Showing papers on "Wind shear published in 1980"

Wind Stress and Heat Flux over the Ocean in Gale Force Winds

Abstract: An offshore stable platform has been instrumented with wind turbulence, temperature and wave height sensors. Data from this platform have been analyzed by the eddy correlation method to obtain wind stress and heat flux at wind speeds from 6 to 22 m s−1 in a deep-water wave regime, significantly extending the range of available measurements. The sea surface drag coefficient increases gradually with increasing wind speed. Sensible heat fluxes have been observed over a much wider range than previously available. Heat flux coefficients are higher in unstable than stable conditions, but are not seen to increase with increasing wind speed.

A new evaluation of the wind stress coefficient over water surfaces

Abstract: An analysis of data from numerous investigators, as well as information obtained directly by the authors, indicates that a large portion of the difficulties encountered in the past in establishing a relationship between the wind stress coefficient C10 and the wind velocity U10 can be attributed to computationally induced scatter of the data points. However, plots of the shear velocity u* against U10 reveal clear trends which show that three regions exist in the development of the wind shear stress: (1) a lower region in which the wind waves have not begun to break, and for which C10 is approximately constant; (2) a transitional region, after the onset of breakers, for which C10 varies nonlinearly with U10 and (3) a limiting region for which C10 tends again toward a constant value, and corresponds to a condition of breaker saturation. A single general equation to express C10 as a function of U10 is proposed, which agrees with the above findings. It is shown that in contrast with the perception of previous investigators, Charnock's coefficient α = z0g/u*2, where z0 is the roughness length and g is the acceleration due to gravity, is not constant anywhere in the range of wind velocities 0 < U10 < 40 m/s. Finally, the data indicate that the wind flow boundary in each one of the three regions described above can be classified as having ‘low roughness,’ ‘transitional,’ and ‘high roughness,’ respectively.

Zonal and meridional circulation of the lower atmosphere of Venus determined by radio interferometry

Abstract: The results of the Pioneer Venus differential long-baseline interferometry experiment are presented. The velocity component of the Pioneer probes as they fell to the surface of Venus was calculated from the Doppler shift of the received signal, and the other two orthogonal components were determined by long-baseline interferometry. The ambient wind velocity was about 1 m/s or less near the surface of the planet and about 100 m/s at an altitude of about 65 km at all four probe locations. Strata of high wind shear were found at altitudes of 15, 45, and 60 km. The wind velocity was always directed within a few degrees of due west except at a few km above the surface. The dominant motion of the lower atmosphere seems to be a retrograde zonal rotation, and eddies appear to account for most of the instantaneous meridional velocity. The data suggest that, within the clouds, a thermally driven mean meridional circulation is superimposed upon the much more rapid zonal rotation.

Role of Baroclinic Instability in the Development of Monsoon Disturbances

Abstract: A linear baroclinic stability analysis of the zonal wind representing the mean monsoon situation over India is performed by the use of a multi-level quasi-geostrophic numerical model. An initial value approach is chosen to determine the instability characteristics of the wind. The dependency of the growth rate spectrum on the number of levels in the vertical and on the presence of vertical walls is studied. It is shown that 20 levels in the vertical are sufficient to realize the baroclinic instability of the monsoon mean wind. A shorter unstable wave of wavelength 1500 km and a longer unstable wave of wavelength 4750 km are found to be the most preferred growing waves from the growth rate spectrum. The shorter unstable wave is essentially confined below 500 mb, whereas the longer unstable wave is above 500 mb. It is also shown that the removal of wind shear below (above) the level of the westerly (easterly) jet from the wind profile, shifts the shorter (longer) unstable wave toward higher wavelen...

Lidar Measurement of Wind Velocity Profiles in the Boundary Layer

Abstract: A lidar technique for measuring wind in the atmospheric boundary is presented. Inhomogeneities in ambient aerosol content are used as tracers of the wind. This technique yields both horizontal components of the wind and the wind velocity variance. These results are achieved using a model which assumes an isotropic Gaussian distribution of turbulent velocities. Experimental results comparing lidar wind measurements with winds derived from radar-tracked pilot balloons and tower-mounted anemometers show good agreement. Wind measurements have been obtained at slant range distances up to 6.5 km.

Deep Convection on Day 261 of GATE

Abstract: The structural features of the deep convection observed on September 18, 1974, day 261 of the GARP Atlantic Tropical Experiment (GATE), as the ridge axis of a 700 mb wave passed the center of the GATE B-scale network are reported. Satellite and aircraft maps indicate the presence of clouds penetrating above 2.5 km into the middle troposphere organized in bands about 9 km apart and aligned roughly along the direction of the wind shear in the cloud layer. Radar echoes corresponding to cumulus convection of lifetime, peak height and peak rainfall rates on the orders of 30 min, 6 km and 1.3 mm/h, respectively, were observed to triple in number density as convergence at 950 hPa increased from 1.5 to 3 x 10 to the -5th/sec. The structural features of the radar echoes indicate that the day was similar to a mesoscale precipitation feature of Leary and Houe (1979), with the cluster consisting of many echoes appearing in succession. Data from aircraft penetrations of the deep convection reveal downdrafts accompanying the precipitation and updrafts immediately to their south. Shipboard and rawinsonde observations show that the convective downdrafts brought down air of low pseudo-equivalent potential temperature, with local surface convergence of up to 0.001/sec. Mean wind shears through the cloud layer to the top of a main cloud layer are found to be only 75% greater than those of Malkus (1958) for the Caribbean, with shears just above the cloud base several factors larger.

The spectral characteristics of wind-generated capillary waves

Abstract: An experimental study of wind-generated capillary waves has been carried out in a laboratory wind-wave tunnel. In this tunnel it was possible to generate a stationary homogeneous wave field and to vary the wind over a range of speeds. A technique which makes use of a Preston tube has been used to measure the shear stress at the air-water boundary. Measurements of surface elevation and wave slope spectra in the capillary range of frequencies were obtained. Equations describing the wave spectra equilibrium range under the action of wind shear, surface tension and viscosity are derived. For frequencies beyond 15 hertz the measured spectra are in satisfactory agreement with the derived equations. Comparisons with other existing data have also been made.

Comparison of Mean Wind Speeds and Turbulence at a Coastal Site and Offshore Location

Abstract: Observations of mean wind speed and longitudinal turbulence at a height of 8 m over the Atlantic ocean, 5 km off Long Island, New York, were compared with simultaneous observations at the beach Results were grouped into wind direction classes characteristic of changes in roughness and fetch Mean winds over the ocean were 15–100% higher than those at the beach Changes in turbulence seem to depend on variations in the aerodynamic roughness of the sea surface and the thermal processes that take place over the water A decrease in turbulence over the ocean relative to that at the beach due to a decrease in sea surface roughness for alongshore flows could be predicted reasonably well with a simple logarithmic wind profile relationship

Long‐term measurements of Cn2 in the boundary layer

Abstract: The results of a year-long Cn2 measurement program to determine the feasibility for radar detection of hazardous wind shear are discussed. Monthly and hourly statistics of Cn2 are presented, and diurnal and annual cycles are identified. It is shown that Cn2 can be modeled as a one-parameter log normal random variable.

General motors sulfate dispersion experiment

Abstract: The mean wind field and the wind fluctuations observed near a controlled, heavily traveled level roadway were investigated. The mean wind field was studied in terms of velocity defects relative to the ambient mean wind. The wind fluctuations were studied in terms of energy spectra, velocity cross-spectra, excesses of velocity variances and cross-covariances over their ambient values. Two-point cross-spectra and correlations were also used to study the propagation of wind fluctuations. The influence of traffic on the ambient wind field extended vertically up to at least 4.5m above ground, and horizontally to at least 30 m downwind of the road. The vertical extension was more gradual at high cross-road winds than at low cross-road winds. The wind tended to move up as it approached the road and down as it left the road, except when the cross-road wind was low, where the downward motion was reduced or reversed. This is likely due to the buoyancy effect of the heated exhaust. The traffic effects propagated outward at about 1 m s-1. Thus at cross-road winds of less than 1 ms-1, the influence of traffic could be observed upwind of the road. When intense shear existed between the local wind field and an opposing traffic direction, large turbulent energy production resulted, generating large eddies, in additions to the high frequency (> 0.1 Hz) eddies generated by vehicular motions in the absence of intense shear. The large eddies also increased the mixing volume, so that when the ambient wind opposed the traffic direction on the upwind lanes, the pollutant concentration upwind of the road could be higher than that downwind. While the cross-covariance excesses generally agreed with the gradient transport assumption, disagreements also existed, which may not be surprising in view of the participation of large eddies in the momentum transport.

Wind shear responsive turbulence compensated aircraft throttle control system

Abstract: A turbulence compensated aircraft throttle control system is augmented with switching logic and signal processing to subtantially decrease the magnitude of wind shear-induced airspeed errors and the time required to recover from such airspeed errors The disclosed arrangements include a throttle controller having a speed control channel that is responsive to airspeed error and a turbulence compensation channel that is responsive to aircraft airspeed and longitudinal inertial acceleration of the aircraft A signal representative of the wind shear being encountered by the aircraft is derived within the turbulence compensation channel (or from signals provided thereby) and is processed to form a wind shear compensation signal that is coupled into the speed command channel whenever the airspeed error exceeds a predetermined amount The wind shear compensation signal supplements the airspeed error signal to substantially increase the magnitude of the throttle command signal In the more detailed disclosed arrangements signal coupling between the speed command channel and the turbulence compensation channel aids in controlling system response to changes in selected airspeed and, during an encounter with wind shear, also couples the wind shear compensation signal back into the turbulence compensation channel to cause even further corrective throttle action

Neutral winds in the polar cap

Abstract: The optical Doppler technique of the ground-based remote sensing of the thermospheric wind is reviewed with special consideration given to the admissibility of the assumptions made in the reduction of data from a single station. Examples of observations made in the Auroral Zone indicating substantial wind gradients and vertical winds are given. Such observations indicate the importance of small scale disturbances in the Polar thermosphere. Data from three stations in the Northern Auroral Zone and Polar Cap show how the flow pattern, the wind speed and its variation with magnetic activity strongly suggest that the main source of the momentum of the Polar Thermosphere is the convecting ions in the ionosphere.

The Effects of Evaporation, Water Load and Wind Shear on Cloud Development in a Three-Dimensional Numerical Model

Abstract: Steiner's (1973) three-dimensional cloud model has been updated to incorporate the deep anelastic system of equations and the microphysical parameterization scheme of Kessler (1969) for precipitation growth. The interactions between cloud microphysics and dynamics for an isolated cumulus cloud in a unidirectional shearing environment has been examined in four comparative experiments. The results confirm that 1) the intensity of convection is suppressed by shear except during the initial stage, and 2) evaporative cooling can have a more dominant effect than water loading in limiting the vertical development of a moderate size cumulus.

An airport wind shear detection and warning system using Doppler radar

Abstract: Two numerical models of aircraft performance are developed to relate atmospheric wind signals in wind-shear situations to aircraft response. Both models produce time histories of pitch angle, airspeed, height above the ground, and other parameters for simulating aircraft making precision approaches along a 3-deg glide slope to the runway. Simulations conducted using thunderstorm environment wind data indicate that the horizontal wind component is at least as important as the vertical component in determining aircraft response to wind shear. Obtaining quantitative measurements of aircraft approach or departure quality is found to depend on several factors, including the knowledge of winds along the expected flight track, clear-air detection in a quasi-horizontal path, and a near continuous estimate of approach deterioration due to shear conditions.

Surface meteorology over the gate a-scale

Abstract: A method of analysis for the surface motion field based on a mix of observations from research ships, merchant ships and low-cloud motion vectors from geostationary satellites is proposed in this paper. The proposed scheme permits a turning of wind with height between the cloud wind level and the surface. This turning is incorporated via a successive correction procedure which enables us to utilize fully the low-cloud winds. The method does not depend on the selection of a level of best fit for the cloud winds and thus is not dependent on a predetermined theory. The analysis of the surface wind field is carried out for the 100 days of the GARP Atlantic Tropical Experiment (GATE) for two observation periods per day (00Z, 12Z) over a wide domain. This paper illustrates i) time-averaged geometry of the motion field during GATE, and ii) time-averaged fields of the zonal and the meridional stresses as well as of the curl of the wind stress. Finally, we examine the spectral analysis of the motion field and the stresses over the trade wind belts of the summer and winter hemisphere. Our study emphasizes the importance of two modes of oscillation, one around 3 to 6 days and the other around 10 to 15 days. While the former is identified with African v/aves, the scale and speed of propagation of the latter is determined from x–t diagrams and is identified as a westward propagating wave, whose scale is of the order of 40° longitude and speed of propagation is around 3° longitude/day. This study emphasizes the possible usefulness of this approach during the GARP Global Experiment when a global coverage from geostationary satellites is expected.

Bifurcation and Stability in a Model of Moist Convection in a Shearing Environment

Abstract: The truncated spectral system (model I) of shallow moist two-dimensional convection discussed by Shirer and Dutton (1979) is expanded to eleven coefficients (model II) in order to include a basic wind. Cloud streets, the atmospheric analog of the solutions to model II, are typically observed in an environment containing a shearing basic motion field. Analysis of the branching behavior of solutions to mode II shows that, if the basic wind direction varies with height, very complex temporal behavior is possible as the modified Rayleigh number HR is increased sufficiently. The first convective solution is periodic, corresponding to a cloud band that propagates downwind; but secondary branching to a two-dimensional torus can occur for larger values of HR. Orientation band formulas are derived whose predictions generally agree with the results of previous studies.

The Low-Level Wind Shear Alert System (LLWSAS)

Abstract: : The results of the Low-Level Wind Shear Alert System (LLWSAS) field test and evaluation are reported. The system is a computer controlled anemometer mesonetwork used to detect near-surface wind discontinuities associated with strong thunderstorms and cold fronts -- those that are potentially hazardous to low-flying aircraft. The test and evaluation was conducted at seven major airports in the United States. This report discusses the principal meteorological events that will trigger alarms, the system hardware and software, and characteristics of the system that are unique to each airport. Heavy emphasis is placed on the proper siting of anemometry which was found to be the most important factor influencing high quality system performance. The siting criteria enumerated should serve as a model for future installations. Deviations from the model will severely impact system performance. The origin and justification for the vector difference threshold is discussed as well as LLWSAS by-products: wind gust determination and special displays for Terminal Radar Approach Control Facility (TRACON) rooms.

Wind speed measurement

Abstract: Wind is the large-scale movement of air from one place to another. It is usual for the air to become horizontally stratified, the layers nearer the ground moving more slowly than or in a different direction to those higher up. This change of wind speed with height is known as wind gradient (or wind shear), and it may cause serious problems for aircraft during their approach and landing. Unfortunately, at present there is no satisfactory way of ascertaining the wind gradient situation. The invention provides acceptable apparatus for, and a method of, actually detecting--and preferably measuring--wind gradient at a location, and involves a comparison of the wind speed in the same direction at two or more heights at the location, this comparision being upon the basis of a comparison of the speed of sound in that direction and at those heights, a difference in the apparent speeds indicating the presence of wind gradient. The invention involves: beaming a regular sound wave train between a transmitter/receiver pair positioned and like orientated at each of two or more heights at the location; noting each transceiver pair's received sound wave train phase, and comparing it with its transmitted phase, so as to deduce the wind-caused phase change; and using these deduced phase changes to calculate the actual wind speeds, and thus the relative changes of wind speed with height, in the selected direction.

Short-interval SMS wind vector determinations for a severe local storms area

Abstract: Short-interval SMS-2 visible digital image data are used to derive wind vectors from cloud tracking on time-lapsed sequences of geosynchronous satellite images. The cloud tracking areas are located in the Central Plains, where on May 6, 1975 hail-producing thunderstorms occurred ahead of a well defined dry line. Cloud tracking is performed on the Goddard Space Flight Center Atmospheric and Oceanographic Information Processing System. Lower tropospheric cumulus tracers are selected with the assistance of a cloud-top height algorithm. Divergence is derived from the cloud motions using a modified Cressman (1959) objective analysis technique which is designed to organize irregularly spaced wind vectors into uniformly gridded wind fields. The results demonstrate the feasibility of using satellite-derived wind vectors and their associated divergence fields in describing the conditions preceding severe local storm development. For this case, an area of convergence appeared ahead of the dry line and coincided with the developing area of severe weather. The magnitude of the maximum convergence varied between -10 to the -5th and -10 to the -14th per sec. The number of satellite-derived wind vectors which were required to describe conditions of the low-level atmosphere was adequate before numerous cumulonimbus cells formed. This technique is limited in areas of advanced convection.

Analysis of Surface Wind and Its Gradient In a Mesoscale Wind Observation Network

Abstract: The surface wind is analysed with an optimum interpolation method. Covariances and long-term averages of wind observations are parameterized from data in the fairly dense synoptic wind observation network in the Netherlands. The analysis scheme leads to an rms error of 1.5 m s−1 in wind speed estimates. The error is approximately normally distributed, with a maximum value of 7 m s−1 associated with heavy rainshower conditions. Derived surface wind divergence and vorticity agree quantitatively with independently estimated values.

Atmospheric pressure and velocity fluctuations near the auroral electrojet

Abstract: The low-frequency pressure and velocity perturbations caused by the temporally varying Lorentz force associated with auroral electrojet activity are modeled by calculating the disturbances generated by a two-dimensional, time-dependent current system in a gravitationally stratified, isothermal, windless atmosphere. These calculations provide information about the pattern of gravity waves around the hypothetical electrojet and give estimates of the magnitudes of near-field auroral disturbances in the middle atmosphere. It is suggested that the near-field vertical wind shears may be large enough to affect the development of air turbulence in the auroral zone.