Generation of turbulence in the upper atmosphere by internal gravity waves.
TL;DR: Turbulence in upper atmosphere possibly due to density fluctuations accompanying internal gravity waves was reported in this article, where the authors attributed it to density fluctuation associated with internal gravity wave propagation.
Abstract: Turbulence in upper atmosphere possibly due to density fluctuations accompanying internal gravity waves
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TL;DR: In this article, a review of gravity wave sources and characteristics, the evolution of the gravity wave spectrum with altitude and with variations of wind and stability, the character and implications of observed climatologies, and the wave interaction and instability processes that constrain wave amplitudes and spectral shape are discussed.
Abstract: [1] Atmospheric gravity waves have been a subject of intense research activity in recent years because of their myriad effects and their major contributions to atmospheric circulation, structure, and variability. Apart from occasionally strong lower-atmospheric effects, the major wave influences occur in the middle atmosphere, between ∼ 10 and 110 km altitudes because of decreasing density and increasing wave amplitudes with altitude. Theoretical, numerical, and observational studies have advanced our understanding of gravity waves on many fronts since the review by Fritts [1984a]; the present review will focus on these more recent contributions. Progress includes a better appreciation of gravity wave sources and characteristics, the evolution of the gravity wave spectrum with altitude and with variations of wind and stability, the character and implications of observed climatologies, and the wave interaction and instability processes that constrain wave amplitudes and spectral shape. Recent studies have also expanded dramatically our understanding of gravity wave influences on the large-scale circulation and the thermal and constituent structures of the middle atmosphere. These advances have led to a number of parameterizations of gravity wave effects which are enabling ever more realistic descriptions of gravity wave forcing in large-scale models. There remain, nevertheless, a number of areas in which further progress is needed in refining our understanding of and our ability to describe and predict gravity wave influences in the middle atmosphere. Our view of these unknowns and needs is also offered.
2,206 citations
TL;DR: A review of recent advances in the understanding of gravity wave saturation in the middle atmosphere can be found in this article, where a brief discussion of the studies leading to the identification of the gravity wave effects and their role in middle atmosphere dynamics is presented.
Abstract: This paper provides a review of recent advances in our understanding of gravity wave saturation in the middle atmosphere. A brief discussion of those studies leading to the identification of gravity wave effects and their role in middle atmosphere dynamics is presented first. This is followed by a simple development of the linear saturation theory to illustrate the principal effects. Recent extensions to the linear saturation theory, including quasi-linear, nonlinear, and transient effects, are then described. Those studies addressing the role of gravity wave saturation in the mean circulation of the middle atmosphere are also discussed. Finally, observations of gravity wave motions, distribution, and variability and those measurements specifically addressing gravity wave saturation are reviewed.
575 citations
TL;DR: In this paper, a smoothed time series of daily values of the area A(t) of the main vortex, as it appears on isentropic Q maps, is proposed.
449 citations
TL;DR: In this paper, the authors extended the parameterization developed in its companion paper, Part 1, and discussed the input vertical-wavenumber spectrum of the broad background of gravity waves that Part 1 is designed to treat.
388 citations
TL;DR: In this paper, the authors review the theory and the observational evidence for both types of instabilities in the lower and middle atmosphere and show that convective instabilities predominate for high-frequency wave motions.
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.
326 citations
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TL;DR: In this paper, the proper interpretation of irregular motions in the upper atmosphere has been investigated by a variety of techniques, but their proper interpretation has yet to be established. But their proper meaning has not yet been established.
Abstract: Irregularities and irregular motions in the upper atmosphere have been detected and studied by a variety of techniques during recent years, but their proper interpretation has yet to be established...
1,886 citations
TL;DR: Atomic-to-molecular oxygen concentrations ratio values in upper atmosphere used to determine strength of turbulent mixing in atmosphere, noting definition of turbopause height as mentioned in this paper, which is defined as the height at which the turbopsause threshold is reached.
Abstract: Atomic-to-molecular oxygen concentrations ratio values in upper atmosphere used to determine strength of turbulent mixing in atmosphere, noting definition of turbopause height
186 citations
TL;DR: In this paper, an attempt is made to resolve the apparent motion into various constituents by assuming that the observed drift represents a sum of three types of motion superimposed on one another: a general drift, tidal components, and internal gravity waves.
Abstract: Horizontal motions from 25 sodium cloud experiments are examined in the altitude range from 70 to 190 km. The outstanding characteristics of the apparent motion are pronounced velocity oscillations in the 70- to 130-km layer; they reach a maximum near 105 km and attenuate at greater heights. A quiescent zone appears from 140 to 190 km, where, despite an increase of speed with height, the rate at which velocity changes with elevation is small. An attempt is made to resolve the apparent motion into various constituents by assuming that the observed drift represents a sum of three types of motion superimposed on one another: a general drift, tidal components, and internal gravity waves. The derived quantities seem to explain vertical shear distribution and other phenomena. It is estimated that in the 90- to 125-km layer the contributions of the three constituents to the observed motion are: gravity waves, 40% general drift, 34% tidal components, 26%. Above 180 km, the term representing a sum of the general drift and tidal components assumes a still more dominant role, and at 160 km its contribution to the observed motion is 85%.
128 citations
TL;DR: In this article, the authors derived an equation for the flux Richardson number in terms of the ordinary Richardson number and some non-dimensional ratios connected with the turbulent motion, and showed that the interaction between the temperature and velocity fields imposes on the turbulent Richardson number an upper limit of 0·5, and on the regular Richardson number a limit of about 0·08.
Abstract: Fluctuations of velocity and temperature which occur in a turbulent flow in a stably-stratified atmosphere far from restraining boundaries are discussed using the equations for the turbulent intensity and for the mean square temperature fluctuation. From these, an equation is derived for the flux Richardson number in terms of the ordinary Richardson number and some non-dimensional ratios connected with the turbulent motion. It is shown that the interaction between the temperature and velocity fields imposes on the flux Richardson number an upper limit of 0·5, and on the ordinary Richardson number a limit of about 0·08. If these values are exceeded, no equilibrium value of the turbulent intensity can exist and a collapse of the turbulent motion would occur. Although the analysis applies strictly only to a homogeneous non-developing flow, it should have approximate validity for effectively homogeneous, developing flows, and the predictions are compared with some recent observations of these flows.
96 citations
TL;DR: The maximum amount of heat that is available for transportation downward through any level by eddy conduction can be determined from the absorption of solar energy above that level; any heat losses by infrared radiation or large-scale circulation can only reduce the available heat as mentioned in this paper.
Abstract: The thermodynamic stability of the mesosphere and lower thermosphere requires that there be a downward transport of heat associated with any turbulent mixing. The maximum amount of heat that is available for transportation downward through any level by eddy conduction can be determined from the absorption of solar energy above that level; any heat losses by infrared radiation or large-scale circulation can only reduce the available heat. This evaluation of the available heat can be used to place an upper limit on the vertical eddy diffusivity, and this limit is substantially below values frequently quoted on the basis of vapor or meteor trail studies. The upper limit is about 106 cm2/sec above 80 km and about 4×105 cm2/sec from 50 to 70 km. The fact that diffusive equilibrium distributions of atmospheric constituents prevail no lower than 105 km, where the molecular diffusion coefficient is 106 cm2/sec, indicates that the eddy coefficient at that altitude has the limiting value obtained from thermal considerations.
69 citations