| Summary: | 碩士 === 國立臺灣大學 === 大氣科學研究所 === 106 === This study uses long-term surface and radar observations to analyze the relationship between the precipitation and terrain features as typhoons influence the Taiwan area. In the typhoon environment, the precipitation enhancement over mountains through upslope lifting is well known. However, experience indicates that typhoon-induced heavy precipitation sometimes can be concentrated on the plain areas or the valley regions, which cannot be simply explained by the upslope forcing. The particular aim of this study is not only to investigate the frequency of occurrence for these two types of typhoon precipitation but also to explore knowledge regarding the spatial variability of typhoon precipitation and its possible mechanisms over the complex terrain.
Statistical analyses from 95 typhoons during 1997-2016 show that there are 76.8 (20)% of the typhoon cases whose intense precipitation occurs primarily over mountainous (plain) areas and 3.2% of the typhoon cases have no obvious difference in precipitation intensity between mountainous and plain areas. For cases with precipitation concentrate on the plain area, they are often associated with weak or moderate typhoons. In addition, the precipitation intensity is more sensitive to slopes of lower terrain.
On the other hand, this study investigates the distribution of precipitation intensity over southwestern Taiwan for 50 typhoon cases during 1997~2016. Statistical analysis indicates intense precipitation is concentrated on the valley regions, with more than a half of typhoon cases whose precipitation occurrs primarily over the valley region. Especially, low level upstream winds associate with valley enhancement cases are often characterized by southwesterlies. As an extreme example, Typhoon Morakot (2009), the valley enhancement of precipitation occur as upstream wind directions are more parallel to the valley and there are strong updrafts and convergence along the valley. According to the correlation coefficient analysis, the enhanced precipitation observed is closely related to the convergence and updrafts within the valley but is less related to upslope lifting or seeder-feeder mechanisms.
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