Analysis of rainfall intensity features based on quantitative evaluation indexduring pre-flood season over Fujian Province

Based on quantitative evaluation index, we analyze the day-to-day precipitation data during 1961-2012 from 66 observation stations in Fujian province in order to reveal the multi-scale features of rainfall intensity on the respect of low-frequency characteristics during the pre-flood season. The analysis results show that (1) There were two inter-decadal drifts in the pre-flood season rainfall intensity; one was from strong to weak in the middle of 1970s, and the other was from weak to strong in the early 1990s, with the phase change point at years of 1976 and 1991. (2) During the weak inter-decadal period (from mid 1970s to early 1990s), the variation of inter-annual rainfall intensity was relatively low, while during the strong inter-decadal period (from 1960s to mid 1970s and from early 1990s to the present), the inter-annual fluctuation of rainfall intensity was relatively high. In particular, since 2000 extreme precipitation events have clearly increased, and the persistent heavy rain events occurred more often. (3) The inter-seasonal oscillation of rainfall intensity in the pre-flood season has become evident, and the intensity of the precipitation is proportional to the intensity of low-frequency variation. Namely, during the strong phase years, the low-frequency signals were more evident, and vice verse. (4) In 75 percent of all the strong phase years, significant low-frequency signals are detected, which had three kinds of stable cycles of 30~60 days, 20~30 days and 10~20 days; while in the other 25 percent of the years it is shown an adjustment process in the low-frequency cycle.