Summer Atmospheric Water Cycle under the Transition Influence of the Westerly and Summer Monsoon over the Yarlung Zangbo River Basin in the Southern Tibetan Plateau

Abstract

This study compares the summer atmospheric water cycle, including moisture sources and consumption, in the upstream, midstream, and downstream regions of the Yarlung Zangbo River Basin in the southern Tibetan Plateau. The evolutions of moisture properties under the influence of the westerly and summer southerly monsoon are examined using 5-yr multi-source measurements and ERA5 reanalysis data. Note that moisture consumption in this study is associated with clouds, precipitation, and diabatic heating. Compared to the midstream and downstream regions, the upstream region has less moisture, clouds, and precipitation, where the moisture is brought by the westerly. In early August, the vertical wet advection over this region becomes enhanced and generates more high clouds and precipitation. The midstream region has moisture carried by the westerly in June and by the southerly monsoon from July to August. The higher vertical wet advection maximum here forms more high clouds, with a precipitation peak in early July. The downstream region is mainly affected by the southerly-driven wet advection. The rich moisture and strong vertical wet advection here produce the most clouds and precipitation among the three regions, with a precipitation peak in late June. The height of the maximum moisture condensation is different between the midstream region (325 hPa) and the other two regions (375 hPa), due to the higher upward motion maximum in the midstream region. The diabatic heating structures show that stratiform clouds dominate the upstream region, stratiform clouds and deep convection co-exist in the midstream region, and deep convection systems characterize the downstream region.

摘 要

本研究利用5年平均多源观测数据和ERA5再分析资料,对比了西风和夏季风影响下青藏高原南部雅鲁藏布江流域上、中、下游关键区夏季大气水分循环特征以及水汽来源和消耗结构,其中水汽消耗与云、降水和非绝热加热有关。相比于中游和下游,上游地区水汽、云量和降水量最少,水汽由西风输送,8月初垂直湿平流的加强形成了高云和降水峰值。中游地区水汽在6月由西风输送、在7-8月由东亚夏季风输送,当地较高的垂直湿平流中心有利于形成较多的高云云量,降水峰值出现在7月初。下游地区主要受南风湿平流影响,丰富的水汽和强烈的垂直湿平流使下游地区出现了最多的云量和降水,降水峰值出现在6月末。中游地区地区水汽最大消耗高度(325 hPa)高于另两个区域(375 hPa)。非绝热加热廓线表明上游地区以层状云降水为主,中游地区层状云和深对流共存,下游地区降水基本由深对流系统产生。