摘要(英) |
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. |
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