A preliminary study of the floodcausing rainstorm during the first rainy season in South China in 2008

The characteristics and causes of the floodcausing rainstorm occurred in south China during the first rainy season in 2008 were preliminarily studied in this paper. Additionally, the circulation field of the heavy rainfalls during the first rainy season in 2008 was compared with that of heavy rainfalls occurred in South China during the same period in recent years (such as 1994, 1998, 2005).The conclusions were as follows: (1) According to the influencing systems and the distribution of rain area, the floodcausing rainstorm occurred in South China during the first rainy season in 2008 were divided into four stages. The distribution of precipitation centers in the first stage (May 26 to 30) was scattered. During the second stage (June 6 to 11), the rainfall is in the form of band with several precipitation centers located seperately on the rainband. The rain area of the third stage (June 12 to 14) presented a sheet shape. During the fourth stage, the rain areas were either banded or sheet. (2) The analysis of the averaged circulation field in the middle and lower atmosphere for the four stages indicated that the difference characteristics of circulation field shaped the differences of rain area distribution. (3） The frequent action of the South China Sea monsoon was beneficial to the transportation of moisture vapor to South China, which supplied abundant moisture vapor for heavy rainfalls. (4) During the first rainy season in South China in 2008, there was negative anomaly of geopotential height at 500 hPa in North China, East China, South China and the Bay of Bengal. The negative anomaly in North China, East China and the eastern Tibetan Platean was favorable to cold air activities, but the negative anomaly in the Bay of Bengal was favorable to warm air activities. Both the clod air and the warm air were active during this rainy season, which contributed to the sustainable heavy rainfall in the south part of China. (5) The comparison of the total precipitation from May 15 to June 30 among 2008, 2005, 1998 and 1994 with the climate state indicated that the total precipitation center of 2008 was located at the junctional zone between the Province of Zhejiang, Jiangxi and Anhui. But this precipitation center was more northward than the climate state. And the study showed that the total precipitation of 2008 was the highest. (6) The comparison of circulation field at 500 hPa presented that the circulation field of 2008 was more beneficial to the interaction between cold and warm air in the southern part of China than the other years, leading to the strongest heavy rainfalls among the four years. (7) The comparison of anomaly wind field presented that the anomaly south wind component of 2008 covered a more northward area than the other years, leading to the more northward location of the precipitation center.