The Analysis Of Southwest Vortex Structure And Its Numerical Simulation

Southwest vortex is one of the important synoptic systems that affect the heavy rainfall in China.Applying the National Centers for Environmental Prediction Final Operational Global Analysis,the Interim European Center for Medium-Range Weather Forecasts Re-Analysis(ERA-Interim),the data of Black Body Temperature(TBB)from MTS-2 satellite,rainfall data from conventional and encrypting automatic stations in China Meteorological Administration,and the mesoscale Advanced Research Weather Research and Forecasting(WRF)Model,this paper performs diagnostic analyses and numerical simulation to the Southwest vortex during 17-18 July 2013.The results are as follows:(1)The Southwest vortex affecting the heavy rainfall during 17-18 July 2013 is different from the classical understanding of Southwest vortex.Unlike the former understanding that Southwest vortex is merely a single mesoscale vortex,two low centers are contained within the Southwest vortex,or the Southwest vortex has binary vortices structure,one of which is located in the northeast of Sichuan Province and another appears in the south of Sichuan Basin.(2)The binary vortices structure within the Southwest vortex is well performed by the mesoscale WRF model.Two rain centers produced by the gauged data and simulation also reflect the binary vortices structure.The results of numerical simulation show that vortex-A(higher latitude)forms at 1800 UTC 17 July 2013,with a lifespan of approximate 11 hours,while vortex-B(lower latitude)is generated at 0600 UTC 17 July,remaining for 26 hours.The centers of binary vortices at 700 h Pa correspond with the large value centers of vorticity.There is obvious convergence of airflow in the center,leading to strong vertical motions over vortices.As to the aspect of thermal structure,warm and moist centers exist within the binary vortices in the lower troposphere.(3)The geopotential height field adjusting to the wind field is the main reason of theformation of binary vortices structure within the Southwest vortex.Vorticity budget shows that the fluctuation of vortex-B is stronger than vortex-A,and the horizontal advection of vorticity plays an important role in the formation of vortex-A in 700 h Pa,indicating that the positive vorticity produced by convergence promotes the development of vortex-A.Stretching effect has an influence on vortex-B,which transports the vorticity to higher level making the vorticity decreasing in the lower level.The formation and activity of the binary vortices are promoted by the potential vorticity on 315 K isentropic surface.Besides,the propagation of positive potential vorticity from middle and higher level of troposphere can also induce the development of cyclonic circulation or vortex in the lower level.(4)The condition of water vapor is also an important reason for the formation of binary vortices structure.The Bay of Bengal,South China Sea and East China Sea are three main vapor sources during the formation of binary vortices.The divergence of water vapor flux has two convergence centers,which are corresponding to the vortices,indicating that the water vapor gathers here and moves upward.Water vapor releases potential heat to heat the air while ascending.The centers of binary vortices present apparent heat sources centers.Time series of regional averaged apparent heat source,divergence and vorticity have greater correlations.There are phase differences among the extreme values of them.The apparent heat source is earlier,followed by divergence and vorticity in the order they appear,showing that the heat source warms the air,leading to the vertical motion,then intensifying the convergence of airflow due to the continuity of the air,resulting the formation of the binary vortices.The sensitivity experiments show that the isolines around vortices in height field could not close and the structure of binary vortices is gone with the cyclonic wind shear disappearing,which illustrates that water vapor is essential to the formation of binary vortices.The binary vortices structure within the Southwest vortex is a unique structure that is generated under the mutual effects of dynamical and thermal factors.