Roles of Upper-Level Descending Inflow in Moat Development in Simulated Tropical Cyclones with Secondary Eyewall Formation

This study investigated the effects of upper-level descending inflow (ULDI) associated with inner-eyewall convection on the formation of the moat in tropical cyclones (TCs) with secondary eyewall formation (SEF). In our numerical experiments, a clear moat with SEF occurred in TCs with a significant ULDI, while no SEF occurred in TCs without a significant ULDI. The eyewall convection developed more vigorously in the control run. A ULDI occurred outside the inner-eyewall convection, where it was symmetrically unstable. The ULDI was initially triggered by the diabatic warming released by the inner eyewall and later enhanced by the cooling below the anvil cloud. The ULDI penetrated the outer edge of the inner eyewall with relatively dry air and prevented excessive solid-phase hydrometeors from being advected further outward. It produced extensive sublimation cooling of falling hydrometeors between the eyewall and the outer convection. The sublimation cooling resulted in negative buoyancy and further induced strong subsidence between the eyewall and the outer convection. As a result, a clear moat was generated. Development of the moat in the ongoing SEF prevented the outer rainband from moving farther inward, helping the outer rainband to symmetrize into an outer eyewall. In the sensitivity experiment, no significant ULDI formed since the eyewall convection was weaker, and the eyewall anvil developed relatively lower, meaning the formation of a moat and thus an outer eyewall was less likely. This study suggests that a better-represented simulation of inner-eyewall convective structures and distribution of the solid-phase hydrometeors is important to the prediction of SEF.