Study Of The Impact Of Tropical Cyclones Over The Bay Of Bengal On Precipitation Caused By Plateau Vortexes

Both the plateau vortexes and the tropical cyclones over the Bay of Bengal are important weather systems affecting plateau precipitation,and the period from April to June is the common active period of them.What influence the tropical cyclones over the Bay of Bengal will have on the plateau vortexes and their precipitation is worth paying attention to in the research of plateau weather.In this paper,the activity regularity of the plateau vortexes and the variation of the active characteristics and precipitation of the vortexes at the tropical cyclones over the Bay of Bengal were analyzed by using the TPV and SL Yearbook and Joint Typhoon Warning Center(JTWC)best-track data.Then this article analyzes a large scale heavy precipitation process over the Tibetan Plateau under the effect of plateau vortexes at duration of storm activity during May26-31,2017,based on the hourly precipitation observation data and reanalysis data.Furthermore,the contribution and physical process to plateau vortex and its precipitation generation of tropical cyclones water vapor on the plateau are studied by using Hybrid Single-Particle Lagrangian Integrated Trajectory(HYSPLIT)model.The following conclusions are obtained:The statistical study from 1998 to 2019 shows that,the overall number of plateau vortexes increases year by year,mainly occurring from April to August,with the highest number in June.From March to October is the frequency of plateau vortexes migration,with the peak in June.The northeastern paths of the plateau vortexes account for36.56% of the total number of vortexes moving out,while eastern paths account for33.48% and southeastern paths account for 17.62%.The number of movements into Gansu and Sichuan accounted for 47.58% and 26.87% respectively.In these 22 years,the proportion of plateau vortexes active with tropical cyclones in November is the largest,while the number of plateau vortexes active with tropical cyclones in May is the largest.The plateau vortexes that occurred during the same period as the tropical cyclones over the Bay of Bengal are stronger and more likely to develop eastward and with more intense precipitation.A large-scale precipitation process over the Tibetan Plateau under the joint effect of a tropical cyclone over the Bay of Bengal and plateau vortex during May 26-31,2017 was analyzed.With the cooperation of the tropical cyclone over the Bay of Bengal and the southern branch though,the water vapor transport jet from the Bay of Bengal to the southeast of the Tibetan Plateau was established,providing water vapor for the low vortex and shear line of the plateau.The cold air behind the India-Burma though formed a cold cushion on the steep slope of the southern Tibetan Plateau,and the warm water vapor from the Bay of Bengal first rose northward along the cold cushion,then sank after ascending to the plateau,and rose northward again near the low vortex and shear line of the plateau,which could increase precipitable water of the plateau surface to the upper troposphere atmosphere.In the meanwhile,frontogenesis was generated by the confluence of tropical cyclone southerly warm water vapor with the cold and dry air in the northern part of the plateau.In the process of frontogenesis,the atmospheric moist baroclinicity increased significantly and the moist isentropic line rose sharply,which promoted the sharp development of vertical vorticity and the enhancement of plateau low vortex.On the one hand,during the northward movement,the anti-cyclonic outflow from the upper layer of the tropical cyclone strengthened the southwest jet in front of the upper trough of the Tibet Plateau,and the enhancement of divergence favored the development of the plateau shear line and the eastward movement of the plateau low vortex,resulting in a large scale heavy precipitation.Based on HYSPLIT,the water vapor in the coexisting phase of plateau vortex and tropical cyclones over the Bay of Bengal was tracked in three typical cases.The water vapor in the lower layers of the storm is lifted up after being involved in the cyclone circulation,and spread north to the southeastern Tibetan Plateau.Then the water vapor further converges northward through the plateau vortex cyclonic circulation and shear line.Thus,it can affect the activity of low vortex and precipitation on the plateau.