拉萨夏季大气可降水量的演变特征及其与降水的关系

根据拉萨站近40 a(1969—2008年)探空观测资料以及同期的地面降水资料,分析了拉萨近40 a夏季大气可降水量和地面降水的演变特征及其关系.结果表明:该站夏季大气可降水量和降水存在显著的正相关关系,两者存在相同的年际和年代际变化,均具有准3 a年际振荡和准11 a左右的年代际振荡;近40 a来两者均呈现出上升趋势,其中降水的增加趋势明显,其增幅大于可降水量的增幅.进一步通过对拉萨夏季降水转化率分析得知,拉萨夏季平均降水转化率约为26.06%,但存在明显的年际差异,夏季降水转化率最大值约为最小值的3倍;夏季降水转化率正(负)异常年,拉萨地区低层的辐合和高层的辐散均明显增强(减弱),拉萨地区垂直速度将增加(减弱),从而有(不)利于降水形成.

Evolution characteristics of summer precipitable water and precipitation conversion efficiency in Lhasa

By using the radiosonde data and precipitation data in Lhasa station from 1969 to 2008, the evolution characteristics of precipitable water and precipitation in summer are discussed.The statistic results indicate that there is a significantly positive correlation between precipitable water and precipitation in summer.The precipitable water and precipitation have the same inter-annual variation and inter-decadal variation, having the periodic oscillation of 3-years and quasi-11-years respectively.In addition, both precipitable water and precipitation show increasing trend in the last 40 years.But the increasing of the precipitation is more obvious and the growth of precipitation is greater than that of precipitable water.The analysis of precipitation conversion efficiency showed that the mean summer precipitation conversion efficiency is 26.06% with evident inter-annual difference.The maximum conversion efficiency value is about three times of the minimum value.In the years with high precipitation conversion efficiency, both the convergence in the low levels and divergence in the high levels are strong, resulting in the strong vertical velocity in Lhasa, thus favors the precipitation formation, and vice versa.