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At least 811 records · Page 45

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NASA NTRS1981

SAR imaging of ocean waves - Theory

A SAR imaging integral for a rough surface is derived. Aspects of distributed target imaging and questions of ocean-wave imaging are considered. A description is presented of the results of analyses which are performed on aircraft and a spacecraft data in order to gain an understanding of the SAR imaging of ocean waves. The analyzed data illustrate the effect of radar resolution on the images of azimuthally traveling waves, the dependence of image distortion on the angle which the waves make with the radar flight path, and the dependence of the focusing parameter of the radar matched filter on the ocean wave period for azimuthally traveling waves. A dependence of ocean-wave modulation on significant wave height is also observed. The observed dependence of the modulations of azimuth waves on radar resolution is in contradiction to the hypothesis that these modulations are caused mainly by velocity bunching.

Jain, A.↗

NASA NTRS1980

An investigation of wave sheltering by islands

The results of SAR, HF radar, wave rider buoy, and pressure sensor studies of wave behavior around San Clemente Island off the coast of California are reported. The SAR operated at a frequency of 1.2 GHz corresponding to a wavelength of 25 cm at 25 m resolution. Imagery was corrected for the slant range distortion and two-dimensional fast Fourier transforms were used to obtain two-dimensional wave number spectra. A 1 MW source was employed for the HF radar, with a receiving antenna synthesized by mounting the receiver on a van which drove along straight stretches of an oceanside highway. The measurement of 77 m waves displayed a shadow effect after the island, with an unexpected recovery which implies local wave generation. Reflected energy was determined to be too weak to account for the waves, and a nonlinear transfer of wave energy from waves moving parallel to the island was concluded to be the source of the observed inner wave energy.

Hsiao, S. V.↗

NASA NTRS1982

Surface waves on solar wind tangential discontinuities

It is demonstrated that (tangential) discontinuities in the magnetic field direction can support MHD surface waves. The surface waves are similar to the usual Alfven wave, but there are seven important differences. The first is that the surface waves exhibit a low-frequency cutoff; the second is that the velocity and magnetic field fluctuations are elliptically, and sometimes circularly, polarized. It is noted that they may account for the solar wind helicity spectrum. The third difference is that the surface waves are compressive, although there are special cases where they are noncompressive. The fourth is that the wave vector k, the local normals to the surfaces of constant phase, and the magnetic minimum variance direction do not all coincide. The fifth is that there is a tendency for the minimum variance direction to align itself with the mean magnetic field direction. The sixth difference is that the waves can be intrinsically nonplanar, and the seventh is that equipartition between magnetic and kinetic energies is not obeyed locally. These properties of the surface waves are interpreted to mean that surface waves may be common in the solar wind.

Hollweg, J. V.↗

NASA NTRS1983

Synthetic aperture radar imaging of ocean waves during the Marineland experiment

Wave contrast measurements are used to analyze X and L-band simultaneously obtained synthetic aperture radar (SAR) data of ocean gravity waves collected during the Marineland Experiment (1975). The results of the wave contrast measurements show that the ocean waves imaged by a SAR are more discernible using an X-band frequency than an L-band frequency and when the ocean waves are traveling in the range direction. It is determined that ocean waves can be detected by both X and L-band SAR, provided that the radar surface resolution is a small compared to the ocean wavelength. In addition, wave detection using L-band SAR can be improved by adjusting the focal distance and rotation of the cylindrical telescope in the SAR optical processor to account for wave motion. This adjustment is determined to be proportional to a value that is near the wave phase velocity.

Shuchman, R. A.↗

NASA NTRS1983

Magneto-atmospheric waves

A theoretical treatment of magneto-atmospheric waves is presented and applied to the modelling of waves in the solar atmosphere. The waves arise in compressible, stratified, electrically conductive atmospheres within gravitational fields when permeated by a magnetic field. Compression, buoyancy, and distortion of the magnetic field all contribute to the existence of the waves. Basic linearized equations are introduced to describe the waves and attention is given to plane-stratified atmospheres and their stability. A dispersion relation is defined for wave propagation in a plane-stratified atmosphere when there are no plane-wave solutions. Solutions are found for the full wave equation in the presence of either a vertical or a horizontal magnetic field. The theory is applied to describing waves in sunspots, in penumbrae, and flare-induced coronal disturbances.

Thomas, J. H.↗

NASA NTRS1983

Short-period atmospheric gravity waves - A study of their statistical properties and source mechanisms

Gravity waves for the one year period beginning 19 October 1976 around Palisades, New York, are investigated to determine their statistical properties and sources. The waves have typical periods of 10 min, pressure amplitudes of 3 Pa and velocities of 30 m/s. In general, the largest, amplitude waves occur during late fall and early winter when the upper tropospheric winds directly overhead are fastest and the static stability of the lower troposphere is greatest. Mean wave amplitudes correlate highly with the product of the mean maximum wind speed and the mean low level stratification directly aloft. A distinct diurnal variation of wave amplitudes with the largest waves occurring in the pre-dawn hours is also observed as a result of the increased static stability then. The majority of waves are generated by shear instability; however, a number of waves are generated by distant sources such as nuclear detonations or large thunderstorms. The waves with distant sources can be distinguished on the basis of their generally much higher coherency across the grid and velocities that depart markedly from the wind velocity at any point in the sounding.

Gedzelman, S. D.↗

NASA NTRS1983

Meteorological and dynamical requirements for MST radar networks: Waves

Studies of wave motions using the MST radar have concentrated on single station time series analyses of gravity waves and tides. Since these radars collect high time resolution data they have the potential to become a significant tool for mesoscale research. In addition, radars are operated almost continuously unattended and, consequently, data sets are available for analyzing longer period wave motions such as tides and planetary scale waves. Although there is much to learn from single station data, the possibilities of new knowledge from a network of radars is exciting. The scales of wave motions in the atmosphere cover a broad range. Consequently the choice of a radar network depends to a large extent on the types of wave motions that are studied. There are many outstanding research problems that would benefit from observations from a MST radar network. In particular, there is a strong need for measurements of gravity wave parameters and equatorial wave motions. Some of the current problems in wave dynamics are discussed.

Avery, S. K.↗

NASA NTRS1984

Gravity waves from the stratosphere to the mesosphere, part 1.3B

The propagation of gravity waves from the stratosphere to the mesosphere has important implications both for observers and those who are attempting to parameterize wave breaking in global models. As they propagate from the tropopause to their breaking level (here, assumed to be the mesosphere), gravity waves can encounter a refractive environment since the vertical group velocity is a function of the background wind. They may be focussed or scattered or dissipated before reaching the mesosphere. It is even conceivable that gravity waves may break stop breaking, and begin breaking again at high altitudes with a resultant loss of wave energy in the intervening region. From a modeling viewpoint, the important concern for large-scale flows is the total upward flux of gravity wave (pseudo) momentum entering the stratosphere and mesosphere. The refraction of gravity waves also presents a difficult problem for observers since waves passing through the tropopause may arrive a thousand kilometers upstream in the mesosphere. Since mesosphere - stratosphere - troposphere (MST) radars sense tropospheric and mesospheric conditions most accurately, they are ideally suited to assess the total gravity-wave flux through the tropopause and stratospause. Networks of radars making coordinated measurements may be required to accurately determine the upward flux of momentum as well as the flux convergence between layers.

Schoeberl, M. R.↗

NASA NTRS1984

Nonzonal gravity wave breaking in the winter mesosphere

The steady state gravity wave model of Schoeberl et al. (1983) is extended to compute wave breaking by disturbances originating at the earth's surface. For winter and summer mean zonal wind profiles, no waves reach the mesosphere unless the absolute value of the zonally averaged perturbed zonal velocity minus c is greater than approximately 20 m/sec. Gravity waves with c = 0 can only reach the winter mesosphere if planetary scale waves are present in the troposphere and the lower stratosphere, to provide strong zonal wind channels for upward wave propagation. This results in nonzonal wave breaking in the mesosphere which could provide in situ forcing of planetary waves. Dissipation of gravity waves by molecular viscosity and conduction can provide significant deceleration and heating/cooling in the 85-105 km region.

Schoeberl, M. R.↗

NASA NTRS1985

Geometric effects of global lateral heterogeneity on long-period surface wave propagation

The present investigation has the objective to document examples of anomalous long-period surface wave amplitude behavior and to provide a preliminary appraisal of the effects of global lateral heterogeneity on surface wave propagation from a ray theory perspective. Attention is given to remarkable long-period surface wave anomalies described in literature, an equidistance azimuthal plot centered on the Iranian source region, Rayleigh wave and Love wave spectra for the 256-s period arrivals for the Tabas earthquake, constrained moment tensor and fault model inversion solutions ofr Iranian earthquakes, aspects of surface wave ray tracing, and a table of Rayleigh wave amplitude anomalies for Iranian earthquakes. Surface wave ray-tracing calculations for models of global phase velocity variations proposed by Nakanishi and Anderson (1984) are found to show that large-amplitude anomalies will be observed for Love and Rayleigh waves with periods of 100-250 s.

Lay, T.↗

NASA NTRS1985

The zonal harmonic and the yearly and half-yearly waves on the Southern Hemisphere in FGGE compared with the mean

The dominant component in the mean annual cycle of sea level pressure in the latitudes north of approx. 40 deg S is the yearly wave, whereas to the south of this latitude a half-yearly wave dominantes. In FGGE, however, the usually small yearly wave was large in the Antarctic, owing to the extraordinarily low pressure there in the winter of 1979. The half-yearly wave's amplitude was appreciably above normal in the center of each ocean in middle latitudes, and below normal at high latitudes. The yearly wave in the zonal geostrophic wind was larger than normal over the Antarctic Ocean. Similar results are obtained at 500 mb. The three month mean stationary waves (zonal harmonic waves) were larger than the mean in FGGE. The intra-annual variation of both amplitude and phase of the monthly mean harmonic waves in FGGE was large, which is not surprising as they contain an appreciable transient component, unlike the long term monthly mean waves.

Vanloon, H.↗

NASA NTRS1985

Vacillations induced by interference of stationary and traveling waves

It was shown that there exists a correspondence between interference of the 16-day wave with the stationary planetary wave and vacillations in eddy heat and momentum fluxes. There has been speculation on the role such interference may play in stratospheric warmings. The appearance of higher-order normal modes, e.g., the 16-day wave, prior to disturbances in the stratospheric vortex has also been noted. Since amplitudes of the 16-day wave are compared to those of the stationary component, it can significantly alter the instantaneous total wave field. The behavior of the wave number 1 height field at 60 deg N during January 1979 is shown when a large amplitude disturbance retrogressed barotropically around the globe over much of the troposphere and stratosphere. This transient disturbance has been identified with the 16-day wave. More generally, the presence of fluctuating planetary wave activity can, through combined interference, alter the instantaneous planetary wave field as well as the transports it produces. The implications of such interference in a quasi-geostrophic framework has been explored, using the Eliassen-Palm (EP) flux as a diagnostic.

Salby, M. L.↗

NASA NTRS1985

Finite amplitude gravity waves: Harmonics, advective steepening, breaking and saturation

A simple theory is presented which determines details of the breaking and saturation of a gravity wave as it propagates upward in the atmosphere. Breaking and saturation are here due to nonlinear advection analogous to the breaching of a surface wave and to the breaking of a planetary wave. Much simplification is obtained by the assumption that in a wave packet consisting of a primary wave and its harmonics, the primary wave remains dominant. This assumption, referred to a quasi-monochromatic approximation, is suggested by observations. Determined by this approximate theory are: a detailed picture of the waveform as it steepens and breaks; harmonics of the wave; the turbulence generation; and an underlying relationship between superadiabatic lapse rate and saturation by wave-wave interactions.

Weinstock, J.↗

NASA NTRS1986

Production of flickering aurora and field-aligned electron flux by electromagnetic ion cyclotron waves

Recent observations have suggested that flickering aurora is produced by a modulation of the field-aligned component of the electron flux within an auroral arc. It is proposed that a portion of the field-aligned electrons are of ionospheric origin and that these electrons are accelerated and their flux modulated by electromagnetic ion cyclotron waves that occur below the main acceleration region on auroral arc field lines. A model of the electromagnetic ion cyclotron wave shows that the parallel phase velocity of the wave increase as the wave propagates toward the ionosphere. A test particle calculation shows that ionospheric electrons trapped or reflected by the wave are accelerated to energies of several keV and that their flux is modulated at the wave frequency. The relative amplitudes of the model wave electric fields are consistent with the observations of small-scale low-frequency ionospheric and magnetospheric electric fields near auroral arcs of approximately 10 mV/m and 100 mV/m, respectively. The large-amplitude ion cyclotron waves also produce a ponderomotive force and a self-consistent ambipolar electric field. Energy considerations show that the downward energy flux in the electromagnetic ion cyclotron wave can be several percent of the total downward auroral electron energy flux.

Temerin, M.↗

NASA NTRS1986

Evidence of a saturated gravity-wave spectrum throughout the atmosphere

The view adapted here is that the dominant mesoscale motions are due to internal gravity waves and show that previous and new vertical wave number spectra of horizontal winds are consistent with the notion of a saturation limit on wave amplitudes. It is also proposed that, at any height, only those vertical wave numbers m less than m sub asterisk are at saturation amplitudes, where m sub asterisk is the vertical wave number of the dominant energy-containing scale. Wave numbers m less than m sub asterisk are unsaturated, but experience growth with height due to the decrease of atmospheric density. The result is a saturated spectrum of gravity waves with both m sub asterisk decreasing and wave energy increasing with height. This saturation theory is consistent with a variety of atmospheric spectral observations and provides a basis for the notion of a universal spectrum of atmospheric gravity waves.

Fritts, D. C.↗

NASA NTRS1986

Mesoscale waves as a probe of Jupiter's deep atmosphere

Search of the Voyager images of Jupiter reveals a class of mesoscale waves occurring near the extrema of the zonal velocity profile between latitudes 30 deg S and 30 deg N. The average horizontal wavelength is 300 km, compared to an atmospheric scale height of 20 km; the standard deviation about the mean is only 20 percent. The alignment of most wave trains is nearly zonal, i.e., wave crests are north-south. A typical wave packet is 20 wave crests in length and 1300 km wide. Modeling shows that the waves propagate in a duct below the ammonia condensation level (the visible clouds). For selection of one dominant mode, the Richardson number in the duct must be of order unity; this is the first quantitative determination that has ever been made of the stability of Jupiter's atmosphere below its clouds. The duct is topped by a trapping layer, the ammonia clouds, where the Richardson number is less than 0.25. Meridional trapping of the wave packets is due to the shear in the zonal wind. It is speculated that the period of the waves is diurnal and that they do not propagate poleward of latitudes + or - 30 deg because the Coriolis frequency exceeds the duct Brunt frequency. The waves might be excited by diurnal variations of moist convective activity in the ammonia clouds.

Flasar, F. M.↗

NASA NTRS1987

Limitations of spectral measures and observations of the group structure of surface waves

Wave-group structure, which plays a significant role in flux processes and the mechanics of the transfer of wave energy and momentum to the mean current field, is studied. The origin of wave groups, in particular sideband instabilities, the effects of amplitude variations on wave frequency and of wave damping on isolated groups, and flow separation over waves, is discussed. The mechanisms that cause the group formation are compared to the dynamics that result in wave propagation. The bifurcation and shift in wave modulations, and the possible existence of a unique asymptotic statistical state for the wave-modulation field are examined.

Mollo-Christensen, Erik↗

NASA NTRS1987

Density fluctuations induced by nonlinear Alfven waves

The generation of plasma density fluctuations by nonlinear Alfven waves is considered. Of particular interest is the modification to the 'static approximation', describing the relationship between wave intensity and plasma density, when the Alfven wave packet is rapidly steepening. The study concentrates on a forced wave equation for the plasma density, which yields the static approximation in a select approximnation. An analytic solution to this equation is obtained for a reasonable model of a steeping wave packet. The principal results are: (1) the static approximation should give a surprisingly robust description of the coupling between Alfven wave intensity and plasma density; (2) departures of the plasma density perturbation from the static approximation result are determined by a parameter that is a simple function of the wave amplitude and plasma beta; (3) for rapidly evolving wave packets, the position of maximum density change is displaced from that of maximum wave intensity, and (4) the magnitude of the density fluctuation will not be much different than the static approximation estimate.

Spangler, Steven R.↗