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.