藏北高原夏季降水的水汽来源分析

Analysis of Precipitation From Different Water Vapor Sources in Tibetan Plateau

根据GAME-Tibet加强观测期间取得的降水量和δ18O资料,基于来自海洋性气团的水汽形成的降水中δ18O较低、来自局地蒸发形成的降水中δ18O较高这一认识,尝试性地给出了划分不同来源水汽的标准。基于此标准,对研究区域中局地来源水汽和海洋性气团水汽在总降水中所占的比率定量估计。就安多附近平均而言,1998年6～9月海洋气团的直接输送而形成的降水量至多占总降水量的32.06%,而局地蒸发的水汽所形成的降水量至少占总降水量的46.86%。其它至少有21.8%可能来源于季风环流对沿途蒸发水汽的输送。青藏高原中部(如安多等)的降水,可能是海洋性气团携带的水汽经过若干次凝结-降水(降落到地面)-蒸发-再凝结等过程,不断循环并依次将水分通过季风环流向高原中西部推进。

According to the precipitation and δ^(18)O obtained during GAME-Tibet IOP, based on the knowledge that the δ^(18)O in precipitation is lower which is from the ocean air mass and higher from that of the local evaporation, the water vapor sources could be identified from the δ^(18)O value in precipitation. We attempt to give the identification criterion of δ^(18)O value. If we use δ^(18)O< -20‰ as the criterion that the precipitation is formed directly from the ocean air mass, the contribution of ocean air mass may be not underestimated. And if we use δ^(18)O>-13‰ as the criterion that the precipitation is formed directly from the local evaporation, the contribution of local evaporation may be not overestimated. According to such criterion, the proportion of the local-evaporation-formed-precipitation and the ocean-air-mass-formed-precipitation in total precipitation was estimated. The precipitation samples at site NODA, Anduo and AQB basically recorded the precipitation processes in June to early September. On the average of these three sites, the precipitation is 249.76mm. The precipitation formed directly from ocean air mass vapor is not beyond 80.08mm and the precipitation formed directly from local evaporation vapor at least is 117.05mm. That is to say, the proportion of the ocean-air-mass-vapor-formed-precipitation in total precipitation is not beyond 32.06% and the local-evaporation-vapor-formed-precipitation at least is 46.86%. The other 21.8% may come from the transportation of the monsoon circulation to the evaporation vapor on the way. The precipitation in Tibetan Plateau is mainly local rain formed by wet convection from the surface evaporation vapor. The rate of the precipitation formed directly from ocean air mass vapor to the total precipitation in Anduo area is small in summer monsoon.