- 期刊
臺灣北部地區冬季劇烈天氣事件的診斷分析研究
A Diagnostic Study of Severe Weather Events over Northern Taiwan in Winter
摘要
本文利用中央氣象局地面觀測資料與探空資料、歐洲中期天氣預報中心網格資料以及日本氣象廳天氣圖，以探討1998年2月18～21日期間發生於台灣地區之豪（大）雨與冰雹事件之綜觀環境與中尺度對流系統之形成環境與演變。結果顯示，連續數天降雨期間之東北季風較一般冬季者微弱，華中至華南低層受低壓環流影響使南海北部、華南及台灣地區低層有明顯之西南氣流，使台灣附近低層溫度與濕度均較氣候値高，而西太平洋熱帶地區之地面和低對流層為高壓距平區，這些均為聖嬰年冬季東亞地區的環流特徵。不過，台灣附近500hPa以上的溫度與2月氣候值相近，造成底層暖濕而中高層冷乾的配置，使對流容易發展且能持續發展，致使1998年聖嬰年冬季台灣地區有較活躍之對流、較多的雷雨及較多劇烈天氣現象（例如冰雹）發生。此外，2月18～21日高對流層噴流條移進帶來之冷乾空氣疊加在低層暖濕之西南氣流上，造成強烈的溫度平流垂直差異，使大氣不穩定度增加，而18～19日台灣位於高對流層分流槽東，提供綜觀尺度的舉升運動，亦是有利對流之條件。20日上午台灣低層受西南風暖平流影響，環境之條件不穩度與對流不穩度均大。對流系統於中午在台灣西北部海面上陸續形成，各對流系統形成後往東移進陸地。低層西南氣流和對流系統之下衝流前緣在其南邊產生局部輻合，此種冷池前緣的舉升作用，導致新對流系統於舊對流系統之南邊生成，使降雨、落雷及冷池區域以及地面假冷鋒位置隨時間南栘。
並列摘要
This paper presents diagnostic analyses on the synoptic situations, as well as the environmental conditions for formation and evolution of the mesoscale convective systems (MCSs) that produced heavy rainfall and hail events over Taiwan during February 18-21,1998.Data employed included rawinsonde and surface observations of the Central Weather Bureau(CWB), gridded analyses from the European Center for Medium-Range weather Forecasts(ECMWF),and weather maps from the Japan Meteorological Agency(JMA).Results suggest that the East-Asian large-scale circulation during the 6-day period in question exhibited characteristics reminiscent to those during E1 Nino winters. These include 1)a weaker-than-normal northeasterly monsoon wind,2)stronger-than-normal southwesterly flows at low levels from South China Sea to southern China, leading to a warmer and wetter lower troposphere over Taiwan, and 3) high pressure anomaly at low levels over the tropical western Pacific. Conditions above 500hPa over Taiwan, however, were similar to climatology for February. Thus, the coupling of a warm and moist lower troposphere with cold and dry upper layers provided favorable conditions for more active and long-lasting convections and rare severe weather events like hails in Taiwan over the E1 Nino winter of 1998. The upper-tropospheric jet streak moved to a position directly over Taiwan on 18-21 February, and the cold and dry air overlaid with the warm and moist air at low levels, leading to strong differential advection in the vertical and enhanced atmospheric instability. The upper-level diffluent trough west of Taiwan also provided synoptic-scale uplifting and favorable condition for convection on 18-19 February. On 20 February, the atmosphere was both conditionally and convectively unstable in the morning, and MCSs formed offshore of northwestern Taiwan around noon than moved onshore. The downdraft of MCSs produced local convergence with the low-level southwesterly winds at its leading edge and cold pool provided extra forcing force for the formation of new convection. The new convective systems formed to the south of the old systems. Thus, areas with heavy precipitation and lightning, and locations of the pseudo cold front all moved southward with time.