摘要 一個地區降下異常豪雨時，洪水便有可能發生。而短時間內的驟降豪雨所導致的突發洪水，最是危險，2004年的911水災就是如此。其在北台灣及南台灣均造成災害，淹水情況尤其以大台北地區最為嚴重。從9月10日1300UTC至11日0000UTC，短短11小時期間，大台北地區的累積降雨普遍皆高達400毫米，但是中央氣象局的降雨量預報卻呈現嚴重低估的情形。因此本研究便想探討究竟是什麼因素導致台灣北部九一一豪雨的發生。 由中央氣象局的NFS模式觀測分析場資料顯示，在中南部降豪大雨的條件優於北部，並不支持台灣北部會有如此大的豪雨產生。但經由WRF模式的模擬結果中可以看到，除了在中南部有強烈的上升運動極值外，在北部也有強烈的上升運動極值存在，整個台灣也處在高濕度的狀態。將模擬結果經尺度分離後，海平面氣壓場在10日21Z看到一個中尺度低壓在台灣東北方組織發展，至11日03Z逐漸削弱向北移動，再對照850hPa風場，可知此中尺度低壓和位台灣西南方的低壓會造成氣流在北部地區有輻合產生。 故九一一豪雨的產生，推測是由於位於台灣東北及西南外海的低壓的氣旋式環流，在台北地區造成強烈輻合所導致的。此外還將利用渦度收支方程對低壓的發展與維持來討論。 Abstract The occurrence of torrential rain very often will cause flood. A flash flood, especially caused by sudden torrential rainfall, is extremely dangerous. Such a kind of flash flood happened on 11 September 2004 over the Taiwan island. It caused great damages in northern and southern areas, particularly in the Taipei area. From 1300UTC on 10 September to 0000UTC on 11 September, during an 11-hour period, the accumulated precipitation generally exceeded 400mm in the Taipei metropolitan area. However, the forecast of Central Weather Bureau (CWB) underestimated the accumulated rainfall very badly. So the purpose of this study is to figure out what atmospheric mechanisms caused this flood. The observational analyses of the CWB’s NFS model show that favorable conditions for a torrential rainfall to occur in central and southern Taiwan areas are superior to those in northern Taiwan. The analyses even don’t support such a severe heavy rainfall to happen in northern Taiwan. But the simulation results of the WRF model show that there is a strong upward motion over northern Taiwan in addition to central and southern Taiwan areas. The model simulation also shows that the whole Taiwan area is covered with high moisture content. Applying the scale-separation technique to the simulated results for twice, the mesoscale field shows a mesoscale low started to organize and develop to the northeast of Taiwan at 2100UTC on 10 September, and headed to north thereafter. Comparing with the 850hPa wind field, we can realize that the mesoscale low and a low pressure center located to the southeast of Taiwan caused flow convergence in northern Taiwan area. The mechanism that caused the 911 flash flood event in the Taipei area is suggested to be the strong convergence as a result of the cyclonic circulations of low centers located to the northeast and southeast of Taiwan. A vorticity budget analysis was further conducted to investigate the development and maintenance of the mesoscale low center.
