Intensity and Structural Changes and Their Relationship during Rapid Intensification of Tropical Cyclone

Rapid intensification (RI) is an important process in tropical cyclone (TC) evolution. Based on a successful convection-permitting simulation with the Weather Research and Forecast model, the RI of super typhoon Lekima (2019) and the subsequent formation of stationary rainband complex (SBC) are investigated in this work. It is found that, during the RI, the minimum sea-level pressure experiences fast decrease persistently while the maximum axisymmetric tangential velocity strengthens rapidly first (stage I) and then exhibits a comparatively slower increase (stage II). In stage I, both the vertical advection of tangential momentum and the radial absolute vorticity flux enhances above the boundary layer. However, the latter is stronger than the later, leading to the fast increase of tangential velocity. As a contrast, these two terms only fluctuate and the value differences between them are trivial in stage II, resulting the slower enhancement of tangential velocity.

The fast intensification and associated structural changes in stage I are critical to the formation of SBC and subsequently secondary eyewall in Lekima. As the TC intensifies rapidly, the wide and loose eyewall-related rings of vorticity and convection turn to be compact and tight at the end of stage I. Both the deep convection and large vorticity concentrate in narrow annulus. The tight vorticity ring on the one hand favors the development of strong asymmetric perturbations in the inner core via the shear instability processes, and on the one hand, facilitates the radial propagation of vortex Rossby wave (VRW) via producing a beta-skirt with distinct radial vorticity gradient. The quasi-periodic outward propagation of VRW from the eyewall helps the maintenance of intermediate rainbands primarily locating in the downshear-right quadrant in stage II. Meanwhile, a distant rainband originated from the shear-left quadrant reinvigorates after it propagates to the downshear-right quadrant, where huge convective available potential energy has been accumulated in stage I. Due to the convection in the intermediate rainbands, there are evident upper-level inertial instability appearing on the inward side of the distant rainband. Under the effect of upper-level inertial instability and the low-level airflow with low equivalent potential temperature on the outward side, the distant rainband moves inwards and finally merges with the intermediate rainbands, leading to the formation of SBC and finally the secondary eyewall in Lekima.