黄河流域夏季有效降水转化率

利用NCEP/NCAR再分析资料及中国实测雨量资料,分析发现黄河流域夏季整层降水转化率普遍较低,因考虑到整层中应有某一气压层具有最有利的水汽输送、抬升凝结和垂直运动条件,由此将该气压层定义为有效降水转化率层(简称有效层),降水与有效层可降水量之比称为有效降水转化率。通过分析黄河流域夏季各分区垂直速度与水汽等因子,确定上、中、下游夏季有效层分别为地面~500hPa、600~400hPa、850~600hPa。对比分析表明:夏季各区平均水汽收支与降水转化率,各自在整层和有效层上的逐年演变趋势均一致,峰谷相对应,且49年间降水转化率在有效层上的数值均大于整层的。夏季有效降水转化率在青藏高原上空及中游东部最高,高原东北侧最低,旱、涝年与多年平均状况下的分布形式虽然一致,但涝年的数值明显大于旱年。

Research on summer effective precipitation conversion rate over the Yellow River basin

Based on both the NCEP/NCAR reanalysis data and the observed rainfall data,the results show that summer precipitation conversion rate of the whole layer over the Yellow River basin is relatively low.This considers that there should be a certain pressure level which has the most favorable conditions of water vapor transportation,lifting condensation and vertical motion during the entire layer.Thus,this pressure level is defined as the layer of effective precipitation conversion rate(referred to as the effective layer),and the ratio of precipitation to the effective layer’s precipitable water is called the effective precipitation conversion rate.Through analyzing the average factors such as vertical velocity and water vapor in every subarea,the summer effective layers of the upper,middle and lower reaches are identified as ground-500 hPa,600-400 hPa and 850-600 hPa,respectively.It’s proven that the evolutional trends of summer mean water budget and precipitation conversion rate in the whole layer and in effective layer are consistent,peak and valley corresponding,and the values of precipitation conversion rate of the effective level are greater than that of the whole layer during the whole 49 years.The summer effective precipitation conversion rate reaches the highest over the Tibetan Plateau and the eastern part of the middle Yellow River but the lowest over the northeast side of Tibetan Plateau separately.Its distribution forms are similar in drought/flood years and multi-year mean,while the values in flood years are significantly higher than those in drought years.