An Study Of "Train Effect" In The Spiral Cloud Belts Of Typhoon "Fitow" (No.1323)

In this paper,the characteristics and formation of "train effect" in the spiral cloud belt of Typhoon "Fitow"(No.1323)are analyzed and the conclusion are verified by simulation.The results show that there is a "train effect" during persistent rainfall over the south bank of the Bay of Qiantangjiang in Zhejiang.According to the stable position of the rain belt,it can be divided into two "train effect",they both last about 3-4 hours,and the space span is 1-2 longitude distance;The rainstorm area shows the characteristics of band,with high precipitation intensity.The precipitation exceeds 25mm per hour and propagates forward linearly.The average reflectivity of radar echo in the spiral cloud belt of typhoon with over 35dBZ also shows a linear band structure.The trend of precipitation belt and the moving direction of radar echo deviate from the moving direction of typhoon center,and the deviation angle is more than 25 degree.Through the Shuman-Shaprio filtering scale separation,the coastal mesoscale amplitude merging field is separated.The convective clouds in the spiral cloud belt of the typhoon can be enhanced rapidly and maintained in the coastal area.At the back of the spiral cloud belt,there are continuous strong convective cells higher than 40dBZ,which are generated from the mesoscale convergence zone of the coastal area,and develop along the convergence line from the coastal area to the Xiangshan area,forming a continuous linear convective echo with mesoscale system characteristics,and resulting in the "train effect".It is found that the mesoscale convergence belt or convergence center along the spiral cloud belt can maintain for a few hours,which favors for the maintenance of "train effect".From the results of simulation,there are several amplitude merging centers on the amplitude merging zone which are approximately linear.There is also a phenomenon of"displacement" in the convergence zone,which also experienced the strong and weak changes of generation,development,weakening and regeneration.When the low-level jet flows through the southeast coastal complex coastline of Xiangshan,the wind speed in the boundary layer is greatly reduced by friction,the collision between the low-level jet and the coastal terrain results in convergence and uplift,and the increase of wind speed gradient in Xiangshan coastal area results in convergence near the ground.The coastal amplitude center plays an important role in the formation,development and maintenance of the coastal strong convective cell.