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. 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...

Detection of Kelvin‐Helmholtz instability with the Indian mesosphere‐stratosphere‐troposphere radar: A case study

“…and the observed period of the KHI is 12 min, the estimated horizontal wavelength of the billow L x equals 7.2 km. Another way of finding L x for the KHI observed in the shear zone is by using the relation Lx=7.5 AZ [Fritts and Rastogi, 1985]. )•x in this case is the most unstable and hence the fastest growing wavelength within a region of KHI, and AZ is the thickness of the sheared layer.…”

Long-period unstable gravity-waves and associated VHF radar echoes

“…In practise, while the layers would be most often horizontal, a similar, signi®cant minority could be tilted to a range of angles near 6 and 12 , producing a similar e ect on echo powers of beams with h 6 and 12 if the layers are not ®rst destroyed by turbulence. KelvinHelmholtz instabilities (KHI) can develop in regions of su ciently high wind shear, and the tilt angles of their sloping surfaces typically lie in the range 0±30 (Fritts and Rastogi, 1985). If ®ne-scale layered temperature structures which generate the radar echoes are tilted in similar fashion to the background potential temperature surfaces (e.g.…”

“…If ®ne-scale layered temperature structures which generate the radar echoes are tilted in similar fashion to the background potential temperature surfaces (e.g. Fritts and Rastogi, 1985), the most powerful radar echoes will be received from an overtical direction, perpendicular to these tilted layers.…”

The nonlinear effects on the characteristics of gravity wave packets: dispersion and polarization relations

“…A number of theoretical studies of gravity waves have emphasized the importance of gravity waves in transporting energy and momentum from lower atmosphere to the upper middle atmosphere (Lindzen, 1981;Holton, 1982Holton, , 1983Fritts and Rastogi, 1985), which leads to the coupling of di erent layers in atmosphere and plays an important role in energy balance in the global middle and upper atmosphere. Since the 1960s it is clearly understood that when a gravity wave packet propagates in the atmosphere, the background¯ow will be accelerated because of the transient nature of a wave packet, moreover, since the atmospheric density decreases exponentially with increasing height, upgoing gravity waves excited in the troposphere reach such great amplitudes near the mesosphere that they break and cause intensive turbulence (Lindzen, 1981;Fritts, 1984).…”