Showing papers on "Wind shear published in 1998"

An Overview of the HYSPLIT_4 Modelling System for Trajectories, Dispersion, and Deposition

Abstract: The HYSPLIT_4 (HYbrid Single-Particle Lagrangian Integrated Trajectory) model is designed for quick response to atmospheric emergencies, diagnostic case studies, or climatological analyses using previously gridded meteorological data. Calculations may be performed sequentially on multiple meteorological grids, going from fine to coarse resolution using either archive or forecast data fields. Air concentration calculations associate the mass of the pollutant species with the release of either puffs, particles, or a combination of both. The dispersion rate is calculated from the vertical diffusivity profile, wind shear, and horizontal deformation of the wind field. Air concentrations are calculated at a specific grid point for puffs and as cell-average concentrations for particles. The model results are evaluated against ACE balloon trajectories, air concentrations from the ANATEX tracer experiment, radiological deposition from the Chernobyl accident, and satellite photographs of the Rabaul volcanic eruption. One common feature of the model results was their sensitivity to the vertical atmospheric structure; trajectories in terms of their height when near ground-level due to the strong gradients of wind speed and direction, air concentrations with respect to the rate of vertical mixing, and deposition as a result of the vertical distribution of the pollutant.

Nowcasting Thunderstorms: A Status Report

Abstract: This paper reviews the status of forecasting convective precipitation for time periods less than a few hours (nowcasting). Techniques for nowcasting thunderstorm location were developed in the 1960s and 1970s by extrapolating radar echoes. The accuracy of these forecasts generally decreases very rapidly during the first 30 min because of the very short lifetime of individual convective cells. Fortunately more organized features like squall lines and supercells can be successfully extrapolated for longer time periods. Physical processes that dictate the initiation and dissipation of convective storms are not necessarily observable in the past history of a particular echo development; rather, they are often controlled by boundary layer convergence features, environmental vertical wind shear, and buoyancy. Thus, successful forecasts of storm initiation depend on accurate specification of the initial thermodynamic and kinematic fields with particular attention to convergence lines. For these reasons ...

Three-Dimensional Wind Simulation

Abstract: A method for numerically simulating a three-dimensional field of turbulent windspeed (the “Sandia method”) for use in the aerodynamic and structural analyses of wind turbines is presented. The required inputs are single point power spectral densities (PSDs) and the coherence function. Suggestions for appropriate inputs and an example calculation are included. The simulation method is used to obtain “rotationally sampled” PSDs, which are compared with measurements obtained by Pacific Northwest Laboratories. The results show that the Sandia method is capable of producing simulations that agree with the measurements, especially when the coherence function is augmented from the usual form to include the ratio of spatial separation over height raised to the 0.25 power. The method is specialized for horizontal axis wind turbine analysis by phase lagging the simulations at each point in space so that wind speeds are simulated only when the turbine blade passes the point, reducing storage requirements and computation time by about an order of magnitude. For vertical axis applications, where interpolation will be required, the error induced by the interpolation is estimated and eliminated by the addition of white noise.

Radiative–convective equilibrium in a three-dimensional cloud-ensemble model

Abstract: A knowledge of radiative convective interactions is key to an understanding of the tropical climate. In an attempt to address this a cloud-resolving model has been run to a radiative-convective equilibrium state in three dimensions. The model includes a three-phase bulk microphysical scheme and a fully interactive two-stream broadband radiative-transfer scheme for both the infrared and solar radiation. The simulation is performed using a fixed sea surface temperature, and cyclic lateral boundary conditions. No ‘large-scale’ convergence, mean wind shear or background vorticity was imposed. The total integration lasted 70 days, and a statistical equilibrium state was reached at all heights after 30 days of simulation in all model variables. It is seen that some variables, such as vertical mass flux, adjust quickly to their equilibrium values while others, such as column-integrated water amount, domain-mean temperature and convective available potential energy (CAPE) display variation on a longer 30-day time-scale. The equilibrium state had a column-integrated vapour amount of 42.3 kg m−2, a mean temperature of 258.7 K and a pseudo-adiabatic CAPE value of 1900 J kg−1. The equilibrium-state statistics are consistent with tropical observations. The convection does not remain randomly distributed but instead becomes organized, aligning in a band structure associated with high moisture values in the boundary layer. This organization seems to result from interactions between radiation, convection and surface fluxes. The surface-flux feedback is due to higher boundary-layer winds, associated with convection, increasing surface fluxes of moisture locally. Horizontally inhomogeneous radiation can act to make clouds longer lasting and also increase convergence into cloudy region. Replacing the wind-sensitive surface-flux calculation with a linear relaxation to surface values appeared to largely destroy this organization, as did the use of an imposed horizontally uniform radiative-heating rate.

The Role of Environmental Shear and Thermodynamic Conditions in Determining the Structure and Evolution of Mesoscale Convective Systems during TOGA COARE

Abstract: A collection of case studies is used to elucidate the influence of environmental soundings on the structure and evolution of the convection in the mesoscale convective systems sampled by the turboprop aircraft in the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean–Atmosphere Response Experiment (COARE). The soundings were constructed primarily from aircraft data below 5–6 km and primarily from radiosonde data at higher altitudes. The well-documented role of the vertical shear of the horizontal wind in determining the mesoscale structure of tropical convection is confirmed and extended. As noted by earlier investigators, nearly all convective bands occurring in environments with appreciable shear below a low-level wind maximum are oriented nearly normal to the shear beneath the wind maximum and propagate in the direction of the low-level shear at a speed close to the wind maximum; when there is appreciable shear at middle levels (800–400 mb), convective bands form parallel to the shear. With...

The Influence of Midtropospheric Dryness on Supercell Morphology and Evolution

Abstract: This work studies the relationship between midtropospheric dryness and supercell thunderstorm morphology and evolution using a three-dimensional, nonhydrostatic cloud model Environments that differ only in midtropospheric dryness are found to produce supercells having different low-level outflow and rotational characteristics Thunderstorms forming in environments with moderate vertical wind shear, large instability, and very dry midtropospheric air produce strong low-level outflow When this low-level outflow propagates faster than the midlevel mesocyclone, the storm updraft and low-level mesocyclone weaken However, in environments with larger vertical wind shear or with higher-altitude dry midtropospheric air, the low-level outflow is not as detrimental to the supercell This provides a possible explanation for why some environments that appear favorable for the development of strong low-level mesocyclones in supercells fail to do so Downdraft convective available potential energy (DCAPE) is

Traveltimes for infrasonic waves propagating in a stratified atmosphere

Abstract: SUMMARY The tau‐p method of Buland & Chapman (1983) is reformulated for sound waves propagating in a stratified atmosphere under the influence of a height-dependent wind velocity profile. For a given launch angle along a specified azimuth, the ray parameter is redefined to include the influence of the horizontal wind component along the direction of wave propagation. Under the assumption of negligible horizontal wind shear, the horizontal wind component transverse to the ray propagation does not aVect the direction of the wave normal, but displaces the reference frame of the moving wavefront, thus altering the observed incidence azimuth. Expressions are derived for the time, horizontal range, and transverse range of the arriving waves as a function of ray parameter. Algorithms for the location of infrasonic wave sources using the modified tau‐p formulation in conjunction with regional atmospheric wind and temperature data are discussed.

A wind turbine with a wind velocity measurement system

Abstract: The present invention provides a wind turbine with improved efficiency, reduced static as well as dynamic structural loads, reduced noise, and facilitating grid connection with improved power quality. According to the invention these and other features are fulfilled by a wind turbine comprising a wind velocity measurement system for determination of air velocities in front of the wind turbine and having an output for provision of determined air velocity values.

Atlantic Sea Surface Temperatures and Tropical Cyclone Formation.

Abstract: It has long been accepted that interannual fluctuations in sea surface temperature (SST) in the Atlantic are associated with fluctuations in seasonal Atlantic basin tropical cyclone frequency. To isolate the physical mechanism responsible for this relationship, a singular value decomposition (SVD) is used to establish the dominant covarying modes of tropospheric wind shear and SST as well as horizontal SST gradients. The dominant SVD mode of covarying vertical shear and SST gradients, which comprises equatorially confined near-zonal vertical wind shear fluctuations across the Atlantic basin, is highly correlated with both equatorial eastern Pacific SST anomalies (associated with El Nino) and West African Sahel rainfall. While this mode is strongly related to tropical storm, hurricanes, and major hurricane frequency in the Atlantic, it is not associated with any appreciable Atlantic SST signal. By contrast, the second SVD mode of covarying vertical shear and horizontal SST gradient variability, wh...

Mechanisms for the Generation of Mesoscale Vortices within Quasi-Linear Convective Systems

Abstract: Previous idealized simulations of convective systems have demonstrated that the development of mesoscale vortices within quasi-linear convective systems may be a natural consequence of the finite extent of the convective line, as horizontal vorticity is tilted into the vertical at the line ends. However, the source of this horizontal vorticity has not yet been clearly established, either being associated with the ambient shear or else generated within the system. In this paper, results are presented from a series of idealized simulations that demonstrate that the source, strength, and scale of these vortices depends on the strength of the ambient vertical wind shear, the strength of the system-generated cold pool, the scale of the convective line segments, as well as the phase within the life cycle of the convective system. In particular, for systems that develop in an environment with weak-to-moderate shear, a line-end vortex pair is generated primarily via the tilting of horizontal vorticity ge...

Low-Level Potential Vorticity and Cyclogenesis to the Lee of the Alps

Abstract: High-resolution numerical model simulations over the Alpine region are presented that reveal the presence of low-level elongated bands of potential vorticity (PV) downstream of high topography. These PV streamers (or PV banners) occur when the synoptic-scale wind turns into a direction across the Alps. Individual pairs of banners with anomalously positive and negative values of PV can be attributed to flow splitting, either on the scale of the whole of the Alps (primary banners), or on that of individual massifs and peaks of the model topography (secondary banners). The PV bands have amplitudes of up to −2.5 and +5 pvu, a width of 50–150 km, can attain a length of up to 1500 km, and extend in the vertical from the surface up to the 500-hPa level on occasions. The PV banners are associated with zones of enhanced horizontal wind shear. The analysis of daily output from the operational NWP model run of the Swiss Meteorological Institute also demonstrates that such PV streamers are a frequent feature...

Barrier Jets during TAMEX

Abstract: A barrier jet is frequently found along the northwestern coast of Taiwan in the prefrontal southwesterly flow regime during the Taiwan Area Mesoscale Experiment (TAMEX). It has a maximum wind speed of 14 m s−1 at approximately 1 km above the surface with a vertical wind shear approximately 10 × 10−3 s−1 below and 4 × 10−3 s−1 above that altitude. During TAMEX, the southwesterly monsoon flow strengthens over Taiwan when the low-level pressure trough/surface front moves toward the southeastern China coast. The barrier jet is a result of the stably stratified airflow past an island obstacle under a small–Froude number [

Stationary gravity‐wave structure in flows with directional wind shear

Abstract: Most theoretical studies of flow over mountains assume that the wind direction is constant. For two-dimensional orographic ridges this is not an issue, since only the component of the wind across the ridge forces internal gravity waves. A special case of rectilinear flow is when the wind component changes sign (i.e. the wind direction changes discontinuously through 180 degrees): such a point is a critical level for all stationary gravity waves. For isolated three-dimensional mountains the gravity-wave response comprises waves of all azimuthal orientations, and each of these may have a different critical-level height-depending on whether the wind backs or veers with height. Using a combination of ray-tracing methods and stationary-phase solutions, the structure of the stationary, hydrostatic gravity-wave field generated by an isolated mountain is studied for a simple wind field which backs with height. In contrast to the behaviour for unidirectional flow, wave action does not accumulate indefinitely beneath the critical level but is continuously advected downwind by the component of the wind parallel to the phase lines. Instead of the wave energy becoming unbounded following a ray as in the 2D case, the wave energy decays to zero along a trailing wake zone downwind of the mountain along phase lines. This is just an extension of the ‘asymptotic wake’ found in the linear solutions of Smith for constant flow over an isolated mountain. The work described here underlines the desirability of including the effects of selective critical-level absorption in the gravity-wave drag parametrization schemes of numerical weather-prediction models.

A wind-driven gradient drift mechanism for mid-latitude E-region ionospheric irregularities

Abstract: We propose a new explanation for medium- to small-scale irregularities in the mid-latitude E region. We show that the very wind field that forms thin layers of ionization can also create plasma instabilities. To date, the application of gradient drift instability theory has been restricted to free energy sources involving the ambient electric field. We show that a vertical shear in the zonal wind not only leads to layer formation, but also to unstable regions on both sides of the layer. The mechanism thus does not suffer from the problem inherent in electric field-driven processes, namely, that the mapping of electric fields along magnetic field lines leads to stabilizing conditions on one side of the layer, even if the other side is unstable. Evidence is given that anomalous diffusion due to such waves affects the evolution of the layers.

The SEEK chemical release experiment: Observed neutral wind profile in a region of sporadic E

Abstract: Wind measurements made with the chemical release technique during a sporadic E layer event are presented. The data were obtained as part of the Sporadic E Experiment over Kyushu (SEEK) sounding rocket campaign. The winds show a strong maximum in the lower E region approaching 150 m s−1 near 105-km altitude and a large shear below the maximum. The large shear was within a few kilometers altitude of the peak in the electron densities measured on the downleg of the rocket trajectory. Calculations of the Richardson numbers for the wind profile show that the altitude range near the layer was highly unstable.

Evidence of Organized Large Eddies by Ground-Based Doppler Lidar, Sonic Anemometer and Sodar

Abstract: In an experiment investigating the planetary boundary layer (PBL) wind and temperature fields, and PBL inversion height recorded by various instruments, the results reveal the presence of organized large eddies (OLE) or rolls. The measurements by lidars, anemometers, soundings and sodar gave an overview of the characteristics of the rolls and sources of energy production that maintain them. The experimental results obtained on two consecutive days are compared to model outputs. The agreement is excellent, showing that thermal stratification and wind shear are important factors in the structure and dynamics of OLE. A heterodyne Doppler lidar (HDL) is shown to be a useful tool in the study of OLE.

North Atlantic Wind Waves and Wind Stress Fields from Voluntary Observing Ship Data

Abstract: On the basis of the collection of individual marine observations available from the Comprehensive Ocean–Atmosphere Data Set, major parameters of the sea state were evaluated. Climatological fields of wind waves and swell height and period, as well as significant wave height and resultant period are obtained for the North Atlantic Ocean for the period from 1964 to 1993. Validation of the results against instrumental records from National Data Buoy Center buoys and ocean weather station measurements indicate relatively good agreement for wave height and systematic biases in the visually estimated periods that were corrected. Wave age, which is important for wind stress estimates, was evaluated form wave and wind observations. The climatology of wave age indicates younger waves in winter in the North Atlantic midlatitudes and Tropics. Wave age estimates were applied to the calculations of the wind stress using parameterizations from field experiments. Differences between wave-age-based and traditional estimates are not negligible in wintertime in midlatitudes and Tropics where wave-induced stress contributes from 5% to 15% to the total stress estimates. Importance of the obtained effects for ocean circulation and climate variability is discussed.

Wind erosion in agricultural soils: an example of limited supply of particles available for erosion

Abstract: Soil erosion by wind is a complex process since many interacting factors are involved. In addition, wind erosion can show a considerable spatial and temporal variability associated with changes in soil surface conditions. During a wind erosion experiment conducted in August 1995 within an agricultural field of Central Aragon (NE Spain) [Lopez, M.V., Sabre, M., Gracia, R., Arrue, J.L., Gomes, L., 1998. Tillage effects on soil surface conditions and dust emission by wind erosion in semiarid Aragon (NE Spain). Soil Tillage Res. (in press)], a decay in dust emission (vertical dust flux) with an increase in wind speed was observed at the end of the experimental period. A further analysis of the evolution of the vertical flux with time in response to changes in soil erodibility is shown in the present study. The analysis is based on the comparison of the measured flux with the potential flux predicted for identical wind conditions assuming that the supply of erodible material at the soil surface was unlimited. The potential flux was estimated by using the dust emission model developed by Marticorena and Bergametti [Maticorena, B., Bergametti, G., 1995. Modeling the atmospheric dust cycle: 1. Design of a soil-derived dust emission scheme. J. Geophys. Res. 100, pp. 16415–16430]. The model is based on the parameterization of the threshold wind shear velocity as a function of the aggregate size distribution and the roughness length of soil surface. The results indicate that the observed reduction in soil erodibility with time was probably due to variations in the aggregate size distribution and, more precisely, to a limited supply of erodible particles at the soil surface. This study underlines the need to consider the temporal variability of the surface conditions in wind erosion research and derived models.

Katabatic wind fields in coastal areas studied by ERS-1 synthetic aperture radar imagery and numerical modeling

Abstract: Synthetic aperture radar (SAR) images acquired over Mediterranean coastal waters from the first European Remote Sensing Satellite (ERS-1) showing sea surface manifestations of katabatic wind fields are presented. In particular, sea surface roughness patterns generated by katabatic winds blowing from 1800 in high mountains through a broad valley at the western coast of Calabria (southern Italy) into the Gulf of Gioia is investigated. The ERS-1 SAR images show that their areal extent and shape vary strongly, depending on the meteorological conditions. The roughness pattern sometimes has the form of a mushroom, an elongated tongue, a broad blob, or a narrow truncated band. For one event (September 8, 1992) we have simulated the wind field at the sea surface by using a nonhydrostatic mesoscale atmospheric model and then compared it with the wind field derived from the ERS-1 SAR image by using the C band wind scatterometer model CMOD4. The comparison shows that the atmospheric model reproduces quite well the mushroom-like form of the wind field pattern, while the wind speed is obtained somewhat lower than the one inferred from the SAR image. This study demonstrates that SAR images acquired over coastal waters are well suited to study local wind fields in coastal areas and to test numerical models that describe local wind fields which extend from the coast onto the sea.

Observations of Low-Level Baroclinity Generated by Anvil Shadows

Abstract: Low-level cooling beneath the cirrus anvil canopies of supercell thunderstorms is documented in two Verification of the Origins of Rotation in Tornadoes Experiment cases and in the 17 May 1981 Arcadia, Oklahoma, supercell. Surface temperature decreases of 3°C or more occurred beneath the anvils within 45 min of the onset of overcast conditions. Cooling was confined to the lowest few hundred meters of the boundary layer, and believed to be due mainly to a deficit in the energy budget following a reduction of incoming shortwave radiation. In the three cases studied, the vertical wind shear was strong; thus, mixing prevented the formation of an inversion layer. Strong insolation at the ground outside of the anvil shadows coupled with the cooling beneath the cirrus canopies led to corridors of baroclinity along the shadow edges. It is shown that residence times in these baroclinic zones may be long enough for parcels to acquire considerable horizontal vorticity (e.g., ∼10−2 s−1) en route to a storm u...

Preliminary estimates of the vertical profiles of inner and outer scales from White Sands Missile Range, New Mexico, VHF radar observations

Abstract: There are very few reliable results of the inner and outer scales of turbulence in the remote atmosphere. Knowledge of these parameters is of high interest to the propagation and remote sensing communities. Seasonal profiles from 5 to 20 km above mean sea level of the inner scale have been estimated based on the kinematic viscosity and eddy dissipation rates which were determined from 5 years of nearly continuous 49.25-MHz radar observations at White Sands Missile Range, New Mexico. Inner scale values were found to increase from about 1 cm at 5 km to near 7 cm at 19 km altitude. Outer scale profile determinations were made using a method proposed by Tatarskii [1971] that involves vertical wind shear and the eddy dissipation rate, both derived from the longterm VHF radar measurements. The outer scale decreased from about 60 m at 5 km altitude for all seasons to 12–20 m at 15 km (depending on season) and then increased to 22 m at 19 km. Seasonal differences in the inner and outer scales and background meteorological conditions are also presented and discussed.

A comparison of reanalyses in the tropical stratosphere. Part 2: the quasi-biennial oscillation

Abstract: Reanalysis datasets potentially offer the opportunity to examine the tropical quasi-biennial oscillation (QBO) in greater detail than in the past, including the associated meridional circulation and the links with other parts of the atmosphere. For such studies to be useful, the QBO represented by the reanalyses should be realistic. In this work, the QBO in the ERA and NCEP reanalyses is validated against rawinsonde observations from Singapore. Monthly mean data are used. In the lower stratosphere (at 50 hPa and 30 hPa) the ERA QBO is reasonable, although the wind extrema in both phases are too weak and the vertical shear and the temperature anomalies are too small. The NCEP QBO is weaker still. At 10 hPa neither reanalysis system performs well, both systems failing to reproduce the westerlies, possibly because of the proximity of the upper boundary. The Singapore wind is representative of the zonal means in the reanalyses. The weak wind extrema in the reanalyses would not support a wave-mean flow interaction theory of the QBO, because a large portion of the gravity wave spectrum which would be absorbed in reality would be transmitted beyond 10 hPa. The stronger shear zones captured in the ERA data are associated with larger, more realistic temperature perturbations near 30 hPa. The northward velocities in the NCEP data show a more realistic structure than in the ERA reanalysis, where they are dominated by a vertical “gridpoint wave” structure in the lowermost stratosphere. Despite the shortcomings of the reanalyses, the high correlations of the wind at 30 hPa and 50 hPa with the observations at Singapore mean that the reanalyses could potentially be used to examine the effects of the QBO away from the tropical stratosphere. Future reanalyses need to take full account of the wind shears evident in the rawinsonde observations and use models with an adequate resolution to capture these vertical scales.

Sensitivity of a wave model to wind variability

Abstract: A third-generation wave model was applied to the Gulf of California to hindcast wave spectra and to assess model sensitivity to wind variability on the basis of three different numerical simulations: (1) when a synthetic wind field of varying randomness is adopted and when the forcing wind field considered has an input time step of (2) 6 hours and (3) 5 min. In the first idealized simulation the wave field induced by a constant wind field is compared with the result when white noise was added to the originally constant forcing winds. Results from these numerical simulations demonstrate that wave energy increases with wind variability, even though the mean wind is kept constant. In the second and third simulations the forcing wind is averaged for periods of 6 hours and 5 min, which represent relatively low and high timescale variability, respectively. These realistic wind fields were constructed as a blend of detailed in situ measurements and analysis information, representing wind variability in both speed and direction. The results show that high-variability winds induce broad directional wave spectra and secondary peaks with similar magnitude as the main peak (bimodal spectra). With the presence of wind gustiness and a continuous spectrum of wind variability, all source terms are expected to play a significant role in the evolution of the wave spectrum.

On the solar cycle dependence of winds and planetary waves as seen from mid-latitude D1 LF mesopause region wind measurements

Abstract: At the Collm Observatory of the University of Leipzig LF D1 low-frequency total reflection nighttime wind measurements have been carried out continuously for more than two decades. Using a multiple regression analysis to derive prevailing winds, tides and the quasi-2-day wave from the half-hourly mean values of the horizontal wind components, monthly mean values of mesopause wind parameters are obtained that can be analysed with respect to long-term trends and influences of solar variability. The response of the prevailing wind to the 11-year solar cycle differs throughout the year. While in winter no significant correlation between the zonal prevailing wind and solar activity is found, in spring and summer a negative correlation between the TWC can be seen from the measurements. This is connected with stronger vertical gradients of the zonal prevailing wind during solar maximum than during solar minimum. Since the amplitude of the quasi-2-day wave is dependent on the zonal mean wind vertical gradient, this is connected with a positive correlation between solar activity and quasi-two-day wave activity.