Variation of precipitation in Shanghai during the last one hundred years and precipitation differences between city and suburb

Based on the observed data of precipitation during 1893 ∼ 1999 in Shanghai and its suburbs during 1961 ∼ 1999, by means of power spectrum analysis and some statistical methods, including cumulative anomaly percentage, 11-year running average, t-tests for paired samples etc., this paper mainly discusses the space-time features of precipitation in Shanghai. The results obtained are as follows: (1) The variation of precipitation in Shanghai has been obvious in the last one hundred years. There are more rainfall in the 1990s comparing with other decades in the 20th century and less in the 1960s. The anomaly of annual precipitation varies from - 33.4% (1978) to 54.8% (1999). A total of twenty drought and flooding years for the time span of 1893 ∼ 1999, thus the probability of normal annual precipitation is 81.3%. (2) The significant periodic components of inter-annual precipitation are 2a, 3. 2a and 35. 2a. The climate in Shanghai from 1893 ~ 1999 can be divided into six periods which include three dry ones, dwelling from the end of the 19th century, from 1921 ~ 1940 and 1960 ~ 1981, and three wet ones dwelling from 1906 ~ 1920, 1941 ~ 1959 and 1982 to the beginning of the 21st century. The amplitude of variation of dry/wet period has increased since 1960. The recent wet period seems to extend. (3) The effect of rain island influences the time and space distribution of precipitation in Shanghai. It shows that the precipitation differences between the urban and suburban areas (except the newly developed districts of Minhang and Pudong) are significant, with 2-tailed sig. value being less than 0. 01. Annual precipitation increment in the urban area is one and a half times as much as that of the suburbs. Regional precipitation differences in the 1980s and the 1990s have been enlarged by 1% ~ 3% comparing with that in the 1960s and the 1970s. (4) The rain island effect in Shanghai mainly exists from June to September which covers two important rain periods - the plum rain and the typhoon rain. During these months precipitation in the northern part of Shanghai is more than the southern. On the other hand, there is no rain island effect from the end of autumn to next May and the southern Shanghai has more precipitation than the northern. Precipitation from June to September is signified by an obvious long-term trend, while precipitation from October to next May displays fluctuations.