The influence of the island topography on tropical cyclone track deflection and looping motion

Tropical cyclones (TCs) are prone to be deflected when approaching high topography. In some cases, a looping motion may even occur that leads to great changes of wind and rainfall patterns associated with the TC just before its landfall. Therefore, the topography-induced TC track deflection is not only an interesting scientific topic but a very important TC forecasting issue. However, only few literatures have addressed in the unusual looping TC motion. To investigate this issue, in this research, a full-physical model with high resolution (3 km) is used to study the mechanisms leading to the looping motion by a real case, Typhoon Krosa (2007). The results show that the terrain height of Taiwan plays the most important role in Krosa's looping. The surface properties and the detailed shape of Taiwan topography, and model microphysics have slight or much minor impacts on the track prior to its landfall. Based on the sensitivity tests to the model and terrain data resolutions, we infer that model resolution may have more impact on the TC structure and movement, while the variation of terrain resolution is rather insensitive. In addition, to fully investigate how a vortex responds when encountering high topography, idealized experiments are conducted based on the full-physical model with high resolution (3 km) as well. Idealized vortices with similar structure to that of Krosa are embedded in a uniform basic flow. The simulations show that the more the vortex approaches (leaves) the northern (southern) topography, the more southward (northward) deflection occurs. This result is different from the findings of Yeh and Elsberry (1993) which was conducted by a low resolution (45 km) model. Furthermore, both the real-case and the idealized studies show evidences of the channeling effect, which was found to be crucial in leading to Haitaing's (2005) looping motion (Jian and Wu 2008).