Kieu, Chanh QPart I of this dissertation is devoted to a theoretical study of tropical cyclones (TCs), in which a class of exact solutions is obtained. These solutions capture well many important dynamical aspects of the TC development. Major results include: A strong dependence of the TC growth rate on the vertical structure, i.e., the lower the level of the maximal tangential wind, the faster TCs will grow; A much faster TC growth rate inside the radius of the maximal wind than that outside; and The key dynamical roles of the secondary circulation in controlling the evolution and structures of TCs. In particular, the bottom-upward development of the cyclonic flow is demonstrated to be a consequence of the secondary circulation. The new analytical model provides a systematic way to construct the three-dimensional storm structures needed for initialization of TC models. An application of the new theory in deriving the pressure-wind relationship is also presented. In Part II, the genesis of Tropical Storm Eugene (2005) is studied, using a cloud-resolving, multiple-grid simulation with the Weather Research and Forecast (WRF) model. It is shown that the genesis of Eugene is a result of the merger of two mesovortices associated with the ITCZ breakdowns. The simulation captures well the vortex merger as well as Eugene's life-cycle developments. Some key findings include: The merger of mesoscale vortices is critical for the genesis of Eugene; The total potential vorticity associated with the merging vortices increases substantially during the merging phase as a result of the net internal dynamical forcing between the PV condensing and diabatic production and partly from the continuous PV fluxes from the ITCZ; and Cyclonic vorticity grows from the bottom upward during the merger due to deep convection caused by the low-level frictional convergence and latent heating. Without deep convection, little vorticity growth could result from the vortex merger.en-USDissertationAtmospheric SciencesAtmospheric Scienceshurricane theoryvortex theorytropical cyclonescyclogenesisvortex mergerpotential vorticity budget