Showing papers on "Wind shear published in 1985"

Gas exchange-wind speed relation measured with sulfur hexafluoride on a lake.

Abstract: Gas-exchange processes control the uptake and release of various gases in natural systems such as oceans, rivers, and lakes. Not much is known about the effect of wind speed on gas exchange in such systems. In the experiment described here, sulfur hexafluoride was dissolved in lake water, and the rate of escape of the gas with wind speed (at wind speeds up to 6 meters per second) was determined over a 1-month period. A sharp change in the wind speed dependence of the gas-exchange coefficient was found at wind speeds of about 2.4 meters per second, in agreement with the results of wind-tunnel studies. However, the gas-exchange coefficients at wind speeds above 3 meters per second were smaller than those observed in wind tunnels and are in agreement with earlier lake and ocean results.

Lidar observations of vertically organized convection in the planetary boundary layer over the ocean

Abstract: Observations of a convective planetary boundary layer (PBL) were made with an airborne, downward-looking lidar system over the Atlantic Ocean during a cold air outbreak. The lidar data revealed well-organized, regularly spaced cellular convection with dominant spacial scales between two and four times the height of the boundary layer. It is demonstrated that the lidar can accurately measure the structure of the PBL with high vertical and horizontal resolution. Parameters important for PBL modeling such as entrainment zone thickness, entrainment rate, PBL height and relative heat flux can be inferred from the lidar data. It is suggested that wind shear at the PBL top may influence both entrainment and convective cell size.

Radar detection of hazardous small scale weather disturbances

Abstract: The detection and warning of microbursts, low level wind shear, and other weather disturbances, which are hazardous to aircraft operations and to the public at large, are accomplished with either an airport surveillance radar (ASR) or a multi-beam Doppler radar. ASR Doppler systems normally operate to receive one of two relatively large vertical fan beams having different elevation angles but which overlap one another so that they have equal gains at an elevation angle, called the null, at a relatively low angle, for example 5°. Below this null, the low beam antenna gain exceeds that of the high beam, and conversely above it. Accordingly, by subtracting the high beam Doppler spectrum from that on the low beam, a Difference Doppler Spectrum (DDS) is produced which is positive below the null and negative above. The velocity bounds of the positive portion of the DDS provide the wind speed components at the null and at heights near the surface. These wind speed components are then utilized to measure and map radial and horizontal shear, the boundaries of the disturbance and other signatures such as vertical shear and turbulence and the rate of change of all the parameters, thereby permitting the detection of the location and track of the disturbance. A multi-beam Doppler radar can be utilized to perform similar functions of measuring the mean Doppler velocity, Doppler spectral breadth, and reflectivity simultaneously at all elevations. Both systems provide effective enhancements in signal to clutter ratio through pattern recognition and motion detection.

Forecasting Dry Microburst Activity over the High Plains

Abstract: The active dry microburst days during the 1982 JAWS (Joint Airport Weather Studies) Project in Colorado are examined for common characteristics. The environments on these days are shown to have similar thermodynamic structures in the vertical. In the morning, a shallow radiation inversion is capped by a deep, dry-adiabatic boundary layer. Moisture is present at midlevels. By evening the radiation inversion has been replaced by a superadiabatic layer at the surface. Solar heating of the boundary layer is shown to be important for producing an environment favorable for dry microbursts. A model is proposed that can be used by forecasters to issue a “wind shear watch” to the general public and aviation community. Peak downdraft speeds associated with dry microbursts appear to be a result of negative buoyancy, owing to the evaporation of precipitation during the descent below cloud base. These downward velocities are of the same magnitude as the horizontal wind speeds. Entrainment of subcloud air into...

Three-Dimensional Numerical Modeling of Convection Produced by Interacting Thunderstorm Outflows. Part I: Control Simulation and Low-Level Moisture Variations

Abstract: The Klemp–Wilhelmson three-dimensional numerical cloud model is used to investigate cloud development along intersecting thunderstorm outflow boundaries. The model initial environment is characterized by a temperature and moisture profile typically found in strong convective situations, and the initial wind field is prescribed by a constant unidirectional shear 2.9 m s−1 km−1 from 0.8 to 8.9 km, with a constant wind everywhere else. The wind shear vector is perpendicular to the line containing the two initial outflow-producing clouds (which are spaced 16 km apart and are triggered by thermal impulses centered at the top of the boundary layer). The dynamics of the outflow collision are documented using time-dependent, kinematic air parcel trajectories and thermodynamic data. We find that ambient air in the outflow collision region is literally “squeezed” out of the way as the two outflows collide. Some of this air is lifted to saturation, triggering two convective clouds. The upshear member of the...

Postset beam steering and interferometer applications of VHF radars to study winds, waves, and turbulence in the lower and middle atmosphere

Abstract: Spaced antenna VHF radar measurements are utilized for the postset beam steering technique and for interferometer applications to study winds, waves, and turbulence in the stratosphere and mesosphere. Following a brief description of the basic instrumental setup and the off-line analysis method, some results are presented which demonstrate the consistency and applicability of these methods. Vertical velocity oscillations due to gravity waves are analyzed by means of the cross-spectrum technique. It is shown that phase differences of gravity wave oscillations, measured between vertical and two opposite, off-vertical postset beams, change their sign when the beam is swung back and forth. The same features of change of velocity sign are noticed when the mean wind is observed. The mean wind deduced with the postset beam steering technique is similar to the wind deduced with the spaced antenna drift technique and the independent radiosonde winds. Taking these results as justification for the applicability and feasibility of the postset beam steering method, the total wave vector and the intrinsic angular frequency of gravity waves in the stratosphere are deduced. Reasonable arguments are discussed to support the suggestion that these waves were generated in lower stratosphere wind shear regions and were attenuated little when propagating up to the middle stratosphere. Turbulence blobs in the mesosphere are tracked with the interferometer technique. It is shown that these blobs moved horizontally as well as vertically, a feature which cannot unequivocally be detected with conventional Doppler methods.

Prediction of broadband noise from horizontal axis wind turbines

Abstract: A method is presented for predicting the broadband noise spectra of horizontal axis wind turbine generators. It includes contributions from such noise sources as the inflow turbulence to the rotor, the interactions between the turbulent boundary layers on the blade surfaces with their trailing edges, and the wake due to a blunt trailing edge. The method is partly empirical and is based on acoustic measurements of large wind turbines and airfoil models. The predicted frequency spectra are compared with measured data from several machines, including the MOD-OA, MOD-2, WTS-4, and U.S. Windpower Inc. machine. The significance of the effects of machine size, power output, trailing-edge bluntness, and distance to the receiver is illustrated. Good agreement is obtained between the predicted and measured far-field noise spectra.

Long-range transport of air pollution under light gradient wind conditions

Abstract: The long-range transport of air pollution on clew days under light gradient wind conditions is investigated from an analysis of all days with high oxidant concentrations in 1979 at locations in central Japan that are far from pollutant sources. Surface-level wind and pressure distributions over a 300×300 km area were analyzed, together with concentration isopleths of oxidants and suspended particles produced by photochemical reactions. It was found that the transport mechanism consists of. 1) land/sea breezes; 2) a steady onshore wind driven by the diurnal-mean land–sea temperature difference; 3) the generation of a strong thermal low in the inland mountainous region in the daytime; and 4) a subsidence inversion accompanied by a synoptic-scale high pressure system. The last three mechanisms work to combine land/sea breezes and slope and valley winds into one large-scale high-speed wind field that transports pollutants a long distance inland into the mountainous region.

Classification of mesoscale vortices in polar airstreams and the influence of the large-scale environment on their evolutions

Abstract: Satellite imagery was used to detect and study 133 mesoscale cloud vortices which occurred in the northeastern Atlantic Ocean and adjacent seas between 1 December 1981 and 5 January 1982. Only vortices occurring in cold air, separate from synoptic-scale frontal cloud bands, were considered. The complete history of the evolution of each cloud vortex was recorded in terms of its location, size, cloud pattern, and cloud type (shallow or deep; stratiform or convective). Once all vortices were documented, conventional meteorological data and analyses were consulted. One objective of the study was to determine whether satellite imagery could be used to estimate the central pressure of the cyclones accompanying the mesoscale vortices. Other objectives were to determine if there were significant differences in (1) the mesoscale cloud structure or (2) the large-scale environment of vortices which remained insignificant versus those that developed into polar lows. Results indicate that the deficit pressure of the polar vortex could be estimated from satellite imagery and that there were recognizable differences between developing and non-developing vortices. Only a few cloud configurations accompanied polar lows, whereas the occurrence of certain other configurations at early stages in the vortex existence indicated that these vortices would remain insignificant. The synoptic-scale environment appeared to play a considerable role in determining which incipient vortices continued to develop. Moderate baroclinicity (horizontal temperature gradients and vertical wind shear), relatively weak winds, large lapse rates, and favorable vorticity advections accompanied developing polar lows. Non-developing vortices had unfavorable vorticity advections. Of these, some vortices were able to grow initially to moderate disturbances which remained steady thereafter. The apparent difference between these and insignificant vortices was that the steady vortices were more baroclinic and had steeper lapse rates. DOI: 10.1111/j.1600-0870.1985.tb00276.x

Influence of Geostrophic Wind on Atmospheric Nocturnal Cooling

Abstract: A dynamic radiative model was used to study the influence of the geostrophic wind on the nocturnal cooling processes. For weak wind conditions, an important difference appears between the top levels of the inversion and turbulent layers. The dimensionless vertical profile of turbulent heat flux presents an important curvature at the beginning of the night; afterwards this profile varies little during the night.

Structure and evolution of a severe squall line over Oklahoma

Abstract: The structure of a severe squall line that developed in Oklahoma on May 2, 1979, is presented during its growth and part of its mature period. The line was analyzed using radar, satellite, sounding, and surface data for examining, in particular, the cell propagation mechanisms, the three-dimensional structure of the squall line and individual cells during the mature period, the mass and moisture fluxes, and precipitation efficiency. Comparison of the 1979 Oklahoma squall line with other documented squall lines indicates less well organized down-drafts on the rear side at low levels to midlevels, and an absence of low level to midlevel inflow on the rear side. The magnitude of mass and moisture fluxes was comparable to previous squall line cases. At one point, the motion of the individual cells, initially associated with a synoptic scale cold front, takes a sharp rightward turn. The mechanism of the motion turn is explained in terms of conditions created by a combination of wind shear and moisture convergence and lifting.

Automatic Detection of Mesocyclonic Shear with Doppler Radar

Abstract: A region of high wind shear always accompanies large intense tornadoes. Such shear is readily measured by Doppler radar and thus can be used to identify regions in storms where tornadoes are likely to form. An algorithm developed at the National Severe Storms Laboratory uses decision thresholds to discriminate between mesocyclone shear and other shears not associated with organized circulatory flow. A detailed test of these and other thresholds was made by testing the algorithm on storms with and without mesocyclones. The algorithm requires storage and calculations on two consecutive radials of data, and thus is suitable for real time implementation. We have applied the algorithm to data from sixteen mesocyclones to evaluate detection performance. The probability of false alarm was 10%, and the probability of detection was 90%. The algorithm accounts for distortion of the shear pattern caused by the large antenna beam dimensions at long ranges.

Generation of auroral omega bands by shear instability of the neutral winds

Abstract: Thermospheric neutral wind acceleration via ion drag in the conducting E-region of the ionosphere is greatly increased by electron precipitation associated with auroras. This increased acceleration can lead to the development of significant horizontal wind shears, which were found to be unstable to the Kelvin-Helmholtz shear instability. Numerical simulation of the neutral response to an intense, postmidnight, diffuse aurora shows tne formation of an E-region 'jet stream' within the aurora, with peak winds speeds greather than 700 m/s after one hour. It is proposed that this jet stream produces unstable Kelvin-Helmholtz waves, which can drive waves of discrete aurora along the poleward boundary of the preexisting diffuse aurora. It is suggested that such auroral waves, driven by the neutral winds, form eastward propagating waves (omega bands) occasionally observed along the poleward boundary of postmidnight diffuse auroras. It was found that neutral wind shears that develop in response to discrete auroral arcs are unstable; however, the resulting wind waves are not expected to drive significant auroral waves along discrete arcs.

Three-Dimensional Numerical Modeling of Convection Produced by Interacting Thunderstorm Outflows. Part II: Variations in Vertical Wind Shear

Abstract: In this second paper in a series on outflow interactions, we use the three-dimensional model described in Part I to examine the effects of vertical wind shear variations on cloud development along intersecting thunderstorm outflow boundaries. Three wind shear profiles are used in this study: shear only above cloud base, shear only below cloud base, and shear both above and below cloud base. As in Part I, the shear is unidirectional and is oriented perpendicular to the line containing the two initial outflow-producing clouds (which are spaced 16 km apart). Using the environmental thermodynamic structure from the control simulation in Part I, we vary the shear magnitude in each profile and examine the properties of cloud development in the region where the two outflows collide (the outflow collision line or CL). The model results show that the intensity and the time interval between successive cell updraft maxima of the first two clouds along the CL (both of which are triggered by the outflow colli...

Effects of Shear, Stability and Valley Characteristics on the Destruction of Temperature Inversions

Abstract: A dry two-dimensional version of the Colorado State Cloud/Mesoscale Model was used to study the morning, inversion destruction cycle in a variety of deep mountain valley configurations. Eleven simulations were run to examine the effects of valley width, surface heating rate, wind shear above the valley, valley orientation, sidewall slope, initial stability and variable surface albedo on the evolution of the daytime boundary layer in the valley. Each was initiated with a stable layer filling the valley to ridgetop with a neutral layer above the ridge. The model was driven at the lower surface by a sinusoidally varying potential temperature flux which approximates the diurnal heating cycle. All simulations show that the initial inversion layer is destroyed by a combination of three processes; a growing surface based neutral layer over the valley floor, the destabilization of the stable air mass by the recirculation of air warmed over the slopes and the descent of the inversion top by the transport ...

A mixture model for wind shear data

Abstract: Wind shear is known to be an important factor affecting the safety of aircraft during take-off and landing period. Records of its measurement obtained from a civil airlines are presented and discussed. A mixture model with variable proportionality constant is used to describe this data. The method of minimum chi-squared is used to estimate the mixture proportionality constant and the suitability of the model is considered.

Local wind forcing of a coastal sea at subinertial frequencies

Abstract: An analytical, normal mode, two-layer model is presented that gives the response of a coastal region to local wind forcing. The model is linear and is developed in frequency space, the latter allowing one to compare the model responses directly with the results of current meter/wind data spectral and coherence analyses. The theoretical development indicated the requirement of a frictional parameter in both the barotropic and baroclinic modes to balance the wind stress at lower frequencies. Experimentation with the model and current and wind data collected 20 km off the Texas coast showed the requirement for both frictional effects throughout the subinertial frequency range. With appropriate frictional parameters, the model predicts the response of the Texas coastal region to local wind forcing quite well, using data from the fall and winter of 1978–1979. However, during the summer of 1978, the model consistently underpredicted the energy levels of the currents. Since the summer is a period of low wind stress energy for the Texas shelf, this underprediction most likely indicates the presence of nonlocally generated shelf wave phenomena.

Height extrapolation of wind data

Abstract: Various models that are used for height extrapolation of short and long-term averaged wind speeds are discussed. Hourly averaged data from three tall meteorological towers (the NOAA Erie Tower in Colorado, the Battelle Goodnoe Hills Tower in Washington, and the WKY-TV Tower in Oklahoma), together with data from 17 candidate sites (selected for possible installation of large WECS), were used to analyze the variability of short-term average wind shear with at mospheric and surface parameters and the variability of the long-term Weibull distribution parameter with height. The exponents of a power-law model, fit to the wind speed profiles at the three meteorological towers, showed the same variability with anemometer level wind speed, stability, and surface roughness as the similarity law model. Of the four models representing short-term wind data extrapolation with height (1/7 power law, logarithmic law, power law, and modified power law), the modified power law gives the minimum rms for all candidate sites for shortterm average wind speeds and the mean cube of the speed. The modified power-law model was also able to predict the upper-level scale factor for the WKY-TV and Goodnoe Hills Tower data with greater accuracy. All models were not successful in extrapolation of themore » Weibull shape factors.« less

Measurements of the wind-wave energy flux in an opposing wind

Abstract: A wind-wave flume which allows mechanically generated water waves to propagate in opposition to a boundary-layer air flow has been used to measure the air-flow structure in an opposing wind—wave situation. Measurements of the wave-induced pressure closely follow the predictions of potential flow theory, with the pressure in antiphase with the water surface. Hence, in contrast to the following wind situation, there is no appreciable air—water energy flux due to normal stresses. The vertical and horizontal wave-induced velocities deviate slightly in magnitude from the potential flow result while still following it qualitatively. Based on these velocity measurements it is determined that the Reynolds stress—ρa ūū is the dominant term in causing the decay of waves in opposing winds. The predicted rate of decay has a squared dependence on the wave slope and the ratio of wind speed to wave phase speed. © 1985, Cambridge University Press. All rights reserved.

Mixed‐Layer Deepening in Lakes after Wind Setup

Abstract: Laboratory experiments on wind‐driven mixed‐layer deepening in a wind flume are described for the situation where the wind shear stress has become balanced by a streamwise pressure gradient so that the mean mixed‐layer velocity has vanished. The results were obtained at two lengths of the flume, and include visual observations, entrainment rates, density profiles, velocity profiles and profiles of turbulence intensities. An intermediate upwind wedge formed by accumulation of mixed water was observed in all experiments. The entrainment law obtained agrees with Kraus and Turner's relationship, and is almost independent of the length of the flume.

Fine structure in mesospheric wind fluctuations observed by the Arecibo UHF Doppler radar

Abstract: Mesospheric short-period wind fluctuations are investigated, using two sets of daytime wind data obtained on 15 consecutive days in August 1980 and March 1981 by the UHF Doppler radar at Arecibo, Puerto Rico (18.4°N, 66.8°W). The wind fluctuations are found to have a conspicuous recurring structure with periods of 10–20 min on a vertical scale less than a few kilometers. This structure, appearing as a “grill pattern” in the time-height sections of wind velocity, seems to indicate that both upward and downward propagating waves with similar wave parameters exist simultaneously in this height range. The wave energy is shown to be almost equally distributed between the upward and downward propagating waves. The frequency power spectrum has a falling gradient gentler than that of the f−5/3 and even of the f−1 power law (f is frequency). Occurrence of these waves seems to be related to strong wind shears associated with long-period internal inertia-gravity waves present in the mesospheric region.

The Organization of Convection in Narrow Cold-Frontal Rainbands

Abstract: Narrow cold-frontal rainbands are lines of intense precipitation that straddle surface cold fronts Recent observational work has revealed that the rainfall within the band is organized into regularly spaced ellipsoidal cells called precipitation cores The rainband is coincident with a line of intense cyclonic shear associated with a low-level jet that lies ahead of and parallel to the surface cold front Numerous authors have suggested that the organization of cells in the rainband is the result of shear instability of the horizontal wind field To investigate this hypothesis, a linear stability analysis was performed on an idealized frontal zone consisting of a line of convection coincident with a line of cyclonic shear To model the convective processes, the air inside the rainband was assumed to be unstably stratified The presence of horizontal wind shear in an unstably stratified environment resulted in the existence of a mode with a short wave cutoff The coupling between the convective p

Turbulence Below Wind Waves

Abstract: Wind stress at the sea surface induces both surface waves and turbulent shear flow. Laboratory experiments show that 90% of the shear stress below wind waves is supported by eddies of horizontal wave number, k, smaller than $$k\, = \,3/z$$ where z is the distance from the surface. A series of measurements of the horizontal velocity fluctuations in the upper ocean mixed layer has been carried out in water of 77m depth from a fixed platform. Spherical electromagnetic current meters were used to make the measurements and the data analysed on three occasions where the wind velocity exceeds 15ms−1. These measurements support the application of inner law scaling for the stress carrying velocity fluctuations.

Foil clouds as a tool for measuring wind structure and irregularities in the lower thermosphere (92–50 km)

Abstract: Improvements of observation techniques allow a detailed interpretation of results obtained by radar tracking of a descending foil cloud (“chaff”) target. Preposition is that the aerodynamic characteristics of the chaff is properly matched to the environment and that the size of the chaff cloud is kept small during deployment. By this, uplift motions seen at certain height levels could be identified as real motions not caused by radar tracking errors. Further, effects believed to be caused primarily by laminar separation of flow and not by turbulence were observed. This is illustrated by some results obtained during the MAP/WINE campaign winter 1983/1984 over Andenes (Northern Norway).

Wind corners in the winter mesosphere

Abstract: A series of 18 mesospheric wind profiles with an altitude resolution of about 50 m has been obtained by means of radar tracking of chaff clouds. The experiments were performed during during wintertime at 69°N latitude and they cover mostly the 72 to 86 km altitude range. The observed wind shears frequently concentrate the changes of wind direction α with altitude z into shallow layers, which we designate "wind corners". They are characterized by having large directional gradients dα/dz (⩾ 5°/100 m) and by having a small vertical vertical extent ( ⩽ 500 m). Also wind corners separate altitude layers of a few km thickness in which little change of wind direction occurs. Frequently wind corners coincide with local minima in the horizontal wind speed (≤15 m/s).

A Composite Life Cycle of Nonsquall Mesoscale Convective Systems over the Tropical Ocean. Part I: Kinematic Fields

Abstract: The wind fields associated with cloud clusters observed during the Global Atmospheric Research Program's Atlantic Tropical Experiment (GATE) are investigated. A compositing procedure is devised to isolate the cluster circulations. Satellite-observed cloud cover estimates by Cox and Griffith form the basis for the identification and classification of clusters and for the determination of their life cycles. The compositing criteria focus on the upper-tropospheric portions of anvil clouds that are a prominent feature of cloud clusters. The compositing procedure is applied to a set of objectively analyzed upper-air winds for Phase 3 of GATE prepared by K. V. Ooyama and J.-H. Chu. The results show that slow-moving cloud clusters tend to form in regions of relatively small vertical wind shear and that the shear at the cluster center decreases during the cluster life cycle. Squall clusters, on the other hand, have significantly larger lower-tropospheric shear. Changes in the total horizontal wind field ...