Showing papers on "Wind shear published in 1990"

Temporal Changes of the Primary Circulation in Tropical Cyclones.

Abstract: More than 900 radial profiles of in situ aircraft observations collected in 19 Atlantic hurricanes and tropical storms over 13 years confirm that the usual mechanism of tropical cyclone intensification involves contracting maxima of the axisymmetric swirling wind. Radar shows that annuli of convective echoes accompany the wind maxima. These features, called convective rings exist and move inward because latent heat released in the rings leads to descent, adiabatic warming, and rapid isobaric height falls in the area they enclose. The radial change in rate of isobaric height fall is concentrated at the inner edge of the wind maximum, causing the gradient wind to increase there and the maximum to contract. Vigorous convection organized in rings invariably causes well defined, inward moving wind maxima, but when convection is weak, the rings are also weak or even absent. In this case, the swirling wind may be nearly constant with radius and change slowly in time. Hurricanes that have a single, vigor...

Wave Dependence of Sea-Surface Wind Stress

Abstract: Distribution of the wind stress over the oceans is usually estimated by using a bulk formula. It contains the squared 10-m wind speed multiplied by the drag coefficient, which has been assumed in many cases to be a weak function of the 10-m wind speed. Over land the important role of thermal stratification has been clearly recognized, but over the sea the influence of wind waves is less well documented. This paper presents evidence showing the likelihood that the influence of the wind waves can also be large. Charnock proposed an expression for the marine atmospheric boundary layer roughness parameter, z0, which depended only on the wind friction velocity, u☆ and the acceleration of gravity, g. Toba and Koga have recently proposed an alternative expression for flow over growing wind waves, which are in local equilibrium with the wind, given by a form including the wind-wave spectral peak frequency explicity. The criterion for local equilibrium of the wave field with the wind is its consistency wi...

Thunderstorms above Frontal Surfaces in Environments without Positive CAPE. Part I: A Climatology

Abstract: The first of two papers describing thunderstorms that occur above frontal surfaces, frequently in environments without positive convective available potential energy (CAPE), focuses on the climatology of such storms for the conterminous United States. The dataset used consists of 1093 observations made over a 4-year period. The events were selected using conventional network data and a set of criteria that eliminated thunderstorms rooted in the boundary layer. A composite of the dataset shows that the typical “elevated” thunderstorm occurs northeast of an associated surface low-pressure center, and north of a surface warm front in a region with northeasterly surface winds. The planetary boundary layer is generally very stable as determined by comparisons with both the 50-kPa and 85-kPa air. The thunderstorms are usually found in the left exit region of a low-level wind maximum (an area of horizontal deformation). The large-scale environment is strongly baroclinic with large vertical wind shear an...

Some measurements of turbulence over an antarctic ice shelf

Abstract: Profiles of wind speed, temperature and turbulent fluxes of heat and momentum in the lowest 32 m have been measured at a station on an antarctic ice shelf. During the antarctic winter the surface layer often shows strong static stability, with temperature gradients as large as 1 Km−1 in the lowest few metres. the surface inversion is destroyed during periods of high wind speed but the wind profile shows significant deviation from the expected logarithmic form under such conditions. Measurements of stress at 5 m indicate that the roughness length of the snow surface is about lO−4 m. At 5 m height, the variations of the dimensionless wind shear, ϕm, and potential temperature gradient, ϕT, agree with previously determined forms of the Monin-Obukhov similarity functions. Above 5 m, the behaviour of ϕm and ϕT is only qualitatively similar and surface-layer similarity theory does not provide a good description of the profiles. Turbulence length scales have been deduced from vertical velocity power spectra. Under near-neutral conditions, the ratio of turbulence length scale to measurement height is observed to decrease with increasing height of measurement. This observation is consistent with the variation of turbulence length scale with height implied by the measurements of ϕm. It is suggested that the stability of the overlying atmosphere restricts the depth of the turbulent boundary layer and hence the length scales of turbulence within this layer. Increasing stability causes a decrease of turbulence length scales at all levels. The ratios of turbulence kinetic energy to stress and temperature variance to heat flux are examined. Measurements are somewhat scattered, but the distribution of values varies little with height or stability. the form of the distribution suggests that large-scale motions, possibly internal gravity waves, may be playing an important role in boundary-layer processes.

Interactions between Upper and Lower Tropospheric Gravity Waves on Squall Line Structure and Maintenance

Abstract: Using a simplified thermodynamic sounding, and variable vertical wind shear, we investigate the role of gravity waves on the structure and propagation of a simulated two-dimensional squall line. Based on an observed squall line environment, the modeled troposphere has been divided into three distinct thermodynamic layers. These consist of an absolutely stable atmospheric boundary layer (ABL), an elevated well-mixed layer, and an upper tropospheric layer of intermediate stability. We find the mixed layer to have a dual role; it has a reduced stability and thus provides abundant buoyancy for the convective scale updrafts, and it provides an ideal layer to trap mesoβ-scale (20–200 km) wave energy generated in the stable layers. The generated waves thus have a significant and lasting impact on the simulation. We also find this thermodynamic structure to be conducive to both strong surface wind perturbations and long-lived squall lines. Experiments that vary the vertical wind shear profile demonstrate...

Meteorological and Electrical Conditions Associated with Positive Cloud-to-Ground Lightning

Abstract: Meteorological and electrical conditions associated with the occurrence of positive cloud-to-ground (CG) lightning (ie, lightning that lowers positive charge to ground) are examined Results from case studies in winter and summer storms reveal common features and lend support to the tilted dipole hypothesis Lightning bipoles, whose lengths range from the convective scale to the mesoscale, are aligned with the vertical wind shear, with a predominance of negative locations in proximity to the deepest convection and a mixture of positive and negative locations displaced downshear from the deepest convection Comparisons with radar data show that all lightning events am located within a distance of 10–20 km of precipitation extending from the surface to several kilometers above the O°C isotherm Electrostatic field measurements beneath precipitation removed from the deepest convection indicate a positive dipole structure and a tilting deformation by vertical wind shear These observations suggest

Effects of wind shear and turbulence on wind turbine power curves

Abstract: It is a common practice to use wind speeds at hub height in determining wind turbine power curves. Although the possible influence of other variables (sub as turbulence and wind shear) is generally neglected in power curve measurements, we discovered the importance of other variables in an analysis of power curves for three 2.5 MW wind turbines. When the power curves were stratified by turbulence intensity. Such a large sensitivity to turbulence was not expected, and further analyses were conducted to determine if other factors accompanying the change in turbulence level could cause or contribute to the observed sensitivity of the power curves to turbulence. In summary, the sensitivity of the observed power curves was largely due to two factors: (1) an actual sensitivity to turbulence in determining the power curve and (2) the deviation of the disk-averaged velocity from the hub-height velocity under low turbulence conditions that were most prevalent at the site. An examination of the wind shear profiles over the height of the rotor disk revealed that low turbulence conditions were characterized by strong shear in the lower half of the rotor disk and weak or negative shear in the upper half. Implications of this analysis aremore » that significant errors in power curve measurements can result if the effects of wind shear and turbulence are ignored. 7 refs., 6 figs.« less

Stochastic wind simulation for erosion modeling

Abstract: The purpose of this study was to develop a wind simulator to furnish wind direction and sub-hourly wind speed to users of wind speed information, particularly for wind erosion modeling. We analyzed the Wind Energy Resource Information System data to determine scale and shape parameters of the WeibuU distribution for each of the 16 cardinal directions for each month at 704 locations in the United States. We also summarized wind direction distributions, ratio of daily maximum to daily minimum wind speed, and hour of maximum wind speed by month for each location. This summary of historical wind statistics constitutes a compact data base for wind simulation. Equations were formulated and procedures developed and used with the compact data base and a random number generator to simulate wind direction and sub-hourly wind speed. Cumulative wind speed distributions, calculated from the WeibuU parameters, and wind speeds simulated at one-hour intervals for 1000 days agreed well. The model reflects historical day-to-day wind variation and wind speed variations within a day. It will be useful to those needing wind speed and wind direction information and will provide the wind simulator requirements in a wind erosion prediction system.

Momentum Budget over the Pyrénées: The PYREX Experiment

Abstract: Although the qualitative influence of mountains over the atmosphere has been known for a long time, numerous deficiencies, linked to orography, are still noted, either in forecasts by regional models, or in the long-term behavior of climate models. This is why the French and Spanish weather services are undertaking an important field campaign to document the dynamic modifications to the atmospheric flow generated by the Pyrenean range during a 2-month period (October and November 1990) with six intensive observation periods (IOPs) of 2 to 3 days. The experimental strategy is based largely on mesoscale numerical-model results and will help to validate these models. The main focus is on the documentation of clear-air turbulence generated either by breaking mountain waves, by surface roughness, or by the wind shear induced by the lateral-flow deviation around the mountain. Experimental means include several networks of surface stations, radio soundings, constant-level balloons, four wind profilers, ...

Onset of thunderstorms and precipitation over Northern Nigeria

Abstract: An empirical prediction scheme for the start of thunderstorms, and the onset and expected amount of rainfall for any year at Kano, Nigeria, is presented, using 8 years' upper air data. The years investigated include two drought years (1973 and 1974) and 3 years of above average rainfall (1978, 1980, and 1988). Agriculturally sufficient and reliable rainfall begins 5–6 weeks after the vertical wind shear below the African Easterly Jet, ΔUL(surface, 700 hPa), and the mid-tropospheric shear ΔUM (700–400 hPa), simultaneously satisfy the condition −20≤UL≤−5 ms−1 and 0≤UM ≤10 ms−1, as previously put forward by Omotosho. The total precipitation for any year is found to be well correlated with the total moisture anomaly (specific humidity) from the week of the above-critical wind shear to the week of rainfall onset. This makes it possible to estimate the expected rainfall total at the very onset of the rains.

Mixed-Layer Shear Generated by Wind Stress in the Central Equatorial Pacific

Abstract: Sixteen months of wind and current profile observations in the central equatorial Pacific show the response of the upper-ocean shear to local wind forcing. The shear at the ocean surface is significantly correlated with the wind stress in direction but not in magnitude, implying that the vertical eddy viscosity coefficient is proportional to the square of the wind speed or to the stress. Using this variable eddy viscosity coefficient in Stommel's model of the Equatorial Undercurrent, we calculate shears in the mixed layer that compare well with the observations. The flow is downwind on the equator and tends toward and Ekman spiral off the equator.

Global Wind Stress and Sverdrup Circulation from the Seasat Scatterometer

Abstract: Three months of vector wind observations from the Seasat-A satellite scatterometer (SASS) are used to construct gridded fields of monthly average wind stress and wind stress curl over the global ocean. These fields are examined to identify features either poorly resolved or not present in wind stress fields constructed from conventional data. Particular attention is focused on the spatial structures in the high southern latitudes and the tropical regions. The SASS wind stress fields are compared globally with the Hellerman and Rosenstein climatological monthly average surface wind stress fields and with monthly averages of wind stress computed from contemporaneous 1000 mb wind analyses produced by the National Meterological Center. The potential for satellite scatterometry improving the present knowledge of the global wind stress field and making important contributions to ocean modeling is demonstrated by comparison of global maps of the Sverdrup circulation computed from 3-month average SASS an...

On the Wave Dependence of Sea-Surface Wind Stress --A Study by Using Data from an Ocean Data Buoy Station*--

Abstract: Analysis is made of wind and wave data, which were obtained during the passage of Typhoon 8013 at an Ocean Data Buoy Station south of Honshu operated by the Japan Meteorological Agency, in order to investigate the wave dependence of sea-surface roughness parameter in the situation where wind waves are dominant with less significant swells. The data fit better the wave-dependent expression of the wind stress,z 0 σ p/u*=γ, than to Charnock's formula,gz 0/u*2=β, wherez 0 is the roughness length,σ p the angular frequency of the spectral peak of wind waves,u* the friction velocity of air,g the acceleration of gravity,γ andβ are non-dimensional constants. The results are very similar to those of our previous study using data from an oil producing platform in the Bass Strait, Australia, although the type of observation system and the synoptic situation of the winds and wind waves were totally different.

Laplace-normal mixtures fitted to wind shear data

Abstract: A mixture model with Laplace and normal components is fitted to wind shear data available in grouped form. A set of equations is presented for iteratively estimating the parameters of the model using an application of the EM algorithm. Twenty-four sets of data are examined with this technique, and the model is found to give a good fit to the data. Some hypotheses about the parameters in the model are discussed in light of the estimates obtained.

Are Radiosonde Time Scales Appropriate to Characterize Boundary Layer Wind Profiles

Abstract: One approach under investigation for obtaining regional-scale surface fluxes of water vapor, heat, and momentum from complex terrain involves the applicability of flux-profile relationships in the atmospheric boundary layer (ABL). Mean humidity, temperature, and wind speed profiles in the ABL can be measured by means of radiosondes. A disadvantage, however, of this method is the relatively quick passage of the sonde through the ABL compared to the characteristic time scales or memory of the turbulence. Remote sensing with sodar (Sound Detection and Ranging) allows the measurement of mean wind profiles over time scales which are perhaps more appropriate for the turbulence. Wind profiles measured by both radiosondes and sodar over the Landes Forest in southwestern France, as part of the HAPEX-MOBILHY experiment, are compared under conditions of neutral stability. The sodar wind profiles were measured by a team from the CRPE (Centre de Recherches en Physique de l'Environnement, Saint-Maur-des-Fosses...

Airborne Doppler radar detection of low-altitude wind shear

Abstract: NASA and the FAA, as part of a joint research effort aimed at the development of airborne sensor technology for low altitude windshear detection during aircraft takeoffs and landings, are giving attention to the potential usefulness of a microwave Doppler radar operating at X-band or above. A preliminary feasibility study was conducted with a microburst/clutter/radar simulation program. It is found that, using bin-to-bin automatic gain control, clutter filtering, limited detection range, and suitable antenna tilt, the windshear generated from a high-moisture microburst can be detected with 10-65 sec of warning time.

Diabatic Modification of an Extratropical Marine Cyclone Warm Sector by Cold Underlying Water

Abstract: On 25–27 January 1988, the National Oceanic and Atmospheric Administration's Wave Propagation Laboratory, Drexel University, and the Office of Naval Research carried out a combined pre-ERICA research aircraft investigation of a major marine cyclone moving northeastward over the Canadian Maritime Provinces. Flight-level and dropwindsonde observations documented the diabatic modification of the cyclone's warm sector marine boundary layer (MBL) as it moved out over cold underlying water. These observations and results from the Blackadar one-dimensional boundary layer model both show that heat fluxes were directed downward from the warm sector MBL into the cold ocean. Vertical gradients of these downward heat fluxes diabatically cooled the lower portion of the warm sector MBL and generated large static stability within the entire layer. The increase in stable stratification allowed large vertical wind shear to exist within this layer and strong wind speeds to exist at its top. The increase in static ...

Lightning/rainfall relationships during COHMEX

Abstract: Initial results of relationships between cloud-to-ground lightning and radar-derived precipitation measurements in air mass thunderstorms (multi- and single cell) occurring over the Southeastern United States are presented. Buoyancy values computed from soundings are used to estimate wind shear, the energy available to storms, and the ability of the environment to support strong updrafts. Precipitation measurements are utilized for sampling the rainfall history of entire storm life cycles. It is shown that the average rain volume per ground discharge decreases as the buoyancy increases; this is attributed to stronger updrafts elevating mass to higher levels in the storm where more frequent ice particle interactions may play a major role in separating charge leading to enhanced lightning activity. Thus, total lightning rates are expected to correlate better with rainfall production.

Evaluation of a technique to quantify microburst windshear hazard potential to aircraft

Abstract: A wind shear hazard index, known as the F-factor, is investigated for application with look-ahead sensors. Based on data from microburst simulations with the NASA windshear model, the downdraft results in a significant contribution to the wind shear hazard, especially at altitudes above 150 meters. Since most look-ahead wind shear sensors can detect only horizontal shear and cannot measure vertical velocity, a relationship is developed for approximating the total F-factor using information based solely on the horizontal wind shear and altitude. This relationship is then tested using data from several microburst cases. >

Characteristics of thunderstorm-generated low altitude wind shear: a survey based on nationwide terminal Doppler weather radar testbed measurements

Abstract: The characteristics of microbursts and gust fronts, two forms of aviation-hazardous low altitude wind shear, are presented. Data were collected with a prototype terminal Doppler weather radar and a network of surface weather stations in Memphis, Huntsville, Denver, Kansas City, and Orlando. Regional differences and features that could be exploited in detection systems such as the associated reflectivity, surface wind shear, and temperature change are emphasized. >

Atmospheric effects on traffic noise propagation

Abstract: Atmospheric effects on traffic noise propagation have largely been ignored during measurements and modeling, even though it has generally been accepted that the effects may produce large changes in receiver noise levels. Measurement of traffic noise at multiple locations concurrently with measurement of meteorological data is described. Statistical methods were used to evaluate the data. Atmospheric effects on traffic noise levels were shown to be significant, even at very short distances; parallel components of the wind (which are usually ignored) were important at second row receivers; turbulent scattering increased noise levels near the ground more than refractive ray bending for short-distance propagation; and temperature lapse rates were not as important as wind shear very near the highway. A statistical model was developed to predict excess attenuations due to atmospheric effects.

Stochastic prediction techniques for wind shear hazard assessment

Abstract: The threat of low-altitude wind shear has prompted development of aircraft-based sensors that measure winds directly on the aircraft's intended flight path. Measurements from these devices are subject to turbulence inputs and measurement error, as well as to the underlying wind profile. Stochastic estimators are developed to process on-board Doppler sensor measurements, producing optimal estimates of the winds along the path. A stochastic prediction technique is described to predict the hazard to the aircraft from the estimates as well as the level of uncertainty of the hazard prediction. The stochastic prediction technique is demonstrated in a simulated microburst wind shear environment. Use of the technique in a decision-making process is discussed. >

An analytical study of the frictional response of coastal currents and upwelling to wind stress

Abstract: In this paper we present several theoretical results concerning currents forced by the wind in coastal regions, for a shallow sea and for a very deep sea. We investigate the time behavior and the spatial structure of the stream function and the momentum components, i.e., the onset, the transient, the asymptotic, and the periodic behavior of the currents and of the upwelling, forced by winds of different spatial and time structures. Results show that the intensity of the along coast jet initially grows linearly under a δ in time wind impulse, quadratically under a Heaviside in time wind impulse, and cubically under a linearly growing wind impulse. The asymptotic state is such that the intensity of the current vanishes if the wind impulse has a finite duration, while the intensity of the sea current has a final finite amplitude if the wind intensity goes to some finite value. If the wind stress is periodic in time, there is upwelling only when the period of the forcing is longer than a characteristic time scale, which is the sum of the inertial period and the friction e-folding time. Otherwise there are waves which propagate away from the region where the wind stress is acting. The spatial structure is such that the upwelling occurs in a horizontal region of the order of the Rossby deformation radius, corrected by the effect of friction (and by the effect of periodicity, when the wind stress is periodic in time). However, the horizontal gradient of the wind stress can be more important than the Rossby deformation radius in determining the horizontal extent of the upwelling region.