A Full-Wave Investigation of the Use of a \u27Cancellation Factor\u27 in GW-Airglow Interaction Studies

Atmospheric gravity waves (GWs) perturb minor species involved in the chemical reactions of airglow emissions in the mesopause region of the earth\u27s atmosphere. The so-called \u27Cancellation Factor\u27 (CF) is defined as a transfer function relating the amplitude of airglow brightness fluctuation to the amplitude of GW-induced fluctuation in temperature [Swenson and Gardner, 1998]. This transfer factor can be used to determine GW fluxes and the forcing effects of GWs on the mean state through airglow observations, because GW fluxes are proportional to the square of GW amplitude. Numerical models [Walterscheid et al., 1987; Schubert et al., 1991] have previously shown that the airglow relative brightness fluctuation can be much larger than the brightness-weighted relative temperature fluctuation (that is, Krassovsky\u27s ratio is much greater than 1). Analytical expressions of the CF in the OH nightglow were derived by Swenson and Gardner [1998] and later used by Swenson and Liu [1998]. We introduce the full-wave model [Hickey et al., 1997, 1998] describing GW propagation in a non-isothermal, windy, and viscous atmosphere (combined with the chemical reaction scheme for the OH (8, 3) Meinel emission) to derive fluctuations in the OH nightglow from which an equivalent CF is calculated. Extensive comparisons between our CF and that of Swenson and colleagues show under what atmospheric conditions and which range of GW parameters the CF would be expected to provide a good measure of GW amplitude. This thesis consists of four chapters that deal with the calculations and comparisons of the CFs in the OH nightglow from both the analytical and numerical models under various atmospheric conditions. In the first chapter the general subject of internal GWs is introduced for the non-specialist of this field. It reviews the historical theory and observation of atmospheric GWs, and also emphasizes the role of atmospheric GWs in producing the reversal of global temperature gradients at the mesopause. At the end of this chapter, the motivation for calculating the CF is introduced. In the second chapter numerical models of GW-driven fluctuations in the OH nightglow are described in detailed in three developing stages. The Walterscheid et al. [1987] model incorporated a five-reaction photochemical scheme and the complete dynamics of linearized acoustic GWs in an isothermal and motionless atmosphere, but only calculated Krassovsky\u27s ratio for an infinitesimally thin airglow emission layer. Hickey\u27s [1988] model was extended to include the dynamical effects of internal GWs propagating in a viscous, thermally conducting, and rotating (though windless) isothermal atmosphere. The model of Schubert, Walterscheid & Hickey [1991] investigated how the characteristics of the OH nighglow from an extended emission region were modified by eddy momentum and eddy thermal diffusivities. In the rest of the second chapter the full-wave model [Hickey et al., 1997, 1998] along with the chemical reaction scheme for the OH (8, 3) Meinel emission as well as the analytical model of Swenson and Gardner [1998] are introduced. The third chapter commences with a comparison of the CFs derived from the analytical model of Swenson and Gardner [1998] with the CFs calculated with the full-wave model numerically. Much of the work involves the development of computer programs and the plots of data outputs. The analysis and discussion begin with the assumption of an ideal atmosphere, which is isothermal, quasiadiabatic, and motionless, and later continue to that of a more realistic atmosphere (non-isothermal, dissipative, and with meridional and zonal winds). In the case including the influence of mean winds, we employ wind profiles representative of December 15 and GWs traveling in the eastward direction. These comparisons allow us to determine the accuracy of the calculations and the validity of the assumptions used in the analytically derived CF of Swenson & Gardner [1998]. In the last chapter we summarize the advantage and disadvantage in both approaches. The more accurate calculation of the CF in the OH nightglow under a more realistic atmosphere provides a better understanding of GW effects on the mesospheric dynamics. The CF can be used by optical experimenters to relate their airglow observations to GW energy and momentum fluxes in the stated altitude region

Simulated ducting of high-frequency atmospheric gravity waves in the presence of background winds

A new nonlinear and time-dependent model is used to derive the total perturbation energy flux of two gravity wave packets propagating from the troposphere to the lower thermosphere. They are excited by a heat source and respectively propagate in an eastward and westward direction in the presence of a zonal wind. Analysis of the refractive index, the power spectra and the total perturbation energy flux allows us to correctly interpret the ducting characteristics of these two wave packets. In our study the wind acts as a directional filter to the wave propagations and causes noticeable spectral variations at higher altitudes. We are the first that time-resolve the total perturbation energy flux influenced by the winds and the simulations have immediate impacts to the airglow observations on certain wave spectra

Simulated ducting of high-frequency atmospheric gravity waves in the presence of background winds

A new nonlinear and time-dependent model is used to derive the total perturbation energy flux of two gravity wave packets propagating from the troposphere to the lower thermosphere. They are excited by a heat source and respectively propagate in an eastward and westward direction in the presence of a zonal wind. Analysis of the refractive index, the power spectra and the total perturbation energy flux allows us to correctly interpret the ducting characteristics of these two wave packets. In our study the wind acts as a directional filter to the wave propagations and causes noticeable spectral variations at higher altitudes. We are the first that time-resolve the total perturbation energy flux influenced by the winds and the simulations have immediate impacts to the airglow observations on certain wave spectra

Ionospheric Gravity Waves Driven by Oceanic Gravity Waves in Resonance: A Modeling Study in Search of Their Spectra

Ionospheric observations associated with the 2011 Tohoku tsunami have revealed gravity waves having spectral characteristics that depend on their proximity to the epicenter. There is a preponderance of medium-scale waves in the vicinity of the epicenter, a significant bifurcation into short- and long-period waves over the Hawaiian archipelago, and a narrow and rich spectrum of waves over the West Coast and inland of the United States (U.S.). Guided by these previous observations, we consider wave sources as triads of nonlinearly interacting oceanic gravity waves, whose wave parameters satisfy resonant conditions. These waves are simulated using a 2-D nonlinear model describing gravity wave propagation in order to explain the observations of tsunamigenic traveling ionospheric disturbances (TIDs) associated with the Tohoku event

Numerical modeling of a gravity wave packet ducted by the thermal structure of the atmosphere

[1] A time-dependent and fully nonlinear numerical model is employed to solve the Navier-Stokes equations in two spatial dimensions and to describe the propagation of a Gaussian gravity wave packet generated in the troposphere. A Fourier spectral analysis is used to analyze the frequency power spectra of the wave packet, which propagates through and dwells within several thermal ducting regions. The frequency power spectra of the wave packet are derived at several discrete altitudes, which allow us to determine the evolution of the packet. This spectral analysis also clearly reveals the existence of a stratospheric duct, a mesospheric and lower thermospheric duct, and a duct lying between the tropopause and the lower thermosphere. In addition, we determine the spatially localized wave kinetic energy density and the horizontally averaged, time-resolved, normalized vertical velocity. Examination of these diagnostic variables allows us to better understand the process of wave ducting and the vertical transport of wave energy among multiple thermal ducts. The spectral analysis allows us to unambiguously identify the ducted wave modes. These results compare favorably with those derived from a full-wave model

Time-resolved ducting of atmospheric acoustic-gravity waves by analysis of the vertical energy flux

A new 2-D time-dependent model is used to simulate the propagation of an acoustic-gravity wave packet in the atmosphere. A Gaussian tropospheric heat source is assumed with a forcing period of 6.276 minutes. The atmospheric thermal structure creates three discrete wave ducts in the stratosphere, mesosphere, and lower thermosphere, respectively. The horizontally averaged vertical energy flux is derived over altitude and time in order to examine the time-resolved ducting. This ducting is characterized by alternating upward and downward energy fluxes within a particular duct, which clearly show the reflections occurring from the duct boundaries. These ducting simulations are the first that resolve the time-dependent vertical energy flux. They suggest that when ducted gravity waves are observed in the mesosphere they may also be observable at greater distances in the stratosphere

Time-resolved Ducting of Atmospheric Acoustic-gravity Waves by Analysis of the Vertical Energy Flux

A new 2-D time-dependent model is used to simulate the propagation of an acoustic-gravity wave packet in the atmosphere. A Gaussian tropospheric heat source is assumed with a forcing period of 6.276 minutes. The atmospheric thermal structure creates three discrete wave ducts in the stratosphere, mesosphere, and lower thermosphere, respectively. The horizontally averaged vertical energy flux is derived over altitude and time in order to examine the time-resolved ducting. This ducting is characterized by alternating upward and downward energy fluxes within a particular duct, which clearly show the reflections occurring from the duct boundaries. These ducting simulations are the first that resolve the time-dependent vertical energy flux. They suggest that when ducted gravity waves are observed in the mesosphere they may also be observable at greater distances in the stratosphere

Numerical Modeling of a Gravity Wave Packet Ducted by the Thermal Structure of the Atmosphere

A time-dependent and fully nonlinear numerical model is employed to solve the Navier-Stokes equations in two spatial dimensions and to describe the propagation of a Gaussian gravity wave packet generated in the troposphere. A Fourier spectral analysis is used to analyze the frequency power spectra of the wave packet, which propagates through and dwells within several thermal ducting regions. The frequency power spectra of the wave packet are derived at several discrete altitudes, which allow us to determine the evolution of the packet. This spectral analysis also clearly reveals the existence of a stratospheric duct, a mesospheric and lower thermospheric duct, and a duct lying between the tropopause and the lower thermosphere. In addition, we determine the spatially localized wave kinetic energy density and the horizontally averaged, time-resolved, normalized vertical velocity. Examination of these diagnostic variables allows us to better understand the process of wave ducting and the vertical transport of wave energy among multiple thermal ducts. The spectral analysis allows us to unambiguously identify the ducted wave modes. These results compare favorably with those derived from a full-wave model