西北地区5-9月极端干期长度的概率特征分析

  应用西北地区131个测站1961-2000年逐日降水资料,统计逐年5-9月极端干期长度,分析近40 a年平均极端干期长度的空间分布特征以及极端干期长度与降水量的关系,采用概率加权法(PWM)估计其分布函数的参数,探讨用Gumbel分布拟合我国西北地区极端干期长度的适用性,进而给出我国西北地区极端干期长度的概率以及≥20 d、≥50 d极端干期长度的重现期概率特征。结果表明:西北地区近40 a平均极端干期长度总体上自东南向西北递增,但极端干期最短的地方并不在降水量最大的陕南地区,而在海拔较高的青海南部以及甘肃甘南高原。极端干期长度与5-9月降水总量呈负相关。≥20 d界限的极端干期长度的概率与50 a一遇极端干期长度的空间分布基本相似,降水较少的南疆、青海西部、河西走廊西段,基本上每年多雨期都存在20 d以上的连续无雨期,而祁连山以南的青海、甘肃中南部、宁夏南部、陕西中南部≥20 d极端干期长度的概率不到0\^1。

Analysis on Probability of Extreme Drought Period from May to September over Northwest China

The extreme drought period from May to September was counted form the daily precipitation data of 131 stations in Northwest China from 1961 to 2000.Further,we have studied the distribution of extreme drought period over Northwest China and have analyzed its correlation with total precipitation.The distribution of extreme drought period over Northwest China was simulated by the Gumbel Distribution Model by adopting Probability Weighted Method(PWM) to estimated the parameters of distribution function.The results showed that the spatial distribution of mean extreme drought period and the distribution of annual precipitation in recent 40 years were generally reverse over the Northwest China,but the region with the shortest extreme drought period didn't superpose the most precipitation region in south Shaanxi;the shortest extreme drought period located in south Qinghai and south Gansu plateau,with high sea level elevation.The relationship between the extreme drought period and the total precipitation from May to September is negative.The distribution of extreme drought period once in 50 years and the distribution of probability more than 20-day extreme drought period is similar.