Intensity and Structural Changes of Numerically Simulated Typhoon Faxai (1915) before Landfall

Abstract

 A numerical simulation for Typhoon Faxai (1915), which made landfall with the central pressure of 960 hPa in the Kanto region of Japan was conducted using a non-hydrostatic numerical model with a 1 km grid spacing. Faxai sustained a symmetric structure until the landfall and caused severe damage due to strong winds. The simulation successfully simulated the realistic track and intensity of Faxai for 48 h around landfall. The simulated intensity was strong until the time of landfall and the spatial size of the vortex was small. The structure of the simulated Faxai, identified as having an axisymmetric flow field and eyewall, was similar to that of a well-developed tropical cyclone (TC) in the tropics. Around the TC center, the surface latent heat flux was over 300 W m⁻² until landfall, and the vertical wind shear was less than 9 m s⁻¹ between the 1.5 and 12.0 km altitudes, which is relatively weak at midlatitudes. The maximum potential intensity (MPI) was calculated using environmental parameters around the simulated TC. The simulated and best track TC intensities exceeded the MPI for approximately 12 h before landfall, that is, the TC was in a superintense state. The superintensity was mainly caused by the presence of supergradient wind, which in turn resulted from the strong intensity and axisymmetric structure of the typhoon. The simulated TC satisfies the assumptions for the formulation of MPI during the quasi-steady state except for the gradient wind balance, implying that the structure of TC is similar to that of a developed TC in the tropics. The present analyses suggest that the strong intensity of Faxai results from favorable environmental conditions and vortex structure.