Determining The Rainy Season Of The Meiyu And Examining The Anomalous Circulation Of The Rainfall

The Meiyu season is the summer time period during which rainfall is strong over the Yangtze-Huaihe River basins. The EID method, which is recently proposed for detecting the extreme precipitation events, can be used to determine the starting and ending of the Meiyu season. It contains an adjustable parameter a, which varies between 0 and 1. Through giving appropriate values to the parameter, we can find the strong rainfall period of different time scales. For the Meiyu in Nanjing, tests using the EID show that with a value of 0.10 for the parameter, the climatic Meiyu season can be well captured. It starts from June 19 and ends on July 12, and these are consistent with the conventional results. The EID method is also used to determine the strong rainfall period for each of the 62 years. We omit the 12 years, in which the rainfall period is very long or too short, and appears too early or too late. The average of the remaining 50 years indicates that the mean onset date of the strong rainfall is June 24 and the mean end date is July 14, which is also consistent with the conventional results.This study pays particular attention to the Nanjing summer rainfall in 1991,which precipitation was greater than normal and rainfall period lasted longer.Results show that: (1) Compared with the normal year, the atmospheric circulation fields had significant changes. Deeper East Asia trough and the unconspicuous block over the sea of Okhotsk led the cold air southward to the Yangtze-Huaihe River basins.And the Pacific subtropical high is stronger than usual,this circulation situation is favorable for the warm and humid air flow from the west part of subtropical high to the Yangtze-Huaihe River basins. (2) According to the analysis of atmospheric circulation situation, we can see that there are more abnormal water vapor transportation in Yangtze-Huaihe River basins in 1991.In this study, the relative importance of temperature and moisture is analyzed. (3) By defining the value of IT and Iq and analyzing the the relative importance of temperature, we found that the Yangtze-Huaihe River flood disaster was mainly attributed to more-than-nonnal water vapor .