Distinct East–West Distribution of Summer Precipitation over the Qinghai–Xizang Plateau in Relation to the Zonal Shear Line

Summer precipitation over the Qinghai–Xizang Plateau (QXP) exhibits an uneven east–west distribution. The zonal shear line (ZSL) over the QXP is the primary synoptic system influencing summer precipitation, and it is the only synoptic system that can largely span the QXP from east to west, having obvious implications for the distribution of summer precipitation over the QXP. The proximity of the zonal distribution of precipitation to the ZSL and the reasons for this unique proximity are deserving of in-depth investigation. Based on hourly ERA5 reanalysis data spanning the period 1980–2019, 11 cases of a ZSL that caused heavy precipitation over the QXP were composited, and horizontal atmospheric motion was decomposed into rotational and divergent components to diagnose the atmospheric dynamics responsible for the closeness of the precipitation and ZSL. The results demonstrate that obviously heavier precipitation lies in the eastern section of the ZSL (ZSL_ES, east of 88.5°E) than in the western section of the ZSL (ZSL_WS, west of 88.5°E). At upper levels, both the anticyclonic and divergent intensity are stronger in the ZSL_ES than in the ZSL_WS; plus, a deeper convergence layer is present in the mid-lower troposphere of the ZSL_ES, resulting in upward air motion approximately twice the intensity of that in the ZSL_WS. The net water vapor income of the whole layer near the ZSL_ES is 7.7 times that of near the ZSL_WS, as water vapor is found to be transported mainly through the Yarlung Zangbu Grand Canyon and converge on the southern ZSL_ES. The stronger upward velocity lifts more water vapor near the ZSL_ES, resulting in notably greater precipitation than that observed in the ZSL_WS.