Mesopause airglow modulation by ducted short-period gravity waves.

摘要

In this thesis, we examine the excitation and propagation of ducted short-period gravity waves at mesopause altitudes, and the associated wave-induced modulation of nighttime airglow emissions. A new, fully-nonlinear, two-dimensional, numerical model is developed for the simulation of atmospheric gravity waves in a realistic atmosphere, and coupled with photochemical models to allow calculations of wave-induced modulation of mesopause OH NIR and O( 1S) 557.7 nm airglow emissions. Models include transport of relevant minor and major species, and time-dependent chemical kinetics of ozone and hydrogen, which exhibit chemical lifetimes on the order of gravity wave periods at airglow altitudes. The role of dynamics and chemical time dependence on airglow response to realistic short period gravity wave perturbations is then studied. The models allow direct calculation of integrated emission intensities, facilitating detailed comparisons with ground-based airglow imager data.; We first validate our model while exploring three case studies for the excitation of thermally-ducted wave modes, by linear tunneling, and by breaking or nonlinear-propagation of other short period gravity waves. The modeled ducted wave modes are determined by Doppler shift and varying thermal structure. Consistent with past studies, numerically modeled ducted wave properties are found to agree with analytical models for fully-ducted modes in weak, constant and stable wind flow conditions.; Two model case studies of short-period gravity wave events, clearly resembling events observed in experimental airglow data, are presented: First, we examine a front-like ducted wave event, observed during the ALOHA-93 campaign in Hawaii. The hypothetical nonlinear excitation of this ducted wave is modeled, and model airglow intensity modulation is calculated. Agreement, and potential disagreement, between model results and observed image data are found. When vertical transport of O3 is relatively weak, OH and OI emissions are found to emit in anti-phase for equal-phase wave temperature and vertical velocity perturbations. This effect results from the chemical time dynamics associated with the OH excitation reaction, which place dependence principally on H and O3. When steep gradients of O3 lead to strong density perturbations, the OH emission may exhibit opposite response above and below the peak of O3, consistent with the sign of the gradient. The region of the layer exhibiting stronger perturbation will dominate the integrated emission intensity, thereby determining the phase of the intensity measured by a ground-based imager. This reveals strong dependence on the shape of the minor species profiles of O and O3, and also on the local wave perturbation magnitude and packet spatial configuration. Caution is thus needed in the interpretation and modeling of short-period wave signatures in airglow, due to significant variability of minor species profiles and atmospheric structure.; Second, we investigate a wave event observed at Bear Lake Observatory, UT, which also clearly exhibits antiphase perturbations of OH and OI intensity. Simultaneous radar wind data reveals the strong presence of a semidiurnal tidal motion; due to a tidal wind peak ∼50 m/s, the wave is fully-ducted by Doppler shift. Using idealized models of ducted wave forcing, several hypothetical waves are examined, which agree with the observed event. Modeled airglow intensities again suggest that the opposite-phase responses of OH and OI layers may be due to opposite photochemical responses, rather than to opposite wave dynamics at characteristic layer altitudes. Results are again highly sensitive to the actual profiles of minor species, and suggest that the phase of the integrated OH emission is determined by the structure of the wave perturbation with respect to the local O3 density gradients.; Finally, we investigate the effects of dynamic background flow on the propagation of ducted waves. Under idealize