抄録
In this study, the impacts of Typhoon Morakot (2009)'s vortex structure on the extreme rainfall in Taiwan are investigated through an application of piecewise potential vorticity (PV) inversion. The control (CTL) experiment, starting at 0000 UTC 7 August or 15 h before landfall, reproduces the event realistically and is validated against the observations. By altering the PV perturbation inside 750 km from its center, we conduct sensitivity experiments in which the size and/or circulation strength of Morakot is reduced/weakened in the initial field in several different ways.
In the sensitivity tests, particularly those where the initial PV within the inner core (≤ 250 km) is significantly weakened, the storm makes landfall earlier, stays over land longer, and exits Taiwan later. Such track changes are accompanied by a contraction and spin-up of the inner core at 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 receives an overall rainfall amount either comparable to, or even more than (up to +12 %), CTL in all tests. Thus, a weaker 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 were 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 are considerably different than those in CTL, indicating that the vortex structure plays an important role in the rainfall of this region.
