||Taiwan is located over the subtropical western north Pacific Ocean, where there are about 20 typhoons every year. The rainfall brought by typhoons is very important to Taiwan, but it could lead to disasters if the rainfall is too much. In order to reduce the damages by typhoons, we need to study more about extreme-rainfall precipitation typhoon.|
In this study, we used the API(Abnormal Precipitation Index) to classify 81 typhoons (with the centers of typhoons passing the area of 119°E ~125°E and 21°N ~26°N) which invaded Taiwan from 1987 to 2010 as extreme-rainfall typhoons and normal typhoons. In the 81 typhoons, there are 12 extreme-rainfall typhoons with total API from every station above one standard deviation for each track. The typhoon-track classification defined by the Central Weather Bureau is used in this study. For every extreme-rainfall typhoon, we apply the 15-day low-pass filter on the NCEP/FNL 1°× 1° global analysis data to separate the typhoon circulation from the environment. We then use the 5-day composite analysis to examine the synoptic environmental differences between the extreme-rainfall and normal typhoons.
For the synoptic scale environment, the extreme-rainfall typhoons from each type of typhoon tracks have some apparent characteristics. 1) For upstream region of the extreme-rainfall cases, there are higher potential instabilities and the average winds at 850hPa over South China Sea are much closer to the southwesterly than the normal cases. 2) Although extreme-rainfall cases do not always have higher SSTs than normal cases over the South China Sea and eastern area of Philippine, SST is possibly not the necessary condition to extreme rainfall, but higher SST is favorable for typhoon to bring more water vapor from the ocean. 3) Higher low-level relative humidity is conducive to outer rainband’s development and expansion, such as typhoons in Type 1 track. For typhoons with tracks in Types 2, 6, 9, the extreme-rainfall cases have at least one region average higher than normal cases. 4) Almost all extreme-rainfall cases have lower relative humidity at 500hPa and lower relative vorticity at 850hPa over the upstream region. 5) Deep vertical shear and divergence do not have the apparent differences between the extreme-rainfall and normal typhoons.
For the typhoon scale, we can find the potential instability and low-level relative humidity that is higher in the extreme-rainfall cases are same as the synoptic-scale characteristics. And the relative humidity at 500hPa can be used to identify the water vapor’s transmission, like in the track type-6 there are higher relative humidity at the 3rd quadrant. For the type-2, the water vapor mainly transmit from the east-northern area, so the water vapor from the eastern ocean of Taiwan is also important to extreme rainfall that brought by typhoon. There are no apparent water vapor supply for the extreme-rainfall typhoon of type-4, so we can infer that the interaction between the topography and typhoon circulation is important to extreme rainfall case.
Because this study has focused on the synoptic-scale composite analysis, we didn’t discuss the dynamic mechanism. In the future, we will conduct more research on the interaction between extreme-rainfall typhoon and environment by the high resolution numerical model.
計畫成果完整報告，NSC 97-2625-M-008-014, 96頁。
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