本文利用中央氣象局東部地區高時空解析度之氣象／自動雨量站網觀測資料、紅外線衛星雲圖、花蓮探空與雷達資料及 ECMWF0.5° × 0.5°經緯度網格資料，針對 1998 年 5 月 27 日發生於宜蘭縣與花蓮縣之超大豪雨（≧350 mmd-1）事件，進行綜觀形勢與中尺度分析，探討東部地區鋒後產生豪雨之機制，該事件為 1997-2003 年間梅雨季唯一於鋒後出現之超大豪雨個案，降雨強度大且分布範圍廣。
結果顯示，宜蘭與花蓮沿海地區之豪雨、大豪雨及超大豪雨，主要乃因臺灣東部近海發展之組織性中尺度對流系統西移影響而來。臺灣東部近海地區低對流層微弱暖平流與 850 hPa LLJ 之形成，提供上升運動以激發對流發展，並形成組織性中尺度對流系統，影響東部沿海地區產生超大豪雨事件。LLJ 之形成乃因地面與低對流層之南海低壓東移加深發展，且西太平洋高壓發展西伸，產生跨越等高線氣流並增強風速而來。由海上而來之地面與低對流層較強東風，可入侵東部地區內陸，可能乃因地形舉升作用激發對流發展與增強，產生中央山脈東側與北側之豪雨。熱力驅動之局部環流雖在夜間發展，但因受環境氣流與地形影響，白天海風發展受限，可能並未在本超大豪雨個案降水之時空分布扮演角色。

Observational data with high spatial and temporal resolution from the meteorological stations and automatic rainfall stations of the Central Weather Bureau in eastern Taiwan, infrared satellite images, the sounding and radar data of Hua-lian, and ECMWF 0.5°  0.5° grid data were used to study the extremely torrential rain event ( ≥ 350 mm d-1) occurring in Yi-lan and Hua-lian on May 27, 1998. Synoptic and mesoscale analyses were carried out to investigate the mechanism of post-frontal heavy rain occurrence in eastern Taiwan. This case was the only extremely torrential rain event that occurred during the Meiyu season from 1997 to 2003, with a significant intensity of precipitation and a wide coverage.
The results showed that the extremely heavy rain, torrential rain, and extremely torrential rain events in Yi-lan and Hua-lian coastal areas were mainly due to the westward moving of the organized mesoscale convective system, which developed near the coastal area in eastern Taiwan. Weak warm advection in the lower troposphere and the formation of the 850 h Pa LLJ offered the upward motion to trigger convection, and then developed to an organized mesoscale convective system, which influenced the coastal area in eastern Taiwan and resulted in the extremely torrential rain event. The formation of LLJ was because the cyclone at the surface and lower troposphere over the South Sea developed and moved eastward while the anticyclone over the west Pacific Ocean developed and extended westward, producing the cross contour ageostrophic flow and speeded up the wind. The stronger eastern wind of the surface and lower troposphere from the sea could invade the inner land of the eastern Taiwan. It possibly could trigger and enhance the convection due to the topographical lifting effect, and produced heavy rain at the eastern and northern sides of the Central Mountain Range. Though the local circulation driven by the thermal forcing developed at night, it was influenced by the environmental airflow and topography so that the development of the sea breeze was restricted, thus possibly did not play a role in the spatial and temporal distribution of this extremely torrential rain case.