Mechanism of Secondary Eyewall Formation in Tropical Cyclones Revealed by Sensitivity Experiments on the Mesoscale Descending Inflow

An eyewall replacement cycle is often seen in tropical cyclones, when a secondary eyewall forms outside the inner eyewall, and the inner eyewall disappears. Although this cycle significantly affects the intensity of tropical cyclones, the mechanisms of secondary eyewall formation (SEF) are diverse, and most are complementary. Some studies have suggested that dry air inflow and diabatic cooling may have an important role in SEF via the mesoscale descending inflow (MDI). Here, we use numerical experiments to investigate the role of the middle tropospheric dry inflow in SEF. Idealized experiments were conducted using the plane version of the Nonhydrostatic Icosahedral Atmospheric Model. The control experiment produced SEF with a dry air inflow in the middle troposphere and associated MDI. In sensitivity experiments, in which the water vapor in the middle troposphere was increased in the outer areas of the tropical cyclone, the onset of SEF was delayed. These results reveal the two distinct processes of SEF: the angular momentum transport by MDI and the unbalanced dynamics in the boundary layer.