Showing papers on "Wind shear published in 1988"

Coefficients for sea surface wind stress, heat flux, and wind profiles as a function of wind speed and temperature

Abstract: Surface layer coefficients for wind profiles, wind stress, and heat flux in typical open sea conditions are briefly reviewed. Businger-Dyer flux-gradient relationships and a Charnock wind stress formula fit the empirical data and are dimensionally consistent. These have been solved by an iterative method, and the results are presented in a tabular form suitable for climatological calculations from marine wind and temperature data.

Structure and Evolution of Numerically Simulated Squall Lines

Abstract: Using a three-dimensional numerical cloud model, we investigate the effects of vertical wind shear on squall-line structure and evolution over a wide range of shear magnitudes, depths, and orientations relative to the line. We find that the simulated squall lines are most sensitive to the magnitude of the component of shear perpendicular to the line, and that we may reproduce much of the range of observed structures by varying this single parameter. For weak shear, a line of initially upright-to-downshear-tilted short-lived cells quickly tilts upshear, producing a wide band of weaker cells extending behind the surface outflow boundary. For moderate-to-strong shear, the circulation remains upright-to-downshear tilted for longer periods of time, with vigorous, short-lived cells confined to a relatively narrow band along the system's leading edge. At later times, however, these systems may also weaken as the circulation tilts upshear. For strong, deep shears oriented obliquely to the line, the squal...

The GISS Global Climate-Middle Atmosphere Model. Part I: Model Structure and Climatology

Abstract: The GISS global climate model (Hansen et al.) has been extended to include the middle atmosphere up to an altitude of approximately 85 km. The model has the full array of processes used for climate research, i.e., numerical solutions of the primitive equations, calculation of radiative and surface fluxes, a complete hydrologic cycle with convective and cloud cover parameterizations, etc. In addition, a parameterized gravity wave drag formulation has been incorporated, in which gravity-wave momentum fluxes due to flow over topography, wind shear and convection are calculated at each grid box, using theoretical relationships between the grid-scale variables and expected source strengths. The parameterized waves then propagate vertically upward depending on the instantaneous wind and temperature profiles, with waves breaking at levels in which their momentum flux exceed the background saturation value. Radiative damping is also calculated, and the total momentum convergence in each layer is used to ...

Observations of a multi-level turbulence structure in a very stable atmospheric boundary layer

Abstract: Boundary-layer measurements conducted at the Marsta site in Sweden from a winter-time situation (23–25 Feb.) with stable stratification have been analysed. The data comprise wind and temperature profile measurements up to 30 m, turbulence measurements at 2, 6 and 30 m and Doppler acoustic sounder data up to about 150 m. The upwind fetch at the site is flat and free from obstacles to a distance of ca 5 km for the particular sector chosen for the experiment. During the night, a two-layer vertical structure developed. Analysis of power spectra, co-spectra and variances in a shallow and very stable turbulent boundary layer near the ground show that the turbulence is fully developed and follow the universal behaviour. Above, at a height of 30 m, another turbulent layer is produced by increased wind shear near a low-level jet. This turbulent upper layer can be regarded as a layer of free shear flow. At this height, there also exist wave-turbulence interactions at low frequencies which sometimes cause a countergradient heat flux.

Dual-Polarization observations of Microbursts Associated with Intense Convection: The 20 July Storm during the MIST Project

Abstract: A detailed case study of the microbrust-producing storm on 20 July 1986 during the MIST Project is presented, together with visual (based on cloud photogrammetry) and radar observations during the life cycle of the storm. In particular, multiparameter radar information is seen to have important implications for operational detection of this wind shear event. Noteworthy is the observation of a small shaft (less than 1 km in horizontal dimensions) of near zero differential reflectivity (ZDR) surrounded by large positive ZDR values in the main precipitation core within a microburst-producing downdraft. This “ZDRZm-Hole-Hole” implies a strong localized downdraft composed of melting hail.

Inertial wind path and sea surface temperature patterns near the Gulf of Tehuantepec and Gulf of Papagayo

Abstract: During the northern hemisphere winter the surface atmospheric pressure on the Gulf of Mexico side of Central America is often higher than that on the Pacific side. The resultant southward pressure gradient drives strong southward winds through a gap in the Sierra Madre mountain range at the Isthmus of Tehuantepec. The atmospheric jet is narrow in comparison with the relevant radius of deformation, so when it leaves the coast and flows out over the Gulf of Tehuantepec, it is an inertial jet. Therefore the jet is expected to be deflected into a clockwise inertial circle by the Coriolis force. A similar process should also occur for Central American atmospheric jets through mountain range gaps near the Gulf of Papagayo. Satellite measurements of sea surface temperature together with coastal wind data for the first 41 days of 1986 were examined to see if the expected wind path influenced sea surface temperature (SST) near the Gulf of Tehuantepec and the Gulf of Papagayo. Consecutive daily satellite images when the coastal southward Tehuantepec wind increased suggest that the initial development of cold surface water in a clockwise loop is due to the wind mixing the shallow thermocline surface water along its inertial path. A simple 1½-layer analytical ocean model suggests, in agreement with observations, that the wind can drive surface quasi-geostrophic currents of ∼1 m s−1. Since the wind path is circular, the wind actually generates a clockwise rotating ocean eddy with the approximate scale of the wind inertial circle. The satellite sea surface temperature measurements also suggest that the wind-driven surface ocean current is unstable. In addition, stronger gradients in SST to the right of the expected wind flow are consistent with advection of sea surface temperature by ocean surface Ekman flow.

A simple, analytic 3-dimensional downburst model based on boundary layer stagnation flow

Abstract: A simple downburst model is developed for use in batch and real-time piloted simulation studies of guidance strategies for terminal area transport aircraft operations in wind shear conditions. The model represents an axisymmetric stagnation point flow, based on velocity profiles from the Terminal Area Simulation System (TASS) model developed by Proctor and satisfies the mass continuity equation in cylindrical coordinates. Altitude dependence, including boundary layer effects near the ground, closely matches real-world measurements, as do the increase, peak, and decay of outflow and downflow with increasing distance from the downburst center. Equations for horizontal and vertical winds were derived, and found to be infinitely differentiable, with no singular points existent in the flow field. In addition, a simple relationship exists among the ratio of maximum horizontal to vertical velocities, the downdraft radius, depth of outflow, and altitude of maximum outflow. In use, a microburst can be modeled by specifying four characteristic parameters, velocity components in the x, y and z directions, and the corresponding nine partial derivatives are obtained easily from the velocity equations.

Measurements of the angle between the wind vector and wind stress vector in the surface layer over the North Sea

Abstract: Measurements of surface layer turbulence, in addition to full meteorological and oceanography quantities, over the North Sea indicated that the stress vector on long time scales is often aligned with a direction slightly different from the mean wind flow. When stratifications were near neutral, the angle difference between the stress and wind exhibited a dependence on the heat and momentum fluxes. In general, the stress vector was observed to be to the left of the flow during stable stratifications, while for unstable stratifications, it was to the right. This finding was consistent using two independent sets of wind stress data, i.e., from MARSEN (1979) and the North Sea Platform Winter Exercise (1985).

An upper E region quarterdiurnal tide at Arecibo

Abstract: Studies of the time-height trajectories of ionosphere E region layers at Arecibo have revealed not only the usual diurnal sporadic-E and semidiurnal intermediate layer structures but also that on occasion, extra layers form and descend through the E region. In particular, during January 1981, four layers per day were observed to descend through the upper E region. This four-per-day layer system was remarkably coherent over the 3 days of observation and suggests the existence of a 6-hour period or quarterdiurnal tidal wind system in the upper E region. Numerical simulations of ion trajectories in a realistic model atmosphere with diurnal, semidiurnal, and quarterdiurnal tidal wind systems of appropriate amplitude and phase structures confirm that the presence of a quarterdiurnal tide is sufficient to explain the observations.

A study of wind speed modification and internal boundary-layer heights in a coastal region

Abstract: Wind profile data within the first two kilometres of a coast have been used to study the wind field modification downstream of this surface discontinuity The land area is generally very flat, having an overall roughness length of 004 m A wind model, suitable for practical applications and inexpensive to run, has been tested against the data and was found to give satisfactory results Knowing the climatological statistics of wind and stratification, eg, at the coast, the model may thus be used to estimate, on a climatological basis, how the wind field is modified with distance inland, at least in areas with only minor topography This type of information is of great importance when locating wind turbines It is in these cases also important to know the statistics of the internal boundary-layer (IBL) height, as the turbulence intensity may be quite different in and above the IBL, which in turn may influence load and fatigue calculations Using the wind profile data, the IBL height was clearly discernible in the majority of cases Having very unstable stratification over land, the IBL height could, however, not be determined from the wind profiles, as the wind in these cases did not decrease inland This result was also obtained using the wind model A simple model of the type z IBL = a · x b, was instead tested, and was shown to give reasonable results

Mesoscale spiral vortex embedded within a Lake Michigan snow squall band: High resolution satellite observations and numerical model simulations

Abstract: It is known that Great Lakes snow squall convection occurs in a variety of different modes depending on various factors such as air-water temperature contrast, boundary-layer wind shear, and geostrophic wind direction. An exceptional and often neglected source of data for mesoscale cloud studies is the ultrahigh resolution multispectral data produced by Landsat satellites. On October 19, 1972, a clearly defined spiral vortex was noted in a Landsat-1 image near the southern end of Lake Michigan during an exceptionally early cold air outbreak over a still very warm lake. In a numerical simulation using a three-dimensional Eulerian hydrostatic primitive equation mesoscale model with an initially uniform wind field, a definite analog to the observed vortex was generated. This suggests that intense surface heating can be a principal cause in the development of a low-level mesoscale vortex.

A systematic error in MST/ST radar wind measurement induced by a finite range volume effect: 1. Observational results

Abstract: Wind measurement by MST/ST radars may be accompanied by a systematic error due to a finite range volume effect which works when a thin turbulent layer is simultaneously located in several adjacent range volumes. The error occurs when the layer coincides with a cross section through the range volume which is not symmetric with respect to the center of the beam. The finite range volume effect appears as a false vertical shear of horizontal wind in a vertical scale of the order of a few hundred meters, even if the ambient wind field is uniform. The false wind shear sometimes exceeds 40 ms−1 km−1 in magnitude or the critical value to induce the Kelvin-Helmholtz instability. Also the effect leads to a false temporal variation of the wind measurement, although the wind field does not change at all. The false wind shear with a magnitude less than 40 ms−1 km−1 cannot be discriminated from a true one in the observed data. It seems hard to indicate directly that the finite range volume effect appears as theoretically conceived. Judging from wind velocity and echo intensity data obtained by the MU radar in Japan, this effect appears quite frequently in the atmosphere. The small vertical scale wind shear as well as the temporal variation found only at a specific range should be treated with great care except when the ambient wind field is weak, where the finite range volume effect is not so important.

Observations of complex terrain flows using acoustic sounders: Echo interpretation

Abstract: We examine methods for the interpretation of sodar facsimile records obtained in the study of complex terrain flows. Acoustic scattering theory is presented first and then interpreted using a simpolified second-order turbulence closure scheme. The use of this theory suggests the strong sensitivity of acoustic scatter to changes in the wind shear. With this introduction, detailed sodar facsimile records, temperature and wind profiles, and model calculations follow. Characteristic scattering patterns are described for simple drainage jets, complex basin flows, convection with a capping inversion, stratus, and dynamical instabilities. Examples are also shown of bistatic facsimile records detailing the strong temporal and spatial variability in small-scale turbulence.

Comparative morphology of the vertical ozone profile in the Antarctic spring

Abstract: The form and development of the vertical distribution of ozone are examined during the Antarctic winter and spring by comparing profiles selected from a series of balloon-borne ozonesonde ascents which were launched from Halley Bay in 1987. The progress of the ozone depletion is followed from its onset in late August until the deep minimum in mid-October, when nearly all the ozone has been destroyed between 90 and 50 mbar. The presence of structure in the unperturbed profile is discussed, and it is shown that individual profiles are susceptible to the effects of vertical motion and wind shear. After the spring equinox, the vertical distribution of ozone consistently displays a minimum in mixing ratio, in contradiction to theories which purport to explain the spring development by transport alone. The depletion process is restricted to a finite vertical range of some 12 km, with a sharp upper boundary at about 20 mbar.

Observations of a Storm Containing Misocyclones, Downbursts, and Horizontal Vortex Circulations

Abstract: The multiple-Doppler technique is used to derive the three-dimensional wind fields of circulations produced by a multicellular storm observed on June 30, 1982, in Colorado that included four downbursts, three misocyclones, a misoanticyclone, and horizontal vortex circulations within a relatively small area. It is shown that horizontal vortex circulations, or rotors, form along the edges of the downdrift and propagate away from their source region. Strongest surface winds are associated with the rotors. The results of this investigation indicate that the interrelationships exist between the downburst and the production of intense rotation about horizontal and vertical axes.

Flight path responsive aircraft wind shear alerting and warning system

Abstract: A wind shear warning system (10) monitors the flight path of an aircraft and wind shear (16) in the vicinity of the aircraft and generates an advisory or cautionary message (34) for the pilot based both on the magnitude (20, 22) of the wind shear and the flight path of the aircraft. When the aircraft is flying a normal flight path, a wind shear warning is generated only when relatively high negative wind shears are present. If the flight path of the aircraft is not normal, for example, if the aircraft is below the glide slope (18) or descending too rapidly (14), a cautionary alert is given at lower values of wind shear. The alert or warning (54) is selected to provide the pilot with information defining the nature of the hazard or potential hazard being encoutered.

A parameterization of the stable atmospheric boundary layer

Abstract: Two formulations of the stable atmospheric boundary layer are proposed for use in weather forecasting or climate models. They feature the log-linear profile near the surface, but are free from the associated critical Richardson number. The diffusion coefficients in the Ekman layer are a natural extension of the surface layer. They are locally determined using wind shear in one case and turbulent kinetic energy in the other. The parameterizations are tested in a one-dimensional model simulating the evolution of the nocturnal boundary layer with and without radiative cooling. Both formulations give very similar results, except near the top of the boundary layer where the transition to the free atmosphere is smoother with the wind shear formulation. A distinctive feature of these schemes is that they retain their simulating skill when resolution is reduced. This is verified for a wide range of situations. In practice, this means that there is no need for a large-scale model to have a level below 50 m or so.

The effect of thin scattering layers on the determination of wind by Doppler radars

Abstract: It has recently been suggested that the observed tendency for intense turbulence to occur in thin layers in the atmosphere introduces an error in horizontal wind measurements with MST radars, even in the absence of wind shear. This effect, called the finite range volume effect, is a consequence of the tilted pulse volume when using the Doppler technique. A model is shown which illustrates that this effect may be very important in MST radar measurements when wide antenna beam widths are used. These model results are supported by observations performed with the MU radar in Japan, where wind measurements with different beam widths have been performed. The impact of these measurements on MST radar design and possible methods to minimize the error are discussed.

Method for producing a warning of the existence of low-level wind shear and aircraftborne system for performing same

Abstract: A method for producing a warning of the existence of low-level wind shear and an aircraftborne system for performing the method utilizes measured values of airspeed and temperature, to determine a temperature based hazard factor relating to the existence of a wind shear threat to the aircraft and to issue a warning whenever the hazard factor exceeds a predetermined threshold level based upon the performance characteristics of the aircraft. Temperatures may be sensed locally, such as by a resistive temperature sensor or, remotely, such as by an infrarred temperature detector. Furthermore, in accordance with preferred embodiments, an inertial reactive wind shear detector is utilized as an integrated safety measure. Moreover, the threshold level may be varied depending on such factors as aircraft altitude.

Wind shear detection system

Abstract: A wind shear detection system (30) compares airspeed (40) with a composite signal derived from signals representative of longitudinal acceleration (32), normal acceleration (34), angle of attack (36) and flight path angle to generate a shear signal. An enhanced version of the system is also compensated for roll angle (70), radio altitude (64) and flap position. In a modified system, the accelerations are calculated along the velocity vector of the aircraft rather than along the horizontal axis to compensate for inaccuracies that could occur under extreme flight conditions such as high bank angle turns and dynamic manoeuvre.

Wind Spectra And Gust Factors Over Water

Abstract: Spectra of winds in hurricane conditions are needed for the analysis of compliant structures intended for deep water. Spectra of wind measurements made in Hurricanes Eloise and Frederic as well as in North Sea gales were calculated and compared to various models. All of the spectra were reduced to a single form using a nondimensionalisation based on total variance. The variance, in turn, could be accurately predicted given the mean wind speed and significant wave height. A model spectrum which behaves like isotropic turbulence in the high frequency limit and tends to a constant at low frequencies fit the averaged data well. Given the model spectrum and the assumption of Gaussian turbulence, it is possible to calculate gust factors for any averaging period and wind speed.

Comma Cloud Development and Associated Rapid Cyclogenesis over the Gulf of Alaska: A Case Study Using Aircraft and Operational Data

Abstract: The NOAA P-3 aircraft was used to collect data in a genesis region for mesoscale comma clouds over the Gulf of Alaska. Aircraft measurements in the genesis region showed that rainbands with spacings of 65–75 km and orientations along the mean wind shear were present. Possible mechanisms for the formation of the rainbands, including conditional symmetric instability (CSI) and modified wave-CISK were investigated, but the data did not allow the formation of the rainbands to be conclusively ascribed to a particular mechanism. The existence of low static stability in the genesis region was also documented and its role in mesoscale comma-cloud development explored. Careful analysis of images from NOAA polar orbiter and GOES satellites together with synoptic analyses made it possible to trace the life cycles of several mesoscale comma clouds as the genesis region moved across the Gulf of Alaska. As the genesis region approached a preexisting polar frontal cloud band, a wave cyclone formed on the front ...

An Analysis of Wind Fluctuation Statistics Collected under Stable Atmospheric Conditions at Three Sites in Alberta, Canada

Abstract: Wind fluctuation data collected under stable atmospheric conditions at two prairie sites and a site located near the Rocky Mountain foothills have been analyzed. Results of the analysis show a marked tendency for horizontal fluctuation angles to vary inversely with wind speed. In contrast, vertical fluctuation angles tended to be invariant with wind speed. Atmospheric turbulence was much greater at the foothills site than at the prairie sites. This was mainly due to the fact that standard donations of vertical wind angles were almost twice as great. Standard deviations of horizontal fluctuation angles were only about 20% greater.

A case study of Kelvin-Helmholtz waves within an off-shore stable boundary layer: Observations and linear model

Abstract: A case study of Kelvin-Helmholtz waves which were observed by two aircraft in a warm off-shore stable boundary-layer flow over the North Sea is presented. During the one-hour flight mission within an area of 40 × 40 km2, the waves were intermittent both in space and time. They were centered around two levels, at 90 and 330m, where inflection points in the mean profile of the cross-wave wind component occurred together with Richardson numbers smaller than the critical value of 0.25. Observed wave amplitudes were on the order of 0.1 K for the potential temperature, 0.15ms-1 for the vertical wind component, 0.3ms-1 for the cross-wave wind component and 0.15ms-1 for the along-wave wind component. Horizontally averaged vertical wave transports were down-gradient. Based on the observed wind and temperature profiles, wave simulations with a linear model are performed. Different diffusion coefficient estimates are tested. The model produces two types of Kelvin-Helmholtz waves with maximum amplitudes at the above mentioned two heights. The modeled wavelengths are about 30% shorter than the observed ones. Adjusting the modeled to the observed temperature variations, the modeled vertical wind variance and the vertical transports agree well with the observations, whereas the modeled horizontal wind variances are smaller than the observed ones.

On the rate of thinning of thin liquid films on a rotating disk

Abstract: Thin liquid films are often used to coat rotating magnetic recording disks so as to enhance their durability. These rotating films are subject to inertial and wind shear forces that induce radial motion thereby thinning them. An experiment is discussed here to study the relative importance of inertial forces and wind shear. For a 28‐nm‐thick layer of perfluoropolyether, air shear dominates inertial effects. Furthermore, the thinning of these films without wind shear does not deviate from the classical models of nearly parallel, slow viscous flow using the bulk viscosity and no slip.