Showing papers on "Wind shear published in 1977"

Meteorological results from the surface of Mars: Viking 1 and 2

Abstract: The meteorology equipment carried by the Viking landers was intended to measure atmospheric temperature, wind speed, wind direction, and pressure. During the summer months, the winds were a few meters per second, with a complex hodograph and the Lander-1 site, dominated by counterclockwise turning of the wind, and a simpler hodograph at the Lander-2 site, marked by clockwise turning of the wind. With advancing season, the repetitive wind pattern began to break down, and protracted northeasterly winds were recorded on several occasions (some of which are associated with lower than normal temperatures). Examples are given of wind and temperature traces over short periods, illustrating the effects of convection, static stability, and lander interference. A theoretical argument, based on the horizontal scale dictated by heating of slopes and on vertical mixing of momentum, is presented to explain the different sense of wind rotation at the two lander sites.

A Realistic Model of the Wind-Induced Ekman Boundary Layer

Abstract: A physically realistic and general model for the vertical eddy viscosity in a homogeneous fluid is proposed. For an infinitely deep ocean the vertical eddy viscosity increases linearly with depth from a value of zero at the free surface. Based on this model a general theory is developed for the drift current resulting from a time-varying surface shear stress. Explicit expressions are given for the temporal development of the drift current in the vicinity of the free surface and for the steady-state response to a suddenly applied uniform shear stress. The steady-state solution predicts the effective Ekman layer depth to be proportional to the square root of the wind shear stress and reproduces the experimentally observed logarithmic velocity deficit near the free surface. The angle between the surface drift current and the wind stress is found to be somewhat smaller (of the order 10°) than predicted by Ekman's classical solution. For the unsteady response to a suddenly applied wind stress the pres...

Tropical Cyclone Minimum Sea Level Pressure/Maximum Sustained Wind Relationship for the Western North Pacific

Abstract: Determining the proper relationship between the minimum sea level pressures and maximum sustained winds in tropical cyclones has been a long-standing problem The major obstacle has been the lack of sufficient ground truth, ie, actual measurements of maximum wind speeds in tropical cyclones with a wide range of central pressures In this study 28 years of maximum wind measurements made at coastal and island stations in the western North Pacific were collected and analyzed Because of problems in measuring and interpreting sustained surface wind speeds, only recorded peak gust values were used These peak gust values were reduced to a standard anemometer level of 10 m using a power law relationship and then converted to 1 min sustained wind speeds using gust factors representative of an overwater environment The sample was restricted to cases where it was reasonably certain that the station experienced the cyclone's winds during its passage The resulting equation,where pc is the minimum sea le

A wind-tunnel investigation of the wind speed and turbulence characteristics close to the ground over various escarpment shapes

Abstract: A wind-tunnel investigation of the wind flow over two-dimensional forward-facing escarpments is reported as part of a continuing research programme into the effects of local topography on the wind flow close to the ground. Four sharp-edged escarpments with their slopes varying between a cliff and a 4:1 gradient, were placed normal to a simulated neutrally-stable rural boundary layer which was modelled to a scale of 1:300. The resulting flows close to the surfaces of the escarpments were measured with a hot-wire anemometer. The modifications to the mean wind speed, turbulence intensity and energy spectra over the escarpments are described. The results indicate the extent and magnitude of the modification to the flow and suggest that significant changes in turbulence characteristics only occur in the wake region close behind the crest, where a shift of energy to higher frequencies is evident.

Dependence of the wind profile power law on stability for various locations

Abstract: Recent environmental regulations have increased the need for construction of meteorological towers at power generation facilities. Due to practical and economic considerations, tower heights are usually lower than effluent release heights. At heights where wind speed data are not available, the wind speed is usually estimated from the measured wind speed using the %th wind profile power law and assuming neutral stability conditions. This study examines published data for many locations and shows that the %th wind profile power law is often unrepresentative of actual conditions because the degree of variation of wind speed with height depends greatly on atmospheric stability. The frequency of neutral stability conditions also varies appreciably by site. These two considerations are especially important in dispersion models which extrapolate wind speed at stack height from low level wind speed data.

Reducing Wind Erosion with Barriers

Abstract: WHEN wind shear forces on the soil surface exceed the soil's resistance to those forces, soil particles detach and are transported by the wind. Barriers ob-struct the wind and reduce the wind's speed, thus, reducing wind erosion. Application of a model, developed for evaluating ef-fectiveness of a barrier for reducing the capacity of local wind to cause erosion, illustrated: (a) Wind erosion forces are reduced more than windspeed. (b) Properly oriented barriers give much more protection when pre-ponderance of wind erosion forces in prevailing wind erosion direction is high than when preponderance is low. (c) When preponderance of wind erosion forces is low, barrier orientation is almost inconsequential. (d) Because of seasonal variation of wind direction and speed, need for and amount of protection also vary seasonally. Many trees, shrubs, tall growing crops, and grasses, and slat-fences all can be used as barriers to reduce wind erosion.

A Comparative Study on Parameterization of Vertical Turbulent Exchange Processes

Abstract: A one-dimensional version of the Techniques Development Laboratory boundary layer model has been adopted to simulate the O'Neill fifth period and the Wangara day 32 experiment data. Fourteen parameterization schemes for the vertical turbulent exchange processes in the atmospheric boundary layer are examined. These include 1) conventional K theories based on Blackadar's mixing length with wind shear and stability functions, 2) O'Brien K formula with time-varying mixed layer heights governed by the surface heat and momentum fluxes based on the work of Deardorff, and 3) a turbulent energy closure model. Numerical results were compared to observations. In general, it was found that all the models perform well in the simulation of the Wangara day 32 data but less satisfactorily in the O'Neill fifth-period simulation. The most satisfactory simulation in wind speeds was reproduced by the turbulent energy model. For temperatures the O'Brien K formula topped by the mixed layer heights performs the best, p...

Energy Dissipation Within Intermittent Clear Air Turbulence Patches

Abstract: Mean dissipation rates of kinetic energy in the lower stratosphere are estimated from data obtained with an instrumented, superpressure, constant-level balloon launched to measure the vertical wind shear at 200 mb. The flight record shows that while the balloon was located above a mountainous region localized patches of turbulence were encountered. The record is divided into 20 periods. The structure function is calculated over each period and, under the hypothesis of an inertial subrange, the mean dissipation is in turn estimated over each period. Our calculations indicate that the mean dissipation rate of kinetic energy can vary to a great extent over distances as short as a few tens of kilometers. Thus, when the shear layer breaks down, the mean dissipation rate can be 10 times higher than just before or after the event. It can also be 100 times higher within moderately intense clear air turbulence patches than in very weak intensity turbulence zones. These results show the sporadic nature of ...

On the Wavelength of Maximum Baroclinic Instability

Abstract: A four-level quasigeostrophic model of a baroclinic atmosphere is used to examine the instability of short (∼2000 km) baroclinic waves. It is determined that only a slight decrease in the low-level static stability or increase in the low-level wind shear relative to the stale stability and wind shear in the middle and upper troposphere can mean the difference between the maximum growth rate occurring at a wavelength of 4000 km (∼wavenumber 7) or 2000 km (∼wavenumber 15). Similar changes of static stability in the upper troposphere relative to the middle and lower troposphere have very little effect on the growth-rate spectrum. This effect of vertical variations in the static stability and wind shear on the growth-rate spectrum is consistent with the structure of the short wavelengths. Wavelengths 4000 km extend through the depth of the troposphere. Therefore, changes in the static stability of the basic zonal flow near the earth's...

Neutral atmospheric waves in the thermosphere and tropospheric weather systems

Abstract: Neutral atmospheric density profiles between 135 and 225 km obtained from a cold cathode ion gauge onboard the Atmosphere Explorer-C spacecraft frequently exhibit considerable wave-like structure, which is interpreted to be internal gravity waves. We have attempted to correlate the occurrence probability and amplitude of these waves with several parameters of geophysical importance. Only weak correlations are found with common parameters, such as Kp, for which strong correlations with the neutral atmospheric density are known to exist. However, the waves do appear over certain preferred geographical locations which tend to coincide with regions of high wind speeds and windshear in the upper troposphere. This relationship leads us to suggest that the majority of the waves observed in our data originate in the troposphere and subsequently propagate upwards into the thermosphere.

Electric fields by gravity wave winds in the equatorial ionosphere

Abstract: The electric fields engendered by winds of gravity wave origin are theoretically calculated for the dynamo region of the equatorial ionosphere. A general expression for the efficiency parameter, which is a measure of the effectiveness in producing the total electric field, is derived for a two-dimensional case, and the results are compared with those obtained from the one-dimensional treatment of the problem by Kato (1973). The efficiency parameter evaluated by Kato (1973) at the center of the electrojet is higher by about a factor of 5 than that computed here, owing to some approximations made by him. The dependence of the efficiency parameter on the horizontal scale size of the gravity wave wind and the effectiveness of the east-west wind shear in producing the electron density stratifications are discussed.

Sources of low-level wind shear around airports

Abstract: Some potential sources of low-level wind shear in and around airports and their likely effects are probed and analyzed. Wind shear over flat terrain with near-homogeneous surface properties (roughness, specific heat), the turning layer, shear flows over inhomogeneous terrain (airport + urban areas), thunderstorms, turbulent flowfields over bluff bodies (individual buildings), and recirculating wake flow downstream of three-dimensional block bodies are among the topics covered. Overshoot or undershoot of runways, and induced moments (pitch, roll, yaw) in takeoff and landing, and other potential hazards traceable to wind shear patterns at low heights are discussed, with emphasis on mean flow or steady-state wind shear (time-averaged, say 2-min averaged, wind fields). Wind tunnel studies and V/STOL operations are included in the study.

An acoustic sounder for the remote probing of the lower atmosphere

Abstract: Details are given of an atmospheric acoustic sounder which can be used in both a rural area and a noisy town environment. The instrument has been used for the measurement of various parameters of the turbulent boundary layer including the temperature and velocity structure parameters CT2 and Cv2 as well as vertical wind speeds. A summary of some of the results obtained using this instrument is presented.

A field study of wind over a simulated block building

Abstract: A full-scale field study of the wind over a simulated two-dimensional building is reported The study develops an experiment to investigate the structure and magnitude of the wind fields A description of the experimental arrangement, the type and expected accuracy of the data, and the range of the data are given The data are expected to provide a fundamental understanding of mean wind and turbulence structure of the wind field around the bluff body Preliminary analysis of the data demonstrates the reliability and completeness of the data in this regard

Wind structure in strong winds below 150 m

Abstract: In a discussion of wind fluctuations it is suggested that the structure of winds below 150 M is largely determined by the large-scale pressure gradient and the characteristics of turbulence. Two main forms of atmospheric turbulence are described as mechanical turbulence, driven by the vertical shear and the wind; and heat convection, generally driven by heating from below. The author considers that in strong winds, mechanical turbulence is frequently far stronger than heat convection, and that the latter can be neglected. This review of the subject is therefore concerned with wind properties under conditions of dominant mechanical turbulence, but the question of whether or not heat convection can be neglected is also discussed by reference to the richardson number. The review discusses the distribution of mean wind over homogeneous terrain in strong winds; the effect of stability on wind profiles by reference to the monin-obukhov theory; the effect of terrain change on wind speed, in that when air moves from rough to smooth terrain the surface stress immediately drops so that just above the ground the air accelerates; wind variability; and cross spectra of wind speed as described by cospectrum and quadrature spectrum or statistics derived from them. /TRRL/

Application of doppler weather radar to turbulence measurements which affect aircraft

Abstract: : Analysis of thunderstorm turbulence hazardous to aircraft operation and coordinated Doppler radar observations indicate a high potential for Doppler radar utilization particularly the mean velocity spectrum breadth observations in defining severe turbulence areas. The mean Velocity Processor (MVP, the first real-time display of Doppler radar data) and the Multi-moment Ling Display (MMD), both developed at NSSL, are utilized with the radars to study vortex motion, turbulence, and wind shear areas. In addition, the Plan Shear Indicator (PSI) developed by the Air Force Cambridge Research Laboratory (AFCRL) was also employed. A number of severe convective storms were penetrated by an instrumented aircraft directed into areas which analysis inferred to be turbulent. Aircraft recorded turbulence and concurrent Doppler data are compared. Utilization of the spectrum breadth calculated from the mean velocity data as a turbulence signature is discussed. Vortex motion signature is also defined. (Author)

The Dulles Airport Pressure Jump Detector Array for Gust Front Detection

Abstract: Wind shear has long been recognized as one of the major aviation hazards in the airport environment. A principal source of dangerous wind shear is the thunderstorm gust front, a cold air outflow from the thunderstorm downdraft. The gust front is particularly hazardous not only because of the large surface wind shears associated with it, but also because of its highly localized character. Often the down-draft or downburst region producing such fronts is of the order of a few kilometers or less in dimension. As a result, vertically profiling wind shear detection techniques such as the hybrid acoustic-microwave radar system described in the companion paper by Hardesty et al. (1977) do not provide adequate total protection. In this paper we describe an array of pressure sensors installed at Dulles International Airport in Washington, D.C., to detect and monitor gust fronts that could endanger aircraft operations. The pressure sensors (designed to respond only to sudden pressure increase) are so inexp...

Comparison of Developing vs. Non-Developing Tropical Disturbances,

Abstract: : Developing and non-developing western North Pacific tropical disturbances are investigated and compared utilizing Defense Meteorological Satellite Program (DMSP) and rawinsonde data. Quantitative measurements of deep convection and cirrus amounts from the satellite visual and infrared data indicates that no large deep-convective difference exists between these two classes of disturbances. Genesis appears to be related to the special positioning of the deep convection and surrounding wind fields. When genesis occurs, the deep convection appears to act indirectly to warm the tropical disturbance by means of dynamically forced subsidence. Documentation is provided to support this hypothesis. Relative vorticity, vertical shear, and characteristics of the cirrus level outflow patterns were determined to be parameters which displayed the largest difference between the two classes of developing and non-developing disturbances. It is hypothesized that the potential for tropical disturbance genesis is primarily related to: the large-scale surrounding relative vorticity at lower and upper tropospheric levels, the strength of the disturbance's dynamically forced subsidence, and magnitude of disturbance ventilation or wind blow-through.

A theoretical analysis of airplane longitudinal stability and control as affected by wind shear

Abstract: The longitudinal equations of motion with wind shear terms were used to analyze the stability and motions of a jet transport. A positive wind shear gives a decreasing head wind or changes a head wind into a tail wind. A negative wind shear gives a decreasing tail wind or changes a tail wind into a head wind. It was found that wind shear had very little effect on the short period mode and that negative wind shear, although it affected the phugoid, did not cause stability problems. On the other hand, it was found that positive wind shear can cause the phugoid to become aperiodic and unstable. In this case, a stability boundary for the phugoid was found that is valid for most aircraft at all flight speeds. Calculations of aircraft motions confirmed the results of the stability analysis. It was found that a flight path control automatic pilot and an airspeed control system provide good control in all types of wind shear. Appendixes give equations of motion that include the effects of downdrafts and updrafts and extend the longitudinal equations of motion for shear to six degrees of freedom.

Flow due to a two-dimensional heat source with cross flow in the atmosphere

Abstract: This paper presents a model describing the flow field due to a two-dimensional infinitely long heat source in a stably-stratified atmosphere with a uniform wind cross-flow. The governing equation which is valid for any wind shear is non-linear in its general form and is solved for uniform flow only. The energy equation is not solved but a “source” function is assumed. The results of this model exhibit some of the observed mass fire characteristics such as the inflow window, the pinch effect, the upstream downdraft and a strong dynamic convection column. Blocking and the formation of eddies are shown to be the most important characteristics of the onset of mass fire conditions.

The thermal structure of small lakes: The influence of a modified wind speed

Abstract: The thermal structure of a lake is determined primarily by the wind shear, which induces turbulence in the lake. In the case of a lake of small areal extent, numerical models employing observed or climatological functions for the wind speed give unacceptable results. It is shown that if the wind speed is modified by a function of the lake surface area, then the annual stratification of a small lake can be realistically described. This technique is illustrated for the case of Farmoor Reservoir in England.

The Dulles Airport Acoustic-Microwave Radar Wind and Wind Shear Measuring System

Abstract: Wind shear has been recognized as a major aviation hazard in the airport environment. A dual, acoustic Doppler microwave Doppler radar system has been installed at Dulles International Airport near Washington, D.C., to measure the vertical profile of wind from the surface to 510 m in 30 m height increments. The acoustic system gathers data under clear-air conditions, and the microwave radar takes over automatically when precipitation is present. System performance is being assessed by comparing its output with National Weather Service radiosondes and with balloon-borne anemometers and by intercomparing the acoustic- and microwave-measured winds under light precipitation conditions. The dual-sensor system has been operating for several months, registering the passage of fronts, some with potentially hazardous wind shears.

Mean horizontal wind profiles measured in the atmospheric boundary layer about a simulated block building

Abstract: Instrumented wind towers are used to measure the three components of wind about a simulated block building. The mean horizontal wind profiles over the building are compared with wind profiles measured in the absence of the building and the wind speed deficit in the wake of the building is correlated. The turbulence intensity is of the order of 20% in the undisturbed flow whereas the free stream turbulence intensity of wind-tunnel studies is generally not more than 5%. The velocity profiles measured in the undisturbed flow zones support the representation of a neutrally stable atmospheric boundary layer with a logarithmic wind profile.

Model predictions of wind and turbulence profiles associated with an ensemble of aircraft accidents

Abstract: The feasibility of predicting conditions under which wind/turbulence environments hazardous to aviation operations exist is studied by examining a number of different accidents in detail. A model of turbulent flow in the atmospheric boundary layer is used to reconstruct wind and turbulence profiles which may have existed at low altitudes at the time of the accidents. The predictions are consistent with available flight recorder data, but neither the input boundary conditions nor the flight recorder observations are sufficiently precise for these studies to be interpreted as verification tests of the model predictions.

Development of a digital automatic control law for steep glideslope capture and flare

Abstract: A longitudinal digital guidance and control law for steep glideslopes using MLS (Microwave Landing System) data is developed for CTOL aircraft using modern estimation and control techniques. The control law covers the final approach phases of glideslope capture, glideslope tracking, and flare to touchdown for automatic landings under adverse weather conditions. The control law uses a constant gain Kalman filter to process MLS and body-mounted accelerometer data to form estimates of flight path errors and wind velocities including wind shear. The flight path error estimates and wind estimates are used for feedback in generating control surface commands. Results of a digital simulation of the aircraft dynamics and the guidance and control law are presented for various wind conditions.

Transient wind erosion: a study of the nonstationary effect on rate of wind erosion

Abstract: Wind velocities are often nonstationary with an almost cyclical repetition of strong gusts followed by relative calmness. It was demonstrated that the effects of nonstationary wind velocity on soil erosion are significant, and that a transient model is necessary to estimate rates of wind erosion. /Author/