High-resolution Simulation And Helicity Diagnosis Of A Tornado In Foshan City

In this paper,the mesoscale numerical model WRF was used to carry out the 148 m and 48 m high resolution numerical simulation of a strong tornado process in the spiral cloud belt surrounding typhoon "Rainbow" on October 4,2015.The tornadic supercell,TLV(tornado-like vortex)and suction vortex inside tornado and the corresponding physical mechanism were analyzed and studied.The results showed that :(1)The tornado was generated in the remarkable hook echo of the supercell,which was combined with the typical characteristics of the front suspension echo and weak inflow echo.(2)The tornado-like vortex TLV system was analyzed with the simulation results of 148 m resolution,which was mainly manifested as the sharp increase of vertical vorticity on the horizontal plane,the inclined development in the vertical direction,the internal sinking and external rising structure,and the narrow contact funnel-shaped structure corresponding to the low-layer hydrometeors.(3)The model results with a resolution of 48 m successfully simulated the small scale suction vortex system inside the tornado,which was represented as the boundary of UD(updraft)and RFD(rear flank downdraft),and three vertically extended vorticity columns that split successively in the hooked vorticity zone.The vertical circulation structure of strong spiraling exists in the sucker vortex located in the middle of the vorticity zone,which is the source of strong explosive force of tornado.The formation mechanism is the cooperation of strong ground rotation and updraft in UD.(4)The establishment of physical three-dimensional concept map reflects the fine structure of the supercell and TLV: UD in the updraft of the middle part of the supercell extends upward,FFD(forward flank downdraft)and RFD in the downdraft of the front and rear parts of the supercell appear,and the low level hydrometeors under the rear cloud wall touch the ground;Strong convergence occurred at the coupling of RFD and UD,and the formation of TLV system resulted from the surge of cyclones in the lower layer.A smaller scale vortex of suction inside the tornado swirls around the center.(5)Through the diagnosis of the vertical helicity equation,the physical mechanism affecting the tornado and its internal suction vortex was obtained.The main forcing term of the diagnosis equation included the flux divergence term in the horizontal direction of vertical helicity,which represented the separation of vorticity column and the rotation of suction vortex around the center of the tornado.The vertical advection term of the vertical helicity enhanced the suction vortex at the interface between RFD and UD.(6)There are two main differences between the results of this tornado case and the previous study.In terms of tornado supercell structure,in this case,the rear inflow is weak,the outflow is strong,and the front airflow rises to hydrate aggregation.In terms of the tornado formation mechanism,the conversion term of horizontal helicity to vertical helicity,as a secondary forcing term,only occurs at the early stage of tornado formation and has little influence on the development of sucker vortex.The main reason is that the horizontal separation and vertical rise of hooked vorticity zone lead to the formation of sucker vortex.