Low Frequency Characteristics And Cause Analysis Of Strong Rainfall Over Southeastern China From April To June

Based on the daily rainfall data of more than 2400 stations in China from 1979-2013, and the NCEP/NCAR reanalysis data of geopotential height, UV wind, OLR and potential function; empirical orthogonal decomposition (EOF), Power spectrum analysis, linear correlation, regression analysis, Butterworth filter and statistic synthesis of t test method are used to studied the low frequency characteristics of strong rainfall which is above 25mm in the Southeast China from April to June in recent 35 years and the typical year (1999 and 1999). Futher study is on the cause of the anomalies strong rainfall from April to June, The following conclusions were obtained:(1) The high variance areas of filtered 10-30 day component for strong rainfall from April to June in recent 35 years are in the Yangtze River and south China, the center located in the middle of the east Yangtze River, its variance of filtered 10-30 d component for rainfall accounted for more than 40% of the unfiltered rainfall variance.(2)The filtered 10-30 d component for strong rainfall from April to June in recent 35 years over Southeast China has two main modes. The first mode reflects a consistent variation in the region; the second rainfall mode in the region shows the north-south anti-phase variation. Power spectrum analysis showed the main period of the first mode is 10-30 d component.(3) Based on the strong rainfall and filtered 10-30 d component for rainfall, the first Empirical Orthogonal Function (EOF) time series (PCl) of the strong rainfall and interannual variance of filtered 10-30 d component for PCl, the strong typical years of filtered 10-30 d component for strong rainfall are defined in combination with their yearly correlation coefficients of each other. The abnormal of total rainfall in the region is mainly caused by 10-30 d low frequency of the strong rainfall changes. The filtered 10-30 d component for rainfall and strong rainfall variance analysis of anomaly spatial distribution typical years showed that the significant variance centers of total rainfall, strong rainfall and their filtered 10-30 d component for rainfall are all in the middle of the east Yangtze River, which reflects the anomalies variability of rainfall are mainly due to the variation of the filtered 10-30 d component for rainfall in the region.(4) When regional convection is abnormally strong, the geopotential height in low level is negative anomaly, there is a cyclonic circulation and vice versa. When there is a significant positive abnormal at 850hpa in southeast China, that is convergence, there will be anomalous divergence near the tropical western Pacific. On the contrary, when top level 200hpa is negative, the upward movement anomalies ocuured in the eastern hemisphere. The strong rainfall in southeast China between April and June is increasing; whereas reducing.(5) There are main three branches water vapor airflow into Southeast China. They are:① Moisture from the subtropical high over the Western Pacific Ocean ② Moisture from the Bay of Bengal ③ Moisture from the South plateau.(6) The study of potential function, OLR revealed that tropical western Indian Ocean and western Pacific have significant changes ahead of 15 days. Tropical potential function and abnormal convection activities have obvious eastward propagation characteristics, the circulation anomaly is similar to the response of tropical low frequency Rossby wave. The geopotential height analysis showed that, the positive anomalies mainly performance for plateau of spreading easterly at the top, that for the South Asia High of spreading easterly, abnormal high pressure caused upper level divergence anomaly exception conducive to China Southeast regional of low frequency rainfall anomaly. Middle and low level is characterized by the westward and northward spread of Western Pacific Subtropical High, and middle and high the latitudes there are South negative anomalies, also known as cold air of South transmission, the cool and warm air from the South and North met in Southeastern China region, thereby promote regional precipitation anomalies.(7) In 1994 and 1999, the PCl from EOF analysis of strong rainfall over southeast China from April to June is associated with each circulation. It’s similar to the circulation of strong oscillation years, but the centre is bigger.