| dc.description.abstract | The global model FV3GFS is used to simulate Typhoon Lekima (2019) with track deflection when approaching near Taiwan from the southeast during 8th to 9th August 2019. FV3GFS successfully captures the observed northward track deflection of the northwestward Lekima in sensitivity experiments with different physical parameterization schemes among which the old simplified Arakawa-Schubert cumulus scheme performs best. When the typhoon begins to deflect northward, its intensity is enhanced with split flow around the southern end of the topography to converge with the southern flank of the typhoon vortex, thus producing intense southerly radial inflow into the inner vortex at low levels and causing the vortex to move northward, which enhanced typhoon intensity of inner core caused by the radial horizontal advection of mean AM, mean Coriolis force term in angular momentum budget and mean radial advection of MTKE (RMTKE), work rate by mean radial pressure gradient force (WMRP) in kinetic energy budget at low levels is strengthen, but the radial horizontal advection of eddy AM and eddy radial advection of tangential momentum of TKE getting negative made typhoon intensity of outer core enhanced slowly.
From the diagnostics of vorticity budget, the typhoon movement was dominated by horizontal advection that induces a pair of gyres around the vortex center in the asymmetric wavenumber-one flow difference between the simulated flow with and without Taiwan terrain, which tends to rotate the vortex motion vector cyclonically with first northward movement and followed westward movement.
Sensitivity experiments using idealized WRF were used to explore how the track of an approaching typhoon will be changed in response to the topographic effects for faster and slower steering flows over a long mountain range at different orientations from the north. For the mountain at an orientation of 50o (mimicking northwestward typhoons), a northward
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deflection will be induced and becomes more significant and occurs later as the initial meridional departure position of the typhoon is closer to the center of the mountain. There will be no apparent track deflection if the meridional departure exceeds 450 km.The northward track deflection is also greatly reduced as the steering flow speed is doubled to 8 m s-1. When the mountain orientation is 90o (mimicking northward typhoons), the track deflection becomes more apparent, but without a clearer time delay for larger meridional departure distances. When the mountain orientation is 0o (mimicking westward typhoons), the typhoon will first deflect northward as closing to the mountain and then southward near landfall ahead the mountain, and finally rebound back (northward) after passing over the mountain. In wavenumber-one potential vorticity budget analysis, typhoon movement was dominated by horizontal advection, but diabatic heating was also stronger and contribution of typhoon movement caused by diabatic heating was opposite to horizontal advection, which made idealized typhoon move slower. | en_US |