Showing papers by "David C. Fritts published in 1987"

Evidence for a Saturated Spectrum of Atmospheric Gravity Waves.

Abstract: The slope and power spectral density of atmospheric velocity fluctuations versus vertical wavenumber at large wavenumbers are observed to be nearly independent of altitude. It is suggested that such a universality is due to saturation of short vertical-scale fluctuations. A brief review of linear gravity wave saturation theory indicates a physical basis for such spectra. It is demonstrated that observed saturation spectra are not solely due to individually saturated waves but most likely result from amplitude limiting instabilities arising from wave superposition. It is also shown that, while the spectrum is saturated at large wavenumbers, the total kinetic energy per unit mass and the characteristic vertical wavelength increase with altitude. Both of these predictions are consistent with observations.

Mesospheric Momentum Flux Studies at Adelaide, Australia: Observations and a Gravity Wave–Tidal Interaction Model

Abstract: We present here the results of an analysis of gravity wave momentum fluxes in the mesosphere and lower thermosphere, inferred using a dual-beam Doppler radar near Adelaide, Australia during June 1984. Our analysis reveals that over 70% of the momentum flux and of the inferred zonal drag was due to gravity waves with observed periods less than one hour. This suggests that it is the gravity waves with high intrinsic frequencies and small horizontal scales that are most effective at transporting momentum into the middle atmosphere. The temporal variations in the momentum flux and flux divergence due to high-frequency motions were also examined in detail. In addition to daily variability, a strong diurnal modulation was observed to occur. This was found to be correlated with the phase of large-amplitude diurnal tidal motions. As a result of these observations, a gravity wave–tidal interaction model was proposed which accounts for all of the major features of the observed data, including a reduction i...

A Climatology of Gravity Wave Motions in the Mesopause Region at Adelaide, Australia

Abstract: A statistical study of gravity wave motions in the mesosphere and lower thermosphere measured with a MF partial reflection radar located at Buckland Park new Adelaide (35°S, 138°E) in the period November 1933 to December 1984 is presented. The analyses am confined to waves with ground based periods between 1 and 24 h. Time-height cross sections show that the mean square amplitudes u′2 and v′2, of the zonal and meridional perturbation velocities, respectively, vary in a predominantly semiannual manner such that the minima in wave activity coincide with the reversals in the zonal circulation in the middle atmosphere. In most instances, v′2 is greater than u′2 which, together with the small but nonzero u′v′ fluxes shows that the gravity wave field is partially polarized. A technique similar to that used to analyse partially polarized electromagnetic waves suggests that on a seasonal basis, the wave field is polarized by about 10% to 20% but for shorter periods the degree of polarization may be signi...

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 Terrain Effects on the Mesoscale Spectrum of Atmospheric Motions

Abstract: Wind and temperature data collected on commercial aircraft during the Global Atmospheric Sampling Program (GASP) are used to investigate the effects of underlying terrain on mesoscale variability, and the observational results are interpreted within the theories of gravity wave motions and quasi-two-dimensional turbulence. The data show the variances are up to six times larger over mountainous terrain than over oceans or plains, with the most striking differences at horizontal scales from 4 to 80 km. Results were subdivided between the stratosphere and troposphere, and between high- and low-background wind speed cases, and show basically the same response to topography in all cases. The linear theory of gravity waves is found to predict correctly the scaling of wave amplitude with background stability in the case of low-background wind speeds, while the two-dimensional turbulence theory correctly predicts the shape of the variance spectrum and the observed amplitude scales with ϵ as required by t...

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.

Recent Progress in Gravity Wave Saturation Studies

Abstract: This paper will present a brief survey of some of the advances made during the last three years in understanding gravity wave saturation as well as their effects and variability in the lower and middle atmosphere. Our emphasis will be on observational results, though theoretical and modeling studies will be discussed where relevant. We will first present recent evidence of the processes contributing to wave saturation. We will also examine the implications of wave saturation and local turbulence production for wave fluxes of energy and momentum, the turbulent diffusion of heat and constituents, and a saturated spectrum of gravity waves throughout the atmosphere. Finally, some of the recent evidence of geographic and temporal variability of the gravity wave field and of the processes that may contribute to this variability will be reviewed.