Showing papers on "Wind shear published in 1991"

Buoyancy and Shear Characteristics of Hurricane-Tornado Environments

Abstract: This study presents detailed composite profiles of temperature, moisture, and wind constructed for tornado environments in tropical cyclones that affected the U.S. between 1948 and 1986. Winds are composited in components radial and tangential to the tropical cyclone center at observation time. Guided by observed patterns of tornado occurrence, composites are constructed for a variety of different stratifications of the data, including proximity to tornadoes, position relative to the cyclone center, time of day, time after cyclone landfall, cyclone translation speed, and landfall location. The composites are also compared to composite soundings from Great Plains tornado environments. A variety of sounding parameters are examined to see which are most closely related to the tornado distribution patterns. Lower-tropospheric vertical shears are found to be stronger in the tropical cyclone tornado environments than on the Great Plains. Buoyancy for the tropical cyclone tornado cases is much smaller than that seen with Great Plains tornado events and exhibits a weak negative correlation with tornado outbreak severity.

On ENSO Physics

Abstract: Two extended integrations of general circulation models (GCMs) are examined to determine the physical processes operating during an ENSO cycle. The first integration is from the Hamburg version of the ECMWF T21 atmospheric model forced with observed global sea surface temperatures (SST) over the period 1970–85. The second integration is from a Max Planck Institut model of the tropical Pacific forced by observed wind stress for the same period. Both integrations produce key observed features of the tropical ocean-atmosphere system during the 1970–85 period. The atmospheric model results show an eastward propagation of information from the western to eastern Pacific along the equator, although this signal is somewhat weaker than observed. The Laplacian of SST largely drives the surface wind field convergence and hence determines the position of large scale precipitation-condensation heating. This statement is valid only in the near-equatorial zone. Air-sea heat exchange is important in the planetary boundary layer in forcing the wind field convergence but not so important to the main troposphere, which is heated largely by condensation heating. The monopole response seen in the atmosphere above about 500 mb is due to a combination of factors, the most important being adiabatic heating associated with subsidence and tropic-wide variations in precipitation. The models show the role of air-sea heat exchange in the ocean heat balance in the wave guide is one of dissipation/damping. Total air-sea heat exchange is well represented by a simple Newtonian cooling parameterization in the near-equatorial region. In the wave guide, advection dominates the oceanic heat balance with meridional advection being numerically the most important in all regions except right on the equator. The meridional term is largely explained by local Ekman dynamics that generally overwhelm other processes in the regions of significant wind stress. The principal element in this advection term is the anomalous meridional current acting on the climatological mean meridional SST gradient. The eastward motion of the anomalies seen in both models is driven primarily by the ocean. The wind stress associated with the SST anomalies forces an equatorial convergence of heat and mass in the ocean. Outside the region of significant wind forcing, the mass source leads to a convergent geostrophic flow, which drives the meridional heat flux and causes warming to the east of the main wind anomaly. West of the main anomaly the wind and geostrophic divergence cause advective cooling. The result is that the main SST anomaly appears to move eastward. Since the direct SST forcing drives the anomalous wind, surface wind convergence, and associated precipitation, these fields are seen also to move eastward.

Coherent lidar airborne windshear sensor: performance evaluation

Abstract: As part of the NASA/FAA National Integrated Windshear Program, a measurable windshear hazard index has been defined that can be remotely sensed from an aircraft, to give the pilot information about the wind conditions along the flight path. A technology analysis and end-to-end performance simulation measuring signal-to-noise ratios and resulting wind velocity errors for competing coherent laser radar (lidar) systems have been carried out. The results show that a Ho:YAG lidar at a wavelength of 2.1 microns and a CO2 lidar at 10.6 microns can give the pilot information about the line-of-sight component of a windshear threat from his present position to a region extending 2-4 km in front of the aircraft. This constitutes a warning time of 20-40 s, even in conditions of moderately heavy precipitation.

A study of climate and weather variability over the tropical southwest Indian Ocean

Abstract: The climatology and variability of summer convection and circulation over the tropical southwest Indian Ocean is investigated using satellite imagery, routine synoptic observations, outgoing longwave radiation (OLR) data, sea surface temperatures (SST) and areal averaged rainfall departures OLR has a −090 correlation with rainfall departures and the OLR minimum (ITCZ) in January and February lies across the 10°S latitude, extending further south near Madagascar The intensity of ITCZ convection is greatest in the longitudes 20–35°E over northern Zambia and is considerably reduced over the SW Indian Ocean Spatial correlations are analyzed for standardized departures of OLR, rainfall and SST The correlations change sign in a coherent fashion, creating a climatic dipole between southern Africa and the SW Indian Ocean Interannual trends are examined through analysis of January–February zonal and meridional wind indices constructed from significantly correlated variables at Zimbabwe, Madagascar and Mauritius Circulation variability is dominated by quasi-decadal cycles and a trend of inereasing westerly winds Zonal wind shear alternates from easterly (barotropic) to westerly and together with SST appears to regulate the frequency and intensity of tropical cyclogenesis Areally averaged rainfall departures exhibit 625 year cycles in NE Madagascar and 125 and 1875 year cycles in SW Madagascar and Zimbabwe, respectively Summer rainfall and meridional winds in NE Madagascar and Zimbabwe are out of phase and negatively correlated in most summers The presence of synoptic weather systems is assessed using daily Hovmoller-type satellite imagery composites Convective structure is dominated by transient waves in the 10°–20°S latitude band, with periods of 15–20 days common The waves are more prominent in summers with increased easterly shear and contribute to fluctuations in rainfall over SE Africa

Surface wind speeds over the North Sea from special sensor microwave/imager observations

Abstract: The Special Sensor Microwave/Imager (SSM/I) flying as part of the U.S. Defense Meteorological Satellite Program measures millimeter-wave radiation emerging from the Earth-atmosphere system at different directions of polarization that can be used to measure oceanic surface wind speed. The radiative transport within the radiometer channels is studied theoretically for a large set of oceanic/atmospheric situations with respect to surface wind speed variations. Statistical retrieval coefficients for the recoverage of surface wind speeds from the satellite measurements are then derived via regression analysis of the synthetic measurements simulated. The theoretically estimated accuracy for the globally valid passive wind speed retrieval is 1.4 m/s under conditions where no heavy rain hampers the surface-leaving radiation in reaching the satellite. The technique is validated with in situ wind fields over the North Sea being objectively analyzed from routine wind and pressure observations.

Airborne wind shear detection weather radar

Abstract: Radar Wind cisailement the sensing edge comprising a transmitter (28) which emits successive radar beams in air at the front of the aircraft and a receiver (28) which receives the reflected signals. The radar analyzes the received reflected signals in the frequency domain to determine if there is a windshear condition in the air on the front of the aircraft.

Turbulence in the Solar Wind

Abstract: The solar wind is, apparently, a turbulent medium permeated by fluctuations on a broad range of scales characterizing the plasma as a fluid and kinetic entity. Here we briefly review some recent developments and observations on MHD fluctuations in the frequency range between some 10−6 and 10−2 Hz. Spectral analysis reveals that solar wind fluctuations represent ongoing turbulence, often with high Alfvenic correlations of coronal origin, which undergoes considerable spatial and spectral evolution in the radially expanding wind and is coupled to large-scale plasma structures and magnetic field inhomogeneities. Theoretical attempts to model the transport of magnetohydrodynamic fluctuations are discussed. Finally, the dissipation of turbulence and concurrent heating of the wind are addressed.

Parameterization of the Vertical Transport of Momentum by Cumulus Clouds. Part I: Theory

Abstract: A scheme is developed to parameterize the vertical transport of momentum by cumulus clouds, in which the effect of a cloud-induced pressure field is included. In addition, a new form for momentum exchange between clouds and their environment is used. The scheme incorporates a simple cloud model which includes both updraft and downdraft and specifies the cloud dynamic fields to enable us to determine the mean pressure gradient force across the clouds. It is shown that in a typical environment with vertical wind shear, the pressure gradient force is along the direction of the wind shear. The equation governing cloud mean momentum together with the diagnostic equation for cloud-induced pressure field is solved by iteration to evaluate quantitatively the horizontal pressure gradient force across the cloud and determine the cloud mean momentum. Application of the parameterization scheme to GATE convective events will be presented in Part II of this paper.

A simple, analytical, axisymmetric microburst model for downdraft estimation

Abstract: A simple analytical microburst model was developed for use in estimating vertical winds from horizontal wind measurements. It is an axisymmetric, steady state model that uses shaping functions to satisfy the mass continuity equation and simulate boundary layer effects. The model is defined through four model variables: the radius and altitude of the maximum horizontal wind, a shaping function variable, and a scale factor. The model closely agrees with a high fidelity analytical model and measured data, particularily in the radial direction and at lower altitudes. At higher altitudes, the model tends to overestimate the wind magnitude relative to the measured data.

Numerical Simulation of Mesoscale Circulation in Taiwan and Surrounding Area

Abstract: Mesoscale circulation around Taiwan and the surrounding area has been investigated using the Purdue mesoscale model. The numerical results generated in an inviscid atmosphere show: (a) A cyclonic vortex forms in the southeast and a slightly weaker anticyclonic vortex forms in the northeast of Taiwan uner a westerly or southwesterly wind. Subsidence warming also generates a relative low pressure on the southeastern coast. (b) A low pressure associated with a cyclonic flow forms in the northwest and a slightly weaker anticyclonic flow forms in the southwest of Taiwan under an easterly mean flow. The easterly wind tends to turn northeasterly over the Taiwan Strait, with a stronger wind speed, due to the blocking effects of the mountains in Taiwan and along the Chinese coast. (c) Under the existence of an easterly surface wind with a reverse shear, the horizontal temperature advection is not important in the formation of low pressure on the leeside, due to the small length scale of the island of Taiw...

Wind wave cast in the Mediterranean Sea

Abstract: An advanced, third-generation wave model has been repeatedly applied to the Mediterranean Sea. We have studied the accuracy of the results and the factors that control it. The grid resolution, when less than 0.5°, is shown to have in general negligible effect in the open sea. The related accuracy of description of the coastal border can have a relevant effect on the wave field on the side and on the wake of the coastal details, for a distance of a few grid points. The difficulty of correctly evaluating the wind in the Mediterranean basin is dramatically shown by comparing the results obtained using wind fields with different resolution. The crucial role of the orography in shaping the wind fields and the need for higher-resolution models is established. It is found that owing to the strong dependence of wave height on wind speed, for resolution coarser than 70 km the loss of accuracy of the wind fields, even if meteorologically acceptable, makes them useless for an efficient evaluation of the wave conditions. The exception, explained by the local absence of relevant mountain ridges, is the case of a southerly wind in the eastern Mediterranean. The accuracy of wave forecasting is obtained by comparing analysis and forecast fields for a very severe storm. In connection with the preferential west to east flow of most of the Mediterranean storms, and with the scarcity of information on the Atlantic Ocean, the equality of the meteorological forecast rapidly deteriorates beyond 1 or 2 days of forecast. This leads to an even faster deterioration of the wave forecast, whose practical limits in the western Mediterranean can at present be established at 1 day.

Na temperature lidar measurements of gravity wave perturbations of wind, density and temperature in the mesopause region

Abstract: High resolution temperature profiles of the mesopause region above Fort Collins, CO (40.6°N,105°W) were measured with a Na lidar on the nights of March 2–3 and April 15–16, 1990, during the ALOHA-90 campaign. This paper reports the initial scientific analysis of these data which were used to compute (1) the altitude profiles of relative atmospheric temperature perturbations, (2) the mean Brunt-Vaisala frequency in the mesopause region, and (3) the vertical shear variance of horizontal winds. On March 2–3 and April 15–16, the rms temperature perturbations were 5.7% and 7.1%, the average Brunt-Vaisala periods were 4.9 min and 4.6 min, and the wind shear variances were 878 (ms−1/km)² and 967 (ms−1/km)², respectively.

Variations in boundary layer properties associated with clouds and transient weather disturbances at the South Pole during winter

Abstract: Both the increasing concentrations of greenhouse gases and potential changes in cloud distributions are likely to affect the surface energy budget of the polar regions. Changes in the polar atmosphere are linked to dynamical processes that control the transport of mass, heat, and moisture from lower latitudes and in turn, feed back into the global circulation. Radiation and meteorological data collected at the South Pole during the 1986 austral winter are analyzed to gain a better understanding of the relationships between cloud radiative effects, transport processes and the vertical distribution of temperature and wind. An algorithm is developed to characterize the quasi-permanent surface-based temperature inversion and the 'warm' radiatively active layer above it. Mean winter temperature and wind profiles for clear and overcast conditions are combined with surface radiation measurements and synoptic circulation patterns to study the mechanisms that cause periodic weakening of the inversion. Results support previous studies that ascribe this weakening to (1) warm air advection, (2) downward vertical mixing of sensible and latent heat, and (3) longwave cloud radiative heating. The integrity of the inversion depends on the combined effects of all three mechanisms. Parameters representing the intensity of the inversion and the bulk wind shear throughmore » the lower troposphere are suggested as appropriate indices for the detection of climate change in the region of the Antarctic Plateau.« less

Effects of Diabatic Cooling in a Shear Flow with a Critical Level

Abstract: The response of a two-dimensional, stably stratified shear flow to diabatic cooling, which represents the evaporative cooling of falling precipitation in the subcloud layer, is examined using both a linear analytical theory and a nonlinear numerical model. The ambient wind is allowed to reverse its direction at a certain height and the cooling is specified from the surface to a height below the wind reversal level. From a scale analysis of the governing equations a nonlinearity factor of the thermally induced finite-amplitude wave, gQ0l(cpT0U02N), is found. From a scale analysis of the linear system, it is shown that the wind shear can modify the condition in which the upstream propagation of the density current is opposed by the ambient wind. When the shear and the basic wind are of opposite sign, small basic wind is enough to prevent the upstream propagation of the density current. This is because part of the cooling is used to compensate the positive vorticity associated with the positive wind...

A Stochastic Model of Gravity-Wave-Induced Clear-Air Turbulence

Abstract: We examine the consequences of using a vertical wavenumber spectral model to describe variations of vertical profiles of atmospheric variables (horizontal and vertical wind, temperature, and other scalars) about a mean profile. At high wavenumbers the model exhibits a wavenumber to the -3 dependence, which is characteristic of a continuum of internal gravity waves whose amplitudes are controlled by a breaking process. By employing a random phase between wavenumber amplitude components, a reverse fourier transform of the spectrum yields simulated profiles of velocity and thermal variability as well as shear and Brunt–Vaisala frequency variability. Individual components of the vertical shear of the horizontal wind and the Brunt–Vaisala frequency exhibit Gaussian distributions; the square of the magnitude of the shear exhibits a Rice–Nakagami distribution. Assuming regions with Ri < 0.25 are turbulent, we can examine a number of aspects of the occurrence of clear-air turbulent breakdown in the strat...

Parameterization of the Vertical Transport of Momentum by Cumulus Clouds. Part II: Application.

Abstract: The parameterization theory developed in Part I is applied to compute the vertical transport of momentum by cumulus clouds for the average of six convective periods in Phase III of GATE. Special attention is paid to the role of perturbation pressure field in vertical momentum transport. Good agreement between the parameterized and the observed apparent momentum sources is obtained. In general, cumulus convection tends to decrease the vertical wind shear of the environment. It is found that the cloud-mean momentum varies significantly with height in an environment with vertical wind shear, and the pressure gradient force is mostly responsible for this variation. Sensitivity tests of the parameterization scheme show that the cloud-mean momentum obtained is fairly insensitive to some poorly represented parameters in the cloud model. Results using this scheme are compared with those using the Schneider and Lindzen scheme. Appreciable improvement is found with the use of the new scheme.

Wind shear alert system

Abstract: A wind shear alert system for low level horizontal wind shears is disclosed. The wind shear alert system is based upon a network of remote sensor stations placed near runways throughout the geographic area covered by an airport. An airport configuration file supplies numerous parameters related to the geometry and geography of the airport, as well as threshold values for divergence phenomena in the form of microbursts. Selected pairs, edges, and triples, triangles, of said network are monitored specifically to detect a divergence. Data from the various remote sensor stations is filtered to remove short term effects and adjust for missing data. A network mean is computed to which wind field measurement data and divergence analysis is compared. If a divergence in the form of a microburst or generic wind shear is detected, then the relative magnitude of the loss or gain in head wind to approaching aircraft is determined, as well as the location of the head wind. Alarm messages are issued to air traffic controller display screens which are then relayed to the pilot of an arriving or departing aircraft.

Frequency spectra of vertical velocity from Flatland VHF radar data

Abstract: The vertical wind velocity over very flat terrain was observed every 153 s in the troposphere and lower stratosphere by the Flatland radar, near Champaign-Urbana, Illinois. Several hundred frequency spectra were calculated from all accepted 6-hour time series from March through May 1987. By stratifying the spectra in various ways we find the following: (1) The spectra were independent of altitude within the troposphere or lower stratosphere, but the spectra in the two regions differed in amplitude and frequency; (2) At a given altitude the spectra were independent of the wind shear dū/dz, the buoyancy frequency N, and the maximum wind speed below 16 km; (3) The change of spectral shape and amplitude with increasing background wind speed ū was much less than at stations near mountains. The variance of the spectra, equal to twice the vertical kinetic energy per unit mass, roughly doubled as ū increased by 10 m s−1; (4) The spectra were consistent with being due to a spectrum of gravity waves, as indicated by the sharp drop in spectral amplitude near N at small ū and by the fact that the observed change of shape with increasing ū was quite consistent with the change of shape of model Doppler-shifted gravity wave spectra; (5) The results of comparison between the observed and model spectra are consistent with an intrinsic gravity wave spectrum that is invariant with ū, dū/dz, etc., contrary to expectations from gravity wave theory; (6) The results are insensitive to the azimuthal distribution of gravity wave energy, as long as the distribution is roughly symmetrical relative to the mean flow; (7) The resulting characteristic horizontal phase velocity c* of the intrinsic frequency spectrum was about 6 m s−1 in both the troposphere and the stratosphere. The corresponding characteristic vertical wavelengths were about 3300 and 1800 m, respectively, consistent with previous estimates.

The dependence of wind stress on wave height and wind speed

Abstract: Three near-neutral boundary layer data sets were investigated with the aim of finding a dependence of wind stress on both wind speed and significant wave height. The data set most representative of open-ocean wave height, wind speed, and momentum flux conditions, was selected and analyzed by means of the least-squares method to produce a new parameterization for the wind stress as a function of both wind speed and significant wave height. This study shows that the wind stress, and consequently the drag coefficient, decreases with increasing wave height for a fixed wind speed. The study also shows that the curvature of the wind profile decreases with increasing wave height and that the C(DN) = A + BU-bar(10) form for the drag coefficient parameterization is inadequate. A drag coefficient that applies to both smooth and rough flows is proposed. These results are more applicable for open-ocean deep-water conditions and less applicable for sheltered, closed, shallow water sites.

Occurrence of roll circulations in a shallow boundary layer

Abstract: Meteorological measurements taken at the Nasudden wind turbine site during slightly unstable conditions have been analyzed. The height of the convective boundary layer (CBL) was rather low, varying between 60 and 300 m. Turbulence statistics near the ground followed Monin-Obukhov similarity, whereas the remaining part of the boundary layer can be regarded as a ‘near neutral upper layer’. In 55% of the runs, horizontal roll vortices were found. Those were the most unstable runs, with -zi/L > 5. Spectra and co-spectra are used to identify the structures. Three ‘roll indicators’ were identified: (i) a low frequency peak in the spectrum of the lateral component at low level; (ii) a corresponding increase in the vertical component at mid-CBL; (iii) a positive covariance {ovv′w′} together with positive wind shear in the lateral direction (∂V/∂z) in the CBL. By applying these ‘indicators’, it is possible to show that horizontal roll circulations are likely to be a common phenomenon over the Baltic during late summer and early winter.

Nocturnal Planetary Boundary Layer Structure and Turbulence Episodes during the Project STABLE Field Program

Abstract: This paper summarizes the results of meteorological and dispersion measurements during Savannah River Laboratory's Project STABLE (Stable Boundary Layer Experiment) field program. The field program took place at the Savannah River site on three nights during 12–17 April 1988. Meteorological data were collected from a 304-m tower, an array of eight 60-m towers, two sodars, a tethersonde, and a sonic anemometer. Based on the classification scheme of Kurzeja et al. (1991) the first and third nights were classified as unsteady type IV nights because of the passage of microfronts on each night. The second night exhibited a continuous level of high turbulence with a weak surface-based inversion and was classified as a steady type III night. The third night was especially interesting because of the considerable directional wind shear with height and the occurrence of two turbulent episodes. The directional shear may have been related to the passage of a high pressure center during the night. The turbule...

Surface wind convergence as a short-term predictor of cloud-to-ground lightning at Kennedy Space Center: A four-year summary and evaluation

Abstract: Since 1986, USAF forecasters at NASA-Kennedy have had available a surface wind convergence technique for use during periods of convective development. In Florida during the summer, most of the thunderstorm development is forced by boundary layer processes. The basic premise is that the life cycle of convection is reflected in the surface wind field beneath these storms. Therefore the monitoring of the local surface divergence and/or convergence fields can be used to determine timing, location, longevity, and the lightning hazards which accompany these thunderstorms. This study evaluates four years of monitoring thunderstorm development using surface wind convergence, particularly the average over the area. Cloud-to-ground (CG) lightning is related in time and space with surface convergence for 346 days during the summers of 1987 through 1990 over the expanded wind network at KSC. The relationships are subdivided according to low level wind flow and midlevel moisture patterns. Results show a one in three chance of CG lightning when a convergence event is identified. However, when there is no convergence, the chance of CG lightning is negligible.

Sensors and systems to enhance aviation safety against weather hazards

Abstract: The authors describe the physics of adverse weather, the basics of Doppler engineering, and a host of advanced sensing systems-some with the ability to autonomously identify and track storm conditions-for all stages of airplane travel. Three major new Doppler radar systems are discussed: the next generation weather radar, the terminal Doppler weather radar, and the airport surveillance radar with a dedicated weather channel. Other relatively simple new instruments for aviation weather support include the low level wind shear alert system, the Doppler wind profilers, the automated weather observation system, and the automated surface observation system. These systems are designed to perform higher level functions such as detection, characterization, and hazard potential estimation of aviation-significant weather phenomena, as well as their communication and display automatically. >

Simulation of Dispersion in Moderately Complex Terrain

Abstract: In operational applications of dispersion calculations in areas on the meso-γ-scale, which covers roughly the interval 2–20 km, one often uses models that presumes the wind- and turbulence fields to be horizontally uniform over the area concerned. This assumption could be very incorrect in an area with complex terrain. The present model works as follows:

Further study of a saturated gravity wave spectrum in the mesosphere

Abstract: Mesospheric wind profiles with a height resolution of 25 m derived from five foil chaff experiments performed during the last salvo of the MAC/SINE (Middle Atmosphere Cooperation/Summer in Northern Europe) campaign in the summer of 1987 at Andoya Rocket Range, in Northern Norway (69°17′N, 16°01′E), allow us to study the character of the motion spectrum in the mesosphere. The wind data have shown that (1) the average slope of the five vertical wave number spectra was −3.0 ± 0.2 for wavelengths in the range from 6.4 km to 100 m; (2) a dominant vertical wavelength of more than 6.4 km was found; and (3) there was good agreement between the observed spectrum and the saturated spectrum presented by Dewan and Good (1986) and Smith et al. (1987). In this paper, we further show that (1) very large wind shears, as high as 50–90 m s−1 km−1, are observed to be present at various heights (such large shears yield Richardson numbers smaller than 0.25, suggesting that the saturated gravity wave spectrum is associated with the shear instabilities); and (2) when the Richardson number profile is compared with the altitude profile of the mesosphere-stratosphere-troposphere (MST) radar (SOUSY) echo power, the low Richardson numbers are observed in the region of most intense backscatter. It is suggested from our measurements that the enhanced echo power is linked to wave saturation processes.