Showing papers by "David C. Fritts published in 1985"

Convective and dynamical instabilities due to gravity wave motions in the lower and middle atmosphere: Theory and observations

Abstract: Dynamical and convective instabilities are two mechanisms that contribute significantly to the dissipation of larger-scale motions and the generation of turbulence in the middle atmosphere. The former are normally due to enhanced velocity shears and/or a local minimum of the static stability either in the mean flow or associated with low-frequency wave motions. The most common dynamical instability is the Kelvin-Helmholtz (KH) instability which is often manifested in the atmosphere as a series of KH billows. Convective instabilities occur where the lapse rate becomes superadiabatic through the action of gravity waves and appear to predominate for high-frequency wave motions. This paper reviews the theory and the observational evidence for both types of instabilities in the lower and middle atmosphere.

Fluxes of Heat and Constituents Due to Convectively Unstable Gravity Waves

Abstract: We consider the implications of a nonuniform turbulent diffusion due to the IOM saturation of a gravity wave via convective instabilities It is found that both wave and turbulence fluxes of heat can be reduced dramatically, depending on the amplitude of the wave motion and the extent to which the turbulent diffusion is localized These results suggest that previous studies that assumed a uniform turbulent diffusion may have overestimated the beat and constituent fluxes due to gravity wave saturation

Comparison of mesospheric wind spectra with a gravity wave model

Abstract: We present radial wave number spectra of wind fluctuations obtained from heights near the mesopause by the Poker Flat, Alaska, mesosphere-stratosphere-troposphere (MST) radar running in a high spatial resolution mode (300 m). The spectra are of radial wind fluctuations along one vertical and two oblique (15° zenith angle) beams measured at heights of 82–88 km during summer. The oblique wave number spectra have amplitudes that are within a factor of 3 of each other and appear to follow power laws with exponents in the −2 to −2.8 range. In order to infer what portion of the spectral amplitude can be attributed to gravity waves, the ratio of oblique to vertical amplitudes is compared with the ratio predicted by the gravity wave model of VanZandt. The observations are found to be consistent with the model, suggesting that gravity waves are the dominant motion in the high-latitude summer mesosphere.

A numerical study of gravity wave saturation - Nonlinear and multiple wave effects

Abstract: In this study we examine some of the effects of wave-wave interactions and convective adjustment on the propagation of gravity waves in the middle atmosphere. For both a nearly monochromatic wave and a super-position of waves, nonlinear wave-wave interactions, while reducing primary wave amplitudes somewhat, are found to be unable to prevent the formation of convectively unstable layers. In contrast, convective adjustment of the wave field causes significant amplitude reductions, resulting in amplitudes for a spectrum of wave motions that achieve only a fraction of their monochromatic saturation values. Neither process is found to cause a major disruption of the primary wave field. Both wave-wave interactions and convective adjustment are found to excite harmonies of the primary wave motions. Excitation by convective adjustment appears to dominate for a monochromatic wave, whereas both processes become important for a spectrum of wave motions. In each case, the characteristics of the excited wave...