Showing papers on "Atmospheric wave published in 1987"

The Synoptic Setting and Possible Energy Sources for Mesoscale Wave Disturbances

Abstract: Published data on 13 cases of mesoscale wave disturbances and their environment were examined to isolate common features for these cases and to determine possible energy sources for the waves. These events are characterized by either a singular wave of depression or wave packets with periods of 1-4 h, horizontal wavelengths of 50-500 km, and surface-pressure perturbation amplitudes of 0.2-7.0 mb. These wave events are shown to be associated with a distinct synoptic pattern (including the existence of a strong inversion in the lower troposphere and the propagation of a jet streak toward a ridge axis in the upper troposphere) while displaying little correlation with the presence of convective storm cells. The observed development of the waves is consistent with the hypothesis that the energy source needed to initiate and sustain the wave disturbances may be related to a geostrophic adjustment process associated with upper-tropospheric jet streaks.

Effects of Doppler shifting on the frequency spectra of atmospheric gravity waves

Abstract: Spectra of atmospheric motions as a function of observed frequency may depart significantly from the spectra as a function of intrinsic frequency due to a nonzero mean wind. In this paper we examine the effects of Doppler shifting on a model spectrum of atmospheric gravity waves. In order to gain insight into the effects of Doppler shifting, we have derived analytic solutions by approximation of the intrinsic frequency spectra and the gravity wave dispersion relation. Our results reveal that Doppler shifting can have major effects on the observed frequency spectrum of both horizontal and vertical gravity wave energy. For levels of Doppler shifting representative of the lower and middle atmosphere, possible effects include a substantial enhancement of horizontal energy density at higher observed frequencies, a corresponding reduction of the vertical energy density at higher frequencies, and a significant transfer of vertical energy to observed frequencies above the buoyancy frequency. The predicted effects are found to be consistent with some of the features of the observed frequency spectra.

An Investigation of the Vertical Wavenumber and Frequency Spectra of Gravity Wave Motions in the Lower Stratosphere

Abstract: The vertical and oblique velocities of atmospheric motions in the lower stratosphere were analyzed using data obtained on February 1-5, 1986, from the Poker Flat, Alaska, MST radar; two beams of orthogonal polarization were directed vertically, and four oblique beams at 7 deg off-vertical were directed at azimuths of 64, 154, 244, and 334 deg from north. Results indicate that the majority of the energy at gravity wave periods is associated with inertia-gravity wave motions having an upward direction of propagation and dominant vertical wavelengths near 2 km. The results of vertical wavenumber spectra support the saturation hypothesis of Dewan and Good (1986) and Smith et al. (1987), suggesting that saturation processes act to control spectral amplitudes at large wavenumbers.

A Study of Planetary Waves in the Southern Winter Troposphere and Stratosphere. Part I: Wave Structure and Vertical Propagation

Abstract: Planetary wave propagation in the southern winter troposphere and stratosphere is studied in an attempt to trace the origins of upward propagating disturbance. Daily geopotential girds from 1000 to 1 mb are analyzed for two 120-day winter seasons. A cross-correlation analysis technique is developed which allows coherent wave structure to be traced in time. Significant correlations are observed between the troposphere and stratosphere at finite time lags, indicative of vertically propagating waves. The observed vertical propagation time scales between the middle troposphere and middle stratosphere are on the order of 4 days for zonal wavenumber 1 (k=1), 1–2 days for k=2, and 1 day for k=3. The cross-correlation analysis also delineates the meridional and vertical structures of the transient (in time) planetary waves. Zonal wavenumber 1 fluctuations exhibit a vertical out-of-phase relationship between the midlatitude troposphere and atmosphere. Three out-of-phase maxima in latitude are observed in ...

Rossby waves and two-dimensional turbulence in a large-scale zonal jet

Abstract: Homogeneous barotropic beta-plane turbulence is investigated, taking into account the effects of spatial inhomogeneity in the form of a zonal shear flows Attention is given to the case of zonal flows that are barotropically stable and of larger scale than the resulting transient eddy field Numerical simulations reveal that large-scale zonal flows alter the picture of classical beta-plane turbulence It is found that the disturbance field penetrates to the largest scales of motion, that the larger disturbance scales show a tendency to meridional rather than zonal anisotropy, and that the initial spectral transfer rate away from an isotropic intermediate-scale source is enhanced by the shear-induced transfer associated with straining by the zonal flow

Analysis and simulations of a troposphere-stratosphere gravity wave model. I

Abstract: An analytical model is presented that accommodates nonhydrostatic shearing stratified flow over an obstacle, and that can be modified to include a superposed stratosphere with constant wind and higher stability. A simulation code is used in parallel with the analytic calculations to demonstrate a methodology for determining the wavelength and magnitude of gravity wave energy reflected, and that transmitted, by the tropopause.

Effect of nonlinear instability on gravity-wave momentum transport

Abstract: This paper investigates the nonlinear instability of internal gravity waves and the effects of their nonlinear interaction on momentum flux, using simple theoretical and numerical models. From the result of an analysis of parametric instability of a two-dimensional internal gravity wave as discussed by Yeh and Liu (1981) and Klostermeyer (1982), a group trajectory length scale for a gravity wave packet was determined, expressed in terms of the dominant vertical wavelenght and the degree of convective saturation. It is shown that this analysis justifies the Eikonal saturation method for relatively transient packets, that are well below the saturation amplitude, propagating in a slowly varying mean flow. Conversely, linear theory fails for persistent disturbances and trasient wave packets near convective saturation.

Waves in the Overlying inversion of the Convective Boundary Layer

Abstract: Large eddy simulations and a linear theory are used to examine the characteristics of waves trapped in the temperature inversion that bounds the convective boundary layer. These waves are important because they strongly affect the rate of entrainment and the fluxes of momentum and scalars across the interface. lie simulations show that the motions, at least on scales larger than the model grid size, are nearly linear and can be described well by the theory of Carruthers and Hunt. A simple linear model is used to relate the frequency of trapped waves in the inversion to the ratio of the pressure-temperature covariance and temperature variance. These quantities, which are easily obtained from the simulations, are used to estimate the structure of the waves. Comparisons of the wave properties are made between the theory, further simulations and observations, with good agreement.

Observations of an internal gravity wave in the lower troposphere at Halley, Antarctica

Abstract: Observations of internal gravity waves in the stably-stratified atmospheric boundary layer at Halley, Antarctica are presented. These were made on 1 February, 1986 and take the form of temperature measurements from a 30 m mast and a Sodar record. The temperature record shows a clearly defined, dominant wave period of around 11 min. A high-resolution radiosonde ascent made during the period of wave activity exhibits thin layers of low Richardson number and it is suggested that these are regions of dynamic instability where the waves are generated. A linear stability analysis of the radiosonde data supports this idea. It is argued from simple theoretical ideas and by means of a numerical model that only waves with a wavelength greater than a certain critical value are likely to be observed at the surface. The observations are shown to be consistent with this hypothesis.

A Study of Planetary Waves in the Southern Winter Troposphere and Stratosphere. Pad II: Life Cycles.

Abstract: Large-amplitude planetary waves in the southern winter stratosphere are observed to occur episodically, the result of episodic tropospheric forcing. This work is an observational study of the dynamics of the planetary waves, focusing on the evolution through a typical life cycle. Time lag correlations of wave amplitude with the Eliassen-Palm flux vector reveal the characteristic heat and momentum flux patterns associated with wave evolution. Energetic studies clearly show that the stratospheric waves can be understood in terms of a life cycle of vertical propagation from the troposphere, followed by decay from barotropic interactions with the zonal mean flow. Although usually of secondary importance baroclinic decay of stratospheric wave energy is also observed, resulting from equatorward heat flux in the lower stratosphere. Good agreement in the energy balances discounts in situ instability in the stratosphere as a source of wave activity. An average or composite over several clearly propagating...

Vacillations induced by interference of stationary and traveling planetary waves

Abstract: The interference pattern created when a traveling planetary wave propagates over a stationary forced wave is explored. Interference leads to a modulation of all the transport properties of the stationary wave even if the traveling wave is barotropic, as is typical in the troposphere and lower stratosphere. In so doing, the steady uniform stream of wave activity associated with the stationary wave is organized into a series of capsules or wavepackets which propagate with time. Consequently, the signature emerges locally as a series of bursts of wave activity. Rising values of Eliassen-Palm flux at the leading edge of these wavepackets and opposite behavior at the trailing edge induce an alternating mean flow response which captures several aspects of the observed behavior of quasi-periodic disturbances. Synoptic patterns of geopotential and Ertel potential vorticity also resemble behavior observed during disturbed conditions.

Gravity Waves and Convection in Colorado during July 1983

Abstract: The dynamics of gravity-wave/convective-cell interaction is studied using NOAA data collected in NE Colorado during July and August 1983. The pressure fields measured with microbarographs, the tropospheric wind profiles obtained with a UHF wind profiler radar, and precipitation data collected with a 10-cm weather radar for four events (A, B, C, and D) are analyzed. The four disturbances are detected through a substantial depth of the troposphere. It is observed that in event A the wave and convective cells appear to be locked together; in event B, the wave and convective cells commence about the same time, but the wave velocities differ from the cell velocities; and in events C and D, the waves move faster than the maximum wind in the jet and faster than the convective cells. It is suggested that events A and B are generated by wind shear in the jet stream, and the excitation of events C and D depends on mechanisms such as vertical convective motion and acceleration in the jet flow.

Dynamical factors affecting ozone mixing ratios in the Antarctic lower stratosphere

Abstract: An account is given of the climatology and interannual variability of dynamical quantities and ozone mixing ratios during the Southern Hemisphere spring for 1979-1984. The seasonal variation in temperature in the lower stratosphere is repeatable; a steep decrease in zonal mean ozone mixing ratios is observed around 60 deg S toward the South Pole in September which, with time, becomes shallower in association with minor warmings and a final warming. Climatological synoptic charts in the lower stratosphere show the circumpolar circulation in the geopotential height field and the prominence of planetary wave 1 in the temperature and ozone fields. When wave activity is strong, there are weaker westeries, higher temperatures, and higher ozone mixing ratios at high latitudes.

The influence of atmospheric waves on the general circulation of the middle atmosphere

Abstract: To a first approximation, the basic features of the globally averaged structure of the middle atmosphere (such as the warm stratopause and cold mesopause) can be understood on radiative grounds alone. However, dynamical processes must be invoked if the observed latitudinally varying structures of the zonal-mean temperature and wind fields are to be explained. Particularly large departures from a hypothetical radiatively determined state occur in the winter stratosphere (especially in the Northern Hemisphere) and in the upper mesosphere at the solstices. Simple theoretical models indicate that the primary dynamical mechanisms that drive the middle atmosphere away from radiative balance are wave motions, notably large-scale planetary waves and small-scale gravity waves. Much current research is being devoted to understanding the complex transient and irreversible processes by which such waves can influence the zonal-mean state and also lead to the meridional transport of chemical species.

Quasi-stationary planetary waves and temperature reference atmosphere

Abstract: A middle atmosphere temperature reference model from satellite measurements has recently been proposed. Comparisons are made with the Air Force Reference Atmosphere and the Reference model from satellite data. Large temperature deviations between two models at 60 N, 90 W in winter are found and the role of quasi-stationary planetary waves for these deviations is dicussed.

Wave–Wave Interaction of Unstable Baroclinic Waves

Abstract: Two slightly unstable baroclinic waves in the two-layer Phillips model are allowed to interact with each other as well as the mean flow. A theory for small dissipation rates is developed to examine the role of wave–wave interaction in the dynamics of vacillation and aperiodicity in unstable systems. It is shown that the form of the dissipation mechanism as well as the overall dissipation timescale determines the nature of the dynamics. In particular, dissipation proportional to potential vorticity is shown to expunge amplitude vacillation due to wave–mean flow interactions. Wave–wave interaction, however, can yield amplitude vacillation. As the dissipation is decreased, the solutions evolve from steady waves (although propagating) to periodic vacillation until finally at small dissipation rates, chaotic behavior is obtained. This occurs in a range of relative growth rates of the two waves which depends on the strength of the wave–wave and wave–mean flow interactions.

Traveling Waves and Regional Transitions in Blocking Activity in the Northern Hemisphere

Abstract: In preference to the use of persistent height anomalies for determining the presence of blocking, new criteria were developed which take the observed flow conditions into account. These criteria were applied daily at 500 mb, using filtered Northern Hemisphere height maps retaining zonal waves 0–5, beginning in July 1981. Blocking statistics for the 3-yr period through June 1984 are in remarkably close agreement with the results from longer-period published data sets involving the use of a split-jet flow as a principal criterion. Thus this data set, albeit short, is deemed appropriate for investigating the long-wave properties of blocks. During July 1981–June 1984, 47 blocks were observed; the long-wave composition of blocks lasting at least 10 days (24 cases) is examined in detail. Contrary to Austin's (1980) hypothesis that blocking should arise from the constructive interference of stationary long waves with enhanced amplitude but normal phase, it is found that traveling waves (predominantly re...

Quasi-Lagrangian measurements of density surface fluctuations and power spectra in the stratosphere

Abstract: Pressure and temperature data from eight superpressure balloon flights at 26 km in the southern hemisphere stratosphere are analyzed. The balloons, which float on a constant density surface, travel steadily westward during summer and eastward during winter, as expected from local climatology. Two types of fluctuations are observed: neutral buoyancy oscillations (NBO) of around 4 min, and 0.1- to 1-hour oscillations that are characterized as small-amplitude density surface fluctuations. Lapse rates and densities are calculated and found to agree well with the expected values. Examples of wave damping and simultaneous fluctuation at two nearby balloons are presented. Spectral analysis is performed clearly showing the NBO and that the majority of the power is in the mesoscale range. Spectral slopes of power versus frequency are measured to be on the average -2.18 + or - 0.24 for pressure and -1.72 + or - 0.24 for temperature. These slopes are compared to the predictions of turbulence theories and the theory of a universal gravity wave spectrum.

A Numerical Study of the Propagation of Topographic Rossby Waves

Abstract: The propagation of linear barotropic Rossby waves is investiaged numerically over a one-dimensional topography similar to the continental rise and slope. A point source is used to generate waves with periods from 4 to 36 days. The resulting distribution of streamfunction and kinetic energy density is examined. The result show that the propagating of tropographic Rossby waves depends on the wave period. Over the continental rise, waves are generated mainly by low-frequency disturbances at periods of about a month. In addition, the continental slope is a good insulator to these waves. Therefore, deep ocean circulation will not influence motions on the continental shelf. At 36 and 15 days, the steep continental slope is a wave guide, and regions of high energy density generated by local sources may be found. Energy of 36-day waves over the continental shelf cannot penetrate the steep slope. Although waves of periods shorter than a week may reach the lower slope, these waves are trapped by the coast,...

Ground-Based Microwave Radiometric Observations of the Temporal Vanation of Atmospheric Geopotential Height and Thickness

Abstract: Since 1981, the Wave Propagation Laboratory of NOAA has operated a ground-based zenith-viewing microwave radiometer. This radiometer, designed to measure precipitable water vapor, cloud liquid, and temperature profiles, has two moisture-sensing channels and four temperature-sounding channels. Data from this system, taken at Denver, Colorado, are used to derive geopotential heights and thicknesses from the surface (about 830 mbar) to 300 mbar. Time series and spectra of several directly measured and inferred quantities are analyzed for different meteorological situations: a period of unusual calm in surface pressure, a frontal passage, and a gravity wave event. The three cases presented illustrate how rapid variations in meteorological variables can be studied using ground-based radiometers. These radiometers provide temporal continuity not hitherto available. The performance of the radiometer, both in observing a blackbody target and during an unusually calm pressure event, shows high sensitivity to changes in geopotential height and thickness and to integrated water vapor. Consequently, the combination of high temporal resolution and high sensitivity allows unique monitoring of rapidly changing conditions, such as frontal passages and gravity wave events. Comparisons of these data with various sources of ground truth, including radiosondes, satellite cloud observations, and arrays of microbarographs, show excellent agreement.

Dynamic aspects of the Southern-Hemisphere medium-scale waves during the southern summer season

Abstract: The role of medium-scale waves on three dynamic aspects of the Southern-Hemisphere general circulation is examined using data generated by the FGGE analyses of the ECMWF. The momentum and sensible heat transports by the medium-scale waves are discussed. The effects of medium-scale waves on atmospheric circulation of the Southern Hemisphere during the summer, in particular the vacillation of atmospheric energetics, are investigated. The horizontal and vertical structures and the transport properties of this wave regime and their relation to downstream development in the Southern Hemisphere are analyzed. It is observed that medium-scale waves supply about a half of the total eddy transport of sensible heat and momentum; the wave regime contributes to the time average of various energy contents and energetic components of atmospheric motion during the southern summer; and the wave regime is amplified during the developing stages of downstream development.

Wave Structure and Evolution In Baroclinic Flow Regimes

Abstract: A uniform potential vorticity model, modified by two Ekman layers of different strengths, is used to study analytically and numerically the structure and evolution of strongly nonlinear, baroclinic waves. the model consists of a single zonal wave with its lowest two meridional modes, and of the mean flow correction with its lowest four meridional modes. Each meridional mode consists of a baroclinic and a barotropic pattern. Travelling steady waves, amplitude vacillation and structural vacillation are found in the model. Steady waves are energetically balanced between baroclinicity and Ekman dissipation through nonlinearity. In a typical amplitude vacillation, the wave potential energy vacillates with time, via the interference of nonlinear barotropic and baroclinic patterns of the lowest meridional mode of the wave. In a typical structural vacillation, the distribution of wave kinetic energy in the meridional direction vacillates with time, via interference of the lowest two nonlinear meridional modes of the wave. the results are qualitatively consistent with those observed in annulus experiments. the mechanisms of amplitude and structural vacillations may be responsible for blocking in middle and high latitudes.

The effect of a gravity wave drag parameterization scheme on GLA fourth order GCM forecasts

Abstract: Ten-day forecast experiments have been performed to determined whether the introduction of a simple orographic gravity wave drag scheme into a fourth-order GCM would reduce the climate drift of the fine resolution model and improve the model's medium range predictive skill. Error reduction due to the gravity waves is found in stratospheric predictions, where the improvement is confined mainly to the zonal mean component. Improvements are noted in the Northern Hemisphere climatology, where low level westerlies are weakened and shifted poleward, and in the Southern Hemisphere, where the roaring forties and fifties are better simulated.

Tidal activity in the meteor zone over Budrio, Italy

Abstract: A brief survey is presented of the variations with time and height of atmospheric tides observed at Budrio (45 deg N, 12 deg E) in the wind field between 80 and 110 km altitude during the 1978 to 1982 year period. Variations of amplitude maxima mainly of the semidiurnal tide in the winter data of 1979 and 1980 show periodicities of a few days throughout the observing period. Upward propagation of tidal energy during a stratospheric warming in January 1982 is proposed to be inhibited because of instabilities in atmospheric conditions.

Equatorial solitary waves of tropical atmospheric motion in shear flow

Abstract: Starting from the primary equations, the author derives the KdV equation which describes solitary Rossby waves in the tropical atmosphere, and indicates that, because these waves are ageostrophic, they differ from the quasigeostrophic solitary Rossby waves studied by Redekopp et al. Owing to nonlinear action, these waves are also different from traditional linear waves of the tropical atmosphere. The author believes that the stationary tropical atmospheric waves reflect the characteristics of solitary waves in that the energy does not disperse.

The characteristic Rossby frequency

Abstract: The characteristic Rossby frequency is defined for a fixed zonal wavenumber perturbation as the variational integral of the Rayleigh-Ritz method. It is a measure of the time scale of the disturbance. For a disturbance which locally has the shape of an eigenfunction but is not global in extent, the characteristic Rossby frequency is very close to the true eigenvalue, and additionally remains unchanged under linear inviscid dynamics. Results are presented for the shallow water equations, both with and without a mean zonal wind. The characteristic Rossby frequency of a wavenumber 1 perturbation having the shape of the second symmetric Rossby mode but confined to the Northern Hemisphere is close to the corresponding Rossby frequency. This finding is helpful in understanding the behavior of the observed wavenumber 1 pattern of January 1979, which propagated westward with nearly the pure Rossby frequency but was discernible only in the Northern Hemisphere (as discussed by Daley and Williamson).

Wave motions in the middle atmosphere and relation to the solar activity

Abstract: Periodograms of time series of the 30 mb geopotential heights at 60°N and sunspot numbers for different winter seasons were analysed Amplitudes and the phase differences of the solar and atmospheric oscillations with the 27-28 day period testify in favour of the solar nature of the source of these waves in the atmosphere Significant results subject to clear interpretation have been obtained only for a limited number of winters, and there are periods when the 27 day oscillations are absent both in the sunspot members and in the atmospheric parameters

Study of internal gravity waves in the meteor zone

Abstract: An important component of the dynamical regime of the atmosphere at heights near 100 km are internal gravity waves (IGW) with periods from about 5 min to about 17.5 hrs which propagate from the lower atmospheric layers and are generated in the uppermost region of the atmosphere. As IGW propagate upwards, their amplitudes increase and they have a considerable effect on upper atmospheric processes: (1) they provide heat flux divergences comparable with solar heating; (2) they influence the gaseous composition and produce wave variations of the concentrations of gaseous components and emissions of the upper atmosphere; and (3) they cause considerable acceleration of the mean stream. It was concluded that the periods, wavelengths, amplitudes and velocities of IGW propagation in the meteor zone are now measured quite reliably. However, for estimating the influence of IGW on the thermal regime and the circulation of the upper atmosphere these parameters are not as important as the values of wave fluxes of energy, heat, moment and mass.