Mesopause

Abstract: It has been suggested (Lindzen, 1967, 1968a, b; Lindzen and Blake, 1971; Hodges, 1969) that turbulence in the upper mesosphere arises from the unstable breakdown of tides and gravity waves. Crudely speaking, it was expected that sufficient turbulence would be generated to prevent the growth of wave amplitude with height (roughly as (basic pressure)−1/2). This work has been extended to allow for the generation of turbulence by smaller amplitude waves, the effects of mean winds on the waves, and the effects of the waves on the mean momentum budget. The effects of mean winds, while of relatively small importance for tides, are crucial for internal gravity waves originating in the troposphere. Winds in the troposphere and stratosphere sharply limit the phase speeds of waves capable of reaching the upper mesosphere. In addition, the existence of critical levels in the mesosphere significantly limits the ability of gravity waves to generate turbulence, while the breakdown of gravity waves contributes to the development of critical levels. The results of the present study suggest that at middle latitudes in winter, eddy coefficients may peak at relatively low altitudes (50 km) and at higher altitudes in summer and during sudden warmings (70–80 km), and decrease with height rather sharply above these levels. Rocket observations are used to estimate momentum deposition by gravity waves. Accelerations of about 100 m/s/day are suggested. Such accelerations are entirely capable of producing the warm winter and cold summer mesopauses.

Abstract: Preface. 1: The Middle Atmosphere and its Evolution. 1.1 Introduction. 1.2 Evolution of the Earth's Atmosphere. 1.3 Anthropogenic Perturbations. 2: Chemical Concepts in the Atmosphere. 2.1 Introduction. 2.2 Energy Levels of Molecules. 2.3 Thermodynamic Considerations. 2.4 Elementary Chemical Kinetics. 2.5 Heterogeneous and Multiphase Reactions. 2.6 Photolysis Processes. 2.7 Excited Species in the Middle Atmosphere. 3: Dynamics and Transport. 3.1 Introduction. 3.2 Structure of the Atmosphere and Some Observed Dynamical Characteristics. 3.3 Fundamental Description of Atmospheric Dynamics. 3.4 Atmospheric Waves. 3.5 Effects of Dynamics on Chemical Species: Transport. 3.6 Dynamics and Meridional Transport in Two Dimensions: A Conceptual View. 3.7 The Importance of Wave Transience and Dissipation. 3.8 Vertical Transport above the Mesopause. 3.9 Models of the Middle Atmosphere. 4: Radiation. 4.1 Introduction. 4.2 Definitions. 4.3 Extraterrestrial Solar Radiation. 4.4 The Attenuation of Solar radiation in the Atmosphere. 4.5. Radiative Transfer. 4.6 The Thermal Effects of Radiation. 4.7 Photochemical Effects of Radiation. 5: Composition and Chemistry. 5.1 General. 5.2 Oxygen Compounds. 5.3 Carbon Compounds. 5.4 Hydrogen Compounds. 5.5 Nitrogen Compounds. 5.6 Halogen Compounds. 5.7 Stratospheric Aerosols and Clouds. 5.8 Generalized ozone Balance. 6: Ozone Perturbations. 6.1 Introduction. 6.2 The Photochemically-Controlled Upper Stratosphere and mesosphere: 25-75 km. 6.3 Lower Stratospheric Ozone Depletion: Observations and Explanations. 6.4 Summary and Outlook. 7: The Ions. 7.1 Introduction. 7.2 Formation of Ions in the Middle Atmosphere. 7.3 Positive Ion Chemistry. 7.4 Negative Ion Chemistry. 7.5 Effect of Ionic Processes on Neutral Constituents. 7.6 Radio Waves in the Lower Ionosphere. Appendices. Appendix 1: Physical Constants and Other Data. Appendix 2: Conversion Factors and Multiplying Prefixes. Appendix 3: Rate Coefficients forSecond-Order Gas-Phase Reactions. Appendix 4: Rate Coefficients for Gas-Phase Associating Reactions. Appendix 5: Surface Reaction Probability. Appendix 6: Atmospheric Profiles. Figure acknowledgements. Index.

Abstract: The influence of breaking gravity waves on the dynamics and chemical composition of the 60- to 110-km region has been investigated with a two-dimensional dynamical/chemical model that includes a parameterization of gravity wave drag and diffusion. The momentum deposited by breaking waves at mesospheric altitudes reverses the zonal winds, drives a strong mean meridional circulation, and produces a very cold summer and warm winter mesopause, in general agreement with observations. The seasonal variations of the computed eddy diffusion coefficient are consistent with the behavior of mesospheric turbulence inferred from MST radar echoes. In particular, it is found that eddy diffusion is strong in summer and winter but much weaker at the equinoxes and that this seasonal behavior has important consequences for the distribution of chemical species. Comparison between computed atomic oxygen and ozone, and the abundances of these constituents inferred from the 557.7-nm and 1.27-μm airglow emissions, reveals excellent agreement. The consistency between model results and these diverse types of observations lends strong support to the hypothesis that gravity waves play a very important role in determining the zonally averaged structure of the mesosphere and lower thermosphere.

Abstract: [1] We have used the Whole Atmosphere Community Climate Model to produce a small (three-member) ensemble of simulations of the period 1950–2003. Comparison of model results against available observations shows that for the most part, the model is able to reproduce well the observed trends in zonal mean temperature and ozone, both as regards their magnitude and their distribution in latitude and altitude. Calculated trends in water vapor, on the other hand, are not at all consistent with observations from either the HALOE satellite instrument or the Boulder, Colorado, hygrosonde data set. We show that such lack of agreement is actually to be expected because water vapor has various sources of low-frequency variability (heating due to volcanic eruptions, the quasi-biennial oscillation and El Nino–Southern Oscillation) that can confound the determination of secular trends. The simulations also reveal the presence of other interesting behavior, such as the lack of any significant temperature trend near the mesopause, a decrease in the stratospheric age of air, and the rare occurrence of an extremely disturbed Southern Hemisphere winter.