Thesis. 1977. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Microfiche copy available in Archives and Science. / Vita. / Bibliography : leaves 162-165. / Ph.D.

Thesis. 1976. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Microfiche copy available in Archives and Science. / Bibliography: leaves 122-123. / by Charles A. Moo. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1980. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND LINDGREN. / Vita. / Bibliography : leaves 203-209. / by Douglas Scott Luther. / Ph.D.

The deep convection from monsoons is known to be a major source of gravity waves in the Earth's summer troposphere. While propagating through the middle atmosphere, these waves can carry their momentum up to the mesosphere, following either a vertical or an oblique path. This upward and oblique propagation of gravity waves refers to the latitudinal propagation, away from their low-latitude tropospheric source and towards the polar summer mesosphere. Their dissipation in this atmospheric region plays an important role in the global dynamical structure of the middle atmosphere and yet, the oblique propagation of gravity waves is not included in the present global models. Understanding the influence of the obliquely propagating monsoon gravity waves on the polar summer mesosphere, as well as the hemispheric and seasonal variations of this phenomenon, can improve the gravity-wave parameterization schemes used in the global models. My dissertation relies upon the atmosphere theory and the gravity-wave observations, first, to perform an observational analysis of the oblique propagation of gravity waves in the summer hemisphere. In response to temperature anomalies in the winter northern stratosphere, the distribution of the gravity-wave pseudomomentum flux in the opposite summer mesosphere appeared to be altered. This in turn changes the gravity-wave oblique propagation and its influence on the temperature variations seen in the polar mesospheric clouds. After the development of a 4-D non-hydrostatic ray-tracing model for the simulation of the gravity-wave propagation, my dissertation explores the hemispheric and seasonal differences in the propagation and dissipation of more than 40,000 gravity waves from the low-latitude troposphere. These ray-tracing simulations show the southern hemisphere to be more conducive to both the vertical and the oblique propagation of tropospheric to mesospheric gravity waves. This analysis also highlighted a strong wave filtering at the northern tropopause where a significant number of gravity waves were vertically reflected before reaching the stratosphere. / Doctor of Philosophy / The propagation of waves throughout the Earth's atmosphere is a key phenomenon to understanding the global atmosphere dynamics. These atmospheric waves are known to change the temperature, the pressure, the density and the composition of the middle atmosphere. As a wave propagates upward, the density of the atmospheric background exponentially decreases, resulting in an exponential increase in the wave amplitude and thus, an exponential increase in the energy carried by the wave. When the wave breaks, this energy is released and transferred to the background flow. Gravity waves are part of the atmospheric wave spectrum that is of interest to the scientific community. While small-scale gravity waves can form from tropospheric instabilities such as an unbalanced flow over the mountains or a deep convection from monsoon or thunderstorms, they can propagate up to the upper mesosphere where they can break and transfer a significant amount of energy to the background flow. Although the significant role of these gravity waves in the coupling mechanisms between atmospheric regions is without dispute, their horizontal scale is too small to be resolved by most of the global-scale atmospheric models. The deep convection from monsoon regions is known to be a major source of mesospheric GWs and previous studies on summer northern hemisphere have shown that monsoon GWs tend to propagate obliquely from the low-latitude stratopause up to the high-latitude mesopause. We focus the observational study on the summer southern hemisphere and the Inter-Hemispheric Coupling (IHC) between the summer mesopause, where Polar Mesospheric Clouds (PMCs) form, and the winter stratosphere where sudden warmings occur. PMCs are excellent indicators of atmospheric changes. Their correlations with wind, temperature and GW pseudomomentum flux highlight the consequences of anomalies in the winter stratosphere, such as warmings, on the oblique propagation of GWs that influence the PMC formation in the summer southern hemisphere. After the computation of a ray-tracing model for the simulation of the gravity-wave propagation, a hemispheric and seasonal comparison of the tropospheric to mesospheric gravity-wave propagation based on four simulations highlights the spectral nature of this phenomenon.

Time series of mesosphere/lower thermosphere half-hourly winds over Collm (51.3°N, 13.0°E) have been obtained from 1984 – 2008 by low frequency spaced receiver measurements and from 2004 to date by very high frequency meteor radar Doppler wind observations in the height range 82 – 97 km. From half-hourly differences of zonal and meridional winds, gravity wave (GW) proxies have been calculated that describe amplitude variations in the period range of 1 – 3 hours. After applying corrections to account for instrumental differences, GW climatology and time series have been obtained. The mean GW activity in the upper mesosphere shows maximum amplitudes in summer, while in the lower thermosphere GWs maximize in winter. Positive/negative long-term trends are visible in winter/summer. Interannual and quasi-decadal variations of GW amplitudes are also visible, but these are intermittent. / Zeitreihen von halbstündlichen Winden der Mesosphare/unteren Thermosphare über Collm (51,3 ¨ °N, 13,0°E) wurden von 1984 bis 2008 durch LF-Driftmessungen und von 2004 bis heute durch VHF-Meteorradarmessungen im Höhenbereich von 82 – 97 km gewonnen. Aus halbstündlichen Differenzen von zonalen und meridionalen Winden wurden Proxies für Schwerewellen (GW) berechnet, welche Amplitudenvariationen im Periodenbereich von 1 – 3 Stunden beschreiben. Nach Korrekturen zur Berücksichtigung instrumenteller Unterschiede wurden GW-Klimatologie und Zeitreihen erstellt. Die mittlere GW-Aktivitat in der oberen Mesosphäre zeigt Maxima im Sommer, wahrend in der unteren Thermosphäre GW-Maxima im Winter auftreten. Positive/negative Langzeittrends sind im Winter/Sommer sichtbar. Interannuale und quasi-dekadische Variationen der GW-Amplituden sind ebenfalls sichtbar, aber nicht durchgehend erkennbar.

Surface gravity waves alter the turbulence of the bottom atmosphere and the upper ocean. Accordingly, they can affect momentum flux, heat fluxes, gas exchange and atmospheric mixing. However, in most state-of-the-art Earth System Models (ESMs), surface wave influences are not fully considered or even included. Here, applying surface wave influences into ESMs is investigated from different aspects. Tuning parameterisations for including instantaneous wave influences has difficulties to capture wave influences. Increasing the horizontal resolution of models intensifies storm simulations for both atmosphere-wave coupled (considering the influence of instantaneous wave-induced stress) and stand-alone atmospheric models. However, coupled models are more sensitive to the horizontal resolution than stand-alone atmospheric models. Under high winds, wave states have a big impact on the sea spray generation. Introducing a wave-state-dependent sea spray generation function and Charnock coefficient into a wind stress parameterisation improves the model performance concerning wind speed (intensifies storms). Adding sea spray impact on heat fluxes improves the simulation results of air temperature. Adding sea spray impact both on the wind stress and heat fluxes results in better model performance on wind speed and air temperature while compared to adding only one wave influence. Swell impact on atmospheric turbulence closure schemes should be taken into account through three terms: the atmospheric mixing length scale, the swell-induced momentum flux at the surface, and the profile of swell-induced momentum flux. Introducing the swell impact on the three terms into turbulence closure schemes shows a better performance than introducing only one of the influences. Considering all surface wave impacts on the upper-ocean turbulence (wave breaking, Stokes drift interaction with the Coriolis force, Langmuir circulation, and stirring by non-breaking waves), rather than just one effect, significantly improves model performance. The non-breaking-wave-induced mixing and Langmuir circulation are the most important terms when considering the impact of waves on upper-ocean mixing. Accurate climate simulations from ESMs are very important references for social and biological systems to adapt the climate change. Comparing simulation results with measurements shows that adding surface wave influences improves model performance. Thus, an accurate description of all important wave impact processes should be correctly represented in ESMs, which are important tools to describe climate and weather. Reducing the uncertainties of simulation results from ESMs through introducing surface gravity wave influences is necessary.

The complexity of numerical ocean circulation models requires careful checking with a variety of test problems. The purpose of this paper is to develop a test problem involving Rossby and gravity waves in a two-layer fluid in a channel. The goal is to compute very accurate solutions to this test problem. These solutions can then be used as a part of the checking process for numerical ocean circulation models. Here, Chebychev pseudo-spectral methods are used to solve the governing equations with a high degree of accuracy. Chebychev pseudo-spectral methods can be described in the following way: For a given function, find the polynomial interpolant at a particular non-uniform grid. The derivative of this polynomial serves as an approximation to the derivative of the original function. This approximation can then be inserted to differential equations to solve for approximate solutions. Here, the governing equations reduce to an eigenvalue problem with eigenvectors and eigenvalues corresponding to the spatial dependences of modal solutions and the frequencies of those solutions, respectively. The results of this method are checked in two ways. First, the solutions using the Chebychev pseudo-spectral methods are analyzed and are found to exhibit the properties known to belong to physical Rossby and gravity waves. Second, in the special case where the two-layer model degenerates to a one-layer system, some analytic solutions are known. When the numerical solutions are compared to the analytic solutions, they show an exponential rate of convergence. The conclusion is that the solutions computed using the Chebychev pseudo-spectral methods are highly accurate and could be used as a test problem to partially check numerical ocean circulation models. / Graduation date: 2012

Gravity waves have a significant dynamic effect in the mesosphere. In particular, they drive the mesospheric circulation and are the reason that the summer polar mesosphere is cooler than the winter polar mesosphere. This thesis examines whether the effects of gravity waves are largely determined by filtering effects which allow only gravity waves with certain properties to propagate into the atmosphere. The filtering of gravity waves above Scott Base, Antarctica is examined using a radiosonde derived gravity wave source function, an MF-radar derived mesospheric gravity wave climatology, and a model derived filtering function. Least squares fitting of the source function and filtering function to the observed mesospheric gravity wave climatology allows us to determine which gravity wave phase velocities and propagation direction are likely to be present in the mesosphere and the relative importance of filtering and sources in this region. It is concluded the blocking of eastward gravity waves is important in winter and westward waves in summer.

Gravity waves have a significant dynamic effect in the mesosphere. In particular, they drive the mesospheric circulation and are the reason that the summer polar mesosphere is cooler than the winter polar mesosphere. This thesis examines whether the effects of gravity waves are largely determined by filtering effects which allow only gravity waves with certain properties to propagate into the atmosphere. The filtering of gravity waves above Scott Base, Antarctica is examined using a radiosonde derived gravity wave source function, an MF-radar derived mesospheric gravity wave climatology, and a model derived filtering function. Least squares fitting of the source function and filtering function to the observed mesospheric gravity wave climatology allows us to determine which gravity wave phase velocities and propagation direction are likely to be present in the mesosphere and the relative importance of filtering and sources in this region. It is concluded the blocking of eastward gravity waves is important in winter and westward waves in summer.

Thesis (Ph.D.)--University of Melbourne, School of Physics, 2007. / Typescript. Includes bibliographical references (leaves 275-308).