Evolution of the Moat Associated with the Secondary Eyewall Formation in a Simulated Tropical Cyclone

Previous studies have focused on the formation and maintenance of spiral rainbands in the secondary eyewall formation (SEF) of tropical cyclones (TCs). However, the evolution of the moat, a region with weak precipitation separating spiral rainbands from the inner eyewall, is also essential for the SEF. In this study, a semi-idealized numerical experiment is conducted to understand the SEF by focusing on the evolution of the moat. In the simulated TC, a secondary eyewall forms around 32 h, and then intensifies and replaces the inner eyewall at 46 h. It is found that the occurrence and subsequent evolution of the moat in the simulated TC are closely associated with the inner-eyewall structure. As the eyewall updraft becomes strong and the eyewall anvil is well developed, the upper-level inflow develops below the eyewall anvil in response to the diabatic warming in the eyewall anvil. The warming-induced inflow causes a drying effect and promotes the sublimation cooling below the anvil, inducing strong subsidence between the inner eyewall and the spiral rainband through the resulting negative buoyancy. Moreover, the resulting subsidence is enhanced by the compensated downward motion in the outer edge of the inner eyewall. Further analysis indicates that the rapidly decreasing vertical shear of environmental wind and the rapid filamentation zone outside the inner eyewall also play important role in the axisymmetrization of the rainband and the moat subsidence. Our results demonstrate that an intense inner eyewall with a wide upper-level anvil is favorable for the SEF in an environment with decreasing vertical wind shear.