Characteristics Of Persistent Precipitation In The Middle-lower Reach Of The Yangtze River And Analysis Of Its Relationship With Low Frequency Oscillation

Together using NCEP/NCAR daily reanalysis data and daily observed precipitation data in China during the period 1958-2007, persistent abnormal rain is defined in this paper. Based on the definition, characteristics of persistent abnormal rain in the middle-lower reach of the Yangtze River valley in summer are studied as well as features of the circulation during the rain period. Then the relationship between low-frequency oscillation and persistent rain is studied. After series of composite analysis, special low-frequency signals are found in the low-frequency atmosphere circulation before persistent rain starts. They have good correlationship with the precipitation 20～30 days later. More details are as follows.Firstly, the definition of persistent abnormal rain is raised. During 1958-2007, persistent abnormal rain happened 30 times in the middle-lower reach of the Yangtze River valley from May to September and they almost happened in El-Nino year or the next year of El-Nino year. In the last 50 years, there is a trend that persistent abnormal rain happened more frequently. During persistent rain period, subtropical high in the Western Pacific is more powerful and locate in the South China Sea. Dry and cold north wind meets wet and warm south wind in the Yangtze River valley.Secondly, features of the low-frequency circulation during the rain period are discussed to look for the relationship between low frequency oscillation and the persistent abnormal rain. Some important phenomena are found. When the rain is on, the position of Western Pacific subtropical high is more westward and more southward and in terms of low-frequency circulation, there is a low-frequency anticyclone in South China and the South China Sea on middle and low levels. At the same time, a divergence center locates in the middle-lower reach of the Yangtze River valley on high levels. The configuration is easy to trigger precipitation in the middle-lower reach of the Yangtze River valley.Thirdly, features of the low-frequency circulation before the rain period starts are discussed to look for signals. On middle and low levels a low-frequency wave is found in a line from Pacific central regions to South China. On high levels there is a low-frequency wave that propagates from the area north to Japan to the middle-lower reach of the Yangtze River valley. Factors of the two areas have good correlationship with the precipitation 20～30 days later. They can help to improve extended-range forecast skill of persistent abnormal rain in the middle-lower reach of the Yangtze River valley in summer.