A study on the Orographic Effects on the Movement of Typhoon Haitang (2005) in East of Taiwan

Many of the studies (e.g., Wang 1980; Chang 1982; Bender et al. 1987; Yeh 1993; Lin 2002; and others) shown the Central Mountain Range in Taiwan can significantly affect the motion and the structure of an approaching typhoon. The influences include changing the typhoon moving speed and deflecting the system from its original track. In some favorable conditions, one of the secondary vortices induced may enhance to replace the original typhoon center and results a discontinuous track for the typhoon to pass Taiwan. Among those interesting track changes under the effect of high mountains, Wang et al. (1998) collected two cases of westward moving typhoons (i.e., Mary in 1965 and Polly in 1993) of that centers had made a cyclonic loop in east of Taiwan prior to the landfall. Whether those loops were caused by the Taiwan orography had not been studied before. Similar phenomenon of the center looping prior to the landfall was revealed from radar imageries when typhoon Haitang approached Taiwan in 2005. From the numerical model simulations we found that the center looping is related to the orographic effect of Taiwan. When the mountains were included in the numerical model, similar track was simulated as the observation. However, when the terrain was removed, the simulated track of the vortex became to move westward without making the loop. Though the piecewise potential vorticity inversion (Davis1992; Wu et al.2004; and others) we were able to identify that the major factor contribute to the eastward motion of the vortex during the loop is not the environment but the vortex itself, particularly in the lower levels. Through more detail analysis, of using the deviation fields, we found that the vortex is modified when it approaches more close to the terrain. The deviation fields were defined as the differences between the fields with terrain effects included and those of without terrain effects. It shows a cyclonic perturbation vortex can be induced, in the leeside of the Central Mountain Range, Taiwan when a typhoon approaches. The perturbation vortex then sheds off from the terrain and interacts with the typhoon. When the perturbation vortex moves cyclonically from south of Taiwan, east of Taiwan to north of Taiwan, the typhoon moves westward, southward and eastward to complete a loop. The result of the interaction between the terrain induced vortex and the approaching typhoon is in a way similar to the binary interaction of two vortices.