Date of Award

2014

Document Type

Dissertation - Open Access

Degree Name

Doctor of Philosophy in Engineering Physics

Department

Doctoral Studies

Committee Chair

Dr. Jonathan B. Snively

First Committee Member

Dr. Michael P. Hickey

Second Committee Member

Dr. Alan Z. Liu

Third Committee Member

Dr. Frederique Drullion

Abstract

A 2-D nonlinear, compressible numerical model [Snively and Pasko, 2008] is used in conjunction with ray-theory to investigate the long-range propagation, dissipation and interaction of small-scale gravity waves in the Mesosphere and Lower Thermosphere (MLT) region. The research in this thesis is made up of three distinct studies which build upon each other. The first investigates the thermospheric dissipation of three gravity wave packets representing: (1) A quasi-monochromatic packet, (2) A monochromatic, steady state wave, and (3) A spectrally broad packet, as well as an initial condition specified packet. It is found that dissipation due to molecular viscosity and thermal conduction acts to decrease the vertical wavelength of the packet in time (except in the steady-state case, when it remains constant). This is due to the higher frequencies (longer wavelengths) reaching the thermsophere first and dissipating before the lower frequencies (shorter wavelengths), thus the spectral content of the packet shifts from higher frequencies (longer wavelengths) to lower frequencies (shorter wavelengths) in time. At any instant of time, the vertical wavelength increases with altitude in the thermosphere when the wave has reached a steady state.

The second study investigated the potential for long-range propagation of three small-scale wave packets under averaged high latitude conditions. The three packets were chosen to represent wave parameters typically observed over Halley, Antarctica [Nielsen et al., 2009, 2012] and ones that may be considered favorable for long-range propagation [ Snively, 2013]. It was found that the stratosphere provides an efficient region of the atmosphere to trap waves and allow them to propagate large horizontal distances. Ducting in the mesosphere was less likely when considering averaged meridional winds, and it is suggested that waves observed in the mesopause, far from the source region, may be the result of leakage from the stratosphere. It was also shown that leakage from the stratosphere over considerable horizontal distances can lead to a periodic and spatially distributed forcing on the MLT region.

The third and final study investigated the propagation of wave packets through a background wind which was horizontally, and vertically inhomogeneities and also time dependent. Two small-scale wave packets were chosen, such that one was prone to critical level filtering and the other reflection. These waves were propagated through (1) a background wind which was static and varied in the vertical and horizontal directions separately, (2) a background wind representing a medium-scale wave propagating in the direction of propagation of the small-scale wave, and (3) a background wind representing a medium-scale wave propagating against the propagation direction of the small-scale wave. It was found that a purely horizontally inhomogeneous background wind can include a blocking level, where the horizontal group velocity of the small-scale packet goes to zero, if the wind opposes and the horizontal gradient is negative relative to the propagation direction. If the wind gradient is positive then the wind will horizontally accelerate the small-scale packet. Adding a time-dependent phase progression to the medium scale waves acts to significantly reduce the effects of both reflection and critical level filtering of the small-scale packet. Also, a small-scale packet was less likely to experience reflection or critical level filtering if it was propagating against the horizontal phase progression of the medium scale wave. The reduction of critical level filtering and reflection in a time-dependent background is the result of 1) The transient nature of the critical or reflection level, which will progress with the phase of the medium scale wave. 2) The time-dependence of the background wind acts to alter the ground relative frequency of the small-scale wave and avoid satisfying the critical level or reflection conditions. Current parameterization schemes consider time-independent backgrounds which vary in the vertical direction only, and generally do not consider the effects of wave reflection. Understanding how a time-dependent, and horizontally inhomogeneous background effects small-scale wave propagation may be important for future parameterizations as small- scale waves are suggested to contribute significantly to the overall momentum budget of the middle atmosphere.

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