| Summary: | 碩士 === 國立中央大學 === 大氣物理研究所 === 99 === To isolate the orographic effects over Lan-Yang plain and its surrounding areas of northeastern Taiwan due to the northeasterly prevailing wind leading to produce heavy rainfall in Autumn, 14 extremely heavy rainfall (>200 mm day-1) events over northeastern Taiwan are identified when no synoptic disturbances are within 500 km over Taiwan during the period from1995 to 2009. Among there 14 events the maximum accumulated rainfall was 631.5 mm which appeared on October 11, 2009. The mechanism of generating such a very high accumulated rainfall in Lan-Yang plain will be pursued in this study.
The objective of this study is to perform observation analyses of the rainfall data of Central Weather Bureau(CWB) rainfall station, WU-FEN-SHAN(WFS) 6-minute interval radar reflectivity, satellite image and ECMWF/ TOGA ( European Centre for Medium -Range Weather Forecasts/Tropical Ocean and Global Atmosphere) data to examine the heavy rainfall mechanism on October 11 2009. A lots of water vapor flux transported westward before 14 LST to the upstream of northeast Taiwan (at 125 E, 25 N) . The low-level conveged and high-level diverged over northeast Taiwan,also provided a favorable environment for producing and maintaining the heavy rainfall event. Besides, the radar data indicated that northeast rainfall mostly occurred over the southen slopes of Lan-Yang plain and enhanced by east-west oriented rain band from southeast of Taiwan. Daily rainfall maximum 631.5 mm also occurred on southward slope of LY plain and the accumulated rainfall over westward slope of Lan-Yang plain was 15% to that of southward slope.
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Weather Research & Forecasting model (version 2.2.1) is employed to simulate the heavy rainfall event which is initialized on 14 LST 10 October 2009. The mechanism for the development and maintance of heavy rainfall is that the orographic lifting moist air by prevailing easterly wind persistently on the southern slope of Lan-Yang plain. In addition, the low level northeasterly wind due to orographic deflection on prevailing wind over southern Lan-Yang plain also enhances rainfall. Rainfall moves westward following with the prevailing wind, but also moves upstream because of the cool pool which was caused by continuous rainfall on southern slope of Lan-Yang plain and local high pressure disturbance. Over western slope of lan-Yang plain, the orientation of terrain parallels to the prevailing wind. As a result, rainfall accumulation is about 30 % of that in the southern slope of Lan-Yang plain. For the noTaiwan terrain(NT_run) case, there was also an east-west oriented rain band over the adjacent sea of southeast Taiwan. Without the orographic lifting, rainfall is reduced at least one order in Ilan and Suao area. In the filled Lan-Yang plain case in which terrain height lower than 500 mm parallels to coast of Lan-Yang plain. Less rainfall occurred because of the reducing lifting and water vapor from Lan-Yang plain. It is quite obvious that moist air lifted by southern slope of Lan-Yang plain has great contribution to heavy rainfall.
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