| Atmospheric gravity waves play important roles in determining the dynamics structure, thermal structure and large-scale circulation of the atmosphere. Vertically propagating gravity waves generated by various sources in the troposphere could transport and deposit energy and momentum, and play a crucial role in determining the momentum and energy budget equations in global model. The most obvious sources include topographic, convection, wind shear, geostrophic adjustment, frontogenesis and baroclinic instability. Wave-wave and wave-flow interactions also play important roles in the generation of gravity waves. Many observations and numerical simulations studies have contributed greatly to our knowledge of gravity waves generation, our understanding of generation mechanisms is still limited, the generation and propagation of gravity waves are still the most concerned problems of scientific research.In this dissertation, aiming at the unsolved problems of gravity wave generation, we investigate the evolution and characteristics of waves excited by thermal source, the relationships between sources' and excited waves' temporal and spatial scales are also analyzed, the jet's filtering and absorbing effect on the excited waves, and contribution of excited waves to drive the background wind are also investigated. We study the characteristics of mid-latitude gravity waves in the troposphere and stratosphere based on the radiosonde observations over U.S. stations, and the generation mechanism of mid-latitude gravity waves is also discussed. The primary results are summarized as follow:1. By using a two-dimensional atmosphere nonlinear dynamic model, we investigate the characteristics of gravity waves excited by local thermal source that oscillates periodically in the stationary background. Numerical results show that the conformation of excited waves are determined by the structure of thermal source; the spatial scale of thermal oscillation determines the corresponding spatial scale of excited waves, and when the thermal source has a Gaussian-varying form in space, the excited waves has a wide spectrum in the corresponding space direction; the total energies provided by thermal source are divided symmetrically to the excited waves that propagate at different directions.2. A temporal and spatial Gaussian-varying thermal forcing is set in an isothermal and compressible atmosphere in this paper, the evolution and characteristics of gravity waves excited by thermal source with wide spectra are analyzed. Gravity waves generated by thermal source have wide spectra in the earlier stage, then the relationship between sources" and excited waves' temporal and spatial scales are investigated, we find that the wavelengths/frequencies of the excited waves are dependent on the temporal and spatial scale of the thermal source; the vertical and temporal scales of the excited waves are more sensitive to the changes of thermal forcing than the horizontal scale of excited waves.3. The evolution and characteristics of gravity waves excited by a temporal and spatial Gaussian-varying thermal source, and their interactions with wind shear are studied. Small scales and low-frequency parts in the excited downwind waves are easily absorbed by the eastward jet, only the large scales and high-frequency parts can propagate through the jet, the maximum amplitude of downwind waves is smaller than that of upwind waves. The eastward jet is enhanced consequently, and the westward background in the mesosphere is enhanced because upwind waves can easily go through the jet and propagate to the mesosphere, this indicates that troposphere jet will influence the wind structure in the mesosphere.4. The characteristics of gravity waves at troposphere and lower stratosphere have been studied based on radiosonde observations over middle latitude U.S. station Del Rio (29.37°N,100.92°W) at1998-2007, wave sources are discussed. Both case and statistical studies indicate that jet is the main source of mid-latitude gravity waves.
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