PV Perspective of Impacts on Downstream Extreme Rainfall Event of a Tibetan Plateau Vortex Collaborating with a Southwest China Vortex

Abstract

An extreme rainfall event occurred over the middle and lower reaches of the Yangtze Basin (MLY) during the end of June 2016, which was attributable to a Tibetan Plateau (TP) Vortex (TPV) in conjunction with a Southwest China Vortex (SWCV). The physical mechanism for this event was investigated from Potential Vorticity (PV) and omega perspectives based on MERRA-2 reanalysis data. The cyclogenesis of the TPV over the northwestern TP along with the lower-tropospheric SWCV was found to involve a midtropospheric large-scale flow reconfiguration across western and eastern China with the formation of a high-amplitude Rossby wave. Subsequently, the eastward-moving TPV coalesced vertically with the SWCV over the eastern Sichuan Basin due to the positive vertical gradient of the TPV-related PV advection, leading the lower-tropospheric jet associated with moisture transport to intensify greatly and converge over the downstream MLY. The merged TPV−SWCV specially facilitated the upper-tropospheric isentropic-gliding ascending motion over the MLY. With the TPV-embedded mid-tropospheric trough migrating continuously eastward, the almost stagnant SWCV was re-separated from the overlying TPV, forming a more eastward-tilted high-PV configuration to trigger stronger ascending motion including isentropic-gliding, isentropic-displacement, and diabatic heating-related ascending components over the MLY. This led to more intense rainfall. Quantitative PV diagnoses demonstrate that both the coalescence and subsequent re-separation processes of the TPV with the SWCV were largely dominated by horizontal PV advection and PV generation due to vertically nonuniform diabatic heating, as well as the feedback of condensation latent heating on the isentropic-displacement vertical velocity.

摘 要

2016 年 6 月底, 长江中下游地区发生的一次极端降雨事件主要归因于高原涡和西南涡的协同影响. 利用 MERRA-2 再分析资料, 本文从位势涡度 (PV) 和垂直速度发展的角度探讨了这一事件发生的内部物理机制. 在中高纬罗斯贝波列的影响下, 我国西部和东部对流层中层发生了大尺度环流重构, 这直接影响了青藏高原西北部上空高原涡以及东部对流层低层西南涡的生成. 随后, 由于高原涡东移导致局地正的 PV 平流随高度的增加而增强, 高原涡在四川盆地东部与西南涡发生垂直合并, 同时引起对流层低空急流的显著加强, 其带来的水汽进一步向长江中下游地区输送和辐合. 合并的高原涡-西南涡系统导致了对流层上层气块沿等熵面滑动引起的上升运动的发展. 随着高原涡嵌入对流层中层的高度槽, 并不断东移, 西南涡移动较小, 并与上层的高原涡再次分离, 形成了向东更加倾斜的大值 PV 结构, 激发了下游更强的上升运动, 包括沿等熵面滑动的垂直速度分量、 等熵面位移导致的垂直速度分量, 以及与非绝热加热有关的垂直速度分量, 最终导致了极端强降水. PV 的定量诊断表明, 在高原涡和西南涡合并与再分离过程中, PV 收支主要受到水平 PV 平流以及垂直非均匀加热的影响, 凝结潜热的释放对等熵位移导致的垂直速度分量存在显著的反馈作用.