A Study on the Synoptic-Dynamical Characteristics of Compact Tropical Cyclones in the Western North Pacific

This study focuses on the synoptic and dynamical characteristic of compact and incompact tropical cyclones (TCs) in the western North Pacific. In order to identify the distinct mechanisms related to the development and maintenance of these two categories of TCs, the Weather Research and Forecast model is used to simulate the compact Typhoon Yutu (2007) and the incompact Typhoon Manyi (2007). Simulation results of Yutu show that the wind speed increases primarily in the inner-core region, where strong relative vorticity and high inertial stability is also located. Comparatively, the inertial stability for Typhoon Manyi is much weaker, which makes it more susceptible to influences from external low-level forcing. Diagnoses of the numerical simulations as well as examination of the synoptic environments that embed the two TCs suggest that compact TCs mainly develop through internal dynamics, whereas that of incompact TCs usually driven by external forcing. Several sets of sensitivity experiments are designed to determine the relative roles of initial vortex structure, environmental flow and humidity in subsequent TC structural evolution. Results show that in environment that favors compact TCs, initial vortex largely determines the later structural development. On the other hand, vortex development is quite sensitive to its initial intensity and radius of maximum wind under environmental flow that more favors incompact TCs. Results of the experiments on environmental humidity show that a humid environment generates large vortex structural changes in incompact TCs. A relatively dry environment brings minimal impacts to the originally compact TC, but can increase the compactness of the originally incompact ones.