Regional Distribution Of Annual Variations Of Dekad Precipitation In China And Study On Forecast

This paper is concerned with the characteristics of dekad precipitation in China based on dekad rainfall data of 188 stations from 1955 to 1998. The annual variations of dekad precipitation are divided into 6 kinds, which include 8 regions in geography: region I ,South-east China; region II a, east of Tibetan Plateau, Sichuan Basin, west of Yungui Plateau and Qinling-Dabashan; region II b, Province Hainan; region III,Huangtu Plateau and north of Hexizoulang; region IV,North Plain, Neimonggu Plateau and North-east China; region V , middle-lower reaches of the Yangtze River; region VIa, east part of Yungui Plateau; region VIb, north of Tibetan Plateau and west of Hexizoulang. The precipitation in each region has regional characteristics besides the common trait of much in summer and little in winter. Compared with annual variation averaged over China, rainfall in region 1 is more in winter and spring but less in July and Autumn; region II a and II b have more rainfall in September and October; rainfall in region III is less from November to March of the next year while more in July and Autumn; region IV has less rainfall in May and June; precipitation in region Vis significantly more from middle-dekad in June to first-dekad in July, which manifests the feature of Meiyu in middle-lower reaches of the Yangtze River; region VIa and VIb have more rainfall in November.The characteristic periods of time are defined which indicate the typical characteristics of rainfall in each region. The significant correlations between the precipitation in characteristic periods of time in differentregions are considerable factors for mid-long term weather forecast. In recent 44 years, the trends of the precipitation in characteristic period of time and annual precipitation in different regions are significant. In some regions, the trend of the precipitation in characteristic period of time can denote the trend of annual precipitation.The Empirical Orthogonal Function(EOF) is employed to analyze dekad rainfall in China in recent 44 years. There are 6 dominant spatial patterns: same distribution allover the country; the seesaw between the regions of south of the Yangtze River and mid-lower reaches of the Yellow River; the seesaw between the regions over the Yangtze River and the Yellow River and the south part of South China; coherent variations over the Yangtze-Huaihe River Basin and North-east China and opposite variations in the regions of North China and North-west China and south-east part of China; the seesaw between the east part and the west part of China; coherent variations over mid-lower reaches of the Yellow River and South China and opposite variations in the Yangtze River Basin. The strength and occurring chances of each pattern all have significant annual period, which indicates that the movements of rain-belts in China have the characteristic of annual variation. The periods of time in which the strength and the occurring chances of each pattern are strong are defined as the concentration periods of time. In the recent half century, each spatial pattern has long-term trend.There are significant statistical correlations among temporal coefficients of spatial patterns of dekad precipitation. Lag correlations and cross correlations can be viewed as one clue to mid-long term weather forecast.