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. meridional circulation and thermal structure in the stratosphere and mesosphere due to the convergence of gravity wave momentum flux [Lindzen, 1981;Matsuno, 1982;Holton, 1982Holton, , 1983Dunkerton, 1982]. In order for the momentum and energy transported by wave motions to have a permanent effect on the background flow, however, these motions The mechanisms that contribute most to the dissipation (and saturation) of the dominant gravity wave and tidal motions in the middle atmosphere are thought to be the dynamical and convective instability [Fritts et al., 1984], though other mechanisms are also expected to contribute to the dissipation of individual wave motions under certain circumstances. These include molecular diffusion, radiative damping, inertial instability, and the cascade of wave energy to small scales via nonlinear wave-wave interactions [Phillips, 1960; Bretherton, 1964; McEwan and Robinson, 1975; Weinstock, 1976]. Of these, molecular diffusion, radiative damping, and nonlinear interactions operate most effectively on wave motions with small spatial scales and vertical group velocities [Pitteway and Hines, 1963; Fels, 1984; Mc-Comas and Bretherton, 1977; McComas and Miiller, 1981; Yeh and Liu, 1981]. Thus typical spatial and temporal scales suggest that these competing processes will be relatively less important than dynamical or convective instabilities in the dissipation ofthe dominant wave motions in the middle atmosphere. The available observational data suggest that con-1247 1248 FRITTS AND RASTOGI: INSTABILITIES DUE TO GRAVITY WAVES vective and dynamical instabilities, because of the scales at which they occur, result in considerable turbulence throughout the atmosphere. This turbulence is likewise expected to play a significant role in the vertical transport of heat and atmospheric constituents. Because of their importance in atmospheric dynamics, this paper will review the theoretical basis for and the observational evidence of dynamical and convective instabilities in the lower and middle atmosphere. A primary objective of this work is to provide the reader with the information needed to distinguish between the two instabilities, and thus between their likely causes and effects, in observationa...

Radar observations show that thin persistent layers of turbulence occur sporadically in the troposphere and stratosphere. Two probabilistic approaches are used to show that the vertical eddy diffusivity due to such layers is of the order of 0.2‐0.3 m²/sec in the lower stratosphere. An actual realization of turbulent layers, derived from the radar observations at Arecibo, is used in a numerical approach to obtain a profile of eddy diffusivity. It is suggested that turbulence plays a significant role in the vertical transport of trace constituents in the stratosphere.

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SA, IDI, and DBS horizontal wind estimators are examined and demonstrated to share common biases under inhomogeneous flow field conditions. The biases can reach magnitudes of several tens of meters per second at mesospheric heights, contaminate gravity wave flow field measurements, cause distortions in measured flow field spectra, and possibly account for large wind measurement errors evidenced in recent experimental studies including the AIDA'89 effort. However, being a linear response to gravity wave flow field fluctuations, the biases are expected to cancel out in long‐term wind and tidal statistics compiled with radar wind data.

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