2002年6 月13 日在梅雨鋒前強西南氣流影響下，苗栗至新竹沿海一帶形成之多胞雷雨，最後組織成東北東-西南西走向之線狀對流系統，該對流系統於0600～1700 LST間在台灣西北部新竹至苗栗沿海陸地降下豪雨，最大降雨量（190 mm）出現在新竹湖口地區。本文即針對此梅雨季豪雨個案，分析綜觀天氣形勢、地面與探空觀測資料、自動雨量站逐時觀測及都卜勒雷達資料，以探討對流系統發展之綜觀環境與伴隨之中尺度現象。12日1200 UTC（即2000 LST）之板橋探空風場顯示，風向由低對流層之西南風隨高度順轉至500hPa 層之西風，垂直風切向量亦呈現順轉特徵，700 hPa 層台灣北部地區有一低層噴流，使低對流層具有顯著垂直風切（6.7 ×10-3s-1），有利扭轉過程於中低對流層形成氣旋式渦旋。北台灣稍弱的CAPE 值僅約798 m-2s-2，但因中低對流層具有適中垂直風切，使Ric值約僅13，接近有利超大胞（supercell）雷雨發展區域。第一波對流降水區係由數個孤立對流胞組成之東北東－西南西走向線狀對流系統而來，其強度和移動與對流系統南側風切輻合帶密切關連。分析結果顯示，環境暖濕西南氣流與對流下衝流降至近地面層所形成之輻散西北冷外流，以及低對流層強西南氣流遭遇雪山山脈阻擋部分順河谷而下轉吹之東南風迴流，三支屬性不同氣流於竹苗沿海陸地匯聚，形成一條東北東－西南南走向之風切輻合帶，有利新對流胞在舊胞西南側形成發展。因此，雖然個別對流胞向東北東移動，但降水中心則往南移動。第二波對流降水主要發生於新竹湖口地區，且呈現滯留狀態。雷達資料顯示，在強烈對流胞區由對流下衝流下降至近地面層所造成之顯著輻散外流中吹向其西南方之冷外流部分，與強西南氣流匯聚產生近地面層輻合帶，有利此後造型對流系統之新對流胞在其西南側發展。當西南氣流與輻合帶強度減弱時，對流系統亦迅速減弱。此外，在對流系統裡1～5 公里高度有一中尺度氣旋式渦旋發展，並於4 公里高度達最強，此可能乃因在具有顯著低層垂直風切與強烈對流上衝流條件下，透過扭轉過程而來。渦旋之增強與中層輻合密切關連，因此當渦旋脫離對流核心區後，在缺乏強對流支持下，迅速減弱。

Under the influence of the strong southwesterly flow in front of the Meiyu front, multi-cellthunderstorms formed along the coastal region of northwestern Taiwan and finally organized into aneast-northeast–west-southwest linear convective system, which produced heavy rainfall on the land along thecoastal region from Miao-li to Hsin-chu at 0600–1700 LST on June 13, 2002. The maximum precipitation of190 mm occurred at Hu-kou, Hsin-chu. This paper aims at this heavy rainfall case of the Meiyu season,analyzing the synoptic situation, surface and sounding data, hourly rainfall and Doppler radar data, toinvestigate the synoptic environment for the convective system to develop and its accompanying mesoscalephenomena.The wind field of Ban-chiao rawinsonde station on 1200 UTC, June 12 showed that the wind directionveered with the height from southwest at low troposphere to west at 500 h Pa, and the vertical wind-shearvector also showed the characteristic of veering. There was a low-level jet in northern Taiwan at the level of700 h Pa, thus an obvious vertical wind-shear (6.7 ×10-3s-1) at low troposphere, which favored the twistingprocess’ forming a cyclonic vortex at the middle-and-low troposphere. The weaker CAPE in northern Taiwanhad a value of only 798 m-2s-2, but with the appropriate vertical wind-shear at the middle-and-low troposphere,the value of Ric was merely about 13, which approached the area favorable for the development of a supercellthunderstorm.The area of the first phase of convective precipitation resulted from a east-northeast–west-southwestlinear convective system, which was composed of several isolated convective cells. Its strength andmovement had a close relationship with the wind-shear convergence zone to the immediate south of theconvective system. The results showed that three airflows with different characteristics—warm and wetenvironmental southwestern airflow, divergent northwestern cold outflow resulting from convectivedowndraft when lowering to near-surface layer, and southeastern returning flow along the river valley turnedfrom part of the strong southwestern airflow of low troposphere when encountering the block by the SnowMountains—joined together at the coastal land in Miao-li and Hsin-chu and formed an east-northeast–south-southwest wind-shear convergence zone, which favored the formation and development of newconvective cells to the southwest of the old cells. Therefore, though individual convective cells moved towardeast-northeast, the center of precipitation moved toward south.The second phase of the convective precipitation occurred mainly at Hu-kou, Hsin-chu and remainedstationary. The radar data showed that part of the cold outflow that blew to the southwest out of the obviousdivergent outflow resulting from the convective downdraft in the region of strong convective cells whenlowering to near-surface layer joined together with strong southwest airflow and resulted in a near-surfaceconvergence zone, which favored the development of the new convective cells to the southwest side of thisback-building-type convective system. When the southwest airflow and the strength of the convergence zoneweakened, the convective system weakened rapidly, too. Besides, in the convective system, there was amesoscale cyclonic vortex developing at the height of 1-5 km, and reached strongest at 4 km high. It waspossibly through the twisting process with the conditions of obvious low-level vertical wind-shear and strongconvective updraft. The strengthening of the vortex had a close relationship with the middle-levelconvergence, therefore, after the vortex left the core region of the convection, it weakened rapidly for the lackof the support of strong convection.