Properties of heavy rainfall in an East Asian monsoon catchment - Analysis of daily rain gauge data

Abstract: The East Asian monsoon has a strong seasonal effect on the rainfall regime of Eastern China. In order to mitigate floods and conserve the water for use throughout the year, dams and other hydro-structures are built. The design of the structures requires knowledge of the rainfall characteristics that influence the inflow to them. In this thesis, the properties of heavy rainfall in a 73 000 km2 area located in the upper reaches of the Huai River Basin are examined. Daily data from 285 rain gauges, 1950-1987, were analysed together with a selection of meteorological maps. Rainfall events exceeding 80 mm/day were shown to occur from April to October, with the heaviest and longest events occurring during July and August. The isohyetal patterns suggest large convective activity, both embedded in large scale rainfall areas and as isolated events. The isolated convective events have an elliptical shape with an average width of 43 km and a shape factor of 1.6. Most rainfall centres occur in the mountainous and hilly areas, and rainfall amounts as well as number of rainy days increase with altitude. Spatial cross-correlation and empirical orthogonal function analyses suggest that the most commonly occurring daily rainfall events have an elongated shape in a south-west/north-easterly direction, and that the rainfall area rotates anti-clockwise from one day to the next. This suggests that cold fronts on the Mei-Yu front may be the main rainfall generating mechanism in the area; a hypothesis supported by the meteorological maps. The non-uniform rainfall distribution and the eastward movement of cold fronts that coincides with the general direction of river flow result in an increase in the runoff peak compared to a spatially uniform and stationary storm. Autocorrelation analysis of areal daily rainfalls suggests a weak cycle of 7-8 days during June-July, which may correspond to the recurrence interval of cyclones. Both temporal and spatial rainfall patterns were shown to be scale-invariant, which suggests that there are inter-scale regularities in the rainfall generating mechanisms and that it may be possible to model rainfall as a cascade process. Although heavy rainfalls may be most frequently associated with cold fronts, it is typhoons that bring the heaviest rainfall intensities to the area. Probable maximum precipitation (PMP) for 1-3 days duration was estimated for four sub-catchments in the study area using moisture maximisation of typhoon rainfall. The 1- day point PMP was estimated to be 1 200 mm at an altitude of 100 m, which is 19% more than the observed point rainfall.