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Keywords:

  • tropical cyclone track;
  • track deflection;
  • topography;
  • orographic;
  • potential vorticity tendency;
  • PVT;
  • diabatic heating

The effects of topography on tropical cyclone (TC) tracks are investigated using the Weather Research and Forecasting model running on a beta plane with actual topography. Two sets of experiments are performed, one focusing on Taiwan topography and the other on the Philippines. In each set, different parts of the terrain are set to be either present or absent and the TCs are inserted at a location southeast of the mountain so that landfall occurs at the central part of the mountain in the no-terrain (control) cases. Relative to the control case, a pair of terrain-induced gyres is found in all with-terrain experiments. This pair of gyres rotates cyclonically around the TC and the gyre-associated flow near the TC centre causes a northward deflection of the TC track prior to landfall. An examination of the potential vorticity tendencies (PVTs) suggests that, in addition to the PVT produced by this flow, the PVT from diabatic heating cannot be ignored in explaining the TC track during landfall. The asymmetric diabatic heating is significant, which tends to retard the fast displacement of the TC caused by horizontal advection. This diabatic heating distribution results from three different mechanisms: an incursion of dry air from the mountain, low-level convergence induced by the terrain-altered wind field in the southwestern part of the TC and the development of convergence cyclonically inward from the eastern side of the mountain to the TC core. However, the first two mechanisms are apparently missing in the Philippine experiments because of the lower height and the moister air over the mountains on Luzon Island.