Abstract
Studying dynamical mechanisms of perturbation growth in tropical cyclones is important from a perspective of designing ensemble prediction system and adaptive observations for tropical cyclones. In this thesis, the role of perturbations in ensemble forecasting and adaptive observations for tropical cyclones is investigated, especially focusing on tropical cyclone tracks. For this purpose, ensemble initial perturbations from operational numerical weather prediction centers are first diagnosed, followed by a study in which the structure and location of singular vectors computed for tropical cyclone-like vortices in a nondivergent barotropic framework are investigated. The three most significant findings of this study are that 1) perturbations grow in the vicinity of tropical cyclones through both baroclinic and barotropic energy conversion as seen in mid-latitude dynamics, that 2) those energy conversions lead to the modification in tracks of tropical cyclones, and that 3) the structure and the location of growing perturbations are sensitive to initial temperature and wind profiles of the tropical cyclone vortex. The above results (1) and (2) determine a mechanism how the ensemble spread of tropical cyclone tracks evolves with time in ensemble prediction systems at operational numerical weather prediction centers. In addition, those results identify the reason why the ensemble spread differs from one ensemble prediction system to another. The result (3) gives an insight into understanding what sensitivity analysis guidance targeted for tropical cyclones represents and helps to optimize observation network for tropical cyclones.