||Precipitation in Taiwan is influenced by the north-easterly monsoon (September-April) during the cold season and the southwesterly monsoon (May-August) during the warm season. Mei-yu season (mid-May to mid-June) and late summer (mid-July to August) are two major rainy seasons over Taiwan. During Mei-Yu season, heavy rainfall evens are freguently observed in southwestern Taiwan.|
In this study, we investigate the characteristics of heavy rainfall in southwestern Taiwan from 1997 to 2004. The heavy rainfall events over southwestern Taiwan is defined as days in which more than 5 stations record hourly rainfall rate that exceeds 15 mm at least once and daily rainfall accumulation is greater than 130 mm. Besides, one or more rainfall stations record daily rainfall accumulation exceeding 200 mm. Based on rainfall data from about 360 Automatic and Mwtworological Telemetry System (ARMTS) around Taiwan, 13 cases are identified. The current study focuses on the heay rainfall event on 7 June 2003. Because this heavy rainfall event bears 88% heavy rainfall frequency over southwestern Taiwan. The frequency of daily rainfall amount between 130 and 200 mm over southwestern Taiwan is around 30.2%. The frequency of daily rainfall amount between 200 and 350 mm over southwestern Taiwan is around 55.7%. In addition, the orographic moisture flux over southwestern Taiwan is 16.8（m/s.g/kg）. This value is very large. In addition, ten minute time interval radar images over southwestern Taiwan are available.
The synoptic analysis derived from European Center for Medium-Range Weather Forecasts (EC) data indicates that a short trough at 300-hPa located at the area near Guangdong and Fujian (24°N、113°E) at the 300-hPa level at 2000LST on 6 June. This short trough enhanced the development of short wave trough at the 500-hPa level. Meanwhile, upper level short wave trough also strengthened the intensity of the low pressure at 850-hpa level and Low Level Jet (LLJ) as well. Due to the southwest flow, the equivalent potential temperature ( ) and relative humidity (RH) increased. Conseguently convective instability was enchanced.
At 0200LST on 7 June, the upper level jet at 300-hPa was located over the East China Sea. The position of Taiwan was near the right side of the jet enfrance region. The ascenting motion was over southern Taiwan Strait and northern South Chine Sea. It would be beneficial to the development of convection. The LLJ at 850-hPa level was over all the south part of Taiwan. A significant upward motion was between the axis of LLJ and the Mei-Yu front which was located at north part of Taiwan. At this time, the high and moist air were observed in southern Taiwan Strait. Besides, the surface low pressure center (1002 hPa) obtained from the EC data was near the coast of south east China (23.5°N、117°E). In the morning (0800 LST) on 7 June, the jet core at 300 hPa was located at Japan. The ascenting area was from the Taiwan Strait to east of Taiwan. The circulation of the low pressure at 850-hPa was over Taiwan. At this time, the position of the Mei-Yu front was near the north of Taiwan (about 24°N) and the surface low (1002 hPa) was in central Taiwan Strait near western coast of Taiwan. Significant upward motion existed between the axis of LLJ and the Mei-Yu front. The was excending 350 K and RH was greater than 90% in southwestern Taiwan. The ascenting motion from low to upper level helps to produce heavy rainfall.
Satellite images at 1300LST on 5 June show that cloud systems developed in the southwestern China (28°N、105°E), coincided with the low pressure at the 850 hPa level. At 0200LST on 6 June, the cloud system move southeastward to the area near 24°N、110°E where the low center at the 850-hPa level strengthened. Furthermore, the cloud system moved southeastward with the low center of the 850-hPa leve to the southwestern Taiwan at 0200LST on 7 June. At this time, the LLJ, the high and moist air arrived Taiwan. The upward area was also over southern Taiwan. At 0800LST on 7 June, the cloud system to the south of the low pressure in central Taiwan Strait moved to the southwest of Taiwan. From radar echo images, the precipitation system associated with the cloud system and the low pressure moved from southern Taiwan Straits eastwards to the southwest of Taiwan. The radar echo intensity was strengthened over sloped areas.
In order to investigate the relationship between the low level pressure over southeast China coast and the heavy rainfall over southwestern Taiwan, the present study employs the Weather Research and Forecasting Model (WRF) to examine the formation of the low pressure and the formation of heavy rainfall southwest Taiwan.
In the simulation, the middle latitudes trough at upper level and the westerly wind caused the production of the low pressure over east of the Tibeta plateau at 850 hPa. Later this low pressure was strengthened in Fujian and Guangdong. In the early morning the short wave trough at 300 and 500hPa occurred over southern China. These short waves produced
a new low pressure at the 700-hPa level, 850-hPa level, and the surface
over the southeast side of the existing low pressure at the southeastern
China coast. The latent heat from the cloud systems associated with the low center at 700-hPa level amplied the low pressure near surface. Precipitation system associated with the new low pressure moved to the southwest of Taiwan and produced heavy rain over the coastal area. As the precipitation system moved to the sloped areas, the intensity of heavy rainfall incressed due to LLJ and topographic effects.
The simulated rainfall pattern over the southwestern Taiwan was similar to that observed. The position and the magnitude of the simulated daily rainfall accumulation (382.68 mm, exceeding the definition of super heavy rainfall 350 mm/day) were also similar to that observed where the maximum daily rainfall accumulation (379.5 mm) was at MA-CHIA （22.685°N、120.679°E）. But, the simulated maximum hourly rainfall rate was 45.7 mm, only 64% of the observed value. The peak value of the simulated rainfall was one hour late than the observed value (at 0800LST on 7 June).
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