| 摘要(英) |
During southeast monsoon, heavy rainfall ( hourly rainfall rate > 15mm and daily rainfall rate > 50mm ) occur frequently in Taiwan Mei-Yu season(mid-May to mid-June) the windward side of Central Mountain Range (CMR) in southwest of Taiwan. Over lee side of CMR in the eastern and southeast of Taiwan heavy rainfall are less than the windward side of CMR. The distribution of heavy rainfall was affected by orographic effect significantly (Chen C.-S. et. al 2007b).
The pass 30 (1977~2006) years rainfall data exhibited there were 17 years rainfall greater than the average of 30 years. Among these 17 years, there were 6 years the rainfall during late May (15-31 May) greater than early June (1-15 June) (81,83,84,86,88 and 01). However, there were 11 years the rainfall during early June greater than late May (77,79,85,90,93,95,97,98,03,05 and 06). In the years with above average rainfall, rainfall is large in early June. In early June of the last 10 years (97~06), the rainfall in 97, 98, 05 and 06 was significantly. But in 05, rainfall in southwest of Taiwan was particularly large and rainfall was relatively less in northern part of central Taiwan. This fact was differ from other 3 years (97,98 and 06). The difference of these 4 years is evident. There was a blocking high in Okhotsk in 97, 98 and 06, but the blocking high disappeared in 05. The stationary low near the Lake Baikal in 98 and 06, also disappeared in 05. The East Asia trough stretched southward from the center of stationary low to 30 N near north of Taiwan, favorably for the baroclinic systems influencing Taiwan. But in 05, the East Asia trough was to the north of 30 N and stretched south to east of the East China Sea. As a result the baroclinic systems hardly influenced Taiwan. In 97, 98 and 06, the west Pacific subtropical high stretched over Philippines to Indochina. The environment was unfavorable for the development of tropic systems in north South China Sea. In addition, the trough in the north South China Sea was significantly in 05, but disappeared in the other 3 years. It show that the rainfall in 05 influenced by the weather system in South China Sea. Heavy rainfall day (at the least 5 stations the daily rainfall greater then 130mm, and at the least 1 station the daily rainfall greater then 200mm) in southwestern Taiwan occurred during 12~15 June 2005. The total rainfall of these 4 days contributed more than a half of rainfall accumulation of Mei-Yu season in 2005. Characteristic of the precipitation had a pair of rainfall peaks happened in 08LST and 15LST in southwestern Taiwan on 12 June. This is differ from other three days(13~15). In 13 June, there was a rainfall peak in 08LST. Afternoon rainfall peak was no significantly. In 14 June, the two rainfall peaks were in 06LST and 21LST. In 15 June, the characteristic of rainfall was similar to 12 June. The rainfall peaked in 05LST and 15LST, but the second peak was higher than in early morning. The number of station with daily rainfall accumulation exceeding 350mm on the 12 June was higher than other three days. Therefore, in this study we investigate the heavy rainfall case on 12 June 2005. Through the analysis 05 the European Center for Medium-Range Weather Forecasts (EC) data, the analysis Chart of Japan Meteorological Agency (JMA), Satellite images and Outgoing Longwave Radiation data, we found a short-wave trough formed over the northeast of South China Sea on 8 June. Moving northeastward, and caused heavy rainfall in southwest Taiwan. During 12~15 June 2005, Mei-Yu front was located on the offshore of north Taiwan. The heavy rainfall event cause by mesoscale convective systems from the South China Sea.
In this study, there were intense radar echoes formed along the coast in the early morning. There may have convergence in the coast of southwestern Taiwan by southeast wind near surface of southwestern Taiwan and the prevailing southwest wind on the ocean. In 925hPa, because the prevailing southwest wind decelerated and became southerly wind cause by terrain blocking. The convergence between prevailing wind and southerly wind occurred over southwestern Taiwan. Rainfall fell in coast and plane. The peak rainfall moved from coast to slope in the southwest of Taiwan. In order to study the characteristic of double rainfall peak and advance our understanding of the mechanism of heavy rainfall in coast and slope, we use the Weather Research and Forecasting Model (WRF) to serve our objectives.
The model results show that intense coastal convergence was due to the prevailing wind and offshore flow which formed by the decrease of virtual potential temperature. This decrease was due to rainfall in slope and plane in early morning. In the plane, southwest wind converged with the south wind due to orographic effect. Both of the two factors enhanced the rainfall in coast and plane. In the afternoon the convergence in coast wasn’t evident. The rainfall become intense when the convective system moved into southwest of Taiwan. Although the mechanism for rainfall of model results can explain rainfall characteristics, but the magnitude(350mm/day) was less than the observation(508mm/day). In the model results, the convective system peaked in 05 and 12LST, 2~3 hours earlier than observation became of the rapid movement of short wave trough in the simulation results than observed. In addition, no Taiwan terrain test exhibit the orographic effect is important to influence coastal and plane’s rainfall due to the prevailing wind decrease become south wind convergence with prevailing wind. |
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