Diagnostic Analysis And Numerical Simulation On The Explosive Development Of Transformed Tropical Cvclone Bavi (0818)

Transformed tropical cyclone Bavi (0818) explosively deepened over the western North Pacific Ocean and reached its maximum intensity of948hPa. It was a very special case of transformed tropical cyclone redevelopment. Based on NCEP reanalysis data and best track data of tropical cyclones provided by JMA, using synoptic meteorology, extended Zwack-Okossi equation, ω equation, potential vorticity dynamics, kinetic energy budget equation and WRF numerical experiments, the redevelopment process of Bavi is diagnostically analyzed. Results show that when Bavi moved northeastward, mid-latitude frontal zone intruded into Bavi and westerly trough moved over Bavi. Bavi’s circulation overlapped the mid-latitude frontal zone and started500-1000hPa thickness advection. Warm advection occurred in the northeast of Bavi and cold advection occurred in the southwest of Bavi. There was a southwest-northeast moisture channel from low latitude to the northeast of Bavi at low level. Warm advection, positive vorticity advection and large-scale heating forced positive vorticity tendency in the northeast of Bavi, and the pressure dropped in the northeast of Bavi. Cold advection and negative vorticity advection forced negative vorticity tendency in the southwest of Bavi, and the pressure rose in the southwest of Bavi. Bavi’s center moved along northeast which was the direction of maximum isallobaric gradient. In Bavi filter out numerical experiment, mid-latitude frontal zone developed into an extratropical cyclone under high-level trough, but the maximum intensity of the extratropical cyclone was33hPa weaker than observation. This shows that the environment Bavi moved into was conducive to the development of extratropical cyclone, while Bavi moved into the environment and merged with the extratropical cyclone, the development of the extratropical cyclone was greatly enhanced. Before the explosively developing stage of Bavi, large-scale heating was the main forcing term. In the explosively developing stage of Bavi, large-scale heating and warm advection were the main forcing terms. Adiabatic cooling, negative vorticity advection and cold advection prevented cyclone development. Warm advection and positive vorticity had direct effect on the development of ground cyclone by making high-level divergence, and indirect effect on the development of ground cyclone by forcing vertical velocity which was closely related to large-scale heating, in return, large-scale heating could affect warm advection and positive vorticity by feedback effect. Kinetic energy generation was the unique source term of Bavi’s kinetic energy increase. About half of the low-level kinetic energy generation was transported to the high-level, strengthening the high-level jet stream. In return, the development of the high-level jet stream promoted the development of the cyclone. When the500hPa westerly trough merged and the extratropical cyclone related to the north westerly trough incorporated into Bavi, cold advection and sinking motion in the west of Bavi dramatically strengthened. The sinking motion made high potential vorticity in upper troposphere go down, making Bavi develop explosively. The500hPa westerly trough merger was the initiation factor for the explosive redevelopment of Bavi. In latent heat feedback turned off numerical experiment, Bavi did not deepen explosively, but first gradually weakened and slightly enhanced after the500hPa westerly trough merger, finally was50hPa weaker than observation.500hPa westerly trough mergers made vorticity advection and warm advection sudden increase, which was the key to Bavi’s explosive redevelopment. With plenty of moisture transport, diabatic heating had non-linear increase effect to Bavi’s rapid redevelopment. Cyclone development and moisture channel, large-scale heating, the development of warm and cold fronts, the development of high-level trough and high-level jet stream, and the propagation of high potential vorticity from upper troposphere to lower level was a positive feedback process before Bavi reached its maximum intensity. Changing any part of the process would have a significant impact to the whole process.