北台灣夏季雹暴之雙都卜勒雷達分析

Summer Hailstorm Observed by Doppler Radars in Northern Taiwan

林品芳(Pin-Fang Lin) ； 周仲島(Ben Jong-Dao Jou) ； 游政谷(Cheng-Ku Yu)

《大氣科學》 31卷4期 (2003/12) Pp. 333-354

摘要

冰雹事件於副熱帶地區相當罕見，根據過去台灣地區38年（1961~1999）地面測站紀錄顯示，只有6次發生於夏季，其中在1999年8月28日至8月31日連續四日於台北地區午後降下冰雹。這些雷暴系統先在台北南方的斜坡山區形成，而後發展成具組織性之多胞對流系統，到達平地時降下冰雹。本文針對8月29日之雹暴個案，利用台灣北部中央氣象局五分山都卜勒雷達及民用航空局中正機場都卜勒雷達資料進行雙都卜勒風場反演，探討此雹暴之運動場及降水結構特徵。
綜觀環境顯示在台灣東方有一高層冷心低壓，伴隨高層較強東北風，具環境風切隨高度順轉之綜觀環境條件與美國中西大平原類似，有利劇烈風暴發展。除此之外，融解層較台灣地區相同季節之氣候值5200公尺低約350公尺。
回波資料顯示大於55dBZ 之回波值初始發生於高度5~6公里，於30分鐘之內先向上發展至高度10公里，然後往下延伸至近地面。回波拱腔（vault）及胚核簾（embryo curtain）特徵位於雹暴移動方向之前緣。由雙都卜勒風場分析得知，上衝流位於系統移動方向的前緣，此傾斜上衝流之最大垂直速度位於高度7公里處，達18 m (s上標-1)。藉由地面觀測資料指出，此上衝流是由局部環流之海風及因降水之下衝流配合地形所形成的下坡冷外流之輻合所維持。

關鍵字

雹暴 ； 都卜勒雷達 ； 回波拱腔 ； 胚核簾 ； 局部環流 ； 地形效應

並列摘要

Occurrence of hailstorm in subtropical oceanic region is region is unusual. According to the surface station record, in the past 38 years (1961~1999), there was only 6 hail events occurring in Taiwan in summer time. During 28-31 August 1999, hailstorms occurred in Taipei every afternoon. Thunderstorms over the sloping area south of Taipei organized into a multicell convective system and produced hails while reached foothill and plain area. In this study, the kinematic and precipitation structure of the hailstorm occurring on August 29 is examined by using data taken from the dual-Doppler radar network operated by Central Weather Bureau and Civil Aeronautic Administration of Taiwan.
The synoptic conditions showed a cold-core low existed on the east of Taiwan and associated with an upper level northeasterly jet streak. The veering of the wind with pronounced vertical wind shear suggested an environment similar to that of severe storms developing in west plain of US. In addition, the melting layer was about 350 meters (i.e. 4.8km) lower than the climatological value in the same season.
The reflectivity data showed echoes greater than 55 dBZ occurred first at 5-6 km height and penetrated upward to 10 km, and then developed downward to the ground within 30 minutes. The signature of the vault and embryo curtain were identified near the leading edge of the storm. From dual-Doppler wind analysis, a pronounced updraft was found at the leading edge of the storm. The tilting updraft possessed its maximum value of 18 m s-1 at 7 km height. With the help of the surface mesonet observations, near-surface dual-Doppler wind analysis indicated the leading-edge updraft was maintained by the convergence between the upslope sea breeze and the down slope precipitation cold outflow.

並列關鍵字

Hailstorm ； Doppler radar ； Vault ； Embryo curtain ； Sea breeze ； Terrain effect