Study On The Mechanism Of Precipitation Induced By A Plateau Vortex Moving Eastward

Heavy rains occur frequently every year,causing serious flood disasters.However,China has a vast area and various types of rainstorms,and their formation conditions and mechanisms are different,which makes it difficult to forecast.Especially in the western region of China,under the influence of the thermal power of the Qinghai-Tibet Plateau,the convection is active in summer and the rainstorm is frequent.In particular,in the complex terrain area on the southeast side of the plateau,heavy rain is likely to cause geological disasters such as mud-rock flow,mountain torrents and landslides,which will cause serious losses to the national economy and the safety of people’s lives and property.Therefore,it is urgent to conduct theoretical and simulation research on the occurrence and development mechanism of rainstorm in the complex terrain area on the east side of the Plateau,which is also the basis of rainstorm forecast.Based on an eastward heavy precipitation event in Sichuan Province during August 5-6,2019,this paper reveals the phenomenon of precipitation development and transmission caused by multiple vortices relay development on the eastern side of the Plateau.The factors affecting the development of different vortices are analyzed by using the vorticity equation.From the Angle of energy evolution,the reasons for the strengthening of eddy precipitation and the increase of convection are revealed.At the same time,the influence of slope change on the evolution of vortices on the eastern side of the Plateau was evaluated by constructing topography and carrying out sensitivity tests of topographic gradient change.(1)For the vortices in the studied region and period,the northern part of the Plateau Vortex moving to the northeast is called Plateau Vortex 1(PLV1),and the southern part of the Plateau Vortex moving to the east is called Plateau Vortex 2(PLV2),and the southwest vortex(SWV)generated over the Sichuan Basin.PLV1 after over the plateau generated when in 0500 east north to mobile and dissipating,during the 0517PLV2 began in 0508 when gradually strengthened and eastward over the plateau,in at around0522 and southwest vortex merging,SWV generated near the Sichuan basin and around 0511 and PLV2 around 0522 when the merger,after the east together,in 0615 when gradually removed from the simulation area.(1)Based on the analysis of hourly precipitation and eddy evolution during the rainstorm in Sichuan Province during August 5-7,2019,it is found that the evolution of multiple vortices on the eastern side of the Plateau leads to the eastward development and propagation of precipitation.In the study area and precipitation period,there are three vortices: the northeast moving part of the northern part of the Plateau is called Plateau Vortex1(PLV1),and the eastward moving part of the southern part of the Plateau is called Plateau Vortex 2(PLV2),and the southwest vortex(SWV)generated over the Sichuan Basin.PLV1 was generated over the Plateau at 0500 and then moved eastward and northward and dissipated gradually at 0517.PLV2 gradually strengthened over the Plateau at 0508 and moved eastward,and merged with the southwest vortex at about 0522.SWV was generated near the Sichuan Basin at about 0511 and merged with PLV2 at about 0522.Then they moved eastward together and gradually moved out of the simulation area at about 0615.The development and propagation of precipitation are closely related to the intensity evolution of multi-vortex paths.(2)Based on the vorticity equation,the dominant factors affecting the occurrence of different vortices were diagnosed and analyzed,and the similarities and differences of the development mechanisms of the three vortices were analyzed.It is found that the vertical advection term and the tensile term are the main contribution items of PLV1.For PLv2,the dominant term is vertical advection term and tensile term.The main contribution of SWV2 is vertical advection and horizontal advection.Using the energy equation,the evolution characteristics of local total energy anomalies are revealed.It is found that the positive energy anomaly in the east of the convective system indicates the development direction of the convective system,that is,eastward propagation.(3)Based on the sensitivity tests of multi-scale topography,such as the Qinghai-Tibet Plateau,Hengduan Mountains and Sichuan Basin,the effects on the development of vortices are discussed.The results show that the Hengduan Mountains play a leading role in the formation of the southwest vortex,while the Sichuan Basin plays a secondary role in the formation of the southwest vortex.For the propagation of the Plateau Vortex(PLV2),the Sichuan Basin only affects the intensity of the vortex,but does not affect the propagation path of the Plateau Vortex.Once Hengduan Mountains are removed,the Plateau vortex propagation phenomenon will no longer appear.In the steep terrain at the junction of Qinghai-Tibet Plateau,Hengduan Mountains and Sichuan Basin,the sensitivity experiments of topographic slope change show that the steeper the slope is,the faster the propagation speed of the Plateau Vortex(PLV2)is,and the stronger the Southwest Vortex in the basin is.On the other hand,the height of the Plateau vortex development(i.e.,the height at which the strongest vorticity extends vertically)is almost stable at the same height over the terrain,and is less affected by(steep/gentle slope)slope change.