Eyewall reconstruction of Typhoon Fanapi (2010) after encountering the Taiwan terrain

In this study, numerical simulations of Typhoon Fanapi (2010) interacting with Taiwan terrain are conducted using the Weather Research and Forecasting model (WRF; version 3.3.1) on a triply-nested grid (with the finest grid size of 1 km and 55 vertical levels). Typhoon Fanapi made landfall on eastern Taiwan on 0040 UTC 19 September and left Taiwan on 1200 UTC 19 September 2010, producing heavy rainfall and severe floods over southwestern Taiwan. Kinematic and microphysical characteristics within typhoon eyewall and inner rainbands were observed by the operational Doppler radar over Chigu and a research mobile dual-polarmetric radar (TEAM-R) over Kaohsiung. When Fanapi approached eastern Taiwan from the ocean, the low-level eyewall gradually weakened and broke down due to strong friction by steep terrain over the Central Mountain Range (CMR), and a secondary low was formed on the lee side (western Taiwan) by adiabatic subsidence. Above the CMR, the Fanapi vortex continued its westward track with a slightly southward deflection. Control simulation showed that as the Fanapi vortex passed through the CMR to the western foothill, positive vorticity was gradually built up within the lee-side secondary low and transported upward from the surface. Then the Fanapi eyewall was reconstructed with a complete vortex ring, as revealed from radar observations. A series of reduced-terrain experiments are conducted to verify the eyewall reconstruction mechanism. In the absence of Taiwan terrain, Fanapi would continue its westward movement without any deflection and the eyewall remained intact. With only half of Taiwan terrain, the southward track deflection was reduced and the degree of eyewall breakdown over the CMR and eyewall reconstruction over the lee side of the CMR was less obvious. A southerly jet was formed along the western foothill of CMR as the secondary horizontal wind maximum (SHWM) within the principal rainband encountered the CMR. The southerly jet resulted from the strong vertical shear of horizontal wind shear within the SHWM or the tilting of horizontal vortex tube within the SHWM by the downslope winds above the CMR and convective updrafts within the rainband. Through the vorticity-budget analyses and terrain sensitivity experiments, it is found that the "bottom-up" processes is active to reorganize the eyewall when Typhoon Fanapi was over the southwestern plain of the CMR.