A Study on the Extratropical Transition of Typhoon Xangsane (2000)

This paper examined the extratropical transition (ET) of Typhoon Xangsane (2000) when it moved north-northeastward along the east coast of Taiwan from the Bashi Channel to the East China Sea during October 31 - November 1. During this period, the orographic lifting of typhoon circulation brought a total of 1,054 mm rainfall to eastern Taiwan. The interactions of Xangsane and a baroclinic system also brought a total of 984 mm rainfall to northern Taiwan. The torrential rain caused severe flooding and debris flows around the island leading to 64 death. Observations showed that Xangsane maintained a well-defined typhoon structure at 1200UTC 31 October, when it was in the Bashi Channel. However, Xangsane lost its definition as a tropical cyclone 24 hours later when it located in the East China Sea. Focus of this study is placed on the structure changes during this time period and the physical mechanism responsible for the changes.

The NCAR/PSU MM5 was used to simulate the structure changes of Xangsane during the ET process. Results showed that the model can simulate reasonably well the synoptic and mesoscale changes in the circulation patterns, as well as the increase in the vertical wind shear. The track, intensity and cloud pattern of Xangsane were also well simulated. In addition, the development of a meso-vortex located at the intersection of a typhoon rainband and the low-level baroclinic zone, as revealed by the Doppler radial wind, was also well simulated. Analyses of observations and the model results showed that although Xangsane experienced the ET process at lower latitudes, it had the common features of ET described by Klein et al. (2000). A down-shear tilt of the vorticity center occurred during the ET process. Analyzing the air parcel trajectories with respected to the moving typhoon center showed that the air parcels traveled cyclonically around the system center. The air rose on the down-shear side and subsided on the up-shear side leading to a warm core at the mid-lower levels behind the vorticity center.

Analyses also showed that when the vorticity center started to tilt toward the down-shear side, the maximum pressure perturbation (2.5 hPa) occurred near the surface and in front of the system (vorticity) center, whereas the minimum pressure perturbation (0.2 hPa) was at the vorticity center and near the surface. At the same time, a perturbation pressure maximum occurred at altitudes of about 2-4 km and behind the vorticity center. This perturbation pressure maximum, which likely induced by the forced subsidence, moved along the shear direction (closer to the system center) and to a higher altitude as the tilting of the center increased. The vertical momentum budget analysis depicted that this perturbation pressure maximum played a crucial role on the change of the thermal structure during the ET processes. The pressure gradient acceleration above the maximum perturbation pressure tended to enhance the upward motion and help weakening the upper level warm core. On the other hand, the downward pressure acceleration helped building the low-level warm core.