The Role of Outflow‐Layer Inertial Stability in Governing the Radial Location of Secondary Eyewall Formation in Tropical Cyclones

Abstract Secondary eyewall formation (SEF) in tropical cyclones (TCs) emerges from a complex interplay of internal dynamics and environmental influences. Motivated by observations linking low inertial stability in the TC outflow layer to eyewall replacement cycles, we investigate how variations in outflow‐layer inertial stability control both the initiation and radial position of SEF. Idealized simulations reveal that reduced outflow‐layer inertial stability enhances upper‐level divergence and updraft in the TC outer core, fostering the growth of stratiform rainbands. By averaging secondary circulation over the domain grids featuring stratiform precipitation, it is explicitly shown that the strength of the mesoscale descending inflow (MDI) is greater within the widespread and more developed stratiform clouds. Such stratiform‐induced MDI can dynamically and thermodynamically broaden the tangential wind field in the lower altitudes. As a result, the ensuing increase in boundary‐layer inertial stability and inflow supplies greater absolute vorticity influx in the outer‐core region, making the tangential wind tendency peaks and the secondary eyewall intensifies at a larger radius. This study highlights the role of MDI in the coupling between the upper‐ and lower‐tropospheric dynamics.