利用1951-2008 年中国北方348 个站的夏半年逐日降水资料,首先把整个北方分几个区分别建立夏半年最长连续无降水日数时间序列;然后采用经验模态分解(EMD)方法求出该序列的本征模态函数(IMF),分析各序列的不同时间尺度的变化,又用小波变换等方法分析各序列的转折点。结果表明,北方4 个区(东北、华北、西北东部和西北西部)夏半年最长连续无降水日数的变化主要是由前2 个较高频的本征模态函数分量构成,它们的方差贡献占总方差的80%左右,说明3～4 年、7 年左右尺度的振荡对整个序列的变化起主要作用。较低频的分量则反映10～15 年以上的年代际变化。近58 年来,东北、华北、西北东部的最长连续无降水日数都呈增加趋势,北方大部分地区干旱化加剧;而西北西部则呈减少趋势,干旱趋于缓解。近十几年来较明显的变化是东北趋于变干,西北东部趋于变湿。各区最长连续无降水日数显著增加的年份,东北、华北在1992 年前后,西北主要在1960 年前后,相应地,这些地区发生明显的干旱情景。分析所用的时间-空间尺度不同,所得到的结果可能有所不同。
Daily precipitation data from 348 stations in northern China are selected to construct the time series of the periods of the longest consecutive dry days in the warm half year (April-September) that is divided into four sub-regions. The empirical mode decomposition (EMD) method is used to obtain intrinsic mode functions (IMFs) by which variations are analyzed at various time scales. Based on wavelet analysis and other methods, sudden jump points are found. Results show that the variation of the longest consecutive dry period is mainly composed of two higher frequency intrinsic mode functions, and their contribution ratio is 80% to the total variance. This shows that 3-4 year and 7-8 year oscillations play important roles. Lower frequency components reflect the inter-annual variation of about 10 years. There is an increasing trend of the longest consecutive dry period in the warm half year in northeast China, north China, and eastern part of northwest China, and the droughts in most of northern China are increasing, but decreasing in the western part of northwest China. For the last 10 years, it get drier in northeast China, while it gets wetter in the eastern part of northwest China. The significant increase of the longest consecutive dry periods observed in north China in 1992 and in northwest China in 1960 was associated with the droughts. The results obtained may be different from each other owing to the differences in temporal and spatial scales.
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