An Observational Study of the Extremely Heavy Rain Event in Northern Vietnam during 30 October-1 November 2008

Abstract

An extraordinarily heavy rain event occurred in northern Vietnam from 30 October to 1 November 2008. The three consecutive days of extremely heavy rain resulted in the worst flooding event in Hanoi, the capital of Vietnam, and its environs in 24 years. Results from analysis of the Japan Meteorological Agency Climate Data Assimilation System (JCDAS) re-analysis data show that a synoptic-scale tropical wave disturbance formed over the South China Sea and traveled northwestward over the eastern coast of the Indochinese Peninsula, brought the extremely heavy rainfall to northern Vietnam. In the mid-latitudes, a belt of surface high extended southeastward from western Mongolia to the East China Sea, causing a persistent northeasterly monsoonal flow on its south-eastern edge along the southern coast of China. The northeasterly flow worked together with the tropical disturbance, creating a strong low-level wind convergence in northern Vietnam. Examination of the structure of the tropical disturbance revealed that the meridional wind structure and cyclonic vorticity of the disturbance extended vertically to approximately 300 hPa, with the maximum fluctuations occurring at the 800-600 hPa layer and shows nearly vertical with height. A remarkable feature of the atmosphere is the large increases in the equivalent potential temperature in the upper troposphere higher than about 500 hPa, with strong southerly winds greater than 15 m s^-1 in the 300-150 hPa layers for the three days in question. In conclusion, the tropical disturbance, which worked together with the persistent Asian winter monsoon, caused the extreme rainfall event.