| Tornado is an extremely destructive natural disaster,and understanding its formation mechanism is crucial for predicting and mitigating its impact.The Jianghuai region is one of the main areas in China where tornadoes occur frequently.In this study,a typical tornado event in this region was selected for numerical simulation,and its formation mechanism was analyzed in depth.An EF2-intensity tornado occurred in Gaoyou County,Jiangsu Province in eastern China in the afternoon of 7 July 2013,within a supercell storm near a Meiyu frontal system,producing destructive winds at the ground for about 20 minutes,injuring 50 people and damaging over a thousand houses.Data from a nearby operational S-band Doppler radar are assimilated using a 4D ensemble Kalman filter at 5 minute intervals into the Advanced Regional Prediction System(ARPS)at 1 km horizontal grid spacing.Simulations are run with a nested50-m grid,capturing the tornado embedded within the supercell storm with reasonable agreement with observations.The tornadogenesis processes within the simulation are analyzed in detail.(1)The environmental factors that influence the formation of tornadoes are analyzed,including the three-dimensional evolution of tornado vortices.It was found that a cold surge within the rear flank downdraft region plays a key role in instigating tornadogenesis when the leading edge of the cold surge approaches a low-level convergence center located underneath the main updraft.(2)Backward trajectories are calculated based on model output at 2 second intervals,and equations for streamwise,crosswise and vertical vorticity components are integrated along the trajectories and contributions from source/sink and conversion terms in the questions are quantitatively calculated.Origins of the parcels feeding the intensifying tornado vortex are identified.The result shows that the tilting of cross-wise horizontal vorticity that has been primarily generated by surface friction is the main source of tornadic vorticity near ground at the genesis stage.(3)Circulations along circuits enclosing the tornado vortex at different levels are integrated backward in time,and contributions of baroclinic and surface frictional generation to the circulation are evaluated.The circulation analyses show that surface drag acting on the nearly horizontal portion of the circuit that is close to the ground is the primary contributor to the final circulation converging into the tornado vortex,while baroclinic generation of circulation associated with the vertical portion of the circuit has opposite signs in the ascending and descending branches of the circuit,resulting in small net circulation generation from baroclinicity.(4)Lagrangian circulations analysis was conducted on the near ground mesocyclone in supercell storm,and the processes of its intensification and descending were analyzed.It was found that the frictional generation of circulation associated with the descending branches of the circuit caused by the downdraft within the cold surge is the main cause of the total circulation growth of the mesocyclone.The generation of the frictional circulation increment also leads to the continuous descending of the mesocyclone.These results provide further evidences on the key importance of surface-friction-generated vorticity/circulation in tornadogenesis,which is consistent with the leading-edge theoretical research in this field in recent years,and in this case,for a realistically simulated tornado in the Jianghuai region for the first time. |
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