2012年6月11日22時(1400UTC)，台灣北部地區開始降下豪大雨，大雨持續至12 日8時，10小時累積雨量多處超過400毫米，超過中央氣象局訂定的超大豪雨門檻 24小時內累積350毫米，北部地區許多測站降雨紀錄創下歷史新高，更使得北部各地出現淹水災情。此研究使用高時間、高空間解析度的雷達資料，對於降水系統進行分析討論。 利用台灣北部三部雷達(五分山、桃園機場、中央大學)，將雷達觀測的徑向風放入 Liou et al. (2012) 多都卜勒三維風場合成方法，得到完整的三維風場，再將其 u,v,w 放入 Gal-Chen (1978) 熱力反演方法，得到空間中三維溫度與壓力擾動場。風場合成時間從11日2000 LST (1200 UTC) 至 2400 LST (1600 UTC) 每30分鐘合成一次，將這段時間分成三個時期。第一期系統在北部外海，以每小時15公里往台灣北部快速移動。此時系統特徵類似線狀MCS，並且將合成的三維風場進行熱力反演，成功的反演出壓力溫度擾動特徵類似 squall line。第二期系統更靠近台灣陸地，受到地形影響越明顯。登陸前，系統西南側有對流胞受到西南氣流影響，向東北移動與系統結合，此時也觀測到西部沿岸的 barrier jet 和高層風增強，為系統帶來更豐沛的暖濕空氣，並且加強系統強度。2130 LST (1330 UTC)，在系統上出現波狀結構，由剖面來看，在外海則是西南氣流遇到對流系統，就如氣流過山產生重力波的結構。第三期系統登陸台灣北部，強回波於台灣北部持續停滯，且其強度並無明顯減弱，故稱為停滯期。強勁的西南風在系統南邊出現低層輻合產生新生對流胞，其受西南氣流影響，被推入系統內維持系統強度，且往南新生對流胞的速度與西南氣流向北的分量平衡，往東的分量遇到地形，使得系統被侷限在北部地區，造成此次持續性的豪大雨。;On 11th June 2012, a Mei-Yu frontal rain band brought over 400mm rainfall to northern Taiwan within 8 hours. The numerical weather predictions were not able to forecast the movement of the rain band and the extreme rainfall. The leading edge was moving southeastward in a fast speed of 15 km hr-1. Once it reached the north coast, the movement of system was almost stationary and brought the most intense rainfall between 22 LST to 24 LST in northern Taiwan. Meanwhile the strong low level southwest wind which was called barrier LLJ was detected to the west coast of the island. (Li and Chen 1998) The purpose of this article is to utilize the Doppler radar network data near northern Taiwan to study the dynamical reasons for the evolution and movement of this torrential rain event. Through a variational multiple Doppler wind synthesis algorithm by Liou et al.2012, the three dimensional winds at different time stages are retrieved. The wind fields were further applied to the calculation of the perturbation pressure and temperature fields. The changes of the vertical motion and the new cell propagation were studied in detail. The interactions between the low level jet, the Mei-Yu front and the high terrain was also discussed. The fast motion of leading edge: the cold pool dynamics of the Mei-Yu front and the stronger convergence from the southwest Low level barrier Jet promoted the convection of the central part of the leading edge. The stationary movement of system on land: The propagation speed toward south was balanced by the southwest barrier jet. Hence the newly formed convection was pushed back to northeast and joined the old system. The interaction between the front, the terrain and the jet was clearly shown by the radar analyses. It can explain the movement and evolution of different stages of this system and the event of the extremely torrential rainfall in northern Taiwan.
