近50年长江流域降水日数的演变趋势

通过分析不同强度降水量（大于75百分位和大于95百分位降水，下同）对应降水日数，研究了长江流域1951~2000年逐年和年代际降水日数变化趋势。大于75百分位的降水日数在上游以及中游的北岸增加趋势最显著，四川盆地是唯一显示减少趋势的地区。同样，大于95百分位的降水日数在中游和下游也表现出十分明显的增加趋势，呈现减少趋势的仍然是四川盆地，并略向其北方延伸。详细分析每10年的平均降水日数的距平发现，大于75百分位降水日数最大的正距平集中在中游的1980s、1990s和下游的1980s。最大的负距平也是在中游地区，发生在1950~1979年。因此，中游的降水日数增加的幅度最大。对于大于95百分位降水日数，长江流域中游和下游的变化趋势也是一致的，在1960s 和1970s的负距平后，都出现较大的正距平。上游降水日数的年际变化要小于中下游。比较不同百分位降水日数的变化趋势，可将长江流域1950~2000年降水日数的变化趋势分为3种类型：（1）在大于75百分位降水日数增加的同时，大于95百分位降水日数却有所减少；（2）大于75和大于95百分位降水日数同时呈减少的趋势；（3）大于75和大于95百分位降水日数同时呈增加趋势。

FREQUENCY OF RAINFALL DAYS IN THE YANGTZE CATCHMENT IN LAST 50 YEARS

By using different percentiles(larger than 75 or 95 percentiles)the frequency of precipitation days in the Yangtze River catchment from 1950 to 2000 was explored in this paper.The positive(increasing)trends of the yearly precipitation days at the 75th percentile appeared in most of the Yangtze River catchment,especially in the northern regions of upper and middle reaches of the catchment.The Sichuan basin was the only region with negative trends.For the 95th percentile,the negative trend regions were still mainly in Sichuan basin but extend to northern regions and there were also obviously increasing trend centers in middle and lower reaches of the Yangtze River catchment.On the decadal time scale,the most significant positive trends at the 75th percentiles were in the middle reaches in 1980s and 1990s.The lower reaches showed significant positive trends in 1980s.Those positive phases greatly contributed to the positive trends of the whole catchment during the last two decades.The most significant negative phase also occurred in the middle reaches during the earlier three decades.For this case,it seems that the precipitation days in the middle reaches are more sensitive to changes than other regions in the Yangtze River catchment.Take the Yangtze River catchment as a whole,a positive trend is very obvious for this percentile.For the 95th percentile,the trends between the middle and lower reaches of the Yangtze River catchment are coherent:after two decades(1960s and 1970s)of negative phases,the most significant increasing trend was shown in the last two decades.On the other side,the fluctuation of the precipitation days in the catchment of the upper reaches over the 50 years was somewhat smoother than for other regions.For the whole catchment,the precipitation days in the 1950s and in the 1990s were at positive phase that indicated the frequent heavier precipitation events in those periods.The precipitation days at the 75th percentile increase more obviously than that at the 95th percentile in the whole catchment compared the 75th with the 95th percentiles.This result indicated that an increasing precipitation in the recent two decades was mainly caused by excessive precipitation.The heavier precipitation events occurred mainly in the upper reaches in 1960s,in the middle and lower reaches in the 1950s,1980s and 1990s.According to the different trends at different percentiles,the variation of yearly precipitation days can be classified as the following types:Type 1:the increasing trend at 75th percentile but decreasing trend at 95th percentile.Type 2:decreasing trends of yearly precipitation days for both percentiles.Type 3:increasing trends of yearly precipitation days for both percentiles.