Numerical Simulation of Tornadoes Generated by a Typhoon near a Coastal Line

Tropical cyclones (hereafter, TC) as typhoons or hurricanes frequently spawn tornadoes. McCaul (1991) reported that approximately 59% of hurricanes that hit the United States during the period of 1948-86 produced at least one tornado. Most of the reported tornades are produced within two areas of TCs: near the core of the storm and in the outer rainbands (Gentry 1983). McCaul (1991) noted that shear and helicity are most favorable for tornadoes in the right-front quadrant. Nearly all hurricane-induced tornadoes occur on land within 200 km of the coast (Nolvan and Gray 1974), because surface wind speeds over land areas were reduced by friction while the flow at slightly higher levels remained strong and was not affected by friction (Gentry 1983). Spratt et al. (1997) used the observation by the WSR88D and revealed that many of the tornadoes are related to intense, persistent cells within the outer rainbands. McCaul and Weisman (1996) simulated shallow supercell storms in landfalling hurricane environments and found that the storm’s updrafts were produced by the strong upward dynamic pressure gradient force and relatively weak buoyancy and the vertical vorticity of the mesocyclone was generated by tilting and stretching of the horizontal vorticity, which was generated by the strong vertical shear. TC-spawned tornadoes, however, have not understood completely because of the lack of three-dimensional data such as multiple Doppler radars observations. It is important for the comprehension of TC-spawned tornadoes to obtain high-resolution threedimensional data by multiple Doppler radars or realistic numerical simulations.