台灣北部伴隨冰雹之春季颮線個案研究

Case Study of a Spring Squall Line Accompanied by Hailstorm Occurrence in Northern Taiwan

陳泰然(George Tai-Jen Chen) ； 周鴻祺(Hung-Chi Chou) ； 楊進賢(Jinn-Shang Yang) ； 陳熙揚(Shi-Yang Chen)

《大氣科學》 27卷3期 (1999/09) Pp. 257-271

摘要

1998年春、夏季在台灣地區出現多起罕見的冰雹或龍捲風等劇烈天氣事件，其中4月12日下午兩點左右，在北二高大溪－龍潭路段出現冰雹，當日北部地區亦出現豪大雨現象。本文即針對伴隨此次冰雹／豪雨事件之春季颮線，分析綜觀天氣圖、地面測站資料、探空資料、自動雨量站的逐時雨量資料及單都卜勒雷達資料，以了解其綜觀環境和中尺度過程。綜觀形勢顯示，12日0000 UTC鋒面位於華南，台灣受太平洋高壓邊緣微弱南南東風影響，CAPE大且中低對流層垂直風切顯著，午後颮線於台灣西北部陸地和北部外海低層氣流輻合區形成。雷達降水回波圖顯示，颮線呈北北東－南南西走向，最大長度可達200公里，形成類型屬於破線型。海陸和平地與地形之加熱差異，產生海風及／或上坡風，在颮線形成與發展過程中伴演重要角色。隨後對流降水產生的下衝流冷空氣外流，與海風和盛行西南風在颮線西南側產生輻合，使新對流胞不斷地在盛行氣流上游發展，颮線因而由破線型轉變為後造型，使其生命期得以維持5小時之久。颮線發展期在其南端發展一個強度可達60 dBz之強烈對流區，該區回波出現凹狀結構並有中尺度氣旋伴隨，此對流區產生降雹之劇烈天氣。此外，颮線系統內之對流，呈多單體對流胞的結構特徵。

關鍵字

颮線 ； 海風 ； 上坡風 ； 輻合線 ； 破線型 ； 後造型

並列摘要

A case of spring squall line accompanied by severe weather of heavy rain and hail occurrence on 12 April 1998 over northern Taiwan was selected in this study. Investigations were carried out by using synoptic weather maps, surface observations, radiosonde data, automatic raingauge data, and Doppler radar observations to understand the environmental conditions and mesoscale processes associated with the event.Synoptic situation indicated that a front was located over southern China and Taiwan was influenced by the southwesterlies ahead of the front and the south-southeasterlies of the Pacific high pressure. The afternoon squall line formed in the low-level convergent zone over northern Taiwan and the adjacent ocean under a strong environmental vertical wind shear and large CAPE. Radar echoes indicated that a broken-line type squall line was oriented in a north-northeast and south-southwest direction with a maximum length of 200 km. The sea breeze and upslope wind driven by the differential heating over land/sea and plain/slope played an important role in triggering and maintaining the squall line system.After the convective rainfall occurrence, the cold air outflow associated with downdraft encountered the sea breeze and prevailing southwesterlies over the southwest side of the squall line to produce convergence. This local convergence was responsible for the new convection to develop over the area. Thus, the squall line of the broken-line type was transformed to the back- building type with a lifetime of 5 hours. This is an ordinary-type squall line with multicell structure. Radar echoes also indicated that hail was produced by intense convection with a radar echo of 60 dBz which developed over the southern part of the squall line at its intensifying stage. This convective area had a concave echo pattern accompanied by a mesocyclone.

並列關鍵字

Squall line ； Sea breeze ； Upslope wind ； Convergent line ； Broken-line type ； Back-building type