Structural Changes Preceding the Rapid Intensification of Typhoon Lekima (2019) Under Moderate Vertical Wind Shear

Abstract

AbstractThe early development of Super Typhoon Lekima (2019) under moderate vertical wind shear (VWS) is examined on the basis of a convection‐permitting simulation with the Weather Research and Forecast model. The tropical cyclone (TC) evolves from a tilted vortex to an upright vortex with a meso‐γ‐scale high‐vorticity core and finally to a vortex with a well‐defined eyewall preceding rapid intensification. The shift of the upper‐middle‐tropospheric vortex to the upshear‐left quadrant is critical to the establishment of an upright TC vortex with high vorticity concentrated in the center. As the upper‐middle‐tropospheric vortex reaches the upshear‐left quadrant, it drifts toward the TC center under the horizontal advective effect of VWS, leading to a reduction of TC vortex tilt. The low stability and high humidity below the upper‐middle‐tropospheric vortex together with the fast increase of upper‐middle‐tropospheric vorticity promote the convection around the TC center, which is important to the maintenance of the nearly upright TC vortex and the development of the high‐vorticity core in TC vortex. The evolution of the TC from an upright vortex with a high‐vorticity core to a vortex with a well‐defined eyewall is bound with the convection near the radius of maximum azimuthal‐mean tangential velocity. The convection enhances the boundary‐layer inflow and then the supergradient force above the radially inward side of the strong boundary‐layer inflow. In this way, the convergence within and above the boundary layer is strengthened, favoring the formation of a ring of deep convection and high vorticity. Such processes are similar to those leading up to secondary eyewall formation.