In this study, the impacts of Typhoon Morakot's (2009) vortex structure on the extreme rainfall in Taiwan were investigated using piecewise potential vorticity (PV) inversion. The control (CTL) experiment, starting at 0000 UTC 7 August or 15 h before landfall, reproduced the event realistically and was validated against the observations. By altering the PV perturbation inside 750 km from its center, we conducted sensitivity experiments in which the size and circulation strength of TY Morakot were reduced/weakened in the initial field in several different ways.

In the sensitivity tests, particularly those in which the initial PV within the inner core (≤ 250 km) was significantly weakened, the storm made landfall earlier, stayed over land longer, and exited Taiwan later. Such track changes were accompanied by a contraction and spin-up of the inner core at the early stages of the integration, caused by convection/latent heating within the inner core under large-scale, low-level southwesterly flow. As a result, Taiwan received an overall rainfall amount either comparable to or even more (up to 12 %) than that of the CTL in all tests. Thus, a weaker TY Morakot does not necessarily lead to less total rainfall over Taiwan, and the strong southwesterly flow and its moisture supply were bigger factors than the vortex structure in this event.

On the other hand, the rainfall in the southern Central Mountain Range on 8 August, which was the most-rainy area and period in reality, tended to decrease by up to 40 % with the contraction and a weaker outer circulation. Thus, the rainfall patterns and evolution in the sensitivity tests were considerably different from those in CTL, indicating that the vortex structure plays a significant role in the rainfall in this region.