The Variations Of The Precipitation And Its Cause Analysis In Chongqing

Based on the daily precipitation data in1961～2006from33stations in Chongqing and from12stations in Hubei Provice, using the analysis methods of Mann-Kendall test, wavelet, synthesis, EOF, correlation and Bivariate Wavelet, the characters of Chongqing precipitation variation and its reasons were studied.The results showed that:(1) From the long-term change, the annual precipitation, rain day and the precipitation concentrated period (PCP) revealed decreasing tendency, while the precipitation concentration degree (PCD) with a weak increase. The distribution of PCD and PCP were positive associated with the annual precipitation.The spatial distribution of indices of extreme precipitation revealed consistency.(2) The precipitation anomaly in Chongqing was closely related to the abnormity of atmospheric circulation.The wind field on the lower troposphere revealed absolutely different characters in more and less precipitation years. In more precipitation years, there was a north wind abnormally moving southward in the zontal water vapor transport, while the meridional water vapor by west wind. The cold flow crossed with the warm flow in Yangtze river basin. It was opposite in less precipitation years.(3) The precipitation anomaly in Chongqing was also closely related to the weather systems. The annual precipitation was negatively associated with the northern boundary of subtropical high and the subtropical high ridge, it also had a positive relationship with the westward ridge spot index. The annual precipitation had a negative relationship with the Asian Summer Monsoon and also associated with the Pacific SST.(4) The annual precipitation of main city zone of Chongqing revealed increasing tendency, which was opposite to the average congdition of the whole aera. No Matter in year or seasons, the occurrence frequency of Droughts and floods are high.(5) The annual precipitation, PCD and PCP displayed obvious different interannual and interdecadal variations in the Three Gorges Reservoir, the periodic oscillations of annual precipitation and PCD were basically consistent with each other. After impound of the Three Gorges Reservoir, less precipitation and lower PCD value displayed comparing with that before impounded, while the concentration period was later. After impounded, the distribution of precipitation and PCD had good consistency while the PCD revealed significant differences.After analyzed, it was known that:When the precipitation in the Three Gorges Reservoir was concentrated, the atmospheric circulation background showed that the anomaly of500hPa geo-potential height field was positive in Siberia and the subtropical high over western Pacific moved westwards. The blocking over the East of Uraland area developed and the subtropical high was west and south when there was less precipitation in Chongqing. The anomaly of sea surface temperature had a indicative significance to forecasting the following year precipitation in Chongqing.