Showing papers on "Wind shear published in 1974"

A three-dimensional primitive equation model of cumulonimbus convection

Abstract: The model is described and the results of three integrations presented. Its main novel features are the inclusion of non-hydrostatic terms in a pressure co-ordinate system, and the treatment of flow through the lateral boundaries. The results of the integrations, one with an ambient vertical wind shear and two without, indicate that the cloud dynamics is quite sensitive to assumptions made regarding the cloud microphysics, particularly at the stage when the downdraught is produced.

A Dynamic Roughness Equation and Its Application to Wind Stress Determination at the Air-Sea Interface

Abstract: A dynamic roughness equation including major wind and wave interaction parameters (wind shear velocity, wave height, and phase velocity) is derived. Because this equation implicitly incorporates the effects of wave steepness, relative water depth, and wind duration and fetch, it may be applied to a wide variety of natural conditions. This equation was used to construct a nomogram which can be utilized to determine the wind stress at the sea surface, given phase velocities, wave heights, and the wind speed at any height in the atmospheric boundary layer. The proposed relationships are verified by the available field and laboratory data under near-neutral atmospheric stability conditions from which the appropriate parameters could be determined.

An Experimental Study of Turbulence Structure in the Fair-Weather Trade Wind Boundary Layer

Abstract: Measurements of air velocity, temperature and humidity were made from an aircraft in the fair-weather trade wind boundary layer. On the day of the experiment, the region studied was characterized by north-south bands of cloud-free and moderately clouded areas roughly 40 km in width. Mean winds, turbulence quantities, and thermodynamic variables were measured in both a clear and a partly cloudy region. Production of turbulence energy in the subcloud layer of both regions was mainly from wind shear. In the cloud-free region, the turbulence energy and momentum flux budgets were measured. One of the most striking features was the large region (covering almost two-thirds of the depth of the mixed layer) of negative production of turbulence energy by wind shear. Notwithstanding this unusual feature, the terms of the turbulence energy budget agree quite well with a model developed by Lenschow. In the cloudy area, a layer of strong wind shear was observed near cloud base. This, coupled with corresponding...

Wind Tunnel Simulations of Light and Dark Streaks on Mars

Abstract: Wind tunnel experiments have revealed a characteristic flow field pattern over raised-rim craters which causes distinctive zones of aeolian erosion and deposition. Comparisons of the results with Mariner 9 images of Mars show that some crater-associated dark zones result from wind erosion and that some crater-associated light streaks are depositional.

The Life Cycle of a Thunderstorm in Three Dimensions

Abstract: The results of an isolated three-dimensional thunderstorm simulation are reported. An initial wind shear of 1.54 m sec−1 km−1 is specified in the x direction and symmetry is assumed in the y direction about the central x-z plane. An erect cloud develops from an initial 3.7 km radius impulse as the updraft core is fed from all directions by moist and warm low-level air. After some time rain begins to fall and a downdraft developes downshear of the updraft core cutting off the major low-level supply of warm and moist air. Subsequently, the updraft core begins to tilt downshear and the center of the lower part of the updraft core is shifted away from the central x–z plane. At middle cloud levels wind features reminiscent of flow about a cylinder are present. Both the hydrostatic and dynamic pressure appear to be important in decelerating flow upwind of the cloud. Comparison with a similar two-dimensional simulation indicates that the three-dimensional cloud develops faster, grows taller, lasts longe...

On the analysis of wind wave-induced disturbances in the atmospheric turbulent surface layer

Abstract: Simultaneous measurements of wind velocity, air humidity and sea surface wave-elevation fluctuations obtained on a platform in the open Caspian Sea are analyzed. It is shown that wave noises exist in the spectra and cospectra of the lower part of the atmospheric boundary layer, not only for the frequency of the main energy transporting component of the sea waves, but also in other parts of the spectra, at both lower and higher frequencies. The high frequencies are noncoherent with the sea waves and could be considered as measurement ‘errors’ due to the existence of the waves. A method of elimination of the coherent wave noises from the spectra and cospectra is suggested and the effectiveness of its application is demonstrated. The essential difference between cases of developing and decaying wind waves is demonstrated.

Some Aspects of the Structure of Convective Planetary Boundary Layers

Abstract: It is shown that although Coriolis forces cause large production rates of stress in a convective planetary boundary layer, there is a control mechanism, involving mean wind shear which prevents stress levels from becoming large. Higher-order-closure model calculations are presented which show that the stress profiles are essentially linear, regardless of wind direction, providing the geostrophic wind shear vanishes and the wind speed jump across the capping inversion is negligible. It is shown that it will he very difficult to verify these predicted stress profiles experimentally because of averaging time problems. A simple two-layer model is developed which leads to geostrophic drag and heat transfer expressions in fairly good agreement with Wangara data.

A Multiple Structured Frontal Zone-Jet Stream System as Revealed by Meteorologically Instrumented Aircraft

Abstract: The distribution of wind, temperature, ozone, and turbulence within a multiple structured frontal zone-jet stream system is described using a combination of direct horizontal measurements by meteorologically-instrumented research aircraft and conventional aerological soundings. Results document the spatial continuity of these zones and associated intrusions of ozone-rich stratospheric air. Aircraft turbulence measurements show the zones with strong vertical wind shear and near-critical values of Richardson number are preferred regions of CAT encounter. Diagnostic calculations of the terms of the gradient thermal wind equation illustrate the importance of the sign and magnitude of the air trajectory curvature and its vertical derivative in maintaining regions of strong vertical wind shear and small Richardson number in the presence of weak horizontal thermal gradient.

Planetary Boundary Layer Winds in Baroclinic Conditions

Abstract: Systematic stratifications and analyses of low-level radiosonde data are performed for portions of the eastern half of the United States. The procedures are designed to specify changes in the planetary boundary layer wind profile resulting from variations in baroclinicity. The angle between the winds and isobars, the ageostrophic wind components, the surface stress, and the surface wind speeds are all shown to be functions of the orientation of the thermal wind vector relative to the surface geostrophic wind. These variations are consistent with a mixing-length model of the additional turbulent momentum transport initiated by the vertical shear of the geostrophic wind.

Wind models for flight simulator certification of landing and approach guidance and control systems

Abstract: The definition of a model suitable for certification was the main objective of this report. The model was designed to simplify and reduce the wind model parameters to enable evaluation of a large number of aircraft and control system design parameters. Analytical descriptions of wind phenomena were presented. For those parameters defying analytic description, probabilistic descriptions were sought. A brief analysis of the effects of wind on aircraft motion was conducted. The axes transformations required between wind and turbulence components in their inherent axis system and in the airplane's axis system were shown. Techniques of providing a random process on computers for the representation of turbulence were presented. A simulation model was presented that combines all the foregoing components.

On the low-level wind structure in the Atlantic trade

Abstract: From ship-borne radiosonde and radar wind observations during the Atlantic Trade Wind Experiment 1969 (ATEX) the relation between actual and geostrophic wind in the subcloud layer is determined. The frictional force and the divergence of vertical transport of momentum between the sea surface and 500-m height are computed by the ageostrophic method. Along the surface wind direction, balance between pressure gradient and frictional force is closely established. Perpendicular to the surface wind the expected balance of pressure gradient and Coriolis force is not obtained but a substantial frictional force results from this component. The boundary condition that the stress vanishes at the lowest wind maximum leads to an underestimation of surface stress by about 35 per cent. It is suggested that convective transport of momentum must take place in the subcloud layer.

Two-point velocity statistics over Lake Ontario

Abstract: Cross-spectra between wind speeds on several masts in Lake Ontario have been analyzed. As previously predicted, coherence over water (small intensity of turbulence) between wind speeds measured on masts lined up with the wind appears to be larger than over land, and increases with decreasing Richardson numbers. As a result, in cold air over warm water, wind speed fluctuations are well predictable from upstream measurements. For large angles between the anemometer line and wind, the difference between coherence over land and water disappears. Furthermore, there is no significant difference in vertical coherence between water and land. When the wind is parallel to the anemometer line, small ‘eddies’ travel, in agreement with Taylor's hypothesis, with the local mean wind speed. Large eddies travel significantly faster. Vertical phase delay increases with increasing Richardson number.

Mean speeds of wind waves at short fetch

Abstract: The mean speeds of short wavelength wind-generated waves (wavelengths less than 4 cm) have been obtained from Doppler spectra measured at very short fetch in a wave tank for wind speeds up to 15 m sec−1. These speeds are compared with phase speeds calculated from a first-order perturbation of the wind drift and reasonable agreement is obtained. The measurements show that the difference between mean wave speed and the mean surface drift becomes quite small for wind speeds exceeding 12 m sec−1.

The mesoscale wind field over the ocean

Abstract: This paper summarizes the results of a 13-day study of variability of surface winds during project Jasin, the British Joint Air-Sea Interaction study that took place in the north Atlantic Ocean west of Ireland during September 1972. Observations were made from toroid buoys placed in two nested equilateral triangles 17 and 100 km on a side. The results show a surprising variability in the wind field as revealed in spectra of wind velocity components. Evidence of horizontal roll vortices and inertial oscillations are found in the data. Spectral densities of wind speed are large at a period of 4 days, fall off rapidly with frequency up to 0.1 cph, then decrease approximately proportional to n−1, where n is frequency, up to n = 3 cph.

Application of a Diagnostic Richardson Number Tendency to a Case Study of Clear Air Turbulence

Abstract: An index for the development of clear air turbulence on a synoptic scale is derived using the time rate of change of the Richardson number following the air motion, the so-called diagnostic Ri tendency. The method used in calculing the Ri tendency is a modified version of Roach's derivation, which in application had several undesirable features. The modification consisted in treating separately the effects of the change of stability and of the vertical wind shear, and expressing the change of the vertical wind shear by substitution from the thermal wind equation and then using the well-known frontogenetical function. The time rate of change of Ri was better expressed as the time to reduce Ri to the critical value, assumed to be unity since bulk layers were used. This index was tested on Hardy et al.'s case study of CAT observed at Wallops Island at 1700 GMT 19 February 1970 at the 750-mb level. Considering advection effects, it was found that the 800-mb level results presented have indicated that...

Self-launching glider

Abstract: An aircraft of variable geometry and certain unique features which allow its operation along the surface of water, ice or the ground under sail power provided by tilting the wings up into a cross-wind to attain sufficient speed to be self launched into gliding flight in two modes -- while close to the surface in wind shear, circling and climbing in a regenerative flight path powered by changes in wind speed with altitude until sufficient altitude is attained to soar on rising air currents.

Wind noise under winter ice fields

Abstract: Wind blowing over a snow layer on sea ice produces noise in the water beneath as a result of a multitude of snow grain impacts on the upper boundary of the snow. These impacts are caused by the downwind flow of saltating snow grains, which extract energy from the wind. Saltation of snow grains and the noise commence at a threshold wind speed sufficient to dislodge loose snow grains at the snow-air boundary. The resulting spectrum of the noise in the 400-Hz to 20-kHz band, sensed by a hydrophone in the water below, depends on the acoustic impulse responses of the paths between the snow grain impacts and the hydrophone. As a consequence, above the threshold wind speed the shape of the underice noise spectrum at a given site will remain constant, subject to changes in the snow and ice layers; however, the noise intensity will increase with the wind speed. The application of Bagnold's (1941) theory of blown sand shows that the noise intensity increases as the cube of the wind speed above the threshold speed for saltation. Comparisons with experimental measurements of wind noise intensity versus wind speed support this dependence. The noise intensity is independent of snow grain size and depends primarily on the snow grain flow.

Multiple Contrail Streamers Observed by Radar

Abstract: An unusual case of multiple streamers or filaments with the characteristic mare's tail pattern in vertical section has been observed by radar where the generating elements were condensation trails laid by high-altitude aircraft. The contrails were laid perpendicular to the wind, and, as they drifted, a multitude of streamers formed along each trail. The streamers extended from 9 km to the ground. RHI and PPI photographs at X and S band taken over a 2-hr period show the three-dimensional shape of the streamers due to the wind shear. Doppler measurements were also taken. The resulting velocity spectra are very narrow indicating little or no turbulence. Reflectivity factors were measured at various altitudes and show a decrease in reflectivity with distance from the generating line. Fall velocities based on the slopes of the streamer patterns varied from 0.4 to 1.4 m/sec. In general, the characteristics of the precipitation streamers were quite similar to those previously measured in naturally occurring cloud forms such as cirrus uncinus.

The minimum free-stream wind speed for initiating motion of surface material on Mars

Abstract: Estimates of the minimum free-stream wind speed that is required for initiating the motion of surficial material on Mars have ranged from 30 to about 200 meters per second. Thus the best value for this quantity is not well established. Graphical comparison of much of the pertinent data taken in the laboratory and in the field on Earth provides a minimum value for the Bagnold coefficient of 0.08 and this in turn provides a minimum value for the threshold friction velocity of 1.3 meters per second for initiating motion of particulate matter on Mars at low elevations where the pressure is 7 millibars. The most appropriate value of the ratio of friction velocity to free-stream velocity for putative unstable condition appears to be 0.026. Thus the minimum free-stream wind speed for initiating motion is obtained as 50 meters per second. If the surface material on Mars, however, is less cohesive than that on earth, the minimum value may be smaller.

A method for measurement of temperature profiles in inversions from refractive transmission of sound

Abstract: A method is described for estimating temperature profiles in the lower troposphere during conditions including a surface-based or elevated inversion layer. The method uses acoustic energy transmitted over paths of the order of 10 km in length. Measurements are made at approximately 1-km intervals extending radially outward from the transmitter. The vertical temperature profile is modeled as two constant temperature gradient layers. The first layer extending from the surface to height H1 has a temperature gradient T1′ (usually negative upward). The second-layer temperature gradient T2′ is strongly positive upward. For temperature profiles of this type, ray paths arrive with a high intensity at a caustic, and no rays return to earth between the source and the caustic. The method requires that H1 be determined by some other means such as vertical acdar sounding. The T1′ and T2′ are then simultaneously determined by measuring the range to the caustic and the wave propagation time. Even if the propagation time cannot be measured, useful estimates of T2′ can be obtained from observations of H1 and the caustic distance. For a ground-based inversion no caustic occurs. However, the temperature profile can be determined by measuring the wave propagation time. Since horizontal wind shear produces the same effect on ray paths as a vertical temperature gradient, the proposed method obtains the sum of the effects of the wind shear and the temperature gradient. In conditions including significant wind shear, corrections for it must be made.

Large-Scale Influences Upon the Generation of a Mesoscale Disturbance

Abstract: A case of squall line generation in the National Severe Storms Laboratory (NSSL) network has been examined with the intention of capturing synoptic-scale influences. A telescopic analysis approach was used whereby observations from both synoptic and mesoscale networks were combined. The squall line formed in the warm air behind the surface position of the cold front. Large-scale circulation was responsible for creating a shallow layer (∼1-km thick) of convectively unstable air immediately above this front. Horizontal gradient of low-level moisture, pronounced low-level wind shear, and surface convergence were the large-scale factors that combined to produce the unstable region. Mesoscale analysis showed that vertical velocity in the low levels exhibited a persistent small-scale variation prior to convective activity. The horizontal variation in vertical velocity was ultimately responsible for creating a favored position within the mesonetwork. Conservation of potential temperature and specific hu...

Vorticity, divergence, and vertical velocity in a baroclinic boundary layer with a linear variation of the geostrophic wind

Abstract: The Ekman-Taylor problem for the planetary boundary layer is solved in the case of a thermal wind which varies linearly with height. The upper boundary condition is a vanishing a- geostrophic wind, while the lower boundary condition is continuity of the stress vector across the interface between the planetary boundary layer and the surface layer. The latter condition is used to determine the magnitude and the direction of the wind at the bottom of the Ekman layer. Theoretical hodographs are compared with observed hodographs based on five years of ohserva- tions from Ship N in the Pacific, giving fair agreement. The divergence, the vorticity, and the vertical velocity are calculated through the Ekman layer with emphasis on differences between the classical barotropic and the baroclinic cases; these differences are significant, especially in the vertical velocities as compared to the standard approximation. An extension of the present study to include thermal stratification is desirable.

Effects of turbulence in the atmosphere of Venus on Pioneer Venus radio, phase 1

Abstract: The prediction of the turbulence effects in the Venus atmosphere on Pioneer Venus radio was investigated. A careful investigation based on a theoretical and experimental study of the power spectrum of the Mariner 5 amplitude fluctuations is carried out and the results contribute considerably to our scientific knowledge of turbulence in the atmosphere of Venus. Fully developed turbulence is seen to exist predominantly in the altitude range of 41 - 49 km. This result is consistent with the high wind shear and wind velocities observed by Venera 4 for altitudes higher than 40 km. The outer scale size of turbulence is on the order of 100 m, the structure constant for the dayside atmosphere 3.9 x 10 to the -7 power m to the -1/3rd power, and that for the nightside atmosphere 2.9 x 10 to the -7 power m to the -1/3rd power.

Experimental Investigation of Small Scale Transport Mechanisms in the Stratosphere

Abstract: Transport properties in the stratosphere were measured in an experimental program where smoke trails, deposited by rockets and aircraft, were recorded by time-lapse photography. This allowed winds, wind shears, and the rate of dispersion of the smoke to be calculated. Radial growth of the trails conformed to a turbulent diffusion rate; the effective diffusion coefficient at 20 km was approximately 0.7 nr sec. The spatial turbulence structure was determined from measured wind shears. Average energy spectra were obtained for total vertical profiles and segmented profiles of horizontal winds. A consistent finding was that the energy spectrum dependence exponent was close to n = 2.8. In the case of segmented profile spectra we found that 20% of the sample population showed n < 2.2 (indicating an inertial subrange). This is consistent with the presence of intermittent thin layers of efficient mixing in the 5-18 km height range. We also found an increase of wind shear and amplitude with altitude in this region.

Precipitation dependence on synoptic-scale conditions and cloud seeding

Abstract: Precipitation over South Florida, as indicated by radar, was analyzed in order to determine its dependence on synoptic-scale conditions and cloud seeding. The synoptic-scale parameters which are used are the prevailing flow, the humidity, the stability and the vertical wind shear. We found that the variation of synoptic-scale conditions is, by far, much more important that multiple cloud seeding in determining precipitation. This finding indicates that cloud seeding experiments in Florida which do not take into account the effect of the varying synoptic conditions, can lead to misleading conclusions.

Decrease in Hurricane Winds After Landfall

Abstract: Case studies of three destructive hurricanes: Carla, Camille, and Celia, as they moved inland, were performed to determine the change in horizontal wind speed distribution. Criteria for determining the storm's intensity were employed to describe storms in sparse data regions, i.e., while over the sea, to conform with their eventual description in relatively data-rich regions while over land. These criteria were considered critically as the storm's winds decreased after landfall. Resulting models of peak gust distribution for each storm are shown. Comparisons are made with Malkin's (1959) factors for wind decrease. Although the storms occurred after Malkin's study, his factors are representative of the synoptic maxima in two storms. For peak gusts other than the synoptic maxima, and for all peak gusts in the third storm, other considerations should be made for engineering estimates.