Showing papers on "Wind shear published in 1992"

Salinity-driven Thermocline Transients in a Wind- and Thermohaline-forced Isopycnic Coordinate Model of the North Atlantic

Abstract: An isopycnic-coordinate oceanic circulation model formulated with the aim of simulating thermodynamically and mechanically driven flow in realistic basins is presented. Special emphasis is placed on the handling of diabatic surface processes and on thermocline ventilation. The model performance is illustrated by a 30-year spinup run with coarse horizontal resolution (2° mesh) in a domain with North Atlantic topography extending from 10° to 60°N latitude. The vertical structure encompasses 10 isopycnic layers in steps of 0.2 σ units, capped by a thermodynamically active mixed layer. From an initially isohaline state with isopycnals prescribed by zonally averaged climatology, the model is forced by seasonally varying wind stress, radiative and freshwater fluxes, and by a thermal relaxation process at the surface. After a mechanical spinup time of about 15 years, a quasi-stationary pattern of mean circulation and annual variability ensues, characterized by pronounced subtropical mode-water formation...

Sources of Mesoscale Variability of Gravity Waves. Part II: Frontal, Convective, and Jet Stream Excitation

Abstract: We present studies of four cases of mesoscale variance enhancements of horizontal velocity and temperature due to frontal activity, nonfrontal convection, and wind shear. These data were obtained aboard commercial aircraft during the Global Atmospheric Sampling Program (GASP) in 1978 and 1979 and from the corresponding meteorological analyses and satellite imagery. Additional GASP data were used to permit a statistical assessment of the importance of various sources of enhanced variances. Our results, and those in a companion paper addressing the variance enhancements associated with topography, represent refinements of previous source analyses using the GASP dataset. Significant findings include mean variance enhancements of velocity and temperature due to convection and jet-stream flow ranging from ∼2 to 8 for 64-km and 256-km data segments, and enhancements for individual segments as high as ∼20 to 100. The mean 64-km variance enhancement for all variables and source types, relative to a quies...

The Role of Convectively Generated Rear-Inflow Jets in the Evolution of Long-Lived Mesoconvective Systems

Abstract: In this study, the structure of convectively generated rear-inflow jets and their role in the evolution of long-lived mesoconvective systems are investigated through an analysis of idealized three-dimensional simulations using a nonhydrostatic cloud model. Rear-inflow jets are generated within these systems in response to the upshear-tilting of the convective circulation, as the horizontal buoyancy gradients along the back edge of the expanding system create a circulation that draws midlevel air in from the rear. Within this framework, a wide range of rear-inflow strengths and structures are produced, depending on the magnitude of the ambient convective available potential energy (CAPE) and the vertical wind shear. In general, for environments characterized by weak-to-moderate vertical wind shear and weak-to moderate CAPE, the rear-inflow jet descends and spreads along the surface well behind the leading edge of the gust front, and the subsequent convective activity becomes weaker. However, for e...

An Objective Clear-Air Turbulence Forecasting Technique: Verification and Operational Use

Abstract: An objective technique for forecasting clear-air turbulence (CAT) is described. An index is calculated based on the product of horizontal deformation and vertical wind shear derived from numerical model forecast winds aloft. The forecast technique has been evaluated and is now in operational use at two forecast centers with international aviation responsibilities: the National Meteorological Center (NMC) in Washington, D.C., and the Air Force Global Weather Central (AFGWC) in Omaha, Nebraska. The index is also an operational forecast tool at the Canadian Atmospheric Environment Service (AES), and the National Aviation Weather Advisory Unit (NAWAU) in Kansas City, Missouri, both responsible for domestic aviation forecasts. The AFGWC index also includes horizontal convergence in its calculation. Thresholds were selected empirically by comparing index values with the location and intensity of observed CAT. Verification indicates that the index is quite reliable. The probability of detection (POD) va...

A Numerical Study of the Variability and the Separation of the Gulf Stream, Induced by Surface Atmospheric Forcing and Lateral Boundary Flows

Abstract: A primitive equation regional model is used to study the effects of surface and lateral forcing on the variability and the climatology of the Gulf Stream system. The model is an eddy-resolving, coastal ocean model that includes thermohaline dynamics and a second-order turbulence closure scheme to provide vertical mixing. The surface forcing consists of wind stroll and beat fluxes obtained from the Comprehensive Ocean-Atmosphere Data Set (COADS). Sensitivity studies am performed by driving the model with different forcing (e.g., annual versus zero surface forcing or monthly versus annual forcing). The model climatology, obtained from a five-year simulation of each case, is then compared to observed climatologies obtained from satellite-derived SST and hydrocast data. The experiments in which surface best flux and wind stress were neglected show less realistic Gulf Stream separation and variability, compared with experiments in which annual or seasonal forcing are used. A similar unrealistic Gulf S...

Formation and Diurnal Variation of the Dryline

Abstract: The formation and diurnal evolution of the dryline during fair weather have been investigated through the use of a two-dimensional mesoscale model that includes condensation/evaporation, budget equations of surface energy and moisture field, as well as turbulence and radiation parameterizations.A moderately strong, vertical wind shear was introduced on a sloping terrain, where the soil is very dry on the west side but moist on the east. Initially, a weak easterly geostrophic wind exists to the east but a weak westerly geostrophic wind to the west of the dryline. During daytime, deepening of the mixed layer due to vertical mixing, especially on the west side, forces the dryline to advance eastward. The westerly wind on the west side is maintained by downward transport of westerly momentum due to strong vertical mixing; the easterly wind on the east side is strengthened due to the inland sea-breeze circulation. Therefore, the resulting low-level convergence sustains a strong moisture gradient along...

Validation and assimilation of Seasat altimeter wave heights using the WAM wave model

Abstract: The third-generation wave model, WAM, is presently used to investigate the mutual consistency of the Seasat global data sets of scatterometer winds and altimeter wave heights for the complete Seasat period. While modeled and observed wave heights are in reasonable agreement on the global average, regional differences can be great and on occasion exceed 40 percent. These errors are primarily attributable to forcing wind field deficiencies; the friction velocities of the Goddard Laboratory for Atmospheres are found to be significantly underestimated in the high-wind belt of the Southern Hemisphere. A wave data assimilation scheme is presented in which the wave field is updated without change to the forcing wind field.

Full vector (3-D) inflow simulation in natural and wind farm environments using an expanded version of the SNLWIND (Veers) turbulence code

Abstract: We have recently expanded the numerical turbulence simulation (SNLWIND) developed by Veers [1] to include all three components of the turbulent wind vector We have also configured the code to simulate the characteristics of turbulent wind fields upwind and downwind of a large wind farm, as well as over uniform, flat terrain Veers's original method only simulates the longitudinal component of the wind in neutral flow This paper overviews the development of spectral distribution, spatial coherence, and cross correlation models used to expired the SNLWIND code to include the three components of the turbulent wind over a range of atmospheric stabilities These models are based on extensive measurements of the turbulence characteristics immediately upwind and downwind of a large wind farm in San Gorgonio Pass, California

Unified Program for the Specification of Hurricane Boundary Layer Winds Over Surfaces of Specified Roughness.

Abstract: : A method is developed to specify the surface stress and the wind speed and direction in the planetary boundary layer of a tropical cyclone from meteorological storm parameters available for historical hurricanes. The method is based upon a numerical primitive-equation model of the planetary boundary layer in a moving tropical cyclone. The complete time history of the evolution of the surface wind field is described from a series of characteristic wind field states calculated at discrete times in a storm's history by the steady- state model. A surface drag formulation, based upon a contemporary similarity model (Arya 1977), coupled with a Toughness parameter specification for a water surface consistent with Cardone's (1969) law, is incorporated into the numerical model and found to produce a consistent description of the integrated planetary boundary layer wind, the surface stress and its direction, and the wind speed and direction at anemometer level. The surface winds calculated in several recent hurricanes are found to be in excellent agreement with available, representative surface wind measurements made from offshore platforms and data buoys. Transformations based upon an equilibrium planetary-boundary-layer similarity model are developed to specify the surface wind over terrain of specified roughness, including lake surfaces, from the over-water wind-field solution. Calculated over-land and over-lake winds are compared to the limited measurements available for several recent storms. Hurricanes, Tropical storms, Wind fields, Numerical modeling, Winds.

Results of Sexbierum Wind Farm: double wake measurements

Abstract: In the framework of the JOULE-0064 'Full-scale Measurements in Wind Turbine Arrays' in the period between June-November 1992 measurements have been performed in the Sexbierum Wind Farm The aim of the measurements is to provide data for the validation of wake and wind farm models, which are being developed simultaneously, and to provide input data for wind turbine load calculation programmes The campaign concerned measurement of the wind speed, turbulence and shear stress behind a single wind turbine at distances of 25,55 and 8 rotor diarneters, respectively Besides detailed measurements of the wind field, the power of the turbine was measured A database has been compiled containing 1-minute averaged values of the measured quantities The database was analysed using a two-dimensional bin analysis with respect to the undisturbed wind direction and wind speed The analysis contains horizontal and vertical profiles of the: - U-,V-, and W-component of the wind in the wake; - turbulence intensities in three directions and turbulent kinetic energy in the wake; - shear stresses u'v', u'w' and v'w' in the wake These quantities are presented both in dimensional as in non-dimensionalized form With the data base a useful set of data is created for the validation of wake and wind farm models

Sources of mesoscale variability of gravity waves. I - Topographic excitation. II - Frontal, convective, and jet stream excitation

Abstract: The effect of topography as a source of mesoscale variability was investigated using aircraft measurements of winds and temperature collected during the Global Atmospheric Sampling Program, with results showing marked increases in the variance of zonal and meridional wind speeds and of potential temperature over rough terrain. In addition, four cases of mesoscale variance enhancements of horizontal velocity and temperature due to frontal activity, nonfrontal convection, and wind shear were studied. The implications of these episodic enhancements of variances for the vertical transports of energy and momentum are considered in the framework of the gravity wave theory.

Microburst precursor detector utilizing microwave radar

Abstract: A microburst precursor detector utilizes a multiplicity of radar beams (1-4) and samples radar returns, in each beam, from meteorological radar signal reflectors and processes (16) the signal returns in a statistical manner to determine average radar reflectivity and to extract doppler signal parameters. These parameters are utilized to determine a second set of parameters; average doppler frequency within each radar beam, doppler spectral spread within each radar beam, and the skewness of the doppler spectrum in each beam. The second set of parameters is processed (18-20) to establish the existence of a microburst, predicted surface impact, time to impact, wind shear surface location and track, and the magnitude of the wind shear.

Observations of Planetary Boundary-Layer Structure in the Eastern Equatorial Pacific

Abstract: Atmospheric soundings were collected from a research ship in the eastern equatorial Pacific to determine the kinematic and thermodynamic structure of the planetary boundary layer (PBL) in the vicinity of a strong front in sea surface temperature (SST). These observations have been used to test the hypothesis proposed by Wallace et al. and Hayes et al. that the low-level wind shear was enhanced on the cold side of the front because of greater static stability. The PBL structure was analyzed from a transect along 110°W from 10°N to 5°S, with a focus on the ∼500-m-deep atmospheric mixed layer, as defined with respect to virtual potential temperature (θv). Significant mixed-layer variability was found due to diurnal and other effects, but there were systematic differences in mean structure as a function of latitude. The mixed layer was neutrally stratified over warm water between 6.5°N and 1.5°N and was slightly stably stratified over the cooler water between 1°N and 2°S. The mean total vertical wind...

Annual cycle of the Atlantic North Equatorial Countercurrent

Abstract: An analysis of numerous meridional XBT sections near 28°W reveals that the geostrophic North Equatorial Countercurrent (NECC) continues to flow eastward throughout the year, fastest in fall and slowest in spring. Drifting buoys and historical ship drifts show that the near-surface Countercurrent reverses each spring even when systematic errors due to windage are taken into account. The seasonally fluctuating winds drive an Ekman surface current that is eastward in fall, adding to the geostrophic current, and westward in spring, countering and overwhelming the geostrophic current. The reversal of the Countercurrent in spring occurs in the near-surface layer and is driven by the Northeast Trades. Thus the near-surface velocity in the Countercurrent is determined by a competition between local wind stress and the larger field of wind stress curl, both of which have large seasonal variations in the tropical Atlantic.

Growth of wind waves with fetch observed by the Geosat altimeter in the Japan Sea under winter monsoon

Abstract: The wind speeds and the significant wave heights derived from the Geosat altimeter are validated by comparing with the data obtained at Japanese ocean data buoy stations. A systematic error is found in the altimeter wind speeds at high winds. An experimental equation is derived in order to correct this error for the present study. By using the corrected wind speeds and the wave heights, growth of wind waves with fetch in the Japan Sea under winter monsoon is investigated. The altimeter-derived nondimensional significant wave height and nondimensional fetch show good agreement with the empirical fetch graph formulas. Fetch variation of the wind speeds is also discussed in relation to the evolution of the turbulent boundary layer over wind waves by using the altimeter data.

The structure of the southwest monsoon winds over the Arabian Sea during the late Quaternary: Observations, simulations, and marine geologic evidence

Abstract: Each summer strong southwest winds produced by the Asian monsoon blow across the NW Arabian Sea, creating a broad region of upwelling characterized by cool sea surface temperatures and enriched phytoplankton biomass. The integrated history of the summer plankton blooms is preserved in the form of distinct plankton fossil assemblages and sediment facies, forming a continuous geologic record of wind-driven upwelling and thus changes in the SW monsoon winds. Sediments have been recovered from two regions in the NW Arabian Sea, one located along the Oman continental margin, and another located approximately 400 km offshore. Over the late Quaternary, these two sites record time series of upwelling that are substantially different. To understand the source of the differences, we examined the relationship between wind stress and upwelling at these sites using historical observations and model-simulated wind fields. In the coastal region, upwelling is directly related to the Ekman transport offshore, and is thus linearly proportional to the alongshore wind stress. Farther offshore, open-ocean upwelling is driven by the positive curl in the wind stress. In historical records, we found little correlation between stronger winds and increased open-ocean upwelling. In the region offshore where the sediment cores were recovered, the year with the strongest winds was accompanied by weaker wind stress curl and diminished upwelling (indicated by warmer sea surface temperatures and decreased phytoplankton pigment content) because the region of maximum wind stress curl moved NW that year. In atmospheric general circulation model experiments using a range of summer insolation forcing, we found large changes in the wind stress curl occur over some areas of the NW Arabian Sea; however the offshore core locations lies outside of the region where changes in the SW monsoon drive large changes in open-ocean upwelling. In contrast to the coastal sites where upwelling is directly related to the wind stress, the offshore sites appear to be located in a region where the wind stress curl does not change much over a large range of summer insolation. There is some evidence from both the interannual and model fields that this location is affected more by movements in the position of the low-level jet than by changes in the over-all strength of the SW monsoon. Although more data are required to define these patterns, these results indicate that the time series of open-ocean upwelling observed in the geologic record from offshore sites may differ substantially from the coastal time series.

The Structure of a Subtropical Prefrontal Convective Rainband. Part I: Mesoscale Kinematic Structure Determined from Dual-Doppler Measurements

Abstract: In Part I of this study, structural features of a convective rainband associated with the Mei-Yu front on 25 June 1987 over northwestern Taiwan are investigated. The dynamic and thermodynamic structures and momentum budgets of the convective rainband will be reported in Part II. Dual-Doppler data were collected from CP-4 and TOGA Doppler radars positioned along the northwest coast of Taiwan during the Taiwan Area Mesoscale Experiment (TAMEX). Fields of the system-relative wind and reflectivity were derived in a horizontal domain of 40 km × 36 km using the objective analysis scheme with a 1-km grid spacing in all three directions. There were ten analysis levels in the vertical ranging from 0.8 to 9.8 km. Vertical velocities were computed from the anelastic continuity equation by integrating downward with variational adjustment. Results show that the low-level jet (LLJ) in the boundary layer provides large vertical shear ahead of the cold front. The orientation of the jet is in the direction almost...

Low-level wind-shear alert system

Abstract: The apparatus of the present invention improved low-level wind shear alert system that provides an improved method of identifying the presence and locus of wind shear in a predefined area This system enhances the operational effectiveness of the existing LLWAS system by mapping the two-dimensional wind velocity, measured at a number of locations, to a geographical indication of wind shear events This system can also integrate data and processed information received from a plurality of sources, such as anemometers and Doppler radar systems, to produce low-level wind shear alerts of significantly improved accuracy over those of prior systems In particular, the apparatus of the present invention makes use of the data and processed information produced by the existing Low-Level Wind Shear Alert System as well as that produced by the Terminal Doppler Weather Radar to precisely identify the locus and magnitude of low-level wind shear events within a predetermined area This resultant geographical indication is displayed in color-graphic form to the air traffic control personnel and can also be transmitted via a telemetry link to aircraft in the vicinity of the airport for display therein

Quasigeostrophic turbulence in a three-layer model : effects of vertical structure in the mean shear

Abstract: A three-layer, horizontally homogeneous, quasigeostrophic model is selected as one of the simplest environments in which to study the sensitivity of baroclinic eddy fluxes in the atmosphere to the vertical structure of the basic-state temperature gradients or vertical wind shears. Eddy statistics obtained from the model are interpreted in terms of linear theory and a modified “baroclinic adjustment” hypothesis. Both linear theory and the baroclinic adjustment construction are found to provide useful predictions for the vertical structure of the eddy potential vorticity flux.For equal values of the mean vertical shear, eddy fluxes and energies are greater when the shear is concentrated at lower levels (d2U/dz2 0). Eddy fluxes are more sensitive to lower-than to upper-level mean temperature gradients. This relative sensitivity is a function of γ = f2Λ/(βN2H), where Λ is the mean vertical shear and His the depth of the fluid. It is...

Relationships between deflation and near surface wind velocity in a coastal dune blowout

Abstract: Detailed field measurements were made of the degree of surface level change in a blowout, situated in the coastal dune area of Meijendel, The Netherlands. A formula was established to express the erosivity of the wind for the corresponding 34 measurement intervals, over a period of about 3 years. Having established, for 12 wind sectors, the relationship between wind velocity at the nearest standard weather station and at five locations in the blowout, correlations were derived between the deflation rate in the blowout and the wind erosivity using standard hourly wind data. The winter season, although the most windy, is by far the least effective season: the threshold shear wind velocity is disproportionally increased because the moisture content of the surface sand is high. The soil moisture conditions are described as a function of the daily precipitation and evaporation rate. The two most relevant upper and lower threshold shear wind velocities are determined empirically. During the summer season, deflation rate is even higher than indicated by the net surface level change because deflation is partly compensated by deposition of sand transported by rainwash from the blowout margin. With regard to spatial variation in the deflation rate within the blowout, it appears that locations with relatively high velocities coincide with higher deflation rates. However, the spatial variation is commonly less than is expected from the horizontal wind velocity distribution. In view of this the role of wind direction, blowout morphology and algal coverage of the soil in the deflation rate is discussed.

Doppler Radar Observations of the Development of a Boundary-Layer Nocturnal Jet

Abstract: A single Doppler radar obtained detailed clear-air measurements of the development of a strong boundary-layer nocturnal jet in North Dakota during the summer of 1989. The evolution of the jet was monitored by the radar with a high degree of vertical and temporal resolution using a repetitive sequence of four different elevation scans. A new variation of the velocity-azimuth display (VAD) analysis technique provided vertical profiles of the mean wind components and several turbulence terms. Boundary-layer wind speeds began to increase in the late afternoon, well before sunset, as surface cooling began. Wind speeds accelerated faster after sunset and eventually produced a jet that exceeded 23 m s−1 at about 0.5 km AGL. The wind veered with height and time and followed the expected inertial oscillation pattern. Measured shear stresses, vertical fluxes of momentum, and velocity variances, which were initially large, decreased sharply after the surface began to cool. The directly measured vertical vel...

The Development of Organized Convection In A Simplified Squall-Line Model

Abstract: Squall lines stand out from ordinary cumulonimbus convection because of a special structure that dramatically extends their duration, precipitation output and area of influence. Time-dependent models have identified surface outflow boundaries and low-level environmental wind shear as the crucial elements in squall-line formation and/or maintenance. Steady-state models have shown how properties of the downdraught reservoir, or ‘cold pool’, must be matched to the far-field wind and thermodynamic profiles to sustain the convection. However, the dynamical role of the environmental shear and the nature of the interaction between the positively and negatively buoyant air have remained uncertain. For help with these issues, a time-dependent numerical model is developed with all moist processes either parametrized or based on ad hoc assumptions. the simplifications make it possible to distinguish more clearly than in the past between the formation and maintenance of squall lines and between the initial and disturbed far-field environments. The convective updraught forms within an expanding region of disturbed flow separated from the undisturbed environment by ‘storm fronts’. Forced (or neutral) ascent occurs at a surface ‘gust front’ on the upwind side of the cold pool. We distinguish squall lines from ordinary convection by looking at the coherence between the forced and convective updraughts. It is found that subsidence over the cold pool disorganizes the updraughts, whereas a deep overturning circulation downshear from the gust front has an organizing effect. Hence, factors which allow the initial subsidence to occur ahead of the gust front, namely contrasting lower-and upper-level winds and large potential buoyancy, favour squall-line development. It is argued that, in cases with large convective potential energy, the deep mesoscale circulation is more important in the formation and maintenance of the line than either the vertical shear of the air reaching the cold pool or the strength of the forced updraught.

Flight control in a windshear via nonlinear H/sub infinity / methods

Abstract: The design of the control of an aircraft encountering wind shear after takeoff is treated as a problem of minimizing the H/sub infinity / gain between the wind shear and the vertical acceleration of the aircraft. The method of nonlinear H/sub infinity / control is used to design the control law. For the reason of robustness, no information or assumptions about wind shear structure or intensity are used in the design of the controller. The approximate solution of the Hamilton-Jacobi-Isaacs equation is used to design a nonlinear controller to improve the performance. Simulations under both linear and nonlinear controllers are carried out. The performance of the aircraft is good and the angle of attack is smooth. >