Research on the formation and precipitation of the southwest vortex impacted by the Plateau Vortex

Abstract

This study investigated the source, trajectory, and the precipitation patterns of the SouthWest (SW) vortex, which was linked with the Plateau (P) vortex. Based on the statistical study on the cases, 1) the SW vortex tended to originate at the northeastern and western peripheries of the Sichuan Basin, normally coinciding with the presence of the P vortices in the eastern region of the Tibetan Plateau. 2) Most of the aforementioned vortices may exhibit a longer life span. When these SW vortices remained within or in close proximity to the Basin, they resulted in severe storms of averaging approximately 50 mm per day. Once they departed from the Basin, eastern and southern China experienced rainfall exceeding 100 mm per day. In this regard, this study carried out an composite examination on 8 cases of SW vortices originated within the Basin, focusing on their common structure and precipitation formation mechanism. Subsequent findings were obtained from the composite examination: 1) The SW vortex and the P vortex were attributed from an ‘Ω’ circulation originating from blocking high pressure in the mid-high latitude region. The formation of the SW vortex was notably influenced by the significant action of two air currents. In the lower troposphere, the southwesterly jet of the South Asian monsoon flowed over and around the Yungui Plateau before entering the Basin from the eastern direction. Simultaneously, cold and dry air originating from the north flowed into the Basin at an average altitude of 600 hPa, moving alongside the front of the blocking high. The convergence of the two air current resulted in the horizontal wind shear, facilitating the movement of the cyclone of SW vortex. 2) Regarding the structure, both the SW vortex and the P vortex displayed a shallow synoptic system characterized by a vertical profile in the field of geopotential height below 500 hPa. Meanwhile, wet-cold cores formed around the sources at low altitudes, while the cyclonic eddy circulation from the vorticity extended into the upper troposphere. 3) The analysis on the atmospheric instability and dynamics suggested that the vortices’ eddies generated significant convective instability at lower levels, while the deep eddy circulation resulted in the convergence of water vapor and momentum forcing. The circulation patterns and instability conditions facilitated the occurrence of heavy precipitation associated with the SW vortex. Additionally, the presence of ample water vapor and subsequent latent heat served intensify rainstorm, thereby establishing a positive feedback effect cycle.