Nonlinear behavior in the propagation of atmospheric gravity waves
Franke, Patricia Minthorn
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https://hdl.handle.net/2142/19348
Description
Title
Nonlinear behavior in the propagation of atmospheric gravity waves
Author(s)
Franke, Patricia Minthorn
Issue Date
1996
Doctoral Committee Chair(s)
Kudeki, Erhan
Department of Study
Electrical and Computer Engineering
Discipline
Electrical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Electronics and Electrical
Engineering, Mechanical
Physics, Atmospheric Science
Language
eng
Abstract
Motivated by the need to understand the experimental data from remote sensing systems measuring upper atmospheric parameters, a tool was developed that would allow the simulation of the nonlinear aspects of the propagation of the waves thought to drive the global circulation of the mesosphere-gravity waves. The results from two numerical experiments are presented comparing the dynamics of gravity wave breaking in two different frequency regimes. Analyses of these data are used to test several of the assumptions and implications of the current theories describing the dynamics of this region.
The goal of this study is to delve into understanding the nonlinear aspects of the gravity wave forcing of the upper atmosphere and how these nonlinearities appear in the frequency domain. To aid in the interpretation of the results from the fully nonlinear model, three other models, each containing a different subset of the full set of nonlinear terms, were developed: (i) a linear model containing no nonlinear terms, (ii) a quasilinear model containing the nonlinear terms associated with the wave-mean flow interactions only, and (iii) a fixed mean flow model containing the nonlinear terms associated with wave-wave interactions only.
Plotting the potential temperature and velocity fields in different ways shows the evolution of the system in each frequency regime, as well as the strong effect of the wave-mean flow interactions on wave breaking. The higher frequency regime exhibits a wave field that breaks the effects of which are confined to a very narrow region. Just enough energy is shed to maintain static stability in the system. Similarly, the low frequency also wave breaks, again just maintaining static stability, but now, the influence of the breaking is felt throughout the grid. In this frequency regime the new components generated by the nonlinearities are propagating. Comparisons of the fully nonlinear model results with those from the fixed mean flow model show that when the wave-mean flow interactions are included in the simulation the gravity wave is allowed to break vigorously; when they are excluded the wave field sheds energy but not through wave breaking (unless the forcing is very strong).
The breaking events are also described in the frequency domain. Various spectra are presented illustrating how the nonlinearities move energy from the main forced component into other components. Comparisons between the different models show the relative importance of each set of nonlinear terms in driving different aspects of the energy rearrangement in the spectrum. The wave-wave interactions create higher-order harmonics, some of which may or may not be propagating. The wave-mean flow interactions smear the energy in frequency and vertical wavenumber.
Parameterization of the nonlinear terms is the subject of the final section describing the results. Looking at the use of linear saturation theory and diffusion theory to predict the effect of the nonlinearities on wave amplitude and mean-flow creation, this section investigates the basic assumptions behind some of the more popular theories of gravity wave propagation in the mesosphere. The data show that the parameterizations are not generally valid, except under special circumstances. These circumstances are identified and compared with the appropriate theoretical descriptions looking for consistency with the basic assumptions of each theory. These results contain implications for how the different nonlinear processes work to shape the signatures seen in experimental data.
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