北京城、郊和山区不同强度等级降水变化特征比较

利用北京地区1977-2013年18个站点逐小时降水资料，将小时降水分为弱降水（第50百分位值以下）、中等强度降水（第50至90百分位值）以及强降水（第90百分位值以上）3个等级，对北京地区山区、郊区以及城区夏季不同强度等级降水变化特征进行了深入细致的分析。结果表明，北京地区夏季降水量存在显著的减少趋势，这种减少趋势主要是由于弱降水和中等强度降水的显著减少引起的，强降水没有表现出明显的增多或减少趋势；与郊区相比，2004年之后城区的强降水对夏季总降水量的贡献越来越大而弱降水的贡献减小。在降水日变化上，不同地区、不同等级的降水存在差异。弱降水存在清晨和夜间双峰值特征，中等强度和强降水只存在夜间单峰值特征。清晨峰值时刻，山区、郊区和城区弱降水都表现出一致的显著减少趋势；夜间峰值时刻，山区的各等级降水变化不显著，而在2004年之后，城区弱降水少于郊区，强降水则多于郊区。北京地区降水过程不对称性特征（降水过程峰值前后差异性）十分明显，其中以强降水的不对称性最强，相对于郊区和山区来说，城区强降水过程的不对称性有增大的趋势。

Changes in Classified Precipitation in the Urban,Suburban and Mountain Areas of Beijing

In this paper, based on hourly precipitation observations during 1977-2013 from the Beijing area, hourly precipitation in summer (June-Auguest) is classified into three categories:light (below the 50th percentile values), moderate (the 50th to 95th percentile values), and heavy (above the 95th percentile values). The characteristics of the classified precipitation changes are analyzed and the results reveal that both light and moderate precipitation decreased significantly during the research period which mainly caused the decrease in summer totals, but heavy precipitation showed no pronounced trend. Since 2004, the contribution of heavy rainfall to the summer totals in the urban area increased as compared to the suburban area, which is opposite to light rainfall. There are obvious differences in the diurnal variations of classified precipitation. Light precipitation shows a double peak structure in the early morning and at night, while moderate and heavy rainfall show a single peak at night. Light precipitation at the early morning peak-time decreased significantly in the whole Beijing area. Compared with the suburban area, light precipitation in the urban area occurred less frequently whereas heavy precipitation occurred more frequently at evening peak-time after 2004. The asymmetry of the rainfall is obvious, especially, for heavy precipitation. The asymmetry of heavy precipitation events in the urban area exhibit a significant increasing trend.