Influences of the mid-level vortex on the formation of Tropical Cyclone Toraji (2013)

This study analyzes the influences of the mid-level vortex on the formation of Tropical Cyclone Toraji (2013). A rare case of a tropical cyclone that formed near Taiwan involved a mid-level vortex that was a remnant of Tropical Cyclone Kong-Rey (2013). The piecewise potential vorticity inversion method is applied to examine the contribution of the mid-level vortex to the low-level wind field under quasi-balanced conditions. Numerical sensitivity experiments are conducted to quantify the importance of the mid-level vortex on Toraji formation, in which the mid-level vortex is removed with different removing factors (percentages) from the initial field. The results indicate that mid-level positive potential vorticity anomalies significantly contribute to the low-level positive vorticity before Toraji formation. Furthermore, when the removing factors increase in the sensitivity experiments, either the intensity of the simulated low-level vortex or the development trend of pre-Toraji decreases. However, there is no significant relationship between the convection's magnitude and the intensity of the mid-level vortex. The main difference comes from the mid-level vortex's intensity, which would result in a greater high-level warm core structure and cause stronger vertical mass flux. In summary, the mid-level vortex plays a critical role in the formation of Toraji. It provides a favorable environment for forming the pre-Toraji vortex by maintaining a high-level warm-core structure, leading to the formation of Toraji.