A numerical study of Typhoon Chanchu (0601): The inner core structure evolution and intensity changes around its northward turn

In this study, the innercore evolution and intensity changes of the severe typhoon Chanchu (0601) was analysed using the FY satellite TBB images and the new generation mesoscale weather research and forecasting (WRF) model. This case experienced a sharp turn from a westward to a northward component motion with an end of its intensification. The TBB images showed that the innercore structures of Chanchu become asymmetric after the northward turn. The WRF model was used to simulate Chanchu at high resolution (3 km spacing) with 180 h integration performed. The intensity and track in simulation are close to the observed, and the model reproduces its innercore structure evolution well. The weak wind shear favored rapid intensification before its sharp northward turn. The wavenumber1 asymmetry was dominant with a downshearleft pattern of the innercore emerged as the wind shear was at its peak just after the northward turn. Its intensification then stopped under the negative impact of increasing shear. Fourier decomposition of the reflectivity from the model outputs revealed that the azimuthal propagation speeds of the wavenumber2 asymmetry of the eyewall and inner spiral rainbands were consistent with those from the vortex Rossby waves (VRWs) theory. The VRWs features were used to explore the possible mechanism for the rapid intensification break and intensity maintenance. The analysis shows that the eyewall experienced a breakdown while the VRWs in the eyewall were perturbed by the outer spiral rainbands and vortex tilt occurred when northward turning. The equivalent potential temperature of the inversion level in the eye increased significantly at this time by growing horizontal mixing, which separated the warm, dry air above, from cooler, moist air below, leading to the reduction of the warm, dry air via transportation by the downdrafts along the inner edge of eyewall. As a result, the low level warming weakened and the rapid intensification stopped. And the inward transport of high vorticity, from the eyewall toward the eye owing to the VRWs radial inward propogation, caused an increase in the tangential winds within the radius of maximum wind (RMW), resulting in the RMW to extend inward and the contraction of eyewall. This feature compensated the negative impact of increased shear to some extent and remained the intensity.