| Summary: | 碩士 === 國立臺灣海洋大學 === 海洋環境資訊系 === 104 === The typhoon strength scales used by the Center Weather Bureau is based solely on the wind speed. However, this intensity scale may not be sufficient in describing typhoon impact on the Taiwan coasts. This study analyzed observed data from the Taiwan coasts to define typhoon impact, according to extreme climate factors. This new strength definition can help prevent the destruction of coastal structure and coastal disaster.
Copula function and Compound Extreme Value Distribution (CEVD) were used in this study to calculate the joint probability of significant wave height and water level. Comparing with the traditional method, the copula function can quantify the correlation between different variables. On the other hand, the Compound Extreme Value Distribution solves the problem of shortage in samples by the use of the typhoon occurrence frequency. This study purposed a new definition for the destructive time period by typhoons for a coast with considerations of background wave height and swell simultaneously.
This study employed Typhoon Extreme Value Culling Method (TEVC) and Wave Height Threshold Culling Method (WHTC) to select data for our analysis. The data selected by TEVC needed to be multiplied by simulations due to the lack of samples. The joint probabilities of wave height and water level obtained by the TEVC as well as WHTC are very close. It was found that the result of WHTC is more sensitive to the threshold of wave height. The return period calculated by CEVD was smaller than copula, while the return period calculated by CEVD is similar to that of the traditional method. This study showed that CEVD is more suitable than WHTC.
Lastly, through the hydrologic conditions, we classified and defined typhoon strength into four types: Slight (SL), Moderate Impact (MO), Severe Impact (SE) and Destructive Impact (DI). This new definition can reflect the sea-state during the typhoon period, and much suitable to describe the impact on the coast. In the past 15 years, only 10% of typhoons had the moderate impact strength and 50% of them CWB have issued typhoon warnings. There were 2% of the typhoon which CWB did not issued any warnings, because they were still far away from Taiwan, but their swells were strong enough to cause severe impacts equivalent to typhoons with a 50 years return period. The results showed our method can reasonably determines how severe the impact of the typhoon is, and it can also be used to predict impact strength before the arrival of typhoons.
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