基于百分位方法,首先通过构建气候等概率区间得到了位势高度场极端天气事件区分方法,并给出了相应的预报结果检验评价方案;以此为基础,对2003年夏季亚欧区域的500hPa位势高度场极端天气事件的NCEP集合概率预报效果进行了深入分析,得到如下结论：1）分析时段和区域内的极端事件的发生频率比气候平均状况略偏高。NCEP集合预报系统对异常偏低的极端事件的预报易偏高,但相对比较可靠;对相对异常偏高极端事件的预报频率随着预报时效的增大而减小,时效小于3d时预报频率偏高,时效大于5d时显著偏低,预报可靠性相对差一些。2）EV（economic value,经济价值）分析表明,NCEP集合预报系统对偏高（低）极端事件的概率预报效果明显高于正常天气事件。3）预报命中率分析表明,在高概率阈值下,预报命中率较低,这与预报系统在部分个例中的不发散有关。随着概率阈值的降低,预报命中率稳步提高,说明集合成员的发散使之可以覆盖实况可能出现的区间,从而得到更好的预报效果。%Based on the percentile method,a distinction method for the extreme weather events is proposed by the construction of the climatologically equally likely intervals firstly.Then,the corresponding forecasting evaluation on extreme weather events is performed for 500 hPa geopotential height over Eurasian area in summer 2003.The results are as follows：1）In the analyzed period and region,the frequency of the extreme events is slightly higher than the climatological average situation.In NCEP-EPS,the forecast frequency is easy to become higher for the lower extreme events,but relatively reliable,whereas it is decreased with the extension of lead-time for the higher extreme events.When the lead-time is less than 3 d （more than 5 d）,the forecast frequency is higher（lower） with relatively less reliability.2）EV（economic value） analysis shows that the performance of NCEP-EPS probabilistic forecast for the higher（lower） extreme events is better than that for the normal weather events.3）Forecast hit rate analysis shows that the hit rate is lower in the high-probability threshold.Further analysis points out that it is perhaps related with the lower ensemble spread in some cases.With the decrease of probability threshold,the hit rate steadily increases.It means that the spread of ensemble members can make them to cover all possible region of observation value,thereby to get the better forecasting results.
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