Empirical Orthogonal Function Analysis of Daily Rainfall in the Upper Reaches of the Huai River Basin, China

Abstract

The design and operation of hydro-structures for flood control and water conservation bring a need for improved characterization of precipitation patterns. A 73 000 km2 study area in East Central China is situated in the East Asian monsoon region and experiences a strong seasonality in the rainfall regime. The characteristics of daily rainfall from 230 gauges during 1967–1986 were investigated for four periods in the summer monsoon season using empirical orthogonal function analysis (EOF) and extended empirical orthogonal function analysis (EEOF). The EOF analysis showed that for all four periods most of the variance was explained by an elongated spatial rainfall pattern. The pattern varied in direction, from roughly west-east to southwest-northeast, in the different periods. The zonally oriented patterns were interpreted as being caused by the stationary Mei-Yu front and the southwest-northeast patterns interpreted as cold fronts in cyclones that were developing over the study area. The latitude of the rain belt described by the first mode moved slightly northward with the advance of the East Asian monsoon from the first period, 9–22 June, to the third period, 23 July–5 August, and then withdrew southward again in accordance with the known seasonal movement of the Mei-Yu front. The EEOF analysis was used to show the development of the rainfall area over sequences of three days. During all four periods rainfall intensified on the second day, compared to the first and third days. During the first and last periods, 9–22 June and 5 August–30 September, respectively, there appeared to be little movement in the rainfall. During the second and third periods, the patterns were interpreted as a cold front in a developing cyclone. The results show the connection between the temporal variation in rainfall intensity and the temporal succession of spatial patterns over three day periods and should be used in the construction of design rainfalls for the study area.