Showing papers on "Wind shear published in 1968"

Global view of the origin of tropical disturbances and storms

Abstract: A global observational study of atmospheric conditions associated with tropical disturbance and storm development is presented. This study primarily uses upper air observations which have become available over the tropical oceans in the last decade. Climatological values of vertical stability, low level wind, tropospheric vertical wind shear and other parameters relative to the location and seasons of tropical disturbance and storm development are discussed. Individual storm data are also presented in summary form for over 300 development cases (with over 1,500 individual observation times) for four tropical storm genesis areas. Results show that most tropical disturbances and storms form in regions equatorward of 20° lat. on the poleward side of doldrum Equatorial Troughs where the tropospheric vertical shear of horizontal wind (i.e., baroclinicity) is a minimum or zero. Storm development occurring on the poleward side of 20° lat. in the Northwest Atlantic and North-west Pacific takes place unde...

The Determination of Kinematic Properties of a Wind Field Using Doppler Radar

Abstract: A technique is proposed for the measurement of kinematic properties of a wind field in situations of widespread precipitation, using a single Doppler radar to sense the motion of the precipitation particles. The technique is an extension of ideas put forward by Probert-Jones, Lhermitte, Atlas, Caton and Harrold, and is based upon the Velocity-Azimuth Display (VAD) obtained by scanning the radar beam about a vertical axis at a fixed elevation angle. Harmonic analysis of the VAD permits divergence to be obtained from the magnitude of the “zeroth” harmonic, wind speed and direction to be obtained from the amplitude and phase of the first harmonic, and resultant deformation and the axis of dilatation to be obtained from the amplitude and phase of the second harmonic. Although limitations to the accuracy of this technique are imposed by inhomogeneities in the horizontal distribution of precipitation fall speed and, in the presence of strong vertical wind shear, by elevation angle errors and reflectivi...

Planetary Rossby Waves Propagating Vertically Through Weak Westerly Wind Wave Guides

Abstract: The role of horizontal wind shears in the vertical propagation of planetary Rossby waves is investigated using an adiabatic linear model. We discuss wave guides formed by regions of weak westerly wind. If the wave guide is formed by trapping of waves between strong westerlies and/or the geometric poles, the ducting occurs as a wave propagation in discrete normal modes of the internal wave guide. On the other hand, for wave guides formed by one or more lines of zero wind, waves are absorbed rather than reflected at the zero wind line so that there are no normal modes of the wave guide. Disturbances excited in the lower stratosphere in the equatorial zero wind wave guide will terminate somewhere in the equatorial stratosphere, but eddy motions may be maintained in the tropics at higher levels by leakage from the Aleutian high planetary wave propagating vertically in a polar wave guide. The Aleutian high should he significantly attenuated by such leakage. The theory of zero wind line absorption sugg...

Turbulence and Wind Speed Spectra within a Japanese Larch Plantation

Abstract: Mean horizontal wind speed profiles within and above a plantation of Japanese larch were obtained. A log-profile analysis of above-vegetation wind speeds yielded a wide range of values for the roughness length parameter z0 and the zero-plane displacement height D. The computed Eulerian space scale of turbulence within the vegetation showed deeper penetration of large eddies after needle fall and during high winds. Power spectra showed that at the base of the plantation most of the variation in wind speed was associated with gusts of about 100 m wavelength. Power spectra at the most dense portion of the plantation canopy showed considerable modification due to the tree spacing.

Vertical movement of temperate‐zone sporadic E layers

Abstract: The wind-shear theory of the formation of temperate-zone sporadic E layers is extended to take into account effects associated with a neutral wind profile having a descending phase velocity. It is shown that layers of long-lived ions (presumably of meteoritic origin) that are formed at higher altitudes are carried down with the wind profile to a region roughly 85–95 km in altitude, where they are dumped. Above about 95-km altitude the layers are trapped in portions of the neutral wind profile having a shear of the correct sign according to the simple (equilibrium) form of the wind-shear theory. Below about 95 km the layers are not trapped in this manner, and their position shows no obvious correlation with the sign of the neutral wind shear. The effects of diffusion are discussed, and an expression for the equilibrium width of a sporadic E layer is obtained.

On the Accuracy of Wind Measurements Using an Inertial Platform in an Aircraft, and an Example of a Measurement of the Vertical Mesostructure of the Atmosphere

Abstract: A high quality inertial navigation platform, originally developed for the TSR 2 aircraft, is now being used by the Meteorological Research Flight in order to measure vertical gust velocities and horizontal wind shear from a Canberra PR 3 aircraft. The equations are derived from which the vertical and horizontal winds are calculated, and an assessment is made of the accuracy of measurement which can be expected for vertical gusts and horizontal shear. Measurements of the residual error velocities which occur when the aircraft is made to pitch, roll and yaw are shown to lie within the theoretical limits, and an example of the velocities derived from a straight and level run in smooth air is also described. The use of this equipment to measure the small-scale variations in wind shear and vertical gusting when the aircraft descends through a layer of turbulence is then described, and the results of one of the first of a series of such flights are given. With the present recording system, the extracti...

Excitation of internal waves in a stably-stratified atmosphere with considerable wind-shear

Abstract: The rate of generation of internal waves by a thin turbulent boundary layer was calculated in a previous paper for a stably-stratified atmosphere with no significant wind-shear outside the boundary layer by considering the excitation of normal modes of wave propagation. By using the concept of wave-packets propagating upwards from the boundary layer, the effects of wind-shear can be included. Conditions for the validity of the approximation are given. In general, the spectral distribution of wave-energy at a particular height takes large values in two bands of horizontal wave-number, one band deriving from wave-packets undergoing internal reflexion near that height and the other from wave-packets of very small local frequency that accumulate there. The ‘reflexion’ wave-numbers are dominant if the wind increases with height and the ‘accumulation’ wave-numbers if the wind initially decreases with height. The spectral energy distributions and intensities of the wave-motion are discussed in more detail for an atmosphere of uniform stability and unidirectional wind-shear. The accumulation process may lead to instability or overturning of the waves, and estimates are made of the probable scale and intensity of the ‘clear-air’ turbulence produced. An interesting point is that the rate of energy loss from the boundary layer by radiation of internal waves turns out to be comparable with the rate of production in the outer nine-tenths of the layer, both for atmospheric boundary layers and for the surface layer of the ocean. It seems likely that radiation limits the layer thickness to some extent.

The Variation of Gust Factors with Mean Wind Speed and with Height

Abstract: Records from wind instruments installed on a 1000-ft tower were analyzed for gustiness characteristics. The results show that gust factors decrease with increasing wind speed, decrease with increasing height, and have no obvious relationship to the temperature lapse rate.

Wind Shear and Reflectivity Gradient Effects on Doppler Radar Spectra

Abstract: Other investigators have discussed the effects of wind and reflectivity gradients across the radar beam on Doppler measurements, but have either estimated their magnitude from a simple approximation or set them aside as negligible. This paper deals with the component of the shear vector along the beam. Exact solutions and simple approximations for both the mean and variance of the Doppler spectrum are derived for two types of reflectivity gradients combined with a linear velocity, gradient. In the case of an exponential reflectivity, gradient it is found that the “effective” beam (i.e., the reflectivity-weighed two-way illumination pattern) remains Gaussian with identical beamwidth to the real beam, but its mean is shifted to an angle ϕm on the high-reflectivity side of the actual beam. With a linear velocity profile in the ϕ direction, the approximate solution shows that the mean Doppler velocity, is then shifted to the scatterer velocity found at ϕm. This approximation is shown to be valid for ...

Tower-Induced Errors in Wind Profile Measurements

Abstract: An investigation of the effect of a meteorological tower on the measurement of wind speed was conducted at Brookhaven National Laboratory. Reductions up to 35% occur in the wake. The distribution of the velocity deficit in the wake is nearly Gaussian; corrections based on this distribution have improved wind data in a trial case. A slight dependence on atmospheric stability was detected; the deficit appears to be independent of the wind speed.

Dynamic model of ionospheric wind profiles

Abstract: A statistical analysis of velocity, shear, and wavelength versus altitude has recently been reported for 70 wind profiles observed at midlatitudes between 90- and 150-km altitude. Hines had previously considered theoretically the characteristics of a rotating wind vector with energy supplied from below and viscous loss within the region. The present paper predicts profiles from this model with two assumptions: (1) the viscosity varies from ten-fold molecular viscosity at 90 km (because of turbulence) to its molecular value at all heights above 105 km, and (2) the time constant for supply of energy is semidiurnal. These two assumptions lead to altitude dependence of speed, shear, wavelength, and energy content and dissipation, which are substantially identical to those obtained by the statistical analysis.

The Mesoscale Wind Field in an Occlusion

Abstract: Data from a 10-site mesoscale rawinsonde network are used to document a number of features in the wind field in a mature cyclone of moderate intensity. The general nature of, and some particular phenomena in, the mesoscale wind field are discussed. Fields of divergence and vertical velocity are presented and the ageostrophic nature of the wind shear is demonstrated. Specific features examined include mid-tropospheric oscillations in wind direction, a low-level jet stream, the coincidence of the tropopause with an ageostrophic level of maximum wind, and the jet-front shear zone. In view of these results from AFCRL's project Stormy Spring, some of the capabilities and limitations of mesoscale rawinsonde data are discussed.

Small-scale variations of radio refractive index in the troposphere. Part 1: Relationship to meteorological conditions

Abstract: A description is given of results obtained from soundings made with a spaced-cavity balloon-borne refractometer and 10cm-band vertical-incidence radar. The experiments, carried out at Cardington, Beds., England, in 1964 and 1965, show that at heights up to 2km there is often considerable inhomogeneity in the refractive-index structure, even in nonanticyclonic conditions. Instantaneous differences in refractive index of 5 parts in 106 (5N units) were observed over distances of 11cm within discrete parcels of air in which the quotient of variance by scale size was unusually large, occasionally 10−11m−1. Such parcels were often located at or near the base of a layer of steep mean vertical gradient in refractive index associated with elevated inversions or isothermals. The results are discussed in relation to meteorological factors, such as wind shear and dynamic stability, and to theories of tropospheric scatter based on the concept of homogeneous turbulence.

On the orientation of cloud bands

Abstract: The growth rate of line perturbations in a horizontally stratified saturated fluid is examined for the case of a vertical shear in the mean wind. Certain of these perturbations are unstable for Richardson numbers less than minus two, while perturbations along the direction of the wind shear vector are unstable so long as the static stability is less than zero. It is concluded that cloud bands will tend to be oriented in the wind shear direction. DOI: 10.1111/j.2153-3490.1968.tb00357.x

Spurious Pressure Fluctuations in Wind Tunnels

Abstract: Wall pressure spectra measured in wind tunnels usually exhibit a pronounced rise at low frequencies, in conflict with theoretical predictions and with those experiments carried out without the constraint imposed by the wind tunnel Some wind tunnel experiments are described to show that typically the spurious disturbances arise in the first diffuser and travel upstream as acoustic waves Removal of the diffuser produces a spectrum falling at low frequencies

A further study of Jimsphere wind profiles as related to space vehicle design and operations

Abstract: Statistical characteristics of wind profiles from Jimsphere balloon sensors for space vehicle design

Small-scale wind and temperature structure as evidenced by meteorological rocket systems.

Abstract: Small scale wind and temperature structure from wind and temperature data from Japanese MT 135 and American ARCAS meteorological rocket systems

Direct Computation of Geostrophic Winds from Observed Winds Using the Balance Equation

Abstract: In certain meteorological problems it is desirable to make a grid-point analysis of observed winds. In order to combine a wind analysis with other quantities, or to use the wind analysis in a numerical model, one may wish to know the geostrophic winds (or the heights) that conform to the analyzed winds. Many previous studies have shown that the balance equation accurately describes relationships between winds and the height field. In this paper the balance equation is used to equate geostrophic vorticity at each grid point to a sum of terms evaluated from the wind analysis. Then the geostrophic winds are found by altering an initial guess field of zonal and meridional wind components until the geostrophic vorticity and divergence required at each point are met within a close tolerance. The final wind vectors are a geostrophic field that conforms to the original wind analysis according to the balance equation. Typical examples are shown of geostrophic wind vectors computed by this method in a slop...

A Comparison of Low-Level Trajectories in an Urban Atmosphere

Abstract: Throughout the month of June 1965 tetroons ballasted for 150–300 m altitude were released and radar-tracked in New York City and environs. The study evaluates the skill with which data from both a dense mesoscale network of surface wind observations and a less dense network of balloon-derived wind observations in the planetary boundary layer can be used to reconstruct the tetroon trajectories. Root-mean-square errors in predicting 2- and 4-hr tetroon positions from surface-wind-derived trajectories are reduced by the addition of a vector correction to account for vertical wind shear; this correction also randomizes the direction of the errors. Corrected surface wind trajectories, when compared with the tetroon trajectories, are slightly better than those computed from the balloon-derived winds. The best results obtained yield rms 2-hr prediction errors of 15 km; the median error of this distribution expressed as a percentage of the range of each tetroon flight was 36%.

Analog study of the longitudinal response of a swept-wing transport airplane to wind shear and sustained gusts during landing approach

Abstract: Analog computed time histories and altitude-ground track plots of swept wing transport aircraft longitudinal response to wind shear and sustained gusts during landing approaches

A study of wind and temperature variability at white sands missile range, new mexico,

Abstract: : Four cases of variation of wind and temperature over periods of approximately 12 hours in the first 152 meters of the planetary boundary layer are examined in some detail. An effort is made to explain some observed marked changes in wind and temperature which may not be apparent on the usual synoptic chart because of their mesoscale or microscale characteristic. (Author)

Diurnal Oscillations of the Tropospheric Wind Field above a Low-Level Jet

Abstract: An hypothesis is given for the cause of the observed diurnal oscillations in the mid- and upper-tropospheric wind field that occur in association with oscillations of the boundary layer wind field and low-level, nocturnal jet occurrences. A simple mathematical model of the middle and upper troposphere is solved using perturbation techniques subject to the condition that the three-dimensional velocity is continuous at a plane separating the boundary layer from the atmosphere above, and the vertical motion is zero at the tropopause height. Theoretical results are presented which show good agreement with previously published wind data from the surface to the region of the tropopause.