The Roles of Moat Width and Outer Eyewall Contraction in Affecting the Timescale of Eyewall Replacement Cycle

The timescale of eyewall replacement cycle (ERC) is critical for the prediction of intensity and structure changes of tropical cyclones (TCs) with concentric eyewall (CE) structures. Previous studies have indicated that the moat width can regulate the interaction between the inner and outer eyewalls and has a salient relationship with the ERC timescale. In this study, a series of sensitivity experiments are carried out to investigate the essential mechanisms resulting in the diversity of the duration of CEs using both simple and full-physics models. Results reveal that a larger moat can induce stronger inflow under the same inner eyewall intensity by providing a longer distance for air parcels to accelerate in the boundary layer. Thus, there is greater inward absolute vorticity flux to sustain the inner eyewall. Besides, the equivalent potential temperature (theta e) budget indicates that the vertical advection and surface flux of moist entropy can overbalance the negative contribution from the horizontal advection and lead to an increasing trend of theta e in the inner eyewall. This suggests that the thermodynamic process in the boundary layer is not indispensable to the inner eyewall weakening. It is also found that the contraction rate of the secondary eyewall, which directly influences the moat width, is subject to the activity of outer spiral rainbands. By directly introducing positive wind tendency outside the eyewall and indirectly promoting a vertically tilted eyewall structure, active convection in the outer region will impede or even suspend the contraction of the outer eyewall and hence extend the ERC timescale. The eyewall replacement cycle (ERC), during which the inner eyewall gradually fades away and is replaced by the outer eyewall ultimately, can impose significant influences on the intensity and structure of tropical cyclones. However, few efforts have been devoted to understanding the timescale of the ERC. The idealized simulations of this study highlight the essential role of the moat width (the radial region between the inner and outer eyewall containing suppressed convection) in the decay rate of the inner eyewall. Results show that a wider moat can provide longer traveling distance for air parcels in the boundary layer to obtain larger radial velocity and to bring greater flux of angular momentum inward, which helps the inner eyewall persist. Besides, stronger diabatic heating rates of outer spiral rainbands tend to slow down the contraction of the outer eyewall by introducing larger positive wind tendency outside than inside the outer eyewall, therefore reducing the contraction rate of the outer eyewall and extending the ERC timescale. The boundary layer dynamics by which the moat width regulates the interaction between inner and outer eyewalls is demonstrated/emphasized The inner eyewall is supported by greater inward absolute vorticity flux and maintained for a longer time period when a larger moat exists Active outer rainbands can slow down the contraction of the outer eyewall and increase the timescale of the eyewall replacement cycle