1961—2012年西藏极端降水事件的变化

Change in Extreme Precipitation Events over Tibet from 1961 to 2012

利用18个气象站点1961—2012年逐日降水量资料,计算了10个极端降水指数,采用滑动平均、线性回归、Mann-Kendall非参数检验和Morlet小波分析等方法,分析了西藏极端降水事件的变化规律。结果表明:西藏近52 a连续干旱日数(CDD)呈显著减少趋势,最大5 d降水量也趋于减少但不显著,大雨日数和强降水量的线性趋势不明显,其他6个极端降水指数都表现为增加趋势且不显著。与全球、青藏高原及其周边地区比较,西藏CDD和连续湿日的变幅明显偏大,最大1日降水量、强降水量和极强降水量的变幅明显偏小。除CDD外,极端降水指数的变化趋势与海拔高度呈显著的二次曲线关系,仅有中雨日数的变化趋势与经度呈显著的正相关。在近52 a的时间尺度上各项极端降水指数都存在3~4 a显著周期,多数指数也存在12a、15 a和16 a的周期。在时间转折上,CDD的突变点时间较早,从1974年开始;中雨日数、连续湿日和年总降水量的突变点发生在20世纪80年代末。从空间分布来看,那曲地区西部是极端降水指数变化最为显著的区域,雅鲁藏布江中游大部的多数极端降水指标趋于下降,而山南地区南部、林芝地区东南部的降水极值和降水强度都在增加。

Variations and trends in extreme climate events are more sensitive to climate change than the mean values, and so have received much attention. In this study, the features for ten indices of precipitation extremes over Tibet are examined, broadly based on daily precipitation data during the period 1961- 2012 from 18 meteorological stations. The methods of 10- year smoothing average, linear regression, Mann- Kendall test and continuous wavelet transform were employed to delineate the rate of change, abrupt change points, statistical significance of the trends, and periodicity of extreme precipitation indices. The results show that Consecutive Dry Days(CDD) exhibit significant decreasing trend during the recent 52 years, so does the maximum 5-day precipitation(RX5day), but the variation trend of the latter is not significant.In contrast, there is not an obvious linear variation trend in very wet day precipitation(R95)and number of medium precipitation days(R10mm), whereas other six extreme precipitation indices have non-significant increasing trends. Compared with over the globe, the Qinghai-Tibet Plateau and its surrounding areas, regression slopes of CDD and continuous wet days(CWD)over Tibet are obviously greater, but regression slopes of maximum 1- day precipitation(RX1day), R95 and extremely wet day precipitation(R99) are apparently smaller. Except for CDD, other extreme precipitation indices can be quadratically regressed against altitude. Moreover, R10mm is positively associated with longitude. With regard to the period of variation, almost all extreme precipitation indices vary at three or four years scale, and most indices have such periods as 12 a, 15 a and 16 a. In terms of the change point, CDD occurs in 1974, much earlier than such indices as R10mm, CWD and wet day precipitation(in the late 1980s). From the spatial distribution characteristics of extreme precipitation indices, the variation trend in western Naqu is more distinct, and most of the extreme precipitation indices are observed to be dropping in most parts of the middle reaches in the Yarlung Zangbo River. On the other hand,escalating trends in extreme precipitation value and precipitation intensity are found in southern Shannan and southeastern Nyingchi.