Region Of Strong Shear Research Articles

The beta limit in the Advanced Toroidal Facility due to local ideal MHD instabilities

The marginal pressure profile that is imposed either by ideal ballooning modes or by interchange modes on every nested flux surface of a three-dimensional equilibrium modelling the ATF device is determined for conditions of vanishing net toroidal plasma current. The corresponding limiting values are β = 3.75% and β(0) = 9.89%. Mercier modes dominate in regions of strong stellarator-like magnetic shear (ι′/ι > 0), and non-symmetric ballooning modes (non-symmetric eigenstructures in the extended poloidal angle domain) dominate in regions with strong tokamaklike magnetic shear (ι′/ι < 0).

On the local generation of interplanetary Alfvénic fluctuations

Evidence is gained in favour of local generation of Alfvénic fluctuations in the solar wind. Instabilities developing in regions of strong velocity shear give a local contribution of waves with inward and outward propagation (with respect to the Sun) to be added to the outward travelling waves of solar origin.

An observational case study of a continental mesoscale vortex

An observational case study of a continental mesoscale vortex embedded in a polar air stream has been conducted, with emphasis on observed vortex features and their possible relationship to the polar-low phenomena and accompanying synoptic scale features at the 850 mb, 700 mb and 500 mb levels. The path of this 14–16 December 1983 event (determined from GOES satellite imagery) was over the relatively data-rich region of the midwestern United States, with the mesoscale vortex rotating cyclonically around and to the south of an occluded synoptic-scale surface low pressure system (and 500 mb low). This case study exhibited features common to the polar low with its convective cloud mass, although a distinct surface signature never developed. The mesoscale vortex formed and remained within a strong cyclonic shear region (4 times 10 -5 s -1 ) at the 850 mb level, but was supported by the evolution of a baroclinic pattern as well. Spatial and temporal cross-sections (both horizontal and vertical) revealed a weak low-level warm core capped by a strong cold dome, most pronounced at the 700 mb level and distinctly separate from the synoptic scale low in the surface map and 500 mb height field. This low static stability mesoscale system occurred entirely beneath the 500 mb level with cloud tops near 600 mb, and was associated with a weakly defined 600 km wave at the 700 mb level. The pure CISK model results for wavelength growth by Bratseth and the combined moist baroclinicity and CISK studies by Sardie and Warner correspond best to this continental event. It is suggested that both barotropic and baroclinic instability mechanisms (combined with CISK) can exist cooperatively in support of the development and maintenance of both continental and maritime polar low events. DOI: 10.1111/j.1600-0870.1989.tb00378.x

Propagation of turbulence structures detected by VHF radar

In the high latitude wintertime mesosphere VHF radar measurements usually reveal several turbulence layers at heights between 65 and 85 km which are closely related to strong vertical wind shear. The turbulence layers are superposed by turbulence bursts, which often form sequences with periods similar to those of simultaneously observed velocity oscillations. The horizontal propagation velocity of the resulting turbulence structures can be obtained by cross-correlating the signal power time series measured at three antenna beam positions. A statistical study using a total of 71 events shows that there is a significant correlation between the propagation velocity of turbulence structures and the mean wind, being consistent with the assumption that turbulence is advected by large scale motions. It is suggested that the observed turbulence bursts are due to secondary static instabilities, which for their part are generated by primary Kelvin-Helmholtz instabilities in regions of strong wind shear.

A Case Study of Gravity Waves–Convective Storms Interaction: 9 May 1979

An analysis is presented of a series of severe storms which occurred in the north central United States on 9 May 1979 and whose spatial distribution and movement correlate well with observed gravity waves. Two gravity wave trains of 2.1-3 mb amplitude, 2.5-3.3 h period and 240-265 km horizontal wavelength were isolated through power spectra analysis and cross-correlation techniques applied to National Weather Service barograph traces. The wave trains propagated in the 200 deg direction, which coincided with the jet axis, with a phase velocity of 20-30 m/s and within a 300 km wide band. The storms were identified on enhanced infrared GOES satellite pictures with the help of radar summaries. These convective systems initially developed in Nebraska and propagated north-northeast at 25 m/s, revealing wave-like characteristics with a separation of 300-400 km. The convective systems were closely linked to the observed wave trains with cell intensity, height and associated rainfall maximized at the wave ridge. One of the two wave trains developed in regions of weak or no convection and appeared to initiate more intense convective clusters downstream from the point of origin. It is shown that the characteristics of the wave trains are consistent with those of gravity waves generated in a region of strong vertical shear associated with the jet. It is suggested that the wave trains continue to extract energy from the basic state all along their track through critical level interaction.

The turbulent kinetic energy budget behind a porous barrier: An analysis in streamline co-ordinates

Measurements have been made of the components of turbulent velocity in the wake of a two-dimensional porous fence under neutral atmospheric conditions. They have been analysed within a system of streamline co-ordinates to construct the downstream budgets of turbulent kinetic energy (TKE). Close to the fence, the dominant role of pressure transport in transferring TKE from the wake to higher levels is clearly demonstrated. The maximum perturbations in shearing stress and TKE tend to follow streamlines rather than developing along a fixed height. Whereas shearing stress enhancement is confined to the region of strong shear just downstream of the fence top, TKE is immediately perturbed to at least four fence heights.

Analogies between transitional and turbulent boundary layers

One of the interesting aspects of transitional and turbulent boundary layers is the development of counter-rotating streamwise vortices near the wall. The most regular pattern is found in a boundary layer on a concave wall where the generation mechanism is known to be the Görtler instability. The origin of these vortices in other translational and turbulent boundary layers is presently unknown. Since the counter-rotating vortices are located in a region of strong shear, low-speed fluid is pumped away from the wall which coalesces into regions of low momentum lying between the vortices. As this pumping action continues, localized inflectional velocity profiles become apparent in the transitional and turbulent boundary layers. The oscillations which develop upon these profiles scale with the local thickness and velocity difference in the same manner as the two-dimensional steady free shear layer stability problems. The oscillations grow to large amplitude and break down into new turbulence in both the transitional and turbulent boundary layers.

Conical vortices: A class of exact solutions of the Navier–Stokes equations

A two-parameter family of exact axially symmetric solutions of the Navier–Stokes equations for vortices contained within conical boundaries is found. The solutions depend upon the same similarity variable, equivalent to the polar angle φ measured from the symmetry axis, as flows previously discussed by Long and by Serrin, but are distinct from the cases they treated. The conical bounding stream surfaces of the present solution can be located at any angle φ=φ0, where 0&amp;lt;φ0&amp;lt;π. The flows in all of these cases, when solutions exist, are finite everywhere except at the cone vertex which is a source of axial momentum, but not of volume. Solutions are of three types, flow may be (a) towards the vertex on the axis and away from the vertex at the conical boundary, (b) towards the vertex both on the axis and at the cone, or (c) away from the vertex on the axis and towards it at the bounding cone. In the first and second case, strong shear layers form on the cone walls for high Reynolds numbers. In case (c), a region of strong axial shear and strong axial vorticity forms near the axis, even for low Reynolds numbers. The qualitative nature of the possible solutions is deduced, using methods of argument due to Serrin, and examples of flows are numerically computed for cone half-angles of π/4, π/2 (flows above the plane z=0), and 3π/4. Regions of the parameter space where solutions are proven not to exist are given for the cone half-angles given above, as well as regions where solutions are proven to exist.

Observations of Near-Surface Oceanic Velocity Strain-Rate Variability during and after Storm Events

During August of 1977, a series of repeated profiles of the upper ocean were made at Ocean Station Papa (145°W, 50°N) with a velocity/temperature microstructure profiler. A strong correlation of wind-speed variations with velocity structures in the 20 m deep mixed layer was observed. Rates of dissipation of mechanical energy as high as 5 × 10−2 ergs cm−2 s−1 were observed during wind speeds of 20 m s−1, and as low as instrumental noise (∼10−6 ergs cm−3 s−1) during light winds. A persistent vertical separation of temperature and velocity variance maxima at high wavenumbers was found near the base of the mixed layer, in a region of strong vertical shear, suggesting that show-induced entrainment continued for several days after the storm events subsided.

Wave Transience in a Compressible Atmosphere. Part I: Transient Internal Wave, Mean-Flow Interaction

Abstract Vertically propagating internal waves give rise to mean flow accelerations in an atmosphere due to the effects of wave transience resulting from compressibility and vertical group velocity feedback. Such accelerations appear to culminate in the spontaneous formation and descent of regions of strong mean wind shear. Both analytical and numerical solutions are obtained in an approximate quasi-linear model which describes this effect. The numerical solutions display mean flow accelerations due to Kelvin waves in the equatorial stratosphere. Wave absorption alters the transience mechanism in some significant respects, particularly in causing the upper atmospheric mean flow acceleration to be very sensitive to the precise magnitude and distribution of the damping mechanisms. Part II of this series discusses numerical simulations of transient equatorial waves in the quasi-biennial oscillation. These results are of sufficient qualitative interest to merit attention in this paper, and this is done with t...

Precision of tropospheric/stratospheric winds measured by the Chatanika radar

A method for reducing tropospheric/stratospheric radar backscatter measurements to wind components and for relating uncertainties in these components to uncertainties in Doppler backscatter moments estimation is described. The centroid of the Doppler spectrum is used to estimate the wind component along the radar line of sight, while the variance of this estimator is used to judge the precision that can be attached to the measurement. Error estimates are given in geographical coordinates so that errors in resolved velocities can be related directly to radar measurements. To assess the validity of the procedure, wind profiles derived from Chatanika radar measurements are compared with wind profiles obtained from a series of 12 windsondes. These windsondes were launched nearby (at approximately 45‐min intervals) on April 6 and 7, 1978. Radar measurements include selected horizontal wind profiles (from about 4‐ to 14‐km altitude) and winds at 9.05 km extending over 15 hours. Windsonde and radar‐derived velocities generally agree within the precision of the measurements, but in regions of strong wind shear, differences exceeding the calculated error bars are found. These discrepancies are shown to be caused by several independent scattering layers that often exist within the scattering volume. When these layers have sufficiently different velocities, causing multiple peaks to appear in the Doppler spectrum, and when the relative strengths of these spectral peaks change with azimuth, the centroid no longer accurately reflects the mean motion of the scatterers. Significant errors in resolved velocities can thus occur. Because regions of high reflectivity apparently exist in thin layers separated by regions of low reflectivity, radar backscatter will provide an accurate sampling of the wind field, only if the scattering layers can be accurately located. To overcome these limitations in accuracy, radars that are used to measure tropospheric/stratospheric winds should have range resolutions comparable with the thickness of the scattering layers.

An Investigation of Small Synoptic-Scale Cyclones in Polar Air Streams

Abstract The large-scale environment of small synoptic-scale cyclones that occur in polar air streams over the wintertime North Pacific behind or poleward of major frontal bands is objectively documented by the compositing of meridional (latitude-height) cross sections for 22 cases. The cold air mass cyclones are found to be associated with deep baroclinity throughout the troposphere and are located on the low-pressure side of well-developed jet streams in regions of strong cyclonic wind shear. The lower troposphere is conditionally unstable and in the early stage of development is strongly heated from below by the warmer ocean. Based on the results of the composites, and theoretical considerations, it is concluded that the oceanic polar air cyclones are a baroclinic instability phenomenon whose small scales relate to low values of the Richardson number near the surface and whose large upper level amplitudes relate to the effects of latent heat release on baroclinic wave development. Barotropic instabilit...

A Multiple Structured Frontal Zone-Jet Stream System as Revealed by Meteorologically Instrumented Aircraft

Abstract The distribution of wind, temperature, ozone, and turbulence within a multiple structured frontal zone-jet stream system is described using a combination of direct horizontal measurements by meteorologically-instrumented research aircraft and conventional aerological soundings. Results document the spatial continuity of these zones and associated intrusions of ozone-rich stratospheric air. Aircraft turbulence measurements show the zones with strong vertical wind shear and near-critical values of Richardson number are preferred regions of CAT encounter. Diagnostic calculations of the terms of the gradient thermal wind equation illustrate the importance of the sign and magnitude of the air trajectory curvature and its vertical derivative in maintaining regions of strong vertical wind shear and small Richardson number in the presence of weak horizontal thermal gradient.

Identification of Transverse Kelvin–Helmholtz Turbulence in a Magnetoplasma Column

Transverse Kelvin–Helmholtz turbulence has been observed in a region of strong rotation and shear in a magnetoplasma column produced using a Lisitano structure. The turbulence is identified on the basis of radial density–potential fluctuation phase, wave localization, and steady-state characteristics of the column. A modified wave equation is used to check experimental observations with theory. This is a second type of magnetoplasma column where transverse Kelvin–Helmholtz turbulence has been identified and it supports previous investigations in Q machines.

Mesoscale Wind Structure Revealed by Doppler Radar

Abstract Doppler wind soundings were taken in the lower 4.5 km of the atmosphere at 12 minute intervals during a seven hour period in a snowstorm over eastern Massachusetts. A time-height cross section of the wind revealed numerous small scale, short period changes in the wind structure. Initially there was a region of strong wind shear near 4 km, above the warm front zone. Periodic break-downs in the wind shear appeared to result in a downward transfer of momentum. In this way, winds at lower levels increased by as much as 10 m sec−1 within a half hour period. The series of three breakdowns occurred at about hourly intervals at successively lower levels, until by the end of the period the wind speed at all levels had increased by about a factor of two, and the wind shear zone was confined to the lowest few hundred meters. A time-height cross section of vertical motions indicated that each breakdown was preceded by a down-draft, followed by a turbulent region of successive updrafts and downdrafts of 2 to ...

A Case Study of Jet Stream Clouds

Certain specific cloud forms are frequently associated with strong winds aloft. Investigation has shown that these clouds are usually observed in regions of strong horizontal and vertical wind shear, and in most cases in or above regions where negative absolute vorticity (dynamic instability) and low Richardson's numbers coincide. The small-scale details in jet stream clouds may be the result of turbulent motion in layers brought to or near saturation by large-scale vertical motions in the atmosphere. This paper contains a detailed study of a typical case.

On the Structure of Some Cumulonimbus Clouds which Penetrated the High Tropical Troposphere

Some extremely large oceanic trade-wind cumulonimbus clouds extending upwards of 40,000 ft into a region of strong winds and intense vertical shear have been studied by means of time-lapse photography. A simultaneous still picture of the clouds taken a known distance and direction away from the motion pictures permits triangulation upon the clouds and calculation of the vertical and horizontal motions of several of the individual towers. By means of a nearby radiosonde observation, it is established that the air forming the strongest of these towers must have risen from near cloud base with little or no dilution of its buoyancy by mixing with the clear-air surroundings.&#x0D; The model of Malkus and Scorer (1955) concerning the erosion of cumulus towers is reviewed and tested upon these towers with satisfactory results. A minimum horizontal cloud dimension is apparently required for the production of undiluted towers. The horizontal motion of the clouds relative to the air is also estimated from the model and tested by the observations and the net upward transport of latent heat in water vapor is calculated approximately.