but also the atmospheric ecology of migration when neither sky nor ground is visible. Either inertial navigation or orientation to the earth's magnetic field requires considerably greater sensitivity than any yet demonstrated in birds. Natural or artificial sounds from the surface, or even ground echoes of flight calls, might facilitate orientation under some conditions. Birds communicating by flight calls might detect the differential wind drift experienced by upper and lower members of a flock, and thus determine the direction of wind shear. Under many nmeteorological conditions such patterns of air movement as roll vortices or internal gravity waves produce readily detectable updrafts capable of providing both useful lift and information about the direction of the wind or wind shear.

Structure of Vertical Wind Profiles

Abstract: An exponential relationship between the vertical wind shear and the layer thickness over which this shear is measured has been derived. From an analysis of the Richardson number it appears that shallow layers reaching maximum shearing conditions under the established power law should be turbulent, irrespective of the prevailing thermal stability conditions.

Ground wind characteristics at Kennedy Space Center Final report

Abstract: Mathematical model for quantitative wind and turbulence effects on design and operation of launch vehicles

Response characteristics of meteorological rocket wind reduction techniques

Abstract: The response characteristics of the height-based and time-based wind reduction techniques for meteorological rocketsondes are deduced for the region 20–55 km by use of analytical expressions for position and time as functions of height. It is shown that the height-based method is more consistent in depicting medium-scale perturbations than is the time-based method. This is accomplished, however, at the expense of the simplicity of the time-based system.

Distribution of robin sensed wind shears at 30 to 70 kilometers.

Abstract: : Distributions of wind shear for several layer thicknesses were computed for six stations from a selected sample of ROBIN sensor meteorological rocket soundings. Wind shears in the 30- to 70-km layer are generally weak but very strong shears were occasionally observed. Strongest shears occur at highest altitudes, 60 to 70 km, over latitude stations during the seasons of reversal in zonal flow. There is also evidence that wind shears decrease in strength with increasing latitude. (Author)

Estimates of wind variability between 100 and 900 meters

Abstract: : The temporal wind variability between 100 and 900 meters above ground level was estimated from wind measurements made with an Automatic Pilot Balloon Wind Measuring System. One-hundred-gram balloons were released simultaneously from four observational sites on White Sands Missile Range, New Mexico, at six-minute intervals, for a period of 180 minutes on three consecutive days. These data indicate that wind variability increases with increasing time lag, the relationship between wind variability and time lag interval being described by a power curve. In addition, a linear relationship between wind variability and mean wind speed is suggested. (Author)

Analyzing and Forecasting Clear-Air Turbulence Probabilities Over the United STATES1

Abstract: Pilot reports from special turbulence-reporting periods were used to investigate methods of analyzing and forecasting clear-air turbulence over the United States. Meteorological analyses for the special reporting periods were made objectively by computer using only standard upper air rawinsonde measurements. The wind analyses were built upward from the 400-mb level to the 200-mb level using thermal wind shears to compensate for missing wind data in high-speed portions of the flow. The best meteorological indicators of turbulence were found to be the vertical vector wind shear and the product of wind shear and horizontal deformation. To a somewhat lesser extent, large gradients of relative humidity (at the 400-mb level) and large magnitudes of divergence also tended to be associated with turbulent regions. Turbulence analyses based on both meteorological relationships and pilot reports were made. These analyses are in terms of the probability of encountering significant turbulence (moderate or sev...

A Numerical Experiment on a Disturbance in the Tropical Easterlies

Abstract: A numerical experiment has been performed on a tropical disturbance embedded in easterlies to test the dependency of the vertical coupling on the value of Richardson number in tropics. By a 2-layer primitive equation model in sigma system of coordinates integrations have been performed for a period of 24 hours with two different sets of initial conditions in which the three dimensional structure of the initial easterly pattern is specified through pre-specified wind shear and different values of the Richardson number. The order of magnitude of vertical velocities, the time rate of change of kinetic energy and the phase speed of the disturbances is discussed for both the cases. The preliminary results indicate that, even the tropical motions have appreciable degree of vertical coupling and baroclinic nature in the cases of low Richardson number. The results seem to be of special interest in context of the fact that even the mean monthly values of static stability during summer are found to be very small and even negative in the middle and lower troposphere in the tropics.